Ganymede (moon) Totally Explained

Everything about Ganymede Moon totally explained

Ganymede (GAN-ə-meed, or as Greek Γανυμήδης) is a natural satellite of Jupiter and the largest natural satellite in the Solar System. Completing an orbit in a little more than seven days, it's the seventh satellite and third Galilean satellite from Jupiter.
   Ganymede is composed primarily of silicate rock and water ice. It is a fully differentiated body with an iron-rich, liquid core. A saltwater ocean is believed to exist nearly 200 km below Ganymede's surface, sandwiched between layers of ice. The satellite's name was soon suggested by astronomer Simon Marius, for the mythological Ganymede, cupbearer of the Greek gods and Zeus's beloved. He claimed the right to name the moons; he considered "Cosmian Stars" and settled on "Medicean Stars".
   The French astronomer Nicolas-Claude Fabri de Peiresc suggested individual names from the Medici family for the moons, but his proposal wasn't taken up. tried to name the moons the "Saturn of Jupiter", the "Jupiter of Jupiter" (this was Ganymede), the "Venus of Jupiter", and the "Mercury of Jupiter", another nomenclature that never caught on. From a suggestion by Johannes Kepler, Marius once again tried to name the moons: and completes a revolution every seven days and three hours. Like most known moons, Ganymede is tidally locked, with one face always pointing toward the planet. 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. The superior conjunction between Io and Europa always occurs when Io is at periapsis and Europa at apoapsis. The superior conjunction between Europa and Ganymede occurs when Europa is at periapsis.
   The current Laplace resonance is unable to pump the orbital eccentricity of Ganymede to a higher value. or that it developed after the formation of the Solar System. A possible sequence of the events is as follows: Io raised tides on Jupiter, causing its orbit to expand until it encountered 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 2:1 resonance with Ganymede. Some additional volatile ices such as ammonia may also be present. The exact composition of Ganymede's rock isn't 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 1.05:1.27 in Ganymede, whereas the solar ratio is around 1.8. significantly more than in Ganymede as a whole. Near-infrared spectroscopy has revealed the presence of strong water ice absorption bands at wavelengths of 1.04, 1.25, 1.5, 2.0 and 3.0 μm. The grooved terrain is brighter and has more icy composition than the dark terrain. The analysis of high-resolution, near-infrared and UV spectra obtained by the Galileo spacecraft and from the ground has revealed various non-water materials: carbon dioxide, sulfur dioxide and, possibly, cyanogen, hydrogen sulfate and various organic compounds. Galileo results have also shown magnesium sulfate (MgSO₄) and, possibly, sodium sulfate (Na₂SO₄) on Ganymede's surface. These salts may originate from the subsurface ocean. The distribution of carbon dioxide doesn't demonstrate any hemispheric asymmetry, although it isn't observed near the poles. Impact craters on Ganymede (except one) don't show any enrichment in carbon dioxide, which also distinguishes it from Callisto. Ganymede's carbon dioxide levels were probably depleted in the past. The density of the core is 5.5–6 g/cm³ and the silicate mantle is 3.4–3.6 g/cm³. A subsurface ocean is also present in the other icy Galilean moons, Europa and Callisto. The existence of the oceans is connected with the anomalous behavior of the ice I melting temperature—it decreases with pressure reaching as low as 251 K at 2,070 bar (207 MPa), which is the water–ice I–ice III triple point. However, the lid is probably somewhat thinner—closer to 100 km. In this case the depth of the ocean is around 230 km.

Surface features

The Ganymedian surface is a mix of two types of terrain: very old, highly cratered, dark regions and somewhat younger (but still ancient), lighter regions marked with an extensive array of grooves and ridges. The dark terrain, which comprises about one-third of the surface, contains clays and organic materials that could indicate the composition of the impactors from which Jovian satellites accreted. The tidal flexing of the ice may have heated the interior and strained the lithosphere, leading to the development of cracks and horst and graben faulting, which erased the old, dark terrain on 70% of the surface. Cratering is seen on both types of terrain, but is especially extensive on the dark terrain: it appears to be saturated with impact craters and has evolved largely through impact events. Ganymede may have experienced a period of heavy cratering 3.5 to 4 billion years ago similar to that of the Moon. Another prominent feature on Ganymede are polar caps, likely composed of water frost. The frost extends to 40° latitude.

Atmosphere and ionosphere

In 1972, a team of Indian, British and American astronomers working at Indonesia's Bosscha Observatory claimed that they'd detected a thin atmosphere around the satellite during an occultation, when it and Jupiter passed in front of a star. They estimated that the surface pressure was around 1 μBar (0.1 Pa). The occultation measurements were conducted in the far-ultraviolet spectrum with wavelength shorter than 200 nm; they were much more sensitive to the presence of gases than measurements in the visible spectrum in 1972. No atmosphere was revealed by the Voyager data. The upper limit on the surface particle number density was found to be cm⁻³}}, which corresponds to a surface pressure of less than μBar}}. HST actually observed airglow of atomic oxygen in the far-ultraviolet at the wavelengths 130.4 nm and 135.6 nm. Such an airglow is excited when molecular oxygen is dissociated by electron impacts, These values are in agreement with the Voyager's upper limit set in 1981. The oxygen isn't evidence of life; it's thought to be produced when water ice on Ganymede's surface is split into hydrogen and oxygen by radiation, with the hydrogen then being more rapidly lost due to its low atomic mass. The existence of a neutral atmosphere implies that an ionosphere should exist, because oxygen molecules are ionized by the impacts of the energetic electrons coming from the magnetosphere However, the nature of the ganymedian ionosphere is as controversial as the nature of the atmosphere. Some Galileo measurements found an elevated electron density near the moon, suggesting an ionosphere, while others failed to detect anything. In 1997 spectroscopic analysis revealed the dimer (or diatomic) absorption features of the molecular oxygen. Such an absorption can arise only if the oxygen is in a dense phase. The best candidate is the molecular oxygen trapped in ice. The depth of the dimer absorption bands depends on latitude and longitude, rather than on surface albedo—they tend to decrease with increasing latitude on Ganymede, while the O₃ shows an opposite effect. Laboratory work has found that O₂ wouldn't cluster and bubble but dissolve in ice at Ganymede's relatively warm surface temperature of 100 K.
A search for sodium in the atmosphere, just after such a finding on Europa, turned up nothing in 1997. Sodium is at least 13 times less abundant around Ganymede than around Europa, possibly because of a relative deficiency at the surface or because the magnetosphere fends off energetic particles. Another minor constituent of the ganymedian atmosphere is atomic hydrogen. Hydrogen atoms were observed as far as 3,000 km from the surface of the moon. Their density on the surface is about cm⁻³}}.

Magnetosphere

The Galileo craft made six close flybys of Ganymede from 1995–2000 (G1, G2, G7, G8, G28 and G29) and discovered that Ganymede has a permanent (intrinsic) magnetic moment independent of the Jovian magnetic field. The value of the moment is about T·m³}}, The ganymedian magnetosphere has a region of closed field lines located below 30° latitude, where charged particles (electrons and ions) are trapped, creating a kind of radiation belt.—which fits well with the tenuous oxygen atmosphere of the moon. In the polar cap regions, at latitudes higher than 30°, magnetic field lines are open, connecting Ganymede with Jupiter's ionosphere. In addition, heavy ions continuously precipitate on the polar surface of the moon, sputtering and darkening the ice. The interaction between the ganymedian magnetosphere and Jovian plasma is in many respects similar to that of the solar wind and Earth's magnetosphere.
   In addition to the intrinsic magnetic moment, Ganymede has an induced dipole magnetic field. its intrinsic magnetic field is probably generated in a similar fashion to the Earth's: as a result of conducting material moving in the interior.
   Despite the presence of an iron core, Ganymede's magnetosphere remains enigmatic, particularly given that similar bodies lack the feature. Another explanation is a remnant magnetization of silicate rocks in the mantle, which is possible if the satellite had a more significant dynamo-generated field in the past. The accretion of Ganymede probably took about 10,000 years, much shorter than the 100,000 years estimated for Callisto. Jovian subnebula may have been relatively "gas-starved" when the Galilean satellites formed; this would have allowed for the lengthy accretion times required for Callisto. This hypothesis explains why the two Jovian moons look so dissimilar, despite their similar mass and composition. The convective motions in Callisto have caused only a partial separation of rock and ice. neither of which returned much information about the satellite. Voyager 1 and Voyager 2 were next, passing by Ganymede in 1979. They refined its size, revealing it was larger than Saturn's moon Titan, which was previously thought to have been bigger. The grooved terrain was also seen.
   In 1995, the Galileo spacecraft entered orbit around Jupiter and between 1996 and 2000 made six close flybys to explore Ganymede. while the discovery of the ocean was announced in 2001.
   One proposal to orbit Ganymede was the Jupiter Icy Moons Orbiter. Nuclear fission would have been used to power the craft, which would have been able to study Ganymede in detail. However, the mission was cancelled in 2005 because of budget cuts. There is a proposal to send a dedicated mission to orbit Ganymede, tentatively called The Grandeur of Ganymede. If flown, the orbiter will stay in a low-altitude polar orbit around the moon for at least a year.

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