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

ganesh
Registered: 2005-06-28
Posts: 37,516

### 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

ganesh
Registered: 2005-06-28
Posts: 37,516

### 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

ganesh
Registered: 2005-06-28
Posts: 37,516

### 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

ganesh
Registered: 2005-06-28
Posts: 37,516

### 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.

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

ganesh
Registered: 2005-06-28
Posts: 37,516

### 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

ganesh
Registered: 2005-06-28
Posts: 37,516

### 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

ganesh
Registered: 2005-06-28
Posts: 37,516

### 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

ganesh
Registered: 2005-06-28
Posts: 37,516

### 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

ganesh
Registered: 2005-06-28
Posts: 37,516

### 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.

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

ganesh
Registered: 2005-06-28
Posts: 37,516

### 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%.

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