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## #1 2021-12-05 18:16:20

Jai Ganesh
Registered: 2005-06-28
Posts: 46,163

### Natural Satellite

Natural Satellite

A natural satellite is in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small solar system body (or sometimes another natural satellite). While natural satellites are often colloquially referred to as moons, there is only the Moon of Earth.

In the Solar System, there are six planetary satellite systems containing 207 known natural satellites altogether. Seven objects commonly considered dwarf planets by astronomers are also known to have natural satellites: Orcus, Pluto, Haumea, Quaoar, Makemake, Gonggong, and Eris. As of September 2018, there are 334 other minor planets known to have natural satellites.

A planet usually has at least around 10,000 times the mass of any natural satellites that orbit it, with a correspondingly much larger diameter. The Earth–Moon system is the unique exception in the Solar System; at 3,474 km (2,158 miles) across, the Moon is 0.273 times the diameter of Earth and about 1/80th of it mass. The next largest ratios are the Neptune–Triton system at 0.055 (with a mass ratio of about 1 to 5000), the Saturn–Titan system at 0.044 (with the second mass ratio next to the Earth-Moon system, 1 to 4250), the Jupiter–Ganymede system at 0.038, and the Uranus–Titania system at 0.031. For the category of dwarf planets, Charon has the largest ratio, being 0.52 the diameter of Pluto.

Terminology

The first known natural satellite was the Moon, but it was considered a "planet" until Copernicus' introduction of De revolutionibus orbium coelestium in 1543. Until the discovery of the Galilean satellites in 1610 there was no opportunity for referring to such objects as a class. Galileo chose to refer to his discoveries as Planetæ ("planets"), but later discoverers chose other terms to distinguish them from the objects they orbited.

The first to use the term satellite to describe orbiting bodies was the German astronomer Johannes Kepler in his pamphlet Narratio de Observatis a se quatuor Iouis satellitibus erronibus ("Narration About Four Satellites of Jupiter Observed") in 1610. He derived the term from the Latin word satelles, meaning "guard", "attendant", or "companion", because the satellites accompanied their primary planet in their journey through the heavens.

The term satellite thus became the normal one for referring to an object orbiting a planet, as it avoided the ambiguity of "moon". In 1957, however, the launching of the artificial object Sputnik created a need for new terminology. The terms man-made satellite and artificial moon were very quickly abandoned in favor of the simpler satellite, and as a consequence, the term has become linked primarily with artificial objects flown in space – including, sometimes, even those not in orbit around a planet.

Because of this shift in meaning, the term moon, which had continued to be used in a generic sense in works of popular science and in fiction, has regained respectability and is now used interchangeably with natural satellite, even in scientific articles. When it is necessary to avoid both the ambiguity of confusion with Earth's natural satellite the Moon and the natural satellites of the other planets on the one hand, and artificial satellites on the other, the term natural satellite (using "natural" in a sense opposed to "artificial") is used. To further avoid ambiguity, the convention is to capitalize the word Moon when referring to Earth's natural satellite, but not when referring to other natural satellites.

Many authors define "satellite" or "natural satellite" as orbiting some planet or minor planet, synonymous with "moon" – by such a definition all natural satellites are moons, but Earth and other planets are not satellites. A few recent authors define "moon" as "a satellite of a planet or minor planet", and "planet" as "a satellite of a star" – such authors consider Earth as a "natural satellite of the Sun".

Definition of a moon

There is no established lower limit on what is considered a "moon". Every natural celestial body with an identified orbit around a planet of the Solar System, some as small as a kilometer across, has been considered a moon, though objects a tenth that size within Saturn's rings, which have not been directly observed, have been called moonlets. Small asteroid moons (natural satellites of asteroids), such as Dactyl, have also been called moonlets.

The upper limit is also vague. Two orbiting bodies are sometimes described as a double planet rather than primary and satellite. Asteroids such as 90 Antiope are considered double asteroids, but they have not forced a clear definition of what constitutes a moon. Some authors consider the Pluto–Charon system to be a double (dwarf) planet. The most common[citation needed] dividing line on what is considered a moon rests upon whether the barycentre is below the surface of the larger body, though this is somewhat arbitrary, because it depends on distance as well as relative mass.

Origin and orbital characteristics

The natural satellites orbiting relatively close to the planet on prograde, uninclined circular orbits (regular satellites) are generally thought to have been formed out of the same collapsing region of the protoplanetary disk that created its primary. In contrast, irregular satellites (generally orbiting on distant, inclined, eccentric and/or retrograde orbits) are thought to be captured asteroids possibly further fragmented by collisions. Most of the major natural satellites of the Solar System have regular orbits, while most of the small natural satellites have irregular orbits. The Moon[16] and possibly Charon are exceptions among large bodies in that they are thought to have originated by the collision of two large proto-planetary objects (see the giant impact hypothesis). The material that would have been placed in orbit around the central body is predicted to have reaccreted to form one or more orbiting natural satellites. As opposed to planetary-sized bodies, asteroid moons are thought to commonly form by this process. Triton is another exception; although large and in a close, circular orbit, its motion is retrograde and it is thought to be a captured dwarf planet.

Temporary satellites

The capture of an asteroid from a heliocentric orbit is not always permanent. According to simulations, temporary satellites should be a common phenomenon. The only observed examples are 1991 VG, 2006 RH120, 2020 CD3.

2006 RH120 was a temporary satellite of Earth for nine months in 2006 and 2007.

Tidal locking

Most regular moons (natural satellites following relatively close and prograde orbits with small orbital inclination and eccentricity) in the Solar System are tidally locked to their respective primaries, meaning that the same side of the natural satellite always faces its planet. This phenomenon comes about through a loss of energy due to tidal forces raised by the planet, slowing the rotation of the satellite until it is negligible. The only known exception is Saturn's natural satellite Hyperion, which rotates chaotically because of the gravitational influence of Titan.

In contrast, the outer natural satellites of the giant planets (irregular satellites) are too far away to have become locked. For example, Jupiter's Himalia, Saturn's Phoebe, and Neptune's Nereid have rotation periods in the range of ten hours, whereas their orbital periods are hundreds of days.

Satellites of satellites

No "moons of moons" or subsatellites (natural satellites that orbit a natural satellite of a planet) are currently known. In most cases, the tidal effects of the planet would make such a system unstable.

However, calculations performed after the 2008 detection of a possible ring system around Saturn's moon Rhea indicate that satellites orbiting Rhea could have stable orbits. Furthermore, the suspected rings are thought to be narrow, a phenomenon normally associated with shepherd moons. However, targeted images taken by the Cassini spacecraft failed to detect rings around Rhea.

It has also been proposed that Saturn's moon Iapetus had a satellite in the past; this is one of several hypotheses that have been put forward to account for its equatorial ridge.

Trojan satellites

Two natural satellites are known to have small companions at both their L4 and L5 Lagrangian points, sixty degrees ahead and behind the body in its orbit. These companions are called trojan moons, as their orbits are analogous to the trojan asteroids of Jupiter. The trojan moons are Telesto and Calypso, which are the leading and following companions, respectively, of the Saturnian moon Tethys; and Helene and Polydeuces, the leading and following companions of the Saturnian moon Dione.

Asteroid satellites

The discovery of 243 Ida's natural satellite Dactyl in the early 1990s confirmed that some asteroids have natural satellites; indeed, 87 Sylvia has two. Some, such as 90 Antiope, are double asteroids with two comparably sized components.

Shape

The relative masses of the natural satellites of the Solar System. Mimas, Enceladus, and Miranda are too small to be visible at this scale. All the irregularly shaped natural satellites, even added together, would also be too small to be visible.

Neptune's moon Proteus is the largest irregularly shaped natural satellite; the shapes of Eris' moon Dysnomia and Orcus' moon Vanth are unknown. All other known natural satellites that are at least the size of Uranus's Miranda have lapsed into rounded ellipsoids under hydrostatic equilibrium, i.e. are "round/rounded satellites". The larger natural satellites, being tidally locked, tend toward ovoid (egg-like) shapes: squat at their poles and with longer equatorial axes in the direction of their primaries (their planets) than in the direction of their motion. Saturn's moon Mimas, for example, has a major axis 9% greater than its polar axis and 5% greater than its other equatorial axis. Methone, another of Saturn's moons, is only around 3 km in diameter and visibly egg-shaped. The effect is smaller on the largest natural satellites, where their own gravity is greater relative to the effects of tidal distortion, especially those that orbit less massive planets or, as in the case of the Moon, at greater distances.

Name  :  Satellite of :  Difference in axes  :
km : % of mean diameter
Mimas  :  Saturn  :  33.4 (20.4 / 13.0)  :  8.4 (5.1 / 3.3)
Enceladus  :  Saturn :  16.6  :  3.3
Miranda  :  Uranus  :  14.2  :  3.0
Tethys  :  Saturn  :  25.8  :  2.4
Io  :  Jupiter  :  29.4  :  0.8
The Moon  :  Earth  :  4.3  :  0.1

Geological activity

Of the nineteen known natural satellites in the Solar System that are large enough to have lapsed into hydrostatic equilibrium, several remain geologically active today. Io is the most volcanically active body in the Solar System, while Europa, Enceladus, Titan and Triton display evidence of ongoing tectonic activity and cryovolcanism. In the first three cases, the geological activity is powered by the tidal heating resulting from having eccentric orbits close to their giant-planet primaries. (This mechanism would have also operated on Triton in the past, before its orbit was circularized.) Many other natural satellites, such as Earth's Moon, Ganymede, Tethys and Miranda, show evidence of past geological activity, resulting from energy sources such as the decay of their primordial radioisotopes, greater past orbital eccentricities (due in some cases to past orbital resonances), or the differentiation or freezing of their interiors. Enceladus and Triton both have active features resembling geysers, although in the case of Triton solar heating appears to provide the energy. Titan and Triton have significant atmospheres; Titan also has hydrocarbon lakes. Also Io and Callisto have atmospheres, even if they are extremely thin. Four of the largest natural satellites, Europa, Ganymede, Callisto, and Titan, are thought to have subsurface oceans of liquid water, while smaller Enceladus may have localized subsurface liquid water.

Natural satellites of the Solar System

Of the objects within our Solar System known to have natural satellites, there are 76 in the asteroid belt (five with two each), four Jupiter trojans, 39 near-Earth objects (two with two satellites each), and 14 Mars-crossers. There are also 84 known natural satellites of trans-Neptunian objects. Some 150 additional small bodies have been observed within the rings of Saturn, but only a few were tracked long enough to establish orbits. Planets around other stars are likely to have satellites as well, and although numerous candidates have been detected to date, none have yet been confirmed.

Of the inner planets, Mercury and Venus have no natural satellites; Earth has one large natural satellite, known as the Moon; and Mars has two tiny natural satellites, Phobos and Deimos. The giant planets have extensive systems of natural satellites, including half a dozen comparable in size to Earth's Moon: the four Galilean moons, Saturn's Titan, and Neptune's Triton. Saturn has an additional six mid-sized natural satellites massive enough to have achieved hydrostatic equilibrium, and Uranus has five. It has been suggested that some satellites may potentially harbour life.

Among the objects generally agreed by astronomers to be dwarf planets, Ceres and Sedna havve no known natural satellites. Pluto has the relatively large natural satellite Charon and four smaller natural satellites; Styx, Nix, Kerberos, and Hydra. Haumea has two natural satellites; Orcus, Quaoar, Makemake, Gonggong, and Eris have one each. The Pluto–Charon system is unusual in that the center of mass lies in open space between the two, a characteristic sometimes associated with a double-planet system.

The seven largest natural satellites in the Solar System (those bigger than 2,500 km across) are Jupiter's Galilean moons (Ganymede, Callisto, Io, and Europa), Saturn's moon Titan, Earth's moon, and Neptune's captured natural satellite Triton. Triton, the smallest of these, has more mass than all smaller natural satellites together. Similarly in the next size group of nine mid-sized natural satellites, between 1,000 km and 1,600 km across, Titania, Oberon, Rhea, Iapetus, Charon, Ariel, Umbriel, Dione, and Tethys, the smallest, Tethys, has more mass than all smaller natural satellites together. As well as the natural satellites of the various planets, there are also over 80 known natural satellites of the dwarf planets, minor planets and other small Solar System bodies. Some studies estimate that up to 15% of all trans-Neptunian objects could have satellites.

It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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## #2 2021-12-05 19:18:50

Jai Ganesh
Registered: 2005-06-28
Posts: 46,163

### Re: Natural Satellite

2) Moon

The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia), it is the largest natural satellite in the Solar System relative to the size of its planet, the fifth largest satellite in the Solar System overall, and is larger than any known dwarf planet. The Moon is a planetary-mass object that formed a differentiated rocky body, making it a satellite planet under geophysical definitions of the term. It lacks any significant atmosphere, hydrosphere, or magnetic field. Its surface gravity is about one-sixth of Earth's (0.1654 g); Jupiter's moon Io is the only satellite in the Solar System known to have a higher surface gravity and density.

Orbiting Earth at an average distance of 384,400 km (238,900 mi), or about 30 times Earth's diameter, its gravitational influence slightly lengthens Earth's day and is the main driver of Earth's tides. The Moon's orbit around Earth has a sidereal period of 27.3 days. During each synodic period of 29.5 days, the amount of visible surface illuminated by the Sun varies from none up to 100%, resulting in lunar phases that form the basis for the months of a lunar calendar. The Moon is tidally locked to Earth, which means that the length of a full rotation of the Moon on its own axis causes its same side (the near side) to always face Earth, and the somewhat longer lunar day is the same as the synodic period. That said, 59% of the total lunar surface can be seen from Earth through shifts in perspective due to libration.

The most widely accepted origin explanation posits that the Moon formed about 4.51 billion years ago, not long after Earth, out of the debris from a giant impact between the planet and a hypothesized Mars-sized body called Theia. It then receded to a wider orbit because of tidal interaction with the Earth. The near side of the Moon is marked by dark volcanic maria ("seas"), which fill the spaces between bright ancient crustal highlands and prominent impact craters. Most of the large impact basins and mare surfaces were in place by the end of the Imbrian period, some three billion years ago. The lunar surface is relatively non-reflective, with a reflectance just slightly brighter than that of worn asphalt. However, because it has a large angular diameter, the full moon is the brightest celestial object in the night sky. The Moon's apparent size is nearly the same as that of the Sun, allowing it to cover the Sun almost completely during a total solar eclipse.

Both the Moon's prominence in the earthly sky and its regular cycle of phases have provided cultural references and influences for human societies throughout history. Such influences can be found in language, calendar systems, art, and mythology. The first artificial object to reach the Moon was the Soviet Union's Luna 2 uncrewed spacecraft in 1959; this was followed by the first successful soft landing by Luna 9 in 1966. The only human lunar missions to date have been those of the United States' Apollo program, which landed twelve men on the surface between 1969 and 1972. These and later uncrewed missions returned lunar rocks that have been used to develop a detailed geological understanding of the Moon's origins, internal structure, and subsequent history.

It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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## #3 2021-12-06 18:15:40

Jai Ganesh
Registered: 2005-06-28
Posts: 46,163

### Re: Natural Satellite

3) Io

Io, or Jupiter I, is the innermost and third-largest of the four Galilean moons of the planet Jupiter. Slightly larger than the Moon, Io is the fourth-largest moon in the Solar System, has the highest density of any moon, and has the lowest amount of water (by atomic ratio) of any known astronomical object in the Solar System. It was discovered in 1610 by Galileo Galilei and was named after the mythological character Io, a priestess of Hera who became one of Zeus's lovers.

With over 400 active volcanoes, Io is the most geologically active object in the Solar System. This extreme geologic activity is the result of tidal heating from friction generated within Io's interior as it is pulled between Jupiter and the other Galilean moons—Europa, Ganymede and Callisto. Several volcanoes produce plumes of sulfur and sulfur dioxide that climb as high as 500 km (300 mi) above the surface. Io's surface is also dotted with more than 100 mountains that have been uplifted by extensive compression at the base of Io's silicate crust. Some of these peaks are taller than Mount Everest, the highest point on Earth's surface. Unlike most moons in the outer Solar System, which are mostly composed of water ice, Io is primarily composed of silicate rock surrounding a molten iron or iron sulfide core. Most of Io's surface is composed of extensive plains with a frosty coating of sulfur and sulfur dioxide.

Io's volcanism is responsible for many of its unique features. Its volcanic plumes and lava flows produce large surface changes and paint the surface in various subtle shades of yellow, red, white, black, and green, largely due to allotropes and compounds of sulfur. Numerous extensive lava flows, several more than 500 km (300 mi) in length, also mark the surface. The materials produced by this volcanism make up Io's thin, patchy atmosphere and Jupiter's extensive magnetosphere. Io's volcanic ejecta also produce a large plasma torus around Jupiter.

Io played a significant role in the development of astronomy in the 17th and 18th centuries; discovered in January 1610 by Galileo Galilei, along with the other Galilean satellites, this discovery furthered the adoption of the Copernican model of the Solar System, the development of Kepler's laws of motion, and the first measurement of the speed of light. Viewed from Earth, Io remained just a point of light until the late 19th and early 20th centuries, when it became possible to resolve its large-scale surface features, such as the dark red polar and bright equatorial regions. In 1979, the two Voyager spacecraft revealed Io to be a geologically active world, with numerous volcanic features, large mountains, and a young surface with no obvious impact craters. The Galileo spacecraft performed several close flybys in the 1990s and early 2000s, obtaining data about Io's interior structure and surface composition. These spacecraft also revealed the relationship between Io and Jupiter's magnetosphere and the existence of a belt of high-energy radiation centered on Io's orbit. Io receives about 3,600 rem (36 Sv) of ionizing radiation per day.

Further observations have been made by Cassini–Huygens in 2000, New Horizons in 2007, and Juno since 2017, as well as from Earth-based telescopes and the Hubble Space Telescope.

Orbit and rotation

Io orbits Jupiter at a distance of 421,700 km (262,000 mi) from Jupiter's center and 350,000 km (217,000 mi) from its cloudtops. It is the innermost of the Galilean satellites of Jupiter, its orbit lying between those of Thebe and Europa. Including Jupiter's inner satellites, Io is the fifth moon out from Jupiter. It takes Io about 42.5 hours to complete one orbit around Jupiter (fast enough for its motion to be observed over a single night of observation). Io is in a 2:1 mean-motion orbital resonance with Europa and a 4:1 mean-motion orbital resonance with Ganymede, completing two orbits of Jupiter for every one orbit completed by Europa, and four orbits for every one completed by Ganymede. This resonance helps maintain Io's orbital eccentricity (0.0041), which in turn provides the primary heating source for its geologic activity. Without this forced eccentricity, Io's orbit would circularize through tidal dissipation, leading to a geologically less active world.

Like the other Galilean satellites and the Moon, Io rotates synchronously with its orbital period, keeping one face nearly pointed toward Jupiter. This synchrony provides the definition for Io's longitude system. Io's prime meridian intersects the equator at the sub-Jovian point. The side of Io that always faces Jupiter is known as the subjovian hemisphere, whereas the side that always faces away is known as the antijovian hemisphere. The side of Io that always faces in the direction that Io travels in its orbit is known as the leading hemisphere, whereas the side that always faces in the opposite direction is known as the trailing hemisphere.

From the surface of Io, Jupiter would subtend an arc of 19.5°, making Jupiter appear 39 times the apparent diameter of Earth's Moon.

Geology

Io is slightly larger than Earth's Moon. It has a mean radius of 1,821.3 km (1,131.7 mi) (about 5% greater than the Moon's) and a mass of 8.9319×{10}^{22} kg (about 21% greater than the Moon's). It is a slight ellipsoid in shape, with its longest axis directed toward Jupiter. Among the Galilean satellites, in both mass and volume, Io ranks behind Ganymede and Callisto but ahead of Europa.

It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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## #4 2021-12-09 16:53:06

Jai Ganesh
Registered: 2005-06-28
Posts: 46,163

### Re: Natural Satellite

4) Europa

Europa, or Jupiter II, is the smallest of the four Galilean moons orbiting Jupiter, and the sixth-closest to the planet of all the 79 known moons of Jupiter. It is also the sixth-largest moon in the Solar System. Europa was discovered in 1610 by Galileo Galilei and was named after Europa, the Phoenician mother of King Minos of Crete and lover of Zeus (the Greek equivalent of the Roman god Jupiter).

Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and has a water-ice crust and probably an iron–nickel core. It has a very thin atmosphere, composed primarily of oxygen. Its surface is striated by cracks and streaks, but craters are relatively few. In addition to Earth-bound telescope observations, Europa has been examined by a succession of space-probe flybys, the first occurring in the early 1970s.

Europa has the smoothest surface of any known solid object in the Solar System. The apparent youth and smoothness of the surface have led to the hypothesis that a water ocean exists beneath the surface, which could conceivably harbor extraterrestrial life. The predominant model suggests that heat from tidal flexing causes the ocean to remain liquid and drives ice movement similar to plate tectonics, absorbing chemicals from the surface into the ocean below. Sea salt from a subsurface ocean may be coating some geological features on Europa, suggesting that the ocean is interacting with the sea floor. This may be important in determining whether Europa could be habitable. In addition, the Hubble Space Telescope detected water vapor plumes similar to those observed on Saturn's moon Enceladus, which are thought to be caused by erupting cryogeysers. In May 2018, astronomers provided supporting evidence of water plume activity on Europa, based on an updated analysis of data obtained from the Galileo space probe, which orbited Jupiter from 1995 to 2003. Such plume activity could help researchers in a search for life from the subsurface Europan ocean without having to land on the moon.

The Galileo mission, launched in 1989, provides the bulk of current data on Europa. No spacecraft has yet landed on Europa, although there have been several proposed exploration missions. The European Space Agency's Jupiter Icy Moon Explorer (JUICE) is a mission to Ganymede that is due to launch in 2023 and will include two flybys of Europa. NASA's planned Europa Clipper should be launched in 2024.

Orbit and rotation

Europa orbits Jupiter in just over three and a half days, with an orbital radius of about 670,900 km. With an orbital eccentricity of only 0.009, the orbit itself is nearly circular, and the orbital inclination relative to Jupiter's equatorial plane is small, at 0.470°. Like its fellow Galilean satellites, Europa is tidally locked to Jupiter, with one hemisphere of Europa constantly facing Jupiter. Because of this, there is a sub-Jovian point on Europa's surface, from which Jupiter would appear to hang directly overhead. Europa's prime meridian is a line passing through this point. Research suggests that the tidal locking may not be full, as a non-synchronous rotation has been proposed: Europa spins faster than it orbits, or at least did so in the past. This suggests an asymmetry in internal mass distribution and that a layer of subsurface liquid separates the icy crust from the rocky interior.

The slight eccentricity of Europa's orbit, maintained by the gravitational disturbances from the other Galileans, causes Europa's sub-Jovian point to oscillate around a mean position. As Europa comes slightly nearer to Jupiter, Jupiter's gravitational attraction increases, causing Europa to elongate towards and away from it. As Europa moves slightly away from Jupiter, Jupiter's gravitational force decreases, causing Europa to relax back into a more spherical shape, and creating tides in its ocean. The orbital eccentricity of Europa is continuously pumped by its mean-motion resonance with Io. Thus, the tidal flexing kneads Europa's interior and gives it a source of heat, possibly allowing its ocean to stay liquid while driving subsurface geological processes. The ultimate source of this energy is Jupiter's rotation, which is tapped by Io through the tides it raises on Jupiter and is transferred to Europa and Ganymede by the orbital resonance.

Analysis of the unique cracks lining Europa yielded evidence that it likely spun around a tilted axis at some point in time. If correct, this would explain many of Europa's features. Europa's immense network of crisscrossing cracks serves as a record of the stresses caused by massive tides in its global ocean. Europa's tilt could influence calculations of how much of its history is recorded in its frozen shell, how much heat is generated by tides in its ocean, and even how long the ocean has been liquid. Its ice layer must stretch to accommodate these changes. When there is too much stress, it cracks. A tilt in Europa's axis could suggest that its cracks may be much more recent than previously thought. The reason for this is that the direction of the spin pole may change by as much as a few degrees per day, completing one precession period over several months. A tilt could also affect the estimates of the age of Europa's ocean. Tidal forces are thought to generate the heat that keeps Europa's ocean liquid, and a tilt in the spin axis would cause more heat to be generated by tidal forces. Such additional heat would have allowed the ocean to remain liquid for a longer time. However, it has not yet been determined when this hypothesized shift in the spin axis might have occurred.

Physical characteristics

Europa is slightly smaller than the Moon. At just over 3,100 kilometres (1,900 mi) in diameter, it is the sixth-largest moon and fifteenth-largest object in the Solar System. Though by a wide margin the least massive of the Galilean satellites, it is nonetheless more massive than all known moons in the Solar System smaller than itself combined. Its bulk density suggests that it is similar in composition to the terrestrial planets, being primarily composed of silicate rock.

Internal structure

It is estimated that Europa has an outer layer of water around 100 km (62 mi) thick; a part frozen as its crust, and a part as a liquid ocean underneath the ice. Recent magnetic-field data from the Galileo orbiter showed that Europa has an induced magnetic field through interaction with Jupiter's, which suggests the presence of a subsurface conductive layer. This layer is likely to be a salty liquid-water ocean. Portions of the crust are estimated to have undergone a rotation of nearly 80°, nearly flipping over (see true polar wander), which would be unlikely if the ice were solidly attached to the mantle. Europa probably contains a metallic iron core.

Surface features

Europa is the smoothest known object in the Solar System, lacking large-scale features such as mountains and craters. However, according to one study, Europa's equator may be covered in icy spikes called penitentes, which may be up to 15 meters high, due to direct overhead sunlight on the equator, causing the ice to sublime, forming vertical cracks. Although the imaging available from the Galileo orbiter does not have the resolution needed to confirm this, radar and thermal data are consistent with this interpretation. The prominent markings crisscrossing Europa appear to be mainly albedo features that emphasize low topography. There are few craters on Europa, because its surface is tectonically too active and therefore young. Europa's icy crust has an albedo (light reflectivity) of 0.64, one of the highest of all moons. This indicates a young and active surface: based on estimates of the frequency of cometary bombardment that Europa experiences, the surface is about 20 to 180 million years old. There is currently no full scientific consensus among the sometimes contradictory explanations for the surface features of Europa.

The radiation level at the surface of Europa is equivalent to a dose of about 5400 mSv (540 rem) per day, an amount of radiation that would cause severe illness or death in human beings exposed for a single Earth-day (24 hours). The duration of a Europan day is approximately 3.5 times that of a day on Earth, resulting in 3.5 times bigger radiation exposure.

It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

Online

## #5 2021-12-10 22:14:47

Jai Ganesh
Registered: 2005-06-28
Posts: 46,163

### Re: Natural Satellite

5) Ganymede

Ganymede, a satellite of Jupiter (Jupiter III), is the largest and most massive of the Solar System's moons. The ninth-largest object (including the Sun) of the Solar System, it is the largest without a substantial atmosphere. It has a diameter of 5,268 km (3,273 mi), making it 26% larger than the planet Mercury by volume, although it is only 45% as massive. Possessing a metallic core, it has the lowest moment of inertia factor of any solid body in the Solar System and is the only moon known to have a magnetic field. Outward from Jupiter, it is the seventh satellite and the third of the Galilean moons, the first group of objects discovered orbiting another planet. Ganymede orbits Jupiter in roughly seven days and is in a 1:2:4 orbital resonance with the moons Europa and Io, respectively.

Ganymede is composed of approximately equal amounts of silicate rock and water. It is a fully differentiated body with an iron-rich, liquid core, and an internal ocean that may contain more water than all of Earth's oceans combined. Its surface is composed of two main types of terrain. Dark regions, saturated with impact craters and dated to four billion years ago, cover about a third of it. Lighter regions, crosscut by extensive grooves and ridges and only slightly less ancient, cover the remainder. The cause of the light terrain's disrupted geology is not fully known, but was likely the result of tectonic activity due to tidal heating.

Ganymede's magnetic field is probably created by convection within its liquid iron core, also created by Jupiter's tidal forces. The meager magnetic field is buried within Jupiter's far larger magnetic field and would show only as a local perturbation of the field lines. Ganymede has a thin oxygen atmosphere that includes O, O2, and possibly O3 (ozone). Atomic hydrogen is a minor atmospheric constituent. Whether Ganymede has an ionosphere associated with its atmosphere is unresolved.

Ganymede's discovery is credited to Galileo Galilei, the first to observe it, on January 7, 1610. Its name was soon suggested by astronomer Simon Marius, after the mythological Ganymede, a Trojan prince desired by Zeus (the Greek counterpart of Jupiter), who carried him off to be the cupbearer of the gods. Beginning with Pioneer 10, several spacecraft have explored Ganymede. The Voyager probes, Voyager 1 and Voyager 2, refined measurements of its size, while Galileo discovered its underground ocean and magnetic field. The next planned mission to the Jovian system is the European Space Agency's Jupiter Icy Moon Explorer (JUICE), due to launch in 2023. After flybys of all three icy Galilean moons, it is planned to enter orbit around Ganymede.

Orbit and rotation

Ganymede orbits Jupiter at a distance of 1,070,400 km, third among the Galilean satellites, and completes a revolution every seven days and three hours. Like most known moons, Ganymede is tidally locked, with one side always facing toward the planet, hence its day is also seven days and three hours. Its orbit is very slightly eccentric and inclined to the Jovian equator, with the eccentricity and inclination changing quasi-periodically due to solar and planetary gravitational perturbations on a timescale of centuries. The ranges of change are 0.0009–0.0022 and 0.05–0.32°, respectively. These orbital variations cause the axial tilt (the angle between rotational and orbital axes) to vary between 0 and 0.33°.

Ganymede participates in orbital resonances with Europa and Io: for every orbit of Ganymede, Europa orbits twice and Io orbits four times. Conjunctions (alignment on the same side of Jupiter) between Io and Europa occur when Io is at periapsis and Europa at apoapsis. Conjunctions between Europa and Ganymede occur when Europa is at periapsis. The longitudes of the Io–Europa and Europa–Ganymede conjunctions change with the same rate, making triple conjunctions impossible. Such a complicated resonance is called the Laplace resonance. The current Laplace resonance is unable to pump the orbital eccentricity of Ganymede to a higher value. The value of about 0.0013 is probably a remnant from a previous epoch, when such pumping was possible. The Ganymedian orbital eccentricity is somewhat puzzling; if it is not pumped now it should have decayed long ago due to the tidal dissipation in the interior of Ganymede. This means that the last episode of the eccentricity excitation happened only several hundred million years ago. Because Ganymede's orbital eccentricity is relatively low—on average 0.0015—tidal heating is negligible now. However, in the past Ganymede may have passed through one or more Laplace-like resonances that were able to pump the orbital eccentricity to a value as high as 0.01–0.02. This probably caused a significant tidal heating of the interior of Ganymede; the formation of the grooved terrain may be a result of one or more heating episodes.

There are two hypotheses for the origin of the Laplace resonance among Io, Europa, and Ganymede: that it is primordial and has existed from the beginning of the Solar System; or that it developed after the formation of the Solar System. A possible sequence of events for the latter scenario is as follows: Io raised tides on Jupiter, causing Io's orbit to expand (due to conservation of momentum) until it encountered the 2:1 resonance with Europa; after that the expansion continued, but some of the angular moment was transferred to Europa as the resonance caused its orbit to expand as well; the process continued until Europa encountered the 2:1 resonance with Ganymede. Eventually the drift rates of conjunctions between all three moons were synchronized and locked in the Laplace resonance.

Size

Ganymede is the largest and most massive moon in the Solar System. Its diameter of 5,268 km is 0.41 times that of Earth, 0.77 times that of Mars, 1.02 times that of Saturn's Titan (Solar System's second largest moon), 1.08 times Mercury's, 1.09 times Callisto's, 1.45 times Io's and 1.51 times the Moon's. Its mass is 10% greater than Titan's, 38% greater than Callisto's, 66% greater than Io's and 2.02 times that of the Moon.

It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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## #6 2021-12-18 19:54:45

Jai Ganesh
Registered: 2005-06-28
Posts: 46,163

### Re: Natural Satellite

6) Callisto

Callisto, or Jupiter IV, is the second-largest moon of Jupiter, after Ganymede. It is the third-largest moon in the Solar System after Ganymede and Saturn's largest moon Titan, and the largest object in the Solar System that may not be properly differentiated. Callisto was discovered in 1610 by Galileo Galilei. At 4821 km in diameter , Callisto has about 99% the diameter of the planet Mercury but only about a third of its mass. It is the fourth Galilean moon of Jupiter by distance, with an orbital radius of about 1883000 km. It is not in an orbital resonance like the three other Galilean satellites—Io, Europa, and Ganymede—and is thus not appreciably tidally heated. Callisto's rotation is tidally locked to its orbit around Jupiter, so that the same hemisphere always faces inward. Because of this, there is a sub-Jovian point on Callisto's surface, from which Jupiter would appear to hang directly overhead. It is less affected by Jupiter's magnetosphere than the other inner satellites because of its more remote orbit, located just outside Jupiter's main radiation belt.

Callisto is composed of approximately equal amounts of rock and ices, with a density of about 1.83 g/cc, the lowest density and surface gravity of Jupiter's major moons. Compounds detected spectroscopically on the surface include water ice, carbon dioxide, silicates, and organic compounds. Investigation by the Galileo spacecraft revealed that Callisto may have a small silicate core and possibly a subsurface ocean of liquid water at depths greater than 100 km.

The surface of Callisto is the oldest and most heavily cratered in the Solar System. Its surface is completely covered with impact craters. It does not show any signatures of subsurface processes such as plate tectonics or volcanism, with no signs that geological activity in general has ever occurred, and is thought to have evolved predominantly under the influence of impacts. Prominent surface features include multi-ring structures, variously shaped impact craters, and chains of craters (catenae) and associated scarps, ridges and deposits. At a small scale, the surface is varied and made up of small, sparkly frost deposits at the tips of high spots, surrounded by a low-lying, smooth blanket of dark material. This is thought to result from the sublimation-driven degradation of small landforms, which is supported by the general deficit of small impact craters and the presence of numerous small knobs, considered to be their remnants. The absolute ages of the landforms are not known.

Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide and probably molecular oxygen, as well as by a rather intense ionosphere. Callisto is thought to have formed by slow accretion from the disk of the gas and dust that surrounded Jupiter after its formation. Callisto's gradual accretion and the lack of tidal heating meant that not enough heat was available for rapid differentiation. The slow convection in the interior of Callisto, which commenced soon after formation, led to partial differentiation and possibly to the formation of a subsurface ocean at a depth of 100–150 km and a small, rocky core.

The likely presence of an ocean within Callisto leaves open the possibility that it could harbor life. However, conditions are thought to be less favorable than on nearby Europa. Various space probes from Pioneers 10 and 11 to Galileo and Cassini have studied Callisto. Because of its low radiation levels, Callisto has long been considered the most suitable place for a human base for future exploration of the Jovian system.

Discovery

Callisto was discovered by Galileo in January 1610, along with the three other large Jovian moons—Ganymede, Io, and Europa.

Orbit and rotation

Callisto is the outermost of the four Galilean moons of Jupiter. It orbits at a distance of approximately 1 880 000 km (26.3 times the 71 492 km radius of Jupiter itself). This is significantly larger than the orbital radius—1 070 000 km—of the next-closest Galilean satellite, Ganymede. As a result of this relatively distant orbit, Callisto does not participate in the mean-motion resonance—in which the three inner Galilean satellites are locked—and probably never has.

Like most other regular planetary moons, Callisto's rotation is locked to be synchronous with its orbit. The length of Callisto's day, simultaneously its orbital period, is about 16.7 Earth days. Its orbit is very slightly eccentric and inclined to the Jovian equator, with the eccentricity and inclination changing quasi-periodically due to solar and planetary gravitational perturbations on a timescale of centuries. The ranges of change are 0.0072–0.0076 and 0.20–0.60°, respectively. These orbital variations cause the axial tilt (the angle between rotational and orbital axes) to vary between 0.4 and 1.6°.

The dynamical isolation of Callisto means that it has never been appreciably tidally heated, which has important consequences for its internal structure and evolution. Its distance from Jupiter also means that the charged-particle flux from Jupiter's magnetosphere at its surface is relatively low—about 300 times lower than, for example, that at Europa. Hence, unlike the other Galilean moons, charged-particle irradiation has had a relatively minor effect on Callisto's surface. The radiation level at Callisto's surface is equivalent to a dose of about 0.01 rem (0.1 mSv) per day, which is over ten times higher than Earth's average background radiation.

Composition

The average density of Callisto, 1.83 g/cc, suggests a composition of approximately equal parts of rocky material and water ice, with some additional volatile ices such as ammonia. The mass fraction of ices is 49–55%. The exact composition of Callisto's rock component is not known, but is probably close to the composition of L/LL type ordinary chondrites, which are characterized by less total iron, less metallic iron and more iron oxide than H chondrites. The weight ratio of iron to silicon is 0.9–1.3 in Callisto, whereas the solar ratio is around 1:8.

Callisto's surface has an albedo of about 20%. Its surface composition is thought to be broadly similar to its composition as a whole. Near-infrared spectroscopy has revealed the presence of water ice absorption bands at wavelengths of 1.04, 1.25, 1.5, 2.0 and 3.0 micrometers. Water ice seems to be ubiquitous on the surface of Callisto, with a mass fraction of 25–50%. The analysis of high-resolution, near-infrared and UV spectra obtained by the Galileo spacecraft and from the ground has revealed various non-ice materials: magnesium- and iron-bearing hydrated silicates, carbon dioxide, sulfur dioxide, and possibly ammonia and various organic compounds. Spectral data indicate that Callisto's surface is extremely heterogeneous at the small scale. Small, bright patches of pure water ice are intermixed with patches of a rock–ice mixture and extended dark areas made of a non-ice material.

The Callistoan surface is asymmetric: the leading hemisphere is darker than the trailing one. This is different from other Galilean satellites, where the reverse is true. The trailing hemisphere of Callisto appears to be enriched in carbon dioxide, whereas the leading hemisphere has more sulfur dioxide. Many fresh impact craters like Lofn also show enrichment in carbon dioxide. Overall, the chemical composition of the surface, especially in the dark areas, may be close to that seen on D-type asteroids, whose surfaces are made of carbonaceous material.

It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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## #7 2022-01-01 14:58:48

Jai Ganesh
Registered: 2005-06-28
Posts: 46,163

### Re: Natural Satellite

7) Titan

Titan is the largest moon of Saturn and the second-largest natural satellite in the Solar System. It is the only moon known to have a dense atmosphere, and the only known moon or planet other than Earth on which clear evidence of stable bodies of surface liquid has been found.

Titan is one of seven gravitationally rounded moons in orbit around Saturn, and the second most distant from Saturn of those seven. Frequently described as a planet-like moon, Titan is 50% larger (in diameter) than Earth's Moon and 80% more massive. It is the second-largest moon in the Solar System after Jupiter's moon Ganymede, and is larger than the planet Mercury, but only 40% as massive.

Discovered in 1655 by the Dutch astronomer Christiaan Huygens, Titan was the first known moon of Saturn, and the sixth known planetary satellite (after Earth's moon and the four Galilean moons of Jupiter). Titan orbits Saturn at 20 Saturn radii. From Titan's surface, Saturn subtends an arc of 5.09 degrees and, were it visible through the moon's thick atmosphere, would appear 11.4 times larger in the sky than the Moon from Earth.

Titan is primarily composed of ice and rocky material, which is likely differentiated into a rocky core surrounded by various layers of ice, including a crust of ice Ih and a subsurface layer of ammonia-rich liquid water. Much as with Venus before the Space Age, the dense opaque atmosphere prevented understanding of Titan's surface until the Cassini–Huygens mission in 2004 provided new information, including the discovery of liquid hydrocarbon lakes in Titan's polar regions. The geologically young surface is generally smooth, with few impact craters, although mountains and several possible cryovolcanoes have been found.

The atmosphere of Titan is largely nitrogen; minor components lead to the formation of methane and ethane clouds and heavy organonitrogen haze. The climate—including wind and rain—creates surface features similar to those of Earth, such as dunes, rivers, lakes, seas (probably of liquid methane and ethane), and deltas, and is dominated by seasonal weather patterns as on Earth. With its liquids (both surface and subsurface) and robust nitrogen atmosphere, Titan's methane cycle bears a striking similarity to Earth's water cycle, albeit at the much lower temperature of about 94 K (−179.2 °C; −290.5 °F).

Discovery

Titan was discovered on March 25, 1655, by the Dutch astronomer Christiaan Huygens. Huygens was inspired by Galileo's discovery of Jupiter's four largest moons in 1610 and his improvements in telescope technology. Christiaan, with the help of his elder brother Constantijn Huygens, Jr., began building telescopes around 1650 and discovered the first observed moon orbiting Saturn with one of the telescopes they built. It was the sixth moon ever discovered, after Earth's Moon and the Galilean moons of Jupiter.

Orbit and rotation

Titan orbits Saturn once every 15 days 22 hours. Like Earth's Moon and many of the satellites of the giant planets, its rotational period (its day) is identical to its orbital period; Titan is tidally locked in synchronous rotation with Saturn, and permanently shows one face to the planet. Longitudes on Titan are measured westward, starting from the meridian passing through this point. Its orbital eccentricity is 0.0288, and the orbital plane is inclined 0.348 degrees relative to the Saturnian equator. Viewed from Earth, Titan reaches an angular distance of about 20 Saturn radii (just over 1,200,000 kilometers (750,000 mi)) from Saturn and subtends a disk 0.8 arcseconds in diameter.

The small, irregularly shaped satellite Hyperion is locked in a 3:4 orbital resonance with Titan. A "slow and smooth" evolution of the resonance—in which Hyperion migrated from a chaotic orbit—is considered unlikely, based on models. Hyperion probably formed in a stable orbital island, whereas the massive Titan absorbed or ejected bodies that made close approaches.

Bulk characteristics

Titan is 5,149.46 kilometers (3,199.73 mi) in diameter, 1.06 times that of the planet Mercury, 1.48 that of the Moon, and 0.40 that of Earth. Before the arrival of Voyager 1 in 1980, Titan was thought to be slightly larger than Ganymede (diameter 5,262 kilometers (3,270 mi)) and thus the largest moon in the Solar System; this was an overestimation caused by Titan's dense, opaque atmosphere, with a haze layer 100-200 kilometres above its surface. This increases its apparent diameter. Titan's diameter and mass (and thus its density) are similar to those of the Jovian moons Ganymede and Callisto. Based on its bulk density of 1.88 g/cc, Titan's composition is half ice and half rocky material. Though similar in composition to Dione and Enceladus, it is denser due to gravitational compression. It has a mass 1/4226 that of Saturn, making it the largest moon of the gas giants relative to the mass of its primary. It is second in terms of relative diameter of moons to a gas giant; Titan being 1/22.609 of Saturn's diameter, Triton is larger in diameter relative to Neptune at 1/18.092.

Titan is probably partially differentiated into distinct layers with a 3,400-kilometer (2,100 mi) rocky center. This rocky center is surrounded by several layers composed of different crystalline forms of ice. Its interior may still be hot enough for a liquid layer consisting of a "magma" composed of water and ammonia between the ice Ih crust and deeper ice layers made of high-pressure forms of ice. The presence of ammonia allows water to remain liquid even at a temperature as low as 176 K (−97 °C) (for eutectic mixture with water). The Cassini probe discovered the evidence for the layered structure in the form of natural extremely-low-frequency radio waves in Titan's atmosphere. Titan's surface is thought to be a poor reflector of extremely-low-frequency radio waves, so they may instead be reflecting off the liquid–ice boundary of a subsurface ocean. Surface features were observed by the Cassini spacecraft to systematically shift by up to 30 kilometers (19 mi) between October 2005 and May 2007, which suggests that the crust is decoupled from the interior, and provides additional evidence for an interior liquid layer. Further supporting evidence for a liquid layer and ice shell decoupled from the solid core comes from the way the gravity field varies as Titan orbits Saturn. Comparison of the gravity field with the RADAR-based topography observations also suggests that the ice shell may be substantially rigid.

It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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## #8 2022-02-08 20:36:15

Jai Ganesh
Registered: 2005-06-28
Posts: 46,163

### Re: Natural Satellite

8) Tethys

Tethys, or Saturn III, is a mid-sized moon of Saturn about 1,060 km (660 mi) across. It was discovered by G. D. Cassini in 1684 and is named after the titan Tethys of Greek mythology.

Tethys has a low density of 0.98 g/cc, the lowest of all the major moons in the Solar System, indicating that it is made of water ice with just a small fraction of rock. This is confirmed by the spectroscopy of its surface, which identified water ice as the dominant surface material. A small amount of an unidentified dark material is present as well. The surface of Tethys is very bright, being the second-brightest of the moons of Saturn after Enceladus, and neutral in color.

Tethys is heavily cratered and cut by a number of large faults/graben. The largest impact crater, Odysseus, is about 400 km in diameter, whereas the largest graben, Ithaca Chasma, is about 100 km wide and more than 2000 km long. These two largest surface features may be related. A small part of the surface is covered by smooth plains that may be cryovolcanic in origin. Like all other regular moons of Saturn, Tethys formed from the Saturnian sub-nebula—a disk of gas and dust that surrounded Saturn soon after its formation.

Tethys has been approached by several space probes including Pioneer 11 (1979), Voyager 1 (1980), Voyager 2 (1981), and multiple times by Cassini between 2004 and 2017.

Orbit

Tethys orbits Saturn at a distance of about 295,000 km (about 4.4 Saturn's radii) from the center of the planet. Its orbital eccentricity is negligible, and its orbital inclination is about 1°. Tethys is locked in an inclination resonance with Mimas; however, due to the low gravity of the respective bodies, this interaction does not cause any noticeable orbital eccentricity or tidal heating.

The Tethyan orbit lies deep inside the magnetosphere of Saturn, so the plasma co-rotating with the planet strikes the trailing hemisphere of the moon. Tethys is also subject to constant bombardment by the energetic particles (electrons and ions) present in the magnetosphere.

Tethys has two co-orbital moons, Telesto and Calypso orbiting near Tethys's trojan points L4 (60° ahead) and L5 (60° behind) respectively.

Physical characteristics

Tethys is the 16th-largest moon in the Solar System, with a radius of 531 km. Its mass is 6.17×{10}^{20} kg(0.000103 Earth mass), which is less than 1% of the Moon. The density of Tethys is 0.98 g/cc, indicating that it is composed almost entirely of water-ice.

It is not known whether Tethys is differentiated into a rocky core and ice mantle. However, if it is differentiated, the radius of the core does not exceed 145 km, and its mass is below 6% of the total mass. Due to the action of tidal and rotational forces, Tethys has the shape of triaxial ellipsoid. The dimensions of this ellipsoid are consistent with it having a homogeneous interior. The existence of a subsurface ocean—a layer of liquid salt water in the interior of Tethys—is considered unlikely.

The surface of Tethys is one of the most reflective (at visual wavelengths) in the Solar System, with a visual albedo of 1.229. This very high albedo is the result of the sandblasting of particles from Saturn's E-ring, a faint ring composed of small, water-ice particles generated by Enceladus's south polar geysers. The radar albedo of the Tethyan surface is also very high. The leading hemisphere of Tethys is brighter by 10–15% than the trailing one.

The high albedo indicates that the surface of Tethys is composed of almost pure water ice with only a small amount of darker materials. The visible spectrum of Tethys is flat and featureless, whereas in the near-infrared strong water ice absorption bands at 1.25, 1.5, 2.0 and 3.0 μm wavelengths are visible. No compound other than crystalline water ice has been unambiguously identified on Tethys. (Possible constituents include organics, ammonia and carbon dioxide.) The dark material in the ice has the same spectral properties as seen on the surfaces of the dark Saturnian moons—Iapetus and Hyperion. The most probable candidate is nanophase iron or hematite. Measurements of the thermal emission as well as radar observations by the Cassini spacecraft show that the icy regolith on the surface of Tethys is structurally complex and has a large porosity exceeding 95%.

It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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## #9 2022-03-20 01:32:24

Jai Ganesh
Registered: 2005-06-28
Posts: 46,163

### Re: Natural Satellite

9) Dione

Dione is a moon of Saturn. It was discovered by Italian astronomer Giovanni Domenico Cassini in 1684. It is named after the Titaness Dione of Greek mythology. It is also designated Saturn IV.

Orbit

Dione orbits Saturn with a semimajor axis about 2% less than that of the Moon. However, reflecting Saturn's greater mass (95 times that of Earth), Dione's orbital period is one tenth that of the Moon. Dione is currently in a 1:2 mean-motion orbital resonance with moon Enceladus, completing one orbit of Saturn for every two orbits completed by Enceladus. This resonance maintains Enceladus's orbital eccentricity (0.0047), providing a source of heat for Enceladus's extensive geological activity, which shows up most dramatically in its cryovolcanic geyser-like jets. The resonance also maintains a smaller eccentricity in Dione's orbit (0.0022), tidally heating it as well.

Dione has two co-orbital, or trojan, moons, Helene and Polydeuces. They are located within Dione's Lagrangian points L4 and L5, 60 degrees ahead of and behind Dione respectively. A leading co-orbital moon twelve degrees ahead of Helene was reported by Stephen P. Synnott in 1982.

Physical characteristics and interior

At 1122 km (697 mi) in diameter, Dione is the 15th largest moon in the Solar System, and is more massive than all known moons smaller than itself combined. It is also Saturn's fourth-largest moon. Based on its density, Dione’s interior is likely a combination of silicate rock and water ice in nearly equal parts by mass.

Shape and gravity observations collected by Cassini suggest a roughly 400 km radius rocky core surrounded by a roughly 160 km envelope of H2O, mainly in the form of water ice, but with some models suggesting that the lowermost part of this layer could be in the form of an internal liquid salt water ocean (a situation similar to that of its orbital resonance partner, Enceladus). Downward bending of the surface associated with the 1.5 km high ridge Janiculum Dorsa can most easily be explained by the presence of such an ocean. Neither moon has a shape close to hydrostatic equilibrium; the deviations are maintained by isostasy. Dione's ice shell is thought to vary in thickness by less than 5%, with the thinnest areas at the poles, where tidal heating of the crust is greatest.

Though somewhat smaller and denser, Dione is otherwise very similar to Rhea. They both have similar albedo features and varied terrain, and both have dissimilar leading and trailing hemispheres. Dione's leading hemisphere is heavily cratered and is uniformly bright. Its trailing hemisphere, however, contains an unusual and distinctive surface feature: a network of bright ice cliffs.

Ice cliffs (formerly 'wispy terrain')

When the Voyager space probe photographed Dione in 1980, it showed what appeared to be wispy features covering its trailing hemisphere. The origin of these features was mysterious, because all that was known was that the material has a high albedo and is thin enough that it does not obscure the surface features underneath. One hypothesis was that shortly after its formation Dione was geologically active, and some process such as cryovolcanism resurfaced much of its surface, with the streaks forming from eruptions along cracks in the Dionean surface that fell back as snow or ash. Later, after the internal activity and resurfacing ceased, cratering continued primarily on the leading hemisphere and wiped out the streak patterns there.

This hypothesis was proven wrong by the Cassini probe flyby of December 13, 2004, which produced close-up images. These revealed that the 'wisps' were, in fact, not ice deposits at all, but rather bright ice cliffs created by tectonic fractures (chasmata). Dione has been revealed as a world riven by enormous fractures on its trailing hemisphere.

The Cassini orbiter performed a closer flyby of Dione at 500 km (310 mi) on October 11, 2005, and captured oblique images of the cliffs, showing that some of them are several hundred metres high.

Linear features

Dione features linear 'virgae' that are up to hundreds of km long but less than 5 km wide. These lines run parallel to the equator and are only apparent at lower latitudes (at less than 45° north or south); similar features are noted on Rhea. They are brighter than everything around them and appear to overlay other features such as ridges and craters, indicating they are relatively young. It has been proposed that these lines are of exogenic origin, as the result of the emplacement of material across the surface by low‐velocity impacts of material sourced from Saturn's rings, co‐orbital moons, or closely approaching comets.

Craters

Dione's icy surface includes heavily cratered terrain, moderately cratered plains, lightly cratered plains, and areas of tectonic fractures. The heavily cratered terrain has numerous craters greater than 100 kilometres (62 mi) in diameter. The plains areas tend to have craters less than 30 kilometres (19 mi) in diameter. Some of the plains are more heavily cratered than others. Much of the heavily cratered terrain is located on the trailing hemisphere, with the less cratered plains areas present on the leading hemisphere. This is the opposite of what some scientists expected; Shoemaker and Wolfe proposed a cratering model for a tidally locked satellite with the highest cratering rates on the leading hemisphere and the lowest on the trailing hemisphere. This suggests that during the period of heavy bombardment, Dione was tidally locked to Saturn in the opposite orientation. Because Dione is relatively small, an impact causing a 35 kilometer crater could have spun the satellite. Because there are many craters larger than 35 kilometres (22 mi), Dione could have been repeatedly spun during its early heavy bombardment. The pattern of cratering since then and the bright albedo of the leading side suggests that Dione has remained in its current orientation for several billion years.

Like Callisto, Dione's craters lack the high-relief features seen on the Moon and Mercury; this is probably due to slumping of the weak icy crust over geologic time.

Atmosphere

On April 7, 2010, instruments on board the unmanned Cassini probe, which flew by Dione, detected a thin layer of molecular oxygen ions around Dione, so thin that scientists prefer to call it an exosphere rather than a tenuous atmosphere. The density of molecular oxygen ions determined from the Cassini plasma spectrometer data ranges from 0.01 to 0.09 per cubic cm.

The Cassini probe instruments were unable to directly detect water from the exosphere due to high background levels, but it seems that highly charged particles from the planet's powerful radiation belts could split the water in the ice into hydrogen and oxygen.

Exploration

Dione was first imaged by the Voyager space probes. It has also been probed five times from close distances by the Cassini orbiter. There was a close targeted flyby, at a distance of 500 km (310 mi) on 11 October 2005; another flyby was performed on 7 April 2010 also at a distance of 500 km.[30] A third flyby was performed on 12 December 2011 at a distance of 99 km (62 mi). The following flyby was on 16 June 2015 at a distance of 516 km (321 mi), and the last Cassini flyby was performed on 17 August 2015 at a distance of 474 km (295 mi).

In May 2013, it was announced that NASA's spacecraft Cassini had provided scientists with evidence that Dione is more active than previously realized. Using topographic data, NASA teams deduced that crustal depression associated with a prominent mountain ridge on the leading hemisphere is best explained if there was a global subsurface liquid ocean like that of Enceladus. The ridge Janiculum Dorsa has a height of 1 to 2 km (0.6 to 1.2 miles); Dione's crust seems to pucker 0.5 km (0.3 miles) under it, suggesting that the icy crust was warm when the ridge formed, probably due to the presence of a subsurface liquid ocean, which increases tidal flexing.

It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

Online

## #10 2022-04-18 02:20:41

Jai Ganesh
Registered: 2005-06-28
Posts: 46,163

### Re: Natural Satellite

10) Titania

Titania, also designated Uranus III, is the largest of the moons of Uranus and the eighth largest moon in the Solar System at a diameter of 1,578 kilometres (981 mi). Discovered by William Herschel in 1787, it is named after the queen of the fairies in Shakespeare's A Midsummer Night's Dream. Its orbit lies inside Uranus's magnetosphere.

Titania consists of approximately equal amounts of ice and rock, and is probably differentiated into a rocky core and an icy mantle. A layer of liquid water may be present at the core–mantle boundary. Its surface, which is relatively dark and slightly red in color, appears to have been shaped by both impacts and endogenic processes. It is covered with numerous impact craters reaching up to 326 kilometres (203 mi) in diameter, but is less heavily cratered than Oberon, outermost of the five large moons of Uranus. It probably underwent an early endogenic resurfacing event which obliterated its older, heavily cratered surface. Its surface is cut by a system of enormous canyons and scarps, the result of the expansion of its interior during the later stages of its evolution. Like all major moons of Uranus, Titania probably formed from an accretion disk which surrounded the planet just after its formation.

Infrared spectroscopy conducted from 2001 to 2005 revealed the presence of water ice as well as frozen carbon dioxide on Titania's surface, suggesting it may have a tenuous carbon dioxide atmosphere with a surface pressure of about 10 nanopascals ({10}^{-13 bar}). Measurements during Titania's occultation of a star put an upper limit on the surface pressure of any possible atmosphere at 1–2 mPa (10–20 nbar).

The Uranian system has been studied up close only once, by the spacecraft Voyager 2 in January 1986. It took several images of Titania, which allowed mapping of about 40% of its surface.

Discovery and naming

Titania was discovered by William Herschel on January 11, 1787, the same day he discovered Uranus's second largest moon, Oberon. He later reported the discoveries of four more satellites, although they were subsequently revealed as spurious. For nearly the next 50 years, Titania and Oberon would not be observed by any instrument other than William Herschel's, although the moon can be seen from Earth with a present-day high-end amateur telescope.

All of Uranus's moons are named after characters created by William Shakespeare or Alexander Pope. The name Titania was taken from the Queen of the Fairies in A Midsummer Night's Dream. The names of all four satellites of Uranus then known were suggested by Herschel's son John in 1852, at the request of William Lasse who had discovered the other two moons, Ariel and Umbriel, the year before.

Titania was initially referred to as "the first satellite of Uranus", and in 1848 was given the designation Uranus I by William Lassell, although he sometimes used William Herschel's numbering (where Titania and Oberon are II and IV). In 1851 Lassell eventually numbered all four known satellites in order of their distance from the planet by Roman numerals, and since then Titania has been designated Uranus III.

Shakespeare's character's name is pronounced, but the moon is often pronounced , by analogy with the familiar chemical element titanium. The adjectival form, Titanian, is homonymous with that of Saturn's moon Titan. The name Titania is ancient Greek for "Daughter of the Titans".

Orbit

Titania orbits Uranus at the distance of about 436,000 kilometres (271,000 mi), being the second farthest from the planet among its five major moons after Oberon. Titania's orbit has a small eccentricity and is inclined very little relative to the equator of Uranus. Its orbital period is around 8.7 days, coincident with its rotational period. In other words, Titania is a synchronous or tidally locked satellite, with one face always pointing toward the planet.

Titania's orbit lies completely inside the Uranian magnetosphere. This is important, because the trailing hemispheres of satellites orbiting inside a magnetosphere are struck by magnetospheric plasma, which co-rotates with the planet. This bombardment may lead to the darkening of the trailing hemispheres, which is actually observed for all Uranian moons except Oberon (see below).

Because Uranus orbits the Sun almost on its side, and its moons orbit in the planet's equatorial plane, they (including Titania) are subject to an extreme seasonal cycle. Both northern and southern poles spend 42 years in a complete darkness, and another 42 years in continuous sunlight, with the sun rising close to the zenith over one of the poles at each solstice. The Voyager 2 flyby coincided with the southern hemisphere's 1986 summer solstice, when nearly the entire southern hemisphere was illuminated. Once every 42 years, when Uranus has an equinox and its equatorial plane intersects the Earth, mutual occultations of Uranus's moons become possible. In 2007–2008 a number of such events were observed including two occultations of Titania by Umbriel on August 15 and December 8, 2007.

Composition and internal structure

A round spherical body with its left half illuminated. The surface has a mottled appearance with bright patches among relatively dark terrain. The terminator is slightly to the right from the center and runs from the top to bottom. A large crater with a central pit can be seen at the terminator in the upper half of the image. Another bright crater can be seen at the bottom intersected by a canyon. The second large canyon runs from the darkness at the lower-right side to visible center of the body.

Voyager 2's highest-resolution image of Titania shows moderately cratered plains, enormous rifts and long scarps. Near the bottom, a region of smoother plains including the crater Ursula is split by the graben Belmont Chasma.

Titania is the largest and most massive Uranian moon, and the eighth most massive moon in the Solar System. Its density of 1.71 g/{cm^3} which is much higher than the typical density of Saturn's satellites, indicates that it consists of roughly equal proportions of water ice and dense non-ice components; the latter could be made of rock and carbonaceous material including heavy organic compounds. The presence of water ice is supported by infrared spectroscopic observations made in 2001–2005, which have revealed crystalline water ice on the surface of the moon. Water ice absorption bands are slightly stronger on Titania's leading hemisphere than on the trailing hemisphere. This is the opposite of what is observed on Oberon, where the trailing hemisphere exhibits stronger water ice signatures. The cause of this asymmetry is not known, but it may be related to the bombardment by charged particles from the magnetosphere of Uranus, which is stronger on the trailing hemisphere (due to the plasma's co-rotation). The energetic particles tend to sputter water ice, decompose methane trapped in ice as clathrate hydrate and darken other organics, leaving a dark, carbon-rich residue behind.

Except for water, the only other compound identified on the surface of Titania by infrared spectroscopy is carbon dioxide, which is concentrated mainly on the trailing hemisphere. The origin of the carbon dioxide is not completely clear. It might be produced locally from carbonates or organic materials under the influence of the solar ultraviolet radiation or energetic charged particles coming from the magnetosphere of Uranus. The latter process would explain the asymmetry in its distribution, because the trailing hemisphere is subject to a more intense magnetospheric influence than the leading hemisphere. Another possible source is the outgassing of the primordial CO2 trapped by water ice in Titania's interior. The escape of CO2 from the interior may be related to the past geological activity on this moon.

Titania may be differentiated into a rocky core surrounded by an icy mantle. If this is the case, the radius of the core 520 kilometres (320 mi) is about 66% of the radius of the moon, and its mass is around 58% of the moon's mass—the proportions are dictated by moon's composition. The pressure in the center of Titania is about 0.58 GPa (5.8 kbar). The current state of the icy mantle is unclear. If the ice contains enough ammonia or other antifreeze, Titania may have a subsurface ocean at the core–mantle boundary. The thickness of this ocean, if it exists, is up to 50 kilometres (31 mi) and its temperature is around 190 K (close to the water–ammonia eutectic temperature of 176 K). However the present internal structure of Titania depends heavily on its thermal history, which is poorly known.

Surface features

Among Uranus's moons, Titania is intermediate in brightness between the dark Oberon and Umbriel and the bright Ariel and Miranda. Its surface shows a strong opposition surge: its reflectivity decreases from 35% at a phase angle of 0° (geometrical albedo) to 25% at an angle of about 1°. Titania has a relatively low Bond albedo of about 17%. Its surface is generally slightly red in color, but less red than that of Oberon. However, fresh impact deposits are bluer, while the smooth plains situated on the leading hemisphere near Ursula crater and along some grabens are somewhat redder. There may be an asymmetry between the leading and trailing hemispheres; the former appears to be redder than the latter by 8%. However, this difference is related to the smooth plains and may be accidental. The reddening of the surfaces probably results from space weathering caused by bombardment by charged particles and micrometeorites over the age of the Solar System] However, the color asymmetry of Titania is more likely related to accretion of a reddish material coming from outer parts of the Uranian system, possibly, from irregular satellites, which would be deposited predominately on the leading hemisphere.

Scientists have recognized three classes of geological feature on Titania: craters, chasmata (canyons) and rupes (scarps). The surface of Titania is less heavily cratered than the surfaces of either Oberon or Umbriel, which means that the surface is much younger. The crater diameters reach 326 kilometers for the largest known crater, Gertrude (there can be also a degraded basin of approximately the same size). Some craters (for instance, Ursula and Jessica) are surrounded by bright impact ejecta (rays) consisting of relatively fresh ice. All large craters on Titania have flat floors and central peaks. The only exception is Ursula, which has a pit in the center. To the west of Gertrude there is an area with irregular topography, the so-called "unnamed basin", which may be another highly degraded impact basin with the diameter of about 330 kilometres (210 mi).

Titania's surface is intersected by a system of enormous faults, or scarps. In some places, two parallel scarps mark depressions in the satellite's crust, forming grabens, which are sometimes called canyons. The most prominent among Titania's canyons is Messina Chasma, which runs for about 1,500 kilometres (930 mi) from the equator almost to the south pole. The grabens on Titania are 20–50 kilometres (12–31 mi) wide and have a relief of about 2–5 km. The scarps that are not related to canyons are called rupes, such as Rousillon Rupes near Ursula crater. The regions along some scarps and near Ursula appear smooth at Voyager's image resolution. These smooth plains were probably resurfaced later in Titania's geological history, after the majority of craters formed. The resurfacing may have been either endogenic in nature, involving the eruption of fluid material from the interior (cryovolcanism), or, alternatively it may be due to blanking by the impact ejecta from nearby large craters. The grabens are probably the youngest geological features on Titania—they cut all craters and even smooth plains.

The geology of Titania was influenced by two competing forces: impact crater formation and endogenic resurfacing. The former acted over the moon's entire history and influenced all surfaces. The latter processes were also global in nature, but active mainly for a period following the moon's formation. They obliterated the original heavily cratered terrain, explaining the relatively low number of impact craters on the moon's present-day surface. Additional episodes of resurfacing may have occurred later and led to the formation of smooth plains. Alternatively smooth plains may be ejecta blankets of the nearby impact craters. The most recent endogenous processes were mainly tectonic in nature and caused the formation of the canyons, which are actually giant cracks in the ice crust. The cracking of the crust was caused by the global expansion of Titania by about 0.7%.

It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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## #11 2022-08-16 21:50:43

Jai Ganesh
Registered: 2005-06-28
Posts: 46,163

### Re: Natural Satellite

11) Rhea

Rhea is the second-largest moon of Saturn and the ninth-largest moon in the Solar System. It is the smallest body in the Solar System for which precise measurements have confirmed a shape consistent with hydrostatic equilibrium. It was discovered in 1672 by Giovanni Domenico Cassini.

Discovery

Rhea was discovered by Giovanni Domenico Cassini on 23 December 1672. It was the second moon of Saturn that Cassini discovered, and the third moon discovered around Saturn overall.

Name

Rhea is named after the Titan Rhea of Greek mythology, the "mother of the gods" and wife of Kronos, the Greek counterpart of the god Saturn. It is also designated Saturn V (being the fifth major moon going outward from the planet, after Mimas, Enceladus, Tethys, and Dione).

Cassini named the four moons he discovered (Tethys, Dione, Rhea, and Iapetus) Sidera Lodoicea (the stars of Louis) to honor King Louis XIV. Astronomers fell into the habit of referring to them and Titan as Saturn I through Saturn V. Once Mimas and Enceladus were discovered, in 1789, the numbering scheme was extended to Saturn VII, and then to Saturn VIII with the discovery of Hyperion in 1848.

Rhea was not named until 1847, when John Herschel (son of William Herschel, discoverer of the planet Uranus, and two other moons of Saturn, Mimas and Enceladus) suggested in Results of Astronomical Observations made at the Cape of Good Hope that the names of the Titans, sisters and brothers of Kronos (Saturn, in Roman mythology), be used.

Physical characteristics:

Size, mass, and internal structure

Rhea is an icy body with a density of about 1.236 g/{cm}^3. This low density indicates that it is made of ~25% rock (density ~3.25 g/{cm}^3) and ~75% water ice (density ~0.93 g/{cm}^3). Although Rhea is the ninth-largest moon, it is only the tenth-most massive moon. Indeed, Oberon, the second-largest moon of Uranus, has almost the same size, but is significantly denser than Rhea (1.63 vs 1.24) and thus more massive, although Rhea is slightly larger by volume.

Before the Cassini-Huygens mission, it was assumed that Rhea had a rocky core. However, measurements taken during a close flyby by the Cassini orbiter in 2005 cast this into doubt. In a paper published in 2007 it was claimed that the axial dimensionless moment of inertia coefficient was 0.4. Such a value indicated that Rhea had an almost homogeneous interior (with some compression of ice in the center) while the existence of a rocky core would imply a moment of inertia of about 0.34. In the same year another paper claimed the moment of inertia was about 0.37. Rhea being either partially or fully differentiated would be consistent with the observations of the Cassini probe. A year later yet another paper claimed that the moon may not be in hydrostatic equilibrium meaning that the moment of inertia cannot be determined from the gravity data alone. In 2008 an author of the first paper tried to reconcile these three disparate results. He concluded that there is a systematic error in the Cassini radio Doppler data used in the analysis, but after restricting the analysis to a subset of data obtained closest to the moon, he arrived at his old result that Rhea was in hydrostatic equilibrium and had a moment of inertia of about 0.4, again implying a homogeneous interior.

The triaxial shape of Rhea is consistent with a homogeneous body in hydrostatic equilibrium rotating at Rhea's angular velocity.[ Modelling in 2006 suggested that Rhea could be barely capable of sustaining an internal liquid-water ocean through heating by radioactive decay; such an ocean would have to be at about 176 K, the eutectic temperature for the water–ammonia system. More recent indications are that Rhea has a homogeneous interior and hence that this ocean does not exist.

Surface features

Rhea's features resemble those of Dione, with dissimilar leading and trailing hemispheres, suggesting similar composition and histories. The temperature on Rhea is 99 K (−174 °C) in direct sunlight and between 73 K (−200 °C) and 53 K (−220 °C) in the shade.

Rhea has a rather typical heavily cratered surface, with the exceptions of a few large Dione-type chasmata or fractures (wispy terrain) on the trailing hemisphere (the side facing away from the direction of motion along Rhea's orbit) and a very faint "line" of material at Rhea's equator that may have been deposited by material deorbiting from its rings. Rhea has two very large impact basins on its anti-Cronian hemisphere (facing away from Saturn), which are about 400 and 500 km across. The more northerly and less degraded of the two, called Tirawa, is roughly comparable to the basin Odysseus on Tethys. There is a 48 km-diameter impact crater at 112°W that is prominent because of an extended system of bright rays. This crater, called Inktomi, is nicknamed "The Splat", and may be one of the youngest craters on the inner moons of Saturn. No evidence of any endogenic activity has been discovered.

Its surface can be divided into two geologically different areas based on crater density; the first area contains craters which are larger than 40 km in diameter, whereas the second area, in parts of the polar and equatorial regions, has only craters under that size. This suggests that a major resurfacing event occurred some time during its formation. The leading hemisphere is heavily cratered and uniformly bright. As on Callisto, the craters lack the high relief features seen on the Moon and Mercury. It has been theorized that these cratered plains are up to four billion years old on average. On the trailing hemisphere there is a network of bright swaths on a dark background and few visible craters. It had been thought that these bright areas might be material ejected from ice volcanoes early in Rhea's history when its interior was still liquid. However, observations of Dione, which has an even darker trailing hemisphere and similar but more prominent bright streaks, show that the streaks are actually ice cliffs resulting from extensive fracturing of the moon's surface.[citation needed] The extensive dark areas are thought to be deposited tholins, which are a mix of complex organic compounds generated on the ice by pyrolysis and radiolysis of simple compounds containing carbon, nitrogen and hydrogen.

The January 17, 2006 distant flyby by the Cassini spacecraft yielded images of the wispy hemisphere at better resolution and a lower Sun angle than previous observations. Images from this and subsequent flybys showed that Rhea's streaks in fact are tectonically formed ice cliffs (chasmata) similar to those of Dione.

Formation

The moons of Saturn are thought to have formed through co-accretion, a similar process to that believed to have formed the planets in the Solar System. As the young giant planets formed, they were surrounded by discs of material that gradually coalesced into moons. However, a proposed model for the formation of Titan may also shine a new light on the origin of Rhea and Iapetus. In this model, Titan was formed in a series of giant impacts between pre-existing moons, and Rhea and Iapetus are thought to have formed from part of the debris of these collisions.

It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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## #12 2022-09-02 02:02:13

Jai Ganesh
Registered: 2005-06-28
Posts: 46,163

### Re: Natural Satellite

12) Iapetus

Iapetus s the third-largest natural satellite of Saturn and the eleventh-largest in the Solar System. Discoveries by the Cassini mission in 2007 revealed several unusual features, such as a massive equatorial ridge running three-quarters of the way around the moon and a distinctive color pattern.

Discovery

Iapetus was discovered by Giovanni Domenico Cassini, an Italian-born French astronomer, in October 1671. He had discovered it on the western side of Saturn and tried viewing it on the eastern side some months later, but was unsuccessful. This was also the case the following year, when he was again able to observe it on the western side, but not the eastern side. Cassini finally observed Iapetus on the eastern side in 1705 with the help of an improved telescope, finding it two magnitudes dimmer on that side.

Cassini correctly surmised that Iapetus has a bright hemisphere and a dark hemisphere, and that it is tidally locked, always keeping the same face towards Saturn. This means that the bright hemisphere is visible from Earth when Iapetus is on the western side of Saturn, and that the dark hemisphere is visible when Iapetus is on the eastern side. The dark hemisphere was later named Cassini Regio in his honor.

Name

Iapetus is named after the Titan Iapetus from Greek mythology. The name was suggested by John Herschel (son of William Herschel, discoverer of Mimas and Enceladus) in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope, in which he advocated naming the moons of Saturn after the Titans, brothers and sisters of the Titan Cronus (whom the Romans equated with their god Saturn).

The name has a largely obsolete variant, Japetus with an adjectival form Japetian. These occur because there was no distinction between the letters ⟨i⟩ and ⟨j⟩ in Latin, and authors rendered them differently.

When first discovered, Iapetus was among four Saturnian moons labelled the Sidera Lodoicea by their discoverer Giovanni Cassini after King Louis XIV (the other three were Tethys, Dione and Rhea). However, astronomers fell into the habit of referring to them using Roman numerals, with Iapetus being Saturn V. Once Mimas and Enceladus were discovered in 1789, the numbering scheme was extended and Iapetus became Saturn VII. And with the discovery of Hyperion in 1848, Iapetus became Saturn VIII, which it is still known by today (see naming of moons).

Geological features on Iapetus are named after characters and places from the French epic poem The Song of Roland. Examples of names used include the craters Charlemagne and Baligant, and the northern bright region, Roncevaux Terra. The one exception is Cassini Regio, the dark region of Iapetus, named after the region's and moon's discoverer, Giovanni Cassini.

Orbit

The orbit of Iapetus is somewhat unusual. Although it is Saturn's third-largest moon, it orbits much farther from Saturn than the next closest major moon, Titan. It has also the most inclined orbital plane of the regular satellites; only the irregular outer satellites like Phoebe have more inclined orbits. Because of this distant, inclined orbit, Iapetus is the only large moon from which the rings of Saturn would be clearly visible; from the other inner moons, the rings would be edge-on and difficult to see. The cause of this highly inclined orbit of Iapetus is unknown; however, it is not likely to have been captured. One suggestion for the cause of Iapetus's orbital inclination is an encounter between Saturn and another planet.

Physical characteristics

The low density of Iapetus indicates that it is mostly composed of ice, with only a small (~20%) amount of rocky materials.

Unlike most of the large moons, its overall shape is neither spherical nor ellipsoid, but has a bulging waistline and squashed poles. Its unique equatorial ridge (see below) is so high that it visibly distorts Iapetus's shape even when viewed from a distance. These features often lead it to be characterized as walnut-shaped.

Iapetus is heavily cratered, and Cassini images have revealed large impact basins, at least five of which are over 350 km (220 mi) wide. The largest, Turgis, has a diameter of 580 km (360 mi); its rim is extremely steep and includes a scarp about 15 km (9.3 mi) high. Iapetus is known to support long-runout landslides or sturzstroms, possibly supported by ice sliding.

Overall shape

Current triaxial measurements of Iapetus give it radial dimensions of 746 km × 746 km × 712 km (464 mi × 464 mi × 442 mi), with a mean radius of 734.5 ± 2.8 km (456.4 ± 1.7 mi). However, these measurements may be inaccurate on the kilometer scale as Iapetus's entire surface has not yet been imaged in high enough resolution. The observed oblateness would be consistent with hydrostatic equilibrium if Iapetus had a rotational period of approximately 16 hours, but it does not; its current rotation period is 79 days. A possible explanation for this is that the shape of Iapetus was frozen by formation of a thick crust shortly after its formation, while its rotation continued to slow afterwards due to tidal dissipation, until it became tidally locked.

Equatorial ridge

A further mystery of Iapetus is the equatorial ridge that runs along the center of Cassini Regio, about 1,300 km (810 mi) long, 20 km (12 mi) wide, and 13 km (8.1 mi) high. It was discovered when the Cassini spacecraft imaged Iapetus on December 31, 2004. Peaks in the ridge rise more than 20 km (12 mi) above the surrounding plains, making them some of the tallest mountains in the Solar System. The ridge forms a complex system including isolated peaks, segments of more than 200 km (120 mi) and sections with three near parallel ridges. Within the bright regions there is no ridge, but there are a series of isolated 10 km (6.2 mi) peaks along the equator. The ridge system is heavily cratered, indicating that it is ancient. The prominent equatorial bulge gives Iapetus a walnut-like appearance.

It is not clear how the ridge formed. One difficulty is to explain why it follows the equator almost perfectly. There are at least four current hypotheses, but none of them explains why the ridge is confined to Cassini Regio.

* A team of scientists associated with the Cassini mission have argued that the ridge could be a remnant of the oblate shape of the young Iapetus, when it was rotating more rapidly than it does today. The height of the ridge suggests a maximum rotational period of 17 hours. If Iapetus cooled fast enough to preserve the ridge but remained plastic long enough for the tides raised by Saturn to have slowed the rotation to its current tidally locked 79 days, Iapetus must have been heated by the radioactive decay of aluminium-26. This isotope appears to have been abundant in the solar nebula from which Saturn formed, but has since all decayed. The quantities of aluminium-26 needed to heat Iapetus to the required temperature give a tentative date to its formation relative to the rest of the Solar System: Iapetus must have come together earlier than expected, only two million years after the asteroids started to form.

* The ridge could be icy material that welled up from beneath the surface and then solidified. If it had formed away from the position of the equator at the time, this hypothesis requires that the rotational axis would have been driven to its current position by the ridge.

* Iapetus could have had a ring system during its formation due to its large Hill sphere of ~49 Iapetian radii, and that the equatorial ridge was then produced by collisional accretion of this ring.

* The ridge and the bulge are the result of ancient convective overturn. This hypothesis states that the bulge is in isostatic equilibrium typical for terrestrial mountains. It means that under the bulge there is material of low density (roots). The weight of the bulge is compensated by buoyancy forces acting on the roots. The ridge is also built of less dense matter. Its position along the equator is probably a result of the Coriolis force acting on a liquid interior of Iapetus.

It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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## #13 2022-10-02 21:06:41

Jai Ganesh
Registered: 2005-06-28
Posts: 46,163

### Re: Natural Satellite

13) Oberon

Oberon, also designated Uranus IV, is the outermost major moon of the planet Uranus. It is the second-largest and second most massive of the Uranian moons, and the ninth most massive moon in the Solar System. Discovered by William Herschel in 1787, Oberon is named after the mythical king of the fairies who appears as a character in Shakespeare's A Midsummer Night's Dream. Its orbit lies partially outside Uranus's magnetosphere.

It is likely that Oberon formed from the accretion disk that surrounded Uranus just after the planet's formation. The moon consists of approximately equal amounts of ice and rock, and is probably differentiated into a rocky core and an icy mantle. A layer of liquid water may be present at the boundary between the mantle and the core. The surface of Oberon, which is dark and slightly red in color, appears to have been primarily shaped by asteroid and comet impacts. It is covered by numerous impact craters reaching 210 km in diameter. Oberon possesses a system of chasmata (graben or scarps) formed during crustal extension as a result of the expansion of its interior during its early evolution.

The Uranian system has been studied up close only once: the spacecraft Voyager 2 took several images of Oberon in January 1986, allowing 40% of the moon's surface to be mapped.

Discovery and naming

Oberon was discovered by William Herschel on January 11, 1787; on the same day he discovered Uranus's largest moon, Titania. He later reported the discoveries of four more satellites, although they were subsequently revealed as spurious. For nearly fifty years following their discovery, Titania and Oberon would not be observed by any instrument other than William Herschel's, although the moon can be seen from Earth with a present-day high-end amateur telescope.

All of the moons of Uranus are named after characters created by William Shakespeare or Alexander Pope. The name Oberon was derived from Oberon, the King of the Fairies in A Midsummer Night's Dream. The names of all four satellites of Uranus then known were suggested by Herschel's son John in 1852, at the request of William Lassell, who had discovered the other two moons, Ariel and Umbriel, the year before. The adjectival form of the name is Oberonian.

Oberon was initially referred to as "the second satellite of Uranus", and in 1848 was given the designation Uranus II by William Lassell, although he sometimes used William Herschel's numbering (where Titania and Oberon are II and IV). In 1851 Lassell eventually numbered all four known satellites in order of their distance from the planet by Roman numerals, and since then Oberon has been designated Uranus IV.

Orbit

Oberon orbits Uranus at a distance of about 584,000 km, being the farthest from the planet among its five major moons. Oberon's orbit has a small orbital eccentricity and inclination relative to the equator of Uranus. Its orbital period is around 13.5 days, coincident with its rotational period. In other words, Oberon is a synchronous satellite, tidally locked, with one face always pointing toward the planet. Oberon spends a significant part of its orbit outside the Uranian magnetosphere. As a result, its surface is directly struck by the solar wind. This is important, because the trailing hemispheres of satellites orbiting inside a magnetosphere are struck by the magnetospheric plasma, which co-rotates with the planet. This bombardment may lead to the darkening of the trailing hemispheres, which is actually observed for all Uranian moons except Oberon (see below).

Because Uranus orbits the Sun almost on its side, and its moons orbit in the planet's equatorial plane, they (including Oberon) are subject to an extreme seasonal cycle. Both northern and southern poles spend 42 years in a complete darkness, and another 42 years in continuous sunlight, with the sun rising close to the zenith over one of the poles at each solstice. The Voyager 2 flyby coincided with the southern hemisphere's 1986 summer solstice, when nearly the entire northern hemisphere was in darkness. Once every 42 years, when Uranus has an equinox and its equatorial plane intersects the Earth, mutual occultations of Uranus's moons become possible. One such event, which lasted for about six minutes, was observed on May 4, 2007, when Oberon occulted Umbriel.

Composition and internal structure

Oberon is the second-largest and second-most massive of the Uranian moons after Titania, and the ninth-most massive moon in the Solar System. It is the tenth-largest moon by size however, since Rhea, the second-largest moon of Saturn and the ninth-largest moon, is nearly the same size as Oberon although it is about 0.4% larger, despite Oberon having more mass than Rhea. Oberon's density of 1.63 g/{cm}^3, which is higher than the typical density of Saturn's satellites, indicates that it consists of roughly equal proportions of water ice and a dense non-ice component. The latter could be made of rock and carbonaceous material including heavy organic compounds. The presence of water ice is supported by spectroscopic observations, which have revealed crystalline water ice on the surface of the moon. Water ice absorption bands are stronger on Oberon's trailing hemisphere than on the leading hemisphere. This is the opposite of what is observed on other Uranian moons, where the leading hemisphere exhibits stronger water ice signatures. The cause of this asymmetry is not known, but it may be related to impact gardening (the creation of soil via impacts) of the surface, which is stronger on the leading hemisphere. Meteorite impacts tend to sputter (knock out) ice from the surface, leaving dark non-ice material behind. The dark material itself may have formed as a result of radiation processing of methane clathrates or radiation darkening of other organic compounds.

Oberon may be differentiated into a rocky core surrounded by an icy mantle. If this is the case, the radius of the core (480 km) is about 63% of the radius of the moon, and its mass is around 54% of the moon's mass—the proportions are dictated by the moon's composition. The pressure in the center of Oberon is about 0.5 GPa (5 kbar). The current state of the icy mantle is unclear. If the ice contains enough ammonia or other antifreeze, Oberon may possess a liquid ocean layer at the core–mantle boundary. The thickness of this ocean, if it exists, is up to 40 km and its temperature is around 180 K (close to the water–ammonia eutectic temperature of 176 K). However, the internal structure of Oberon depends heavily on its thermal history, which is poorly known at present.

Surface features and geology

Oberon is the second-darkest large moon of Uranus after Umbriel. Its surface shows a strong opposition surge: its reflectivity decreases from 31% at a phase angle of 0° (geometrical albedo) to 22% at an angle of about 1°. Oberon has a low Bond albedo of about 14%. Its surface is generally red in color, except for fresh impact deposits, which are neutral or slightly blue. Oberon is, in fact, the reddest among the major Uranian moons. Its trailing and leading hemispheres are asymmetrical: the latter is much redder than the former, because it contains more dark red material. The reddening of the surfaces is often a result of space weathering caused by bombardment of the surface by charged particles and micrometeorites over the age of the Solar System. However, the color asymmetry of Oberon is more likely caused by accretion of a reddish material spiraling in from outer parts of the Uranian system, possibly from irregular satellites, which would occur predominately on the leading hemisphere.

Scientists have recognized two classes of geological feature on Oberon: craters and chasmata ('canyons'—deep, elongated, steep-sided depressions which would probably be described as rift valleys or escarpments if on Earth). Oberon's surface is the most heavily cratered of all the Uranian moons, with a crater density approaching saturation—when the formation of new craters is balanced by destruction of old ones. This high number of craters indicates that Oberon has the most ancient surface among Uranus's moons. The crater diameters range up to 206 kilometers for the largest known crater, Hamlet. Many large craters are surrounded by bright impact ejecta (rays) consisting of relatively fresh ice. The largest craters, Hamlet, Othello and Macbeth, have floors made of a very dark material deposited after their formation. A peak with a height of about 11 km was observed in some Voyager images near the south-eastern limb of Oberon, which may be the central peak of a large impact basin with a diameter of about 375 km. Oberon's surface is intersected by a system of canyons, which, however, are less widespread than those found on Titania. The canyons' sides are probably scarps produced by normal faults which can be either old or fresh: the latter transect the bright deposits of some large craters, indicating that they formed later. The most prominent Oberonian canyon is Mommur Chasma.

The geology of Oberon was influenced by two competing forces: impact crater formation and endogenic resurfacing. The former acted over the moon's entire history and is primarily responsible for its present-day appearance. The latter processes were active for a period following the moon's formation. The endogenic processes were mainly tectonic in nature and led to the formation of the canyons, which are actually giant cracks in the ice crust. The canyons obliterated parts of the older surface. The cracking of the crust was caused by the expansion of Oberon by about 0.5%, which occurred in two phases corresponding to the old and young canyons.

The nature of the dark patches, which mainly occur on the leading hemisphere and inside craters, is not known. Some scientists hypothesized that they are of cryovolcanic origin (analogs of lunar maria), while others think that the impacts excavated dark material buried beneath the pure ice (crust). In the latter case Oberon should be at least partially differentiated, with the ice crust lying atop the non-differentiated interior.

It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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## #14 2022-11-03 20:39:28

Jai Ganesh
Registered: 2005-06-28
Posts: 46,163

### Re: Natural Satellite

14) Ariel

Ariel is the fourth-largest of the 27 known moons of Uranus. Ariel orbits and rotates in the equatorial plane of Uranus, which is almost perpendicular to the orbit of Uranus and so has an extreme seasonal cycle.

It was discovered in October 1851 by William Lassell and named for a character in two different pieces of literature. As of 2019, much of the detailed knowledge of Ariel derives from a single flyby of Uranus performed by the space probe Voyager 2 in 1986, which managed to image around 35% of the moon's surface. There are no active plans at present to return to study the moon in more detail, although various concepts such as a Uranus Orbiter and Probe have been proposed.

After Miranda, Ariel is the second-smallest of Uranus's five major rounded satellites and the second-closest to its planet. Among the smallest of the Solar System's 19 known spherical moons (it ranks 14th among them in diameter), it is believed to be composed of roughly equal parts ice and rocky material. Its mass is approximately equal in magnitude to Earth's hydrosphere.

Like all of Uranus's moons, Ariel probably formed from an accretion disc that surrounded the planet shortly after its formation, and, like other large moons, it is likely differentiated, with an inner core of rock surrounded by a mantle of ice. Ariel has a complex surface consisting of extensive cratered terrain cross-cut by a system of scarps, canyons, and ridges. The surface shows signs of more recent geological activity than other Uranian moons, most likely due to tidal heating.

Orbit

Among Uranus's five major moons, Ariel is the second closest to the planet, orbiting at the distance of about 190,000 km. Its orbit has a small eccentricity and is inclined very little relative to the equator of Uranus. Its orbital period is around 2.5 Earth days, coincident with its rotational period. This means that one side of the moon always faces the planet; a condition known as tidal lock. Ariel's orbit lies completely inside the Uranian magnetosphere. The trailing hemispheres (those facing away from their directions of orbit) of airless satellites orbiting inside a magnetosphere like Ariel are struck by magnetospheric plasma co-rotating with the planet. This bombardment may lead to the darkening of the trailing hemispheres observed for all Uranian moons except Oberon. Ariel also captures magnetospheric charged particles, producing a pronounced dip in energetic particle count near the moon's orbit observed by Voyager 2 in 1986.

Because Ariel, like Uranus, orbits the Sun almost on its side relative to its rotation, its northern and southern hemispheres face either directly towards or directly away from the Sun at the solstices. This means it is subject to an extreme seasonal cycle; just as Earth's poles see permanent night or daylight around the solstices, Ariel's poles see permanent night or daylight for half a Uranian year (42 Earth years), with the Sun rising close to the zenith over one of the poles at each solstice. The Voyager 2 flyby coincided with the 1986 southern summer solstice, when nearly the entire northern hemisphere was dark. Once every 42 years, when Uranus has an equinox and its equatorial plane intersects the Earth, mutual occultations of Uranus's moons become possible. A number of such events occurred in 2007–2008, including an occultation of Ariel by Umbriel on 19 August 2007.

Currently Ariel is not involved in any orbital resonance with other Uranian satellites. In the past, however, it may have been in a 5:3 resonance with Miranda, which could have been partially responsible for the heating of that moon (although the maximum heating attributable to a former 1:3 resonance of Umbriel with Miranda was likely about three times greater). Ariel may have once been locked in the 4:1 resonance with Titania, from which it later escaped. Escape from a mean motion resonance is much easier for the moons of Uranus than for those of Jupiter or Saturn, due to Uranus's lesser degree of oblateness. This resonance, which was likely encountered about 3.8 billion years ago, would have increased Ariel's orbital eccentricity, resulting in tidal friction due to time-varying tidal forces from Uranus. This would have caused warming of the moon's interior by as much as 20 K.

Composition and internal structure

Ariel is the fourth-largest of the Uranian moons, and may have the third-greatest mass. It is also the 14th-largest moon in the Solar System. The moon's density is 1.66 g/cm^3, which indicates that it consists of roughly equal parts water ice and a dense non-ice component. The latter could consist of rock and carbonaceous material including heavy organic compounds known as tholins. The presence of water ice is supported by infrared spectroscopic observations, which have revealed crystalline water ice on the surface of the moon, which is porous and thus transmits little solar heat to layers below. Water ice absorption bands are stronger on Ariel's leading hemisphere than on its trailing hemisphere. The cause of this asymmetry is not known, but it may be related to bombardment by charged particles from Uranus's magnetosphere, which is stronger on the trailing hemisphere (due to the plasma's co-rotation). The energetic particles tend to sputter water ice, decompose methane trapped in ice as clathrate hydrate and darken other organics, leaving a dark, carbon-rich residue behind.

Except for water, the only other compound identified on the surface of Ariel by infrared spectroscopy is carbon dioxide (CO2), which is concentrated mainly on its trailing hemisphere. Ariel shows the strongest spectroscopic evidence for CO2 of any Uranian satellite, and was the first Uranian satellite on which this compound was discovered. The origin of the carbon dioxide is not completely clear. It might be produced locally from carbonates or organic materials under the influence of the energetic charged particles coming from Uranus's magnetosphere or solar ultraviolet radiation. This hypothesis would explain the asymmetry in its distribution, as the trailing hemisphere is subject to a more intense magnetospheric influence than the leading hemisphere. Another possible source is the outgassing of primordial CO2 trapped by water ice in Ariel's interior. The escape of CO2 from the interior may be related to past geological activity on this moon.

Given its size, rock/ice composition and the possible presence of salt or ammonia in solution to lower the freezing point of water, Ariel's interior may be differentiated into a rocky core surrounded by an icy mantle. If this is the case, the radius of the core (372 km) is about 64% of the radius of the moon, and its mass is around 56% of the moon's mass—the parameters are dictated by the moon's composition. The pressure in the center of Ariel is about 0.3 GPa (3 kbar). The current state of the icy mantle is unclear. The existence of a subsurface ocean is currently considered possible, though a 2006 study suggests that radiogenic heating alone would not be enough to allow for one.

It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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## #15 2022-12-03 00:42:11

Jai Ganesh
Registered: 2005-06-28
Posts: 46,163

### Re: Natural Satellite

15) Umbriel

Umbriel is a moon of Uranus discovered on October 24, 1851, by William Lassell. It was discovered at the same time as Ariel and named after a character in Alexander Pope's poem The math of the Lock. Umbriel consists mainly of ice with a substantial fraction of rock, and may be differentiated into a rocky core and an icy mantle. The surface is the darkest among Uranian moons, and appears to have been shaped primarily by impacts. However, the presence of canyons suggests early endogenic processes, and the moon may have undergone an early endogenically driven resurfacing event that obliterated its older surface.

Covered by numerous impact craters reaching 210 km (130 mi) in diameter, Umbriel is the second most heavily cratered satellite of Uranus after Oberon. The most prominent surface feature is a ring of bright material on the floor of Wunda crater. This moon, like all moons of Uranus, probably formed from an accretion disk that surrounded the planet just after its formation. The Uranian system has been studied up close only once, by the spacecraft Voyager 2 in January 1986. It took several images of Umbriel, which allowed mapping of about 40% of the moon's surface.

Discovery and name

Umbriel, along with another Uranian satellite, Ariel, was discovered by William Lassell on October 24, 1851. Although William Herschel, the discoverer of Titania and Oberon, claimed at the end of the 18th century that he had observed four additional moons of Uranus, his observations were not confirmed and those four objects are now thought to be spurious.

All of Uranus's moons are named after characters created by William Shakespeare or Alexander Pope. The names of all four satellites of Uranus then known were suggested by John Herschel in 1852 at the request of Lassell. Umbriel is the "dusky melancholy sprite" in Alexander Pope's The math of the Lock, and the name suggests the Latin umbra, meaning shadow. The moon is also designated Uranus II.

Orbit

Umbriel orbits Uranus at the distance of about 266,000 km (165,000 mi), being the third farthest from the planet among its five major moons. Umbriel's orbit has a small eccentricity and is inclined very little relative to the equator of Uranus. Its orbital period is around 4.1 Earth days, coincident with its rotational period. In other words, Umbriel is a synchronous or tidally locked satellite, with one face always pointing toward its parent planet.[6] Umbriel's orbit lies completely inside the Uranian magnetosphere. This is important, because the trailing hemispheres of airless satellites orbiting inside a magnetosphere (like Umbriel) are struck by magnetospheric plasma, which co-rotates with the planet. This bombardment may lead to the darkening of the trailing hemispheres, which is actually observed for all Uranian moons except Oberon  Umbriel also serves as a sink of the magnetospheric charged particles, which creates a pronounced dip in energetic particle count near the moon's orbit as observed by Voyager 2 in 1986.

Because Uranus orbits the Sun almost on its side, and its moons orbit in the planet's equatorial plane, they (including Umbriel) are subject to an extreme seasonal cycle. Both northern and southern poles spend 42 years in complete darkness, and another 42 years in continuous sunlight, with the Sun rising close to the zenith over one of the poles at each solstice. The Voyager 2 flyby coincided with the southern hemisphere's 1986 summer solstice, when nearly the entire northern hemisphere was unilluminated. Once every 42 years, when Uranus has an equinox and its equatorial plane intersects the Earth, mutual occultations of Uranus's moons become possible. In 2007–2008 a number of such events were observed including two occultations of Titania by Umbriel on August 15 and December 8, 2007 as well as of Ariel by Umbriel on August 19, 2007.

Currently Umbriel is not involved in any orbital resonance with other Uranian satellites. Early in its history, however, it may have been in a 1:3 resonance with Miranda. This would have increased Miranda's orbital eccentricity, contributing to the internal heating and geological activity of that moon, while Umbriel's orbit would have been less affected. Due to Uranus's lower oblateness and smaller size relative to its satellites, its moons can escape more easily from a mean motion resonance than those of Jupiter or Saturn. After Miranda escaped from this resonance (through a mechanism that probably resulted in its anomalously high orbital inclination), its eccentricity would have been damped, turning off the heat source.

Composition and internal structure

Umbriel is the third-largest and fourth-most massive of the Uranian moons. Although Umbriel is the 13th-largest moon in the solar system, it is only the 14th-most massive. The moon's density is 1.39 g/cm^3, which indicates that it mainly consists of water ice, with a dense non-ice component constituting around 40% of its mass. The latter could be made of rock and carbonaceous material including heavy organic compounds known as tholins. The presence of water ice is supported by infrared spectroscopic observations, which have revealed crystalline water ice on the surface of the moon. Water ice absorption bands are stronger on Umbriel's leading hemisphere than on the trailing hemisphere. The cause of this asymmetry is not known, but it may be related to the bombardment by charged particles from the magnetosphere of Uranus, which is stronger on the trailing hemisphere (due to the plasma's co-rotation). The energetic particles tend to sputter water ice, decompose methane trapped in ice as clathrate hydrate and darken other organics, leaving a dark, carbon-rich residue behind.

Except for water, the only other compound identified on the surface of Umbriel by the infrared spectroscopy is carbon dioxide, which is concentrated mainly on the trailing hemisphere. The origin of the carbon dioxide is not completely clear. It might be produced locally from carbonates or organic materials under the influence of the energetic charged particles coming from the magnetosphere of Uranus or the solar ultraviolet radiation. This hypothesis would explain the asymmetry in its distribution, as the trailing hemisphere is subject to a more intense magnetospheric influence than the leading hemisphere. Another possible source is the outgassing of the primordial CO2 trapped by water ice in Umbriel's interior. The escape of CO2 from the interior may be a result of past geological activity on this moon.

Umbriel may be differentiated into a rocky core surrounded by an icy mantle. If this is the case, the radius of the core (317 km) is about 54% of the radius of the moon, and its mass is around 40% of the moon's mass—the parameters are dictated by the moon's composition. The pressure in the center of Umbriel is about 0.24 GPa (2.4 kbar).The current state of the icy mantle is unclear, although the existence of a subsurface ocean is considered unlikely.

Surface features

Umbriel's surface is the darkest of the Uranian moons, and reflects less than half as much light as Ariel, a sister satellite of similar size. Umbriel has a very low Bond albedo of only about 10% as compared to 23% for Ariel. The reflectivity of the moon's surface decreases from 26% at a phase angle of 0° (geometric albedo) to 19% at an angle of about 1°. This phenomenon is called opposition surge. The surface of Umbriel is slightly blue in color, while fresh bright impact deposits (in Wunda crater, for instance) are even bluer. There may be an asymmetry between the leading and trailing hemispheres; the former appears to be redder than the latter. The reddening of the surfaces probably results from space weathering from bombardment by charged particles and micrometeorites over the age of the Solar System. However, the color asymmetry of Umbriel is likely caused by accretion of a reddish material coming from outer parts of the Uranian system, possibly, from irregular satellites, which would occur predominately on the leading hemisphere. The surface of Umbriel is relatively homogeneous—it does not demonstrate strong variation in either albedo or color.

Mean radius  :  584.7±2.8 km (0.092 Earths).

It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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## #16 2023-01-01 01:21:06

Jai Ganesh
Registered: 2005-06-28
Posts: 46,163

### Re: Natural Satellite

16) Miranda

Miranda, also designated Uranus V, is the smallest and innermost of Uranus's five round satellites. It was discovered by Gerard Kuiper on 16 February 1948 at McDonald Observatory in Texas, and named after Miranda from William Shakespeare's play The Tempest. Like the other large moons of Uranus, Miranda orbits close to its planet's equatorial plane. Because Uranus orbits the Sun on its side, Miranda's orbit is perpendicular to the ecliptic and shares Uranus' extreme seasonal cycle.

At just 470 km in diameter, Miranda is one of the smallest closely observed objects in the Solar System that might be in hydrostatic equilibrium (spherical under its own gravity). The only close-up images of Miranda are from the Voyager 2 probe, which made observations of Miranda during its Uranus flyby in January 1986. During the flyby, Miranda's southern hemisphere pointed towards the Sun, so only that part was studied.

Miranda probably formed from an accretion disc that surrounded the planet shortly after its formation, and, like other large moons, it is likely differentiated, with an inner core of rock surrounded by a mantle of ice. Miranda has one of the most extreme and varied topographies of any object in the Solar System, including Verona Rupes, a 20-kilometer-high scarp that is the highest cliff in the Solar System, and chevron-shaped tectonic features called coronae. The origin and evolution of this varied geology, the most of any Uranian satellite, are still not fully understood, and multiple hypotheses exist regarding Miranda's evolution.

Discovery and name

Miranda was discovered on 16 February 1948 by planetary astronomer Gerard Kuiper using the McDonald Observatory's 82-inch (2,080 mm) Otto Struve Telescope. Its motion around Uranus was confirmed on 1 March 1948. It was the first satellite of Uranus discovered in nearly 100 years. Kuiper elected to name the object "Miranda" after the character in Shakespeare's The Tempest, because the four previously discovered moons of Uranus, Ariel, Umbriel, Titania and Oberon, had all been named after characters of Shakespeare or Alexander Pope. However, the previous moons had been named specifically after fairies, whereas Miranda was a human. Subsequently, discovered satellites of Uranus were named after characters from Shakespeare and Pope, whether fairies or not. The moon is also designated Uranus V.

Orbit

Of Uranus's five round satellites, Miranda orbits closest to it, at roughly 129,000 km from the surface; about a quarter again as far as its most distant ring. Its orbital period is 34 hours, and, like that of the Moon, is synchronous with its rotation period, which means it always shows the same face to Uranus, a condition known as tidal locking. Miranda's orbital inclination (4.34°) is unusually high for a body so close to its planet – roughly ten times that of the other major Uranian satellites, and 73 times that of Oberon. The reason for this is still uncertain; there are no mean-motion resonances between the moons that could explain it, leading to the hypothesis that the moons occasionally pass through secondary resonances, which at some point in the past led to Miranda being locked for a time into a 3:1 resonance with Umbriel, before chaotic behaviour induced by the secondary resonances moved it out of it again. In the Uranian system, due to the planet's lesser degree of oblateness and the larger relative size of its satellites, escape from a mean-motion resonance is much easier than for satellites of Jupiter or Saturn.

Composition and internal structure

At 1.2 g/{cm}^3, Miranda is the least dense of Uranus's round satellites. That density suggests a composition of more than 60% water ice. Miranda's surface may be mostly water ice, though it is far rockier than its corresponding satellites in the Saturn system, indicating that heat from radioactive decay may have led to internal differentiation, allowing silicate rock and organic compounds to settle in its interior. Miranda is too small for any internal heat to have been retained over the age of the Solar System. Miranda is the least spherical of Uranus's satellites, with an equatorial diameter 3% wider than its polar diameter. Only water has been detected so far on Miranda's surface, though it has been speculated that methane, ammonia, carbon monoxide or nitrogen may also exist at 3% concentrations. These bulk properties are similar to Saturn's moon Mimas, though Mimas is smaller, less dense, and more oblate.

Precisely how a body as small as Miranda could have enough internal energy to produce the myriad geological features seen on its surface is not established with certainty, though the currently favoured hypothesis is that it was driven by tidal heating during a past time when it was in 3:1 orbital resonance with Umbriel. The resonance would have increased Miranda's orbital eccentricity to 0.1, and generated tidal friction due to the varying tidal forces from Uranus. As Miranda approached Uranus, tidal force increased; as it retreated, tidal force decreased, causing flexing that would have warmed Miranda's interior by 20 K, enough to trigger melting. The period of tidal flexing could have lasted for up to 100 million years. Also, if clathrate existed within Miranda, as has been hypothesised for the satellites of Uranus, it may have acted as an insulator, since it has a lower conductivity than water, increasing Miranda's temperature still further. Miranda may have also once been in a 5:3 orbital resonance with Ariel, which would have also contributed to its internal heating. However, the maximum heating attributable to the resonance with Umbriel was likely about three times greater.

Physical characteristics

Mean radius  :  235.8±0.7 km (0.03697 Earths)
Surface area  :  700,000 {km}^2.

It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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## #17 2023-02-02 21:41:59

Jai Ganesh
Registered: 2005-06-28
Posts: 46,163

### Re: Natural Satellite

17) Triton

Triton is the largest natural satellite of the planet Neptune, and was the first Neptunian moon to be discovered, on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, an orbit in the direction opposite to its planet's rotation. Because of its retrograde orbit and composition similar to Pluto, Triton is thought to have been a dwarf planet, captured from the Kuiper belt.

At 2,710 kilometers (1,680 mi) in diameter, it is the seventh-largest moon in the Solar System, the only satellite of Neptune massive enough to be in hydrostatic equilibrium, the second-largest planetary moon concerning its primary (after Earth's Moon), and larger than Pluto. Triton is one of the few moons in the Solar System known to be geologically active (the others being Jupiter's Io and Europa, and Saturn's Enceladus and Titan). As a consequence, its surface is relatively young, with few obvious impact craters. Intricate cryovolcanic and tectonic terrains suggest a complex geological history. Triton has a surface of mostly frozen nitrogen, a mostly water-ice crust, an icy mantle and a substantial core of rock and metal. The core makes up two-thirds of its total mass. The mean density is 2.061 g/{cm}^3, reflecting a composition of approximately 15–35% water ice.

During its 1989 flyby of Triton, Voyager 2 found surface temperatures of 38 K (−235 °C) and also discovered active geysers erupting sublimated nitrogen gas, contributing to a tenuous nitrogen atmosphere less than 1⁄70,000 the pressure of Earth's atmosphere at sea level. Voyager 2 remains the only spacecraft to have visited Triton. As the probe was only able to study about 40% of the moon's surface, future missions (Dubbed "Trident") have been proposed to Nasa via their Discovery Program to revisit the Neptune system with a focus on Triton.

Discovery and naming

Triton was discovered by British astronomer William Lassell on October 10, 1846, just 17 days after the discovery of Neptune. When John Herschel received news of Neptune's discovery, he wrote to Lassell suggesting he search for possible moons. Lassell discovered Triton eight days later. Lassell also claimed for a period to have discovered rings. Although Neptune was later confirmed to have rings, they are so faint and dark that it is not plausible he saw them. A brewer by trade, Lassell spotted Triton with his self-built 61 cm (24 in) aperture metal mirror reflecting telescope (also known as the "two-foot" reflector). This telescope was donated to the Royal Observatory, Greenwich in the 1880s, but was eventually dismantled.

Triton is named after the Greek sea god Triton, the son of Poseidon (the Greek god corresponding to the Roman Neptune). The name was first proposed by Camille Flammarion in his 1880 book Astronomie Populaire, and was officially adopted many decades later. Until the discovery of the second moon Nereid in 1949, Triton was commonly referred to as "the satellite of Neptune". Lassell did not name his discovery; he later successfully suggested the name Hyperion, previously chosen by John Herschel, for the eighth moon of Saturn when he discovered it.

Orbit and rotation

The orbit of Triton (red) is opposite in direction and tilted −23° compared to a typical moon's orbit (green) in the plane of Neptune's equator.
Triton is unique among all large moons in the Solar System for its retrograde orbit around its planet (i.e. it orbits in a direction opposite to the planet's rotation). Most of the outer irregular moons of Jupiter and Saturn also have retrograde orbits, as do some of Uranus's outer moons. However, these moons are all much more distant from their primaries, and are small in comparison; the largest of them (Phoebe) has only 8% of the diameter (and 0.03% of the mass) of Triton.

Triton's orbit is associated with two tilts, the obliquity of Neptune's rotation to Neptune's orbit, 30°, and the inclination of Triton's orbit to Neptune's rotation, 157° (an inclination over 90° indicates retrograde motion). Triton's orbit precesses forward relative to Neptune's rotation with a period of about 678 Earth years (4.1 Neptunian years), making its Neptune-orbit-relative inclination vary between 127° and 173°. That inclination is currently 130°; Triton's orbit is now near its maximum departure from coplanarity with Neptune's.

Triton's rotation is tidally locked to be synchronous with its orbit around Neptune: it keeps one face oriented toward the planet at all times. Its equator is almost exactly aligned with its orbital plane. At present, Triton's rotational axis is about 40° from Neptune's orbital plane, and hence at some point during Neptune's year each pole points fairly close to the Sun, almost like the poles of Uranus. As Neptune orbits the Sun, Triton's polar regions take turns facing the Sun, resulting in seasonal changes as one pole, then the other moves into the sunlight. Such changes were observed in 2010.

Triton's revolution around Neptune has become a nearly perfect circle with an eccentricity of almost zero. Viscoelastic damping from tides alone is not thought to be capable of circularizing Triton's orbit in the time since the origin of the system, and gas drag from a prograde debris disc is likely to have played a substantial role. Tidal interactions also cause Triton's orbit, which is already closer to Neptune than the Moon is to Earth, to gradually decay further; predictions are that 3.6 billion years from now, Triton will pass within Neptune's Roche limit. This will result in either a collision with Neptune's atmosphere or the breakup of Triton, forming a new ring system similar to that found around Saturn.

Capture

The current understanding of moons in retrograde orbits means they cannot form in the same region of the solar nebula as the planets they orbit. Therefore Triton must have been captured from elsewhere in the solar system. Astrophysicists believe it might have originated in the Kuiper belt, a ring of small icy objects extending from just inside the orbit of Neptune to about 50 AU from the Sun. Thought to be the point of origin for the majority of short-period comets observed from Earth, the belt is also home to several large, planet-like bodies including Pluto, which is now recognized as the largest in a population of Kuiper belt objects (the plutinos) locked in resonant orbits with Neptune. Triton is only slightly larger than Pluto and is nearly identical in composition, which has led to the hypothesis that the two share a common origin.

The proposed capture of Triton may explain several features of the Neptunian system, including the extremely eccentric orbit of Neptune's moon Nereid and the scarcity of moons as compared to the other giant planets. Triton's initially eccentric orbit would have intersected the orbits of irregular moons and disrupted those of smaller regular moons, dispersing them through gravitational interactions.

Triton's eccentric post-capture orbit would have also resulted in tidal heating of its interior, which could have kept Triton fluid for a billion years; this inference is supported by evidence of differentiation in Triton's interior. This source of internal heat disappeared following tidal locking and circularization of the orbit.

Two types of mechanisms have been proposed for Triton's capture. To be gravitationally captured by a planet, a passing body must lose sufficient energy to be slowed down to a speed less than that required to escape. An early theory of how Triton may have been slowed was by collision with another object, either one that happened to be passing by Neptune (which is unlikely), or a moon or proto-moon in orbit around Neptune (which is more likely). A more recent hypothesis suggests that, before its capture, Triton was part of a binary system. When this binary encountered Neptune, it interacted in such a way that the binary dissociated, with one portion of the binary expelled, and the other, Triton, becoming bound to Neptune. This event is more likely for more massive companions. This hypothesis is supported by several lines of evidence, including binaries being very common among the large Kuiper belt objects. The event was brief but gentle, saving Triton from collisional disruption. Events like this may have been common during the formation of Neptune, or later when it migrated outward.

However, simulations in 2017 showed that after Triton's capture, and before its orbital eccentricity decreased, it probably did collide with at least one other moon, and caused collisions between other moons.

Physical characteristics

Triton dominates the Neptunian moon system, with over 99.5% of its total mass. This imbalance may reflect the elimination of many of Neptune's original satellites following Triton's capture.

Triton is the seventh-largest moon and sixteenth-largest object in the Solar System and is modestly larger than the dwarf planets Pluto and Eris. It is also the largest retrograde moon in the solar system. It comprises more than 99.5% of all the mass known to orbit Neptune, including the planet's rings and thirteen other known moons, and is also more massive than all known moons in the Solar System smaller than itself combined. Also, with a diameter 5.5% that of Neptune, it is the largest moon of a gas giant relative to its planet in terms of diameter, although Titan is bigger relative to Saturn in terms of mass (the ratio of Triton's mass to that of Neptune is approximately 1:4788). It has a radius, density (2.061 g/{cm}^3), temperature and chemical composition similar to that of Pluto.

Triton's surface is covered with a transparent layer of annealed frozen nitrogen. Only 40% of Triton's surface has been observed and studied, but it may be entirely covered in such a thin sheet of nitrogen ice. Like Pluto's, Triton's crust consists of 55% nitrogen ice with other ices mixed in. Water ice comprises 15–35% and frozen carbon dioxide (dry ice) the remaining 10–20%. Trace ices include 0.1% methane and 0.05% carbon monoxide. There could also be ammonia ice on the surface, as there are indications of ammonia dihydrate in the lithosphere. Triton's mean density implies that it probably consists of about 30–45% water ice (including relatively small amounts of volatile ices), with the remainder being rocky material. Triton's surface area is 23 million {km}^2, which is 4.5% of Earth, or 15.5% of Earth's land area. Triton has an unusually high albedo, reflecting 60–95% of the sunlight that reaches it, and it has changed only slightly since the first observations. By comparison, the Moon reflects only 11%. Triton's reddish color is thought to be the result of methane ice, which is converted to tholins under exposure to ultraviolet radiation.

Because Triton's surface indicates a long history of melting, models of its interior posit that Triton is differentiated, like Earth, into a solid core, a mantle and a crust. Water, the most abundant volatile in the Solar System, comprises Triton's mantle, enveloping a core of rock and metal. There is enough rock in Triton's interior for radioactive decay to maintain a liquid subsurface ocean to this day, similar to what is thought to exist beneath the surface of Europa and several other icy outer Solar System worlds. This is not thought to be adequate to power convection in Triton's icy crust. However, the strong obliquity tides are believed to generate enough additional heat to accomplish this and produce the observed signs of recent surface geological activity. The black material ejected is suspected to contain organic compounds, and if liquid water is present on Triton, it has been speculated that this could make it habitable for some form of life.

It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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## #18 2023-03-01 22:04:42

Jai Ganesh
Registered: 2005-06-28
Posts: 46,163

### Re: Natural Satellite

18) Charon

Charon, known as (134340) Pluto I, is the largest of the five known natural satellites of the dwarf planet Pluto. It has a mean radius of 606 km (377 mi). Charon is the sixth-largest known trans-Neptunian object after Pluto, Eris, Haumea, Makemake and Gonggong. It was discovered in 1978 at the United States Naval Observatory in Washington, D.C., using photographic plates taken at the United States Naval Observatory Flagstaff Station (NOFS).

With half the diameter and one eighth the mass of Pluto, Charon is a very large moon in comparison to its parent body. Its gravitational influence is such that the barycenter of the Plutonian system lies outside Pluto, and the two bodies are tidally locked to each other.

The reddish-brown cap of the north pole of Charon is composed of tholins, organic macromolecules that may be essential ingredients of life. These tholins were produced from methane, nitrogen and related gases which may have been released by cryovolcanic eruptions on the moon, or may have been transferred over 19,000 km (12,000 mi) from the atmosphere of Pluto to the orbiting moon.

The New Horizons spacecraft is the only probe that has visited the Pluto system. It approached Charon to within 27,000 km (17,000 mi) in 2015.

Discovery

Charon was discovered by United States Naval Observatory astronomer James Christy, using the 1.55-meter (61 in) telescope at United States Naval Observatory Flagstaff Station (NOFS). On June 22, 1978, he had been examining highly magnified images of Pluto on photographic plates taken with the telescope two months prior. Christy noticed that a slight elongation appeared periodically. The bulge was confirmed on plates dating back to April 29, 1965. The International Astronomical Union formally announced Christy's discovery to the world on July 7, 1978.

Subsequent observations of Pluto determined that the bulge was due to a smaller accompanying body. The periodicity of the bulge corresponded to Pluto's rotation period, which was previously known from Pluto's light curve. This indicated a synchronous orbit, which strongly suggested that the bulge effect was real and not spurious. This resulted in reassessments of Pluto's size, mass, and other physical characteristics because the calculated mass and albedo of the Pluto–Charon system had previously been attributed to Pluto alone.

Doubts about Charon's existence were erased when it and Pluto entered a five-year period of mutual eclipses and transits between 1985 and 1990. This occurs when the Pluto–Charon orbital plane is edge-on as seen from Earth, which only happens at two intervals in Pluto's 248-year orbital period. It was fortuitous that one of these intervals happened to occur soon after Charon's discovery.

Atmosphere

Charon has no significant atmosphere. There has been speculation about a minuscule atmosphere surrounding the moon, but there has been no indication of anything substantial.

Pluto does have a thin but significant atmosphere, and under some conditions Charon's gravitation pulls some of Pluto's upper atmosphere, specifically nitrogen, from Pluto's ice formations, toward Charon's surface. The nitrogen is mostly caught in the combined center of gravity between the two bodies before reaching Charon, but any gas that does reach Charon is held closely against the surface. The gas is mostly made up of ions of nitrogen, but the amounts are negligible compared to the total of Pluto's atmosphere.

The many spectral signatures of ice formations on the surface of Charon have led some to believe that the ice formations could supply an atmosphere, but atmosphere supplying formations have not been confirmed yet. Many scientists theorize that these ice formations could be concealed out of direct sight, either in deep craters or beneath Charon's surface. Similar to how Pluto transfers atmosphere to Charon, Charon's relatively low gravity, due to its low mass, causes any atmosphere that might be present to rapidly escape the surface into space. Even through stellar occultation, which is used to probe the atmosphere of stellar bodies, scientists cannot confirm an existing atmosphere; this was tested in 1986 while attempting to perform stellar occultation testing on Pluto. Charon also acts as a protector for Pluto's atmosphere, blocking the solar wind that would normally collide with Pluto, damaging its atmosphere. Since Charon blocks these solar winds, its own atmosphere is diminished, instead of Pluto's. This effect is also a serious potential explanation for Charon's lack of atmosphere; when it begins to accumulate, the solar winds shut it down.[clarification needed] Although, it is still possible for Charon to have an atmosphere. As previously stated, Pluto transfers some of its atmospheric gas to Charon, from where it tends to escape into space. Assuming Charon's density is 1.71 g/{cm}^3, which is the rough estimate we currently have, it would have a surface gravity of 0.6 of Pluto's. It also has a higher mean molecular weight than Pluto and a lower exobase surface temperature, so that the gases in its atmosphere would not escape as rapidly from Charon as they do from Pluto.

There has been significant proof of CO2 gas and H2O vapor on the surface of Charon, but these vapors are not sufficient for a viable atmosphere due to their low vapor pressures. Pluto's surface has abundant ice formations, but these are volatile, as they are made up of volatile substances like methane. These volatile ice structures cause a good deal of geological activity, keeping its atmosphere constant, while Charon's ice structures are mainly made up of water and carbon dioxide, much less volatile substances that can stay dormant and not affect the atmosphere much.

Orbit

Charon and Pluto orbit each other every 6.387 days. The two objects are gravitationally locked to one another, so each keeps the same face towards the other. This is a case of mutual tidal locking, as compared to that of the Earth and the Moon, where the Moon always shows the same face to Earth, but not vice versa. The average distance between Charon and Pluto is 19,570 kilometres (12,160 mi). The discovery of Charon allowed astronomers to calculate accurately the mass of the Plutonian system, and mutual occultations revealed their sizes. However, neither indicated the two bodies' individual masses, which could only be estimated, until the discovery of Pluto's outer moons in late 2005. Details in the orbits of the outer moons revealed that Charon has approximately 12% of the mass of Pluto.

Physical characteristics

Charon's diameter is 1,212 kilometres (753 mi), just over half that of Pluto. Larger than the dwarf planet Ceres, it is the twelfth-largest natural satellite in the Solar System. Charon is even similar in size to Uranus's moons Umbriel and Ariel. Charon's slow rotation means that there should be little flattening or tidal distortion, if Charon is sufficiently massive to be in hydrostatic equilibrium. Any deviation from a perfect sphere is too small to have been detected by observations by the New Horizons mission. This is in contrast to Iapetus, a Saturnian moon similar in size to Charon but with a pronounced oblateness dating to early in its history. The lack of such oblateness in Charon could mean that it is currently in hydrostatic equilibrium, or simply that its orbit approached its current one early in its history, when it was still warm.

Based on mass updates from observations made by New Horizons the mass ratio of Charon to Pluto is 0.1218:1. This is much larger than the Moon to the Earth: 0.0123:1. Because of the high mass ratio, the barycenter is outside of the radius of Pluto, and the Pluto–Charon system has been referred to as a dwarf double planet. With four smaller satellites in orbit about the two larger worlds, the Pluto–Charon system has been considered in studies of the orbital stability of circumbinary planets.

It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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## #19 2023-05-03 21:42:29

Jai Ganesh
Registered: 2005-06-28
Posts: 46,163

### Re: Natural Satellite

19) Phobos

Phobos is the innermost and larger of the two natural satellites of Mars, the other being Deimos. The two moons were discovered in 1877 by American astronomer Asaph Hall. It is named after Phobos, the Greek god of fear and panic, who is the son of Ares (Mars) and twin brother of Deimos.

Phobos is a small, irregularly shaped object with a mean radius of 11 km (7 mi). Phobos orbits 6,000 km (3,700 mi) from the Martian surface, closer to its primary body than any other known planetary moon. It is so close that it orbits Mars much faster than Mars rotates, and completes an orbit in just 7 hours and 39 minutes. As a result, from the surface of Mars it appears to rise in the west, move across the sky in 4 hours and 15 minutes or less, and set in the east, twice each Martian day.

Phobos is one of the least reflective bodies in the Solar System, with an albedo of just 0.071. Surface temperatures range from about −4 °C (25 °F) on the sunlit side to −112 °C (−170 °F) on the shadowed side.[9] The defining surface feature is the large impact crater, Stickney, which takes up a substantial proportion of the moon's surface. The surface is also home to many grooves, with there being numerous theories as to how these grooves were formed.

Images and models indicate that Phobos may be a rubble pile held together by a thin crust that is being torn apart by tidal interactions. Phobos gets closer to Mars by about 2 cm per year, and it is predicted that within 30 to 50 million years it will either collide with the planet or break up into a planetary ring.

Discovery

Phobos was discovered by astronomer Asaph Hall on 18 August 1877 at the United States Naval Observatory in Washington, D.C., at about 09:14 Greenwich Mean Time. (Contemporary sources, using the pre-1925 astronomical convention that began the day at noon, give the time of discovery as 17 August at 16:06 Washington mean time, meaning 18 August 04:06 in the modern convention.) Hall had discovered Deimos, Mars's other moon, a few days earlier on 12 August 1877 at about 07:48 UTC. The names, originally spelled Phobus and Deimus respectively, were suggested by Henry Madan (1838–1901), science master at Eton College, based on Greek mythology, in which Phobos is a companion to the god, Ares.

Shklovsky's "Hollow Phobos" hypothesis

In the late 1950s and 1960s, the unusual orbital characteristics of Phobos led to speculations that it might be hollow. Around 1958, Russian astrophysicist Iosif Samuilovich Shklovsky, studying the secular acceleration of Phobos's orbital motion, suggested a "thin sheet metal" structure for Phobos, a suggestion which led to speculations that Phobos was of artificial origin. Shklovsky based his analysis on estimates of the upper Martian atmosphere's density, and deduced that for the weak braking effect to be able to account for the secular acceleration, Phobos had to be very light—one calculation yielded a hollow iron sphere 16 kilometers (9.9 mi) across but less than 6 cm thick. In a February 1960 letter to the journal Astronautics, Fred Singer, then science advisor to U.S. President Dwight D. Eisenhower, said of Shklovsky's theory:

If the satellite is indeed spiraling inward as deduced from astronomical observation, then there is little alternative to the hypothesis that it is hollow and therefore Martian made. The big 'if' lies in the astronomical observations; they may well be in error. Since they are based on several independent sets of measurements taken decades apart by different observers with different instruments, systematic errors may have influenced them.

Subsequently, the systematic data errors that Singer predicted were found to exist, and the claim was called into doubt, and accurate measurements of the orbit available by 1969 showed that the discrepancy did not exist. Singer's critique was justified when earlier studies were discovered to have used an overestimated value of 5 cm/yr for the rate of altitude loss, which was later revised to 1.8 cm/yr. The secular acceleration is now attributed to tidal effects, which create drag on the moon and therefore cause it spiral inward.

The density of Phobos has now been directly measured by spacecraft to be 1.887 g/{cm}^{3}. Current observations are consistent with Phobos being a rubble pile. In addition, images obtained by the Viking probes in the 1970s clearly showed a natural object, not an artificial one. Nevertheless, mapping by the Mars Express probe and subsequent volume calculations do suggest the presence of voids and indicate that it is not a solid chunk of rock but a porous body. The porosity of Phobos was calculated to be 30% ± 5%, or a quarter to a third being empty.

Physical characteristics

Phobos has dimensions of 27 km × 22 km × 18 km, and retains too little mass to be rounded under its own gravity. Phobos does not have an atmosphere due to its low mass and low gravity. It is one of the least reflective bodies in the Solar System, with an albedo of about 0.071. Infrared spectra show that it has carbon-rich material found in carbonaceous chondrites, and its composition shows similarities to that of Mars' surface. Phobos's density is too low to be solid rock, and it is known to have significant porosity. These results led to the suggestion that Phobos might contain a substantial reservoir of ice. Spectral observations indicate that the surface regolith layer lacks hydration, but ice below the regolith is not ruled out.

Unlike Deimos, Phobos is heavily cratered, with one of the craters near the equator having a central peak despite the moon's small size. The most prominent of these is the crater Stickney, a large impact crater some 9 km (5.6 mi) in diameter, which takes up a substantial proportion of the moon's surface area. As with Mimas's crater Herschel, the impact that created Stickney must have nearly shattered Phobos.

Many grooves and streaks also cover the oddly shaped surface. The grooves are typically less than 30 meters (98 ft) deep, 100 to 200 meters (330 to 660 ft) wide, and up to 20 kilometers (12 mi) in length, and were originally assumed to have been the result of the same impact that created Stickney. Analysis of results from the Mars Express spacecraft, however, revealed that the grooves are not in fact radial to Stickney, but are centered on the leading apex of Phobos in its orbit (which is not far from Stickney). Researchers suspect that they have been excavated by material ejected into space by impacts on the surface of Mars. The grooves thus formed as crater chains, and all of them fade away as the trailing apex of Phobos is approached. They have been grouped into 12 or more families of varying age, presumably representing at least 12 Martian impact events. However, in November 2018, following further computational probability analysis, astronomers concluded that the many grooves on Phobos were caused by boulders, ejected from the asteroid impact that created Stickney crater. These boulders rolled in a predictable pattern on the surface of the moon.

Faint dust rings produced by Phobos and Deimos have long been predicted but attempts to observe these rings have, to date, failed. Recent images from Mars Global Surveyor indicate that Phobos is covered with a layer of fine-grained regolith at least 100 meters thick; it is hypothesized to have been created by impacts from other bodies, but it is not known how the material stuck to an object with almost no gravity.

The unique Kaidun meteorite that fell on a Soviet military base in Yemen in 1980 has been hypothesized to be a piece of Phobos, but this has been difficult to verify because little is known about the exact composition of Phobos.

A person who weighs 68 kilogram-force (150 pounds) on Earth would weigh about 40 gram-force (2 ounces) standing on the surface of Phobos.

Named geological features

Geological features on Phobos are named after astronomers who studied Phobos and people and places from Jonathan Swift's Gulliver's Travels.

(1.76941 mEarths).
Mean density : 1.876 g/{cm}^3.
Temperature  :  ≈ 233 K
Apparent magnitude  :  11.8.

It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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## #20 2023-06-06 22:40:18

Jai Ganesh
Registered: 2005-06-28
Posts: 46,163

### Re: Natural Satellite

20) Deimos

Deimos (systematic designation: Mars II)[ is the smaller and outermost of the two natural satellites of Mars, the other being Phobos. Deimos has a mean radius of 6.2 km (3.9 mi) and takes 30.3 hours to orbit Mars. Deimos is 23,460 km (14,580 mi) from Mars, much farther than Mars' other moon, Phobos. It is named after Deimos, the Ancient Greek god and personification of dread and terror.

Discovery and etymology

Deimos was discovered by Asaph Hall III at the United States Naval Observatory in Washington, D.C. on 12 August 1877, at about 07:48 UTC. Hall, who also discovered Phobos shortly afterwards, had been specifically searching for Martian moons at the time.

The moon is named after Deimos, a figure representing dread in Greek mythology. The name was suggested by academic Henry Madan, who drew from Book XV of the Iliad, where Ares (the Roman god Mars) summons Dread (Deimos) and Fear (Phobos).

Origin

The origin of Mars' moons is unknown and the hypotheses are controversial. The main hypotheses are that they formed either by capture or by accretion.

Because of the postulated similarity to the composition of C- or D-type asteroids, one hypothesis is that the moons may be objects captured into Martian orbit from the asteroid belt, with orbits that have been circularized either by atmospheric drag or tidal forces, as capture requires dissipation of energy. The current Martian atmosphere is too thin to capture a Phobos-sized object by atmospheric braking. Geoffrey Landis has pointed out that the capture could have occurred if the original body was a binary asteroid that separated due to tidal forces. The main alternative hypothesis is that the moons accreted in the present position. Another hypothesis is that Mars was once surrounded by many Phobos- and Deimos-sized bodies, perhaps ejected into orbit around it by a collision with a planetesimal.

In 2021, Amirhossein Bagheri (ETH Zurich), Amir Khan (ETH Zurich), Michael Efroimsky (US Naval Observatory) and their colleagues proposed a new hypothesis on the origin of the moons. By analyzing the seismic and orbital data from Mars InSight Mission and other missions, they proposed that the moons are born from disruption of a common parent body around 1 to 2.7 billion years ago. The common progenitor of Phobos and Deimos was most probably hit by another object and shattered to form Phobos and Deimos.

Physical characteristics

Like most bodies of its size, Deimos is highly non-spherical with triaxial dimensions of 15 × 12.2 × 11 km, making it 56% of the size of Phobos. Deimos is composed of rock rich in carbonaceous material, much like C-type asteroids and carbonaceous chondrite meteorites. It is cratered, but the surface is noticeably smoother than that of Phobos, caused by the partial filling of craters with regolith.[citation needed] The regolith is highly porous and has a radar-estimated density of only 1.471 g/{cm}^3.

Escape velocity from Deimos is 5.6 m/s. This velocity could theoretically be achieved by a human performing a vertical jump. The apparent magnitude of Deimos is 12.45.

Named geological features

Only two geological features on Deimos have been given names. The craters Swift and Voltaire are named after writers who speculated on the existence of two Martian moons before Phobos and Deimos were discovered.

Orbital characteristics

Deimos' orbit is nearly circular and is close to Mars's equatorial plane. Deimos is possibly an asteroid that was perturbed by Jupiter into an orbit that allowed it to be captured by Mars, though this hypothesis is still controversial and disputed. Both Deimos and Phobos have very circular orbits which lie almost exactly in Mars' equatorial plane, and hence a capture origin requires a mechanism for circularizing the initially highly eccentric orbit, and adjusting its inclination into the equatorial plane, most likely by a combination of atmospheric drag and tidal forces; it is not clear that sufficient time was available for this to have occurred for Deimos.

As seen from Mars, Deimos would have an angular diameter of no more than 2.5 minutes (sixty minutes make one degree), one twelfth of the width of the Moon as seen from Earth, and would therefore appear almost star-like to the naked eye. At its brightest ("full moon") it would be about as bright as Venus is from Earth; at the first- or third-quarter phase it would be about as bright as Vega. With a small telescope, a Martian observer could see Deimos's phases, which take 1.2648 days (Deimos's synodic period) to run their course.

Unlike Phobos, which orbits so fast that it rises in the west and sets in the east, Deimos rises in the east and sets in the west, slower than Mars' rotation speed. The Sun-synodic orbital period of Deimos of about 30.4 hours exceeds the Martian solar day ("sol") of about 24.7 hours by such a small amount that 2.48 days (2.41 sols) elapse between its rising and setting for an equatorial observer. From Deimos-rise to Deimos-rise (or setting to setting), 5.466 days (5.320 sols) elapse.

Because Deimos's orbit is relatively close to Mars and has only a very small inclination to Mars's equator, it cannot be seen from Martian latitudes greater than 82.7°.

Deimos's orbit is slowly getting larger, because it is far enough away from Mars and because of tidal acceleration. It is expected to eventually escape Mars's gravity.

Solar transits

Deimos regularly passes in front of the Sun as seen from Mars. It is too small to cause a total eclipse, appearing only as a small black dot moving across the Sun. Its angular diameter is only about 2.5 times the angular diameter of Venus during a transit of Venus from Earth. On 4 March 2004 a transit of Deimos was photographed by Mars rover Opportunity, and on 13 March 2004 a transit was photographed by Mars rover Spirit.

Exploration

Overall, its exploration history is similar to those of Mars and of Phobos. Deimos has been photographed close-up by several spacecraft whose primary mission has been to photograph Mars, including in March 2023 during a rare close encounter by the Emirates Mars Mission. No landings on Deimos have been made.

In 1997 and 1998, the proposed Aladdin mission was selected as a finalist in the NASA Discovery Program. The plan was to visit both Phobos and Deimos, and launch projectiles at the satellites. The probe would collect the ejecta as it performed a slow flyby (~1 km/s).[35] These samples would be returned to Earth for study three years later. The principal investigator was Carle M. Pieters of Brown University. The total mission cost, including launch vehicle and operations was \$247.7 million. Ultimately, the mission chosen to fly was MESSENGER, a probe to the planet Mercury.

In 2008, NASA Glenn Research Center began studying a Phobos and Deimos sample-return mission that would use solar electric propulsion. The study gave rise to the "Hall" mission concept, a New Frontiers-class mission currently under further study.

Also, the sample-return mission called Gulliver has been conceptualized and dedicated to Deimos, in which 1 kilogram (2.2 pounds) of material from Deimos would be returned to Earth.

Another concept of sample-return mission from Phobos and Deimos is OSIRIS-REx 2, which would use heritage from the first OSIRIS-REx.

In March 2014, a Discovery class mission was proposed to place an orbiter in Mars orbit by 2021 and study Phobos and Deimos. It is called Phobos And Deimos & Mars Environment (PADME).

Human exploration of Deimos could serve as a catalyst for the human exploration of Mars. Recently, it was proposed that the sands of Deimos or Phobos could serve as a valuable material for aerobraking in the colonization of Mars.

In April 2023, astronomers released close-up global images, for the first time, of Deimos that were taken by the Mars Hope orbiter. Observations reported by this mission contravene the captured asteroid hypothesis and indicate basaltic planetary origin of Deimos.

It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

Online

## #21 2023-06-18 18:25:10

Jai Ganesh
Registered: 2005-06-28
Posts: 46,163

### Re: Natural Satellite

21) Nix

Nix is a natural satellite of Pluto, with a diameter of 49.8 km (30.9 mi) across its longest dimension. It was discovered along with Pluto's outermost moon Hydra on 15 May 2005 by astronomers using the Hubble Space Telescope, and was named after Nyx, the Greek goddess of the night. Nix is the third moon of Pluto by distance, orbiting between the moons Styx and Kerberos.

Nix was imaged along with Pluto and its other moons by the New Horizons spacecraft as it flew by the Pluto system in July 2015. Images from the New Horizons spacecraft reveal a large reddish area on Nix that is likely an impact crater.

Discovery

Nix was discovered by researchers of the Pluto Companion Search Team, using the Hubble Space Telescope. The New Horizons team had suspected that Pluto and its moon Charon might be accompanied with other moons, hence they used the Hubble Space Telescope to search for faint moons around Pluto in 2005. Since Nix's brightness is about 5,000 times fainter than Pluto, long exposure images were taken in order to find it.

The discovery images were taken on 15 May 2005 and 18 May 2005. Nix and Hydra were independently discovered by Max J. Mutchler on 15 June 2005 and by Andrew J. Steffl on 15 August 2005. The discoveries were announced on 31 October 2005, after confirmation by precovering archival Hubble images of Pluto from 2002. The two newly announced moons of Pluto were subsequently provisionally designated S/2005 P 1 for Hydra and S/2005 P 2 for Nix. The moons were informally referred to as "P1" and "P2", respectively by the discovery team.

Naming

The name Nix was approved by the International Astronomical Union (IAU) and was announced on 21 June 2006 along with the naming of Hydra in the IAU Circular 8723. Nix was named after Nyx, the Greek goddess of darkness and night and mother of Charon, the ferryman of Hades in Greek mythology. The two newly named moons were intentionally named that the order of their initials N and H honors the New Horizons mission to Pluto, similarly to how the first two letters of Pluto's name honors Percival Lowell. The original proposal for the naming of Nix was to use the classical spelling Nyx, but to avoid confusion with the asteroid 3908 Nyx, the spelling was changed to Nix, the Coptic spelling of the name. The adjectival form of the name is Nictian (cf. Russian Никта Nikta).

The names of features on the bodies in the Pluto system are related to mythology and the literature and history of exploration. In particular, the names of features on Nix must be related to deities of the night from literature, mythology, and history.

Origin

Pluto's smaller moons, including Nix, were thought to have formed from debris ejected from a massive collision between Pluto and another Kuiper belt object, similarly to how the Moon is believed to have formed from debris ejected by a large collision of Earth. The ejecta from the collision would then coalesce into the moons of Pluto. However, the collisional hypothesis cannot explain how Nix maintained its highly reflective surface.

Physical characteristics

Nix has an elongated shape, with its longest axis measured at 49.8 km (30.9 mi) across and its shortest axis 31.1 km (19.3 mi) across. This gives Nix the measured dimensions of 49.8 km × 33.2 km × 31.1 km (30.9 mi × 20.6 mi × 19.3 mi). It is the third-largest moon of Pluto, being slightly smaller than Hydra.

Early research appeared to show that the surface of Nix is reddish in color. Contrary to this, other studies show that Nix is spectrally neutral, similar to the small moons of Pluto. The neutral spectrum of Nix signifies that water ice is present on its surface. Nix also appeared to vary in brightness and albedo, or reflectivity. The brightness fluctuations were thought to be caused by areas with different albedos on the surface of Nix. Images of Nix from the New Horizons spacecraft show a large reddish area approximately 18 km (11 mi) across, which could explain the two conflicting measurements of Nix's surface color.

The reddish area is thought to be a large impact crater where the reddish material was ejected from underneath Nix's water ice layer and deposited on its surface. In this case, Nix would likely have regoliths originating from the impact. Another explanation suggests that the reddish material may have originated from a collision with Nix and another object with a different composition. However, there were no significant color variations on other impact craters on Nix.

The water ice present on the surface of Nix is responsible for its high reflectivity. Trace amounts of frozen methane may be also present on the surface of Nix, and could be responsible for the presence of reddish material, likely tholins, on its surface. In this case, tholins on the surface of Nix may have originated from the reaction of methane with ultraviolet radiation from the Sun. Derived from crater counting data from New Horizons, the age of Nix's surface is estimated to be at least four billion years old.

Rotation

Nix is not tidally locked and tumbles chaotically similarly to all smaller moons of Pluto; the moon's axial tilt and rotation period vary greatly over short timescales. Due to the chaotic rotation of Nix, it can occasionally flip its entire rotational axis. The varying gravitational influences of Pluto and Charon as they orbit their barycenter causes the chaotic tumbling of Pluto's small moons, including Nix. The chaotic tumbling of Nix is also strengthened by its elongated shape, which creates torques that act on the object. At the time of the New Horizons flyby, Nix was rotating with a period of 43.9 hours retrograde to Pluto's equator with an axial tilt of 132 degrees — it was rotating backwards in relation to its orbit around Pluto. The rotation rate of Nix had increased by 10 percent since Nix was discovered.

Mean density: 1.37 g/{cm}^3.

It appears to me that if one wants to make progress in mathematics, one should study the masters and not the pupils. - Niels Henrik Abel.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

Online

## #22 2023-08-18 22:35:37

Jai Ganesh
Registered: 2005-06-28
Posts: 46,163

### Re: Natural Satellite

22) Hydra

Hydra is a natural satellite of Pluto, with a diameter of approximately 51 km (32 mi) across its longest dimension. It is the second-largest moon of Pluto, being slightly larger than Nix. Hydra was discovered along with Nix by astronomers using the Hubble Space Telescope on 15 May 2005, and was named after the Hydra, the nine-headed underworld serpent in Greek mythology. By distance, Hydra is the fifth and outermost moon of Pluto, orbiting beyond Pluto's fourth moon Kerberos.

Hydra has a highly reflective surface caused by the presence of water ice, similar to other Plutonian moons. Hydra's reflectivity is intermediate, in between those of Pluto and Charon. The New Horizons spacecraft imaged Pluto and its moons in July 2015 and returned multiple images of Hydra.

Discovery

Members of the New Horizons team suspected that Pluto and Charon might be accompanied by other small, distant moons, weakly bound to the Pluto system. They used the Hubble Space Telescope to test this hypothesis. This lead to the discovery of Nix and Hydra – both surprisingly close to Pluto/Charon – and that no significant moons existed in Pluto's extended sphere of influence.

The discovery images were taken on 15 May 2005 and 18 May 2005. Hydra and Nix were independently discovered by Max J. Mutchler on 15 June 2005 and by Andrew J. Steffl on 15 August 2005. The discoveries were announced on 31 October 2005, after confirmation by precovering archival Hubble images of Pluto from 2002. The two newly discovered moons were subsequently provisionally designated S/2005 P 1 for Hydra and S/2005 P 2 for Nix. The moons were informally referred to as "P1" and "P2" respectively, by the discovery team.

Naming

The name Hydra was approved on 21 June 2006 by the International Astronomical Union (IAU) and was announced along with the naming of Nix in the IAU Circular 8723. Hydra was named after the Lernaean Hydra, a nine-headed serpent that battled Heracles in Greek mythology. Particularly, the nine heads of Hydra subtly references Pluto's former ninth planetary status. The two newly named moons were intentionally named that the order of their initials N and H honors the New Horizons mission to Pluto, similarly to how the first two letters of Pluto's name honors Percival Lowell. Hydra's name was also intentionally chosen that its initial H honors the Hubble Space Telescope used by the Pluto Companion Search Team to discover Hydra and Nix.

The names of features on the bodies in the Pluto system are related to mythology and the literature and history of exploration. In particular, the names of features on Hydra must be related to legendary serpents and dragons from literature, mythology, and history.

Origin

Pluto's smaller moons, including Hydra, were thought to have formed from debris ejected from a massive collision between Pluto and another Kuiper belt object, similarly to how the Moon is believed to have formed from debris ejected by a large collision of Earth. The ejecta from the collision would then coalesce into the moons of Pluto. It was thought that Hydra had initially formed at a closer proximity to Pluto, and its orbit had undergone changes through tidal interactions. In this case, Hydra along with the smaller moons of Pluto would have migrated outwards with Charon into their current orbits around the Pluto-Charon barycenter. Through 'tidal damping' by mutual tidal interactions with Charon, Hydra's orbit around the Pluto-Charon barycenter gradually became more circular over time. Hydra is believed to have formed from two smaller objects merging into one single object.

Physical characteristics

Hydra is irregular in shape, measuring 50.9 km (31.6 mi) along its longest axis and its shortest axis measuring 30.9 km (19.2 mi) across. This gives Hydra the measured dimensions of 50.9 km × 36.1 km × 30.9 km (31.6 mi × 22.4 mi × 19.2 mi).

The surface of Hydra is highly reflective due to the presence of water ice on its surface. The surface of Hydra displays a neutral spectrum similarly to Pluto's small moons, although the spectrum of Hydra appears slightly bluer. The water ice on Hydra's surface is relatively pure and shows no significant darkening compared to Charon. One explanation suggests that Hydra's surface is continually refreshed by micrometeorite impacts ejecting darker material from the surface of Hydra. The surface spectrum of Hydra is slightly bluish compared to that of Nix. Explanations for Hydra's bluish color suggest that the surface of Hydra has a higher amount of water ice compared to Nix, which could also explain Hydra's very high geometric albedo, or its reflectivity, of 83 percent.

Derived from crater counting data from New Horizons, the surface of Hydra is estimated to be about four billion years old. Large craters and indentations on Hydra suggest that it may have lost some of its original mass from impact events since its formation.

Rotation

Hydra is not tidally locked and rotates chaotically; its rotational period and axial tilt vary quickly over astronomical timescales, to the point that its rotational axis regularly flips over. Hydra's chaotic tumbling is largely caused by the varying gravitational influences of Pluto and Charon as they orbit around their barycenter. Hydra's chaotic tumbling is also strengthened by its irregular shape, which creates torques that act on the object. At the time of the New Horizons flyby of Pluto and its moons, Hydra's rotation period was approximately 10 hours and its rotational axis was tilted about 110 degrees to its orbit — it was rotating sideways at the time of the New Horizons flyby.

Hydra rotates relatively quickly compared to the rest of Pluto's moons, which all have rotation periods greater than one day. This rapid rotation of Hydra is common among the rotation periods of most Kuiper belt objects. Hydra's surface material could get ejected due to centrifugal forces if it were rotating at a faster rate.

Orbit

Hydra orbits the Pluto-Charon barycenter at a distance of 64,738 km (40,226 mi). Hydra is the outermost moon of Pluto, orbiting beyond Kerberos. Similarly to all of Pluto's moons, Hydra's orbit is nearly circular and is coplanar to Charon's orbit; all of Pluto's moons have very low orbital inclinations to Pluto's equator.

The nearly circular and coplanar orbits of Pluto's moons suggest that they may have gone through tidal evolutions since their formation. At the time of the formation of Pluto's smaller moons, Hydra may have had a more eccentric orbit around the Pluto-Charon barycenter. The present circular orbit of Hydra may have been caused by Charon's tidal damping of the eccentricity of Hydra's orbit, through tidal interactions. The mutual tidal interactions of Charon on Hydra's orbit would cause Hydra to transfer its orbital eccentricity to Charon, thus causing the orbit of Hydra to gradually become more circular over time.

Hydra has an orbital period of approximately 38.2 days and is resonant with other moons of Pluto. Hydra is in a 2:3 orbital resonance with Nix, and a 6:11 resonance with Styx (the ratios represent numbers of orbits completed per unit time; the period ratios are the inverses). As a result of this "Laplace-like" 3-body resonance, it has conjunctions with Styx and Nix in a 5:3 ratio.

Hydra's orbit is close to a 1:6 orbital resonance with Charon, with a timing discrepancy of 0.3%. A hypothesis explaining the near-resonance suggests that the resonance originated before the outward migration of Charon after the formation of all five known moons, and is maintained by the periodic local fluctuation of 5% in the Pluto–Charon gravitational field strength.

The New Horizons spacecraft visited the Pluto system and imaged Pluto and its moons during its flyby on 14 July 2015. At the time of the New Horizons flyby, Hydra was behind Pluto and was further away from New Horizons at closest approach. The larger distance of Hydra from New Horizons resulted in lower resolution images of Hydra. Before the flyby, the Long Range Reconnaissance Imager on board New Horizons performed measurements of Hydra's size, estimating Hydra to be about 45 km (28 mi) in diameter. Hydra's surface composition, reflectivity, and other basic physical properties were later measured by New Horizons during the flyby.

The first detailed image of Hydra was downlinked, or received from the New Horizons spacecraft on 15 July 2015 after the flyby. The first detailed image of Hydra, taken from a distance of 640,000 km (400,000 mi), appeared to show brightness variations and a dark circular feature 10 km (6.2 mi) across. The highest resolution images of Hydra were taken from a distance of 231,000 km (144,000 mi), with an image resolution of 1.2 km (0.75 mi) per pixel. Derived from those images, Hydra was given the approximate size estimate of 55 km × 40 km (34 mi × 25 mi).

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