Ganymede (moon)

Ganymede

True color image taken by the Galileo probe
Discovery
Discovered by:	G. Galilei S. Marius
Discovery date:	January 11, 1610
Orbital characteristics
Periapsis:	1,069,200 km (0.007147 AU)
Apoapsis	1,071,600 km (0.007163 AU)
Mean radius of orbit:	1,070,400 km (0.007155 AU)
Eccentricity:	0.002[1]
Orbital period:	7.15455296 d (0.019588 a)
Avg. orbital speed:	10.880 km/s
Inclination:	2.21° (to the ecliptic) 0.20° to Jupiter's equator)
Satellite of:	Jupiter
Physical characteristics
Mean radius:	2631.2 km (0.413 Earths)
Surface area:	87.0 million km² (0.171 Earths) [2]
Volume:	7.6×1010 km³ (0.0704 Earths)
Mass:	1.4819×1023 kg (0.025 Earths)
Mean density:	1.942 g/cm³
Equatorial surface gravity:	1.428 m/s2 (0.146 g)
Escape velocity:	2.741 km/s (6,130 mph)
Rotation period:	synchronous
Axial tilt:	zero
Albedo:	0.43 ± 0.02[3]
Temperature:	~109 K (−172°C)
Apparent magnitude:	4.61 (opposition) [3]
Atmosphere
Surface pressure:	trace
Composition:	oxygen

Ganymede (pronounced /ˈgænɨmiːd/, Greek Γανυμήδης) is Jupiter's largest natural satellite, and the largest natural satellite in the Solar System; it is larger in diameter than Mercury but has only about half its mass. Ganymede is one of four planetary satellites of Jupiter that could be seen with the naked eye under favorable conditions, if not for the brighter light of nearby Jupiter. Other than possibly the other Galilean Satellites, the only planetary satellite visible with the naked eye is our own Moon. Ganymede's discovery is generally credited to Galileo Galilei who documented its existence in 1610.^[4] The name Ganymede was suggested soon after by Simon Marius, for the cup-bearer of the Greek gods, beloved of Zeus (see Ganymede).^[5] This name and the names of the other Galilean satellites fell into disfavor for a considerable time, and were not revived in common use until the mid-20th century. In much of the earlier astronomical literature, it is simply referred to by its Roman numeral designation (a system introduced by Galileo) as Jupiter III or as the "third satellite of Jupiter".^[6] Ganymede is the only Galilean moon of Jupiter named after a male figure.

Discovery and naming

On January 7, 1610, Galileo Galilei observed three stars near Jupiter.^[7] The next night, he saw that the stars had moved.^[7] A fourth star, Ganymede, was found on January 11.^[7] By January 15, Galileo came to the conclusion that the stars were bodies orbiting Jupiter.^[7] Galileo claimed the right to name the moons; he considered "Cosmian Stars” and settled on “Medicean Stars”.^[6] Nicholas Clade Fabri de Peiresc tried to give the moons individual names, but his system was not used.^[6] Simon Marius, who had originally claimed to have found the Galilean Satellites,^[8] 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, a system which never caught on.^[6] From a suggestion by Johannes Kepler, Simon Marius once again tried to name the moons.^[6]

However, none of these various naming schemes caught on, except to use a numerical system introduced by Galileo. Following the discovery of moons of Saturn, a naming system based on the one Kepler and Marius had created was put to use for Jupiter’s moons.^[6]

Orbit and rotation

Ganymede orbits at a distance of 1,070,400 kilometers, making it the third farthest out of the Galilean satellites.^[9] Ganymede revolves around Jupiter in seven days and three hours. Like most moons, Ganymede is tidally locked, meaning it keeps one face always pointing toward Jupiter. This is because its revolution around Jupiter is the same as its rotation.^[10] Ganymede participates in an orbital resonance with Europa and Io, in which for every orbit of Ganymede, Europa orbits twice and Io orbits four times.^[11] The resonance causes an orbital eccentricity of 0.002.^[1][11] Also due to the orbital resonance as well as Jupiter, tidal heating is present. Tidal heating doesn’t have as much of an effect on Ganymede as it does on Europa and Io, but there are still outcomes of the heating. For example, Ganymede’s interior has differentiated.^[11]

Physical characteristics

Composition

Ganymede's surface is composed of ice and carbonaceous silicates.^[10] The composition differs by layers. The interior has carbonaceous chondrite and other materials formed when Jupiter formed. The interior is the mostly rock, while the middle is a mixture of ice and rock, and the surface is ice.^[12] The distribution of water ice on the surface of Ganymede is also uneven. Infrared pictures from Galileo have shown that in brighter areas water ice is more abundant, while in darker spots water ice is depleted.^[13] Other Galileo results have shown magnesium sulfate on Ganymede's surface.^[10]

Internal structure

Interior of Ganymede

Ganymede is composed of 49 to 59% silicate rock, and its density is consistent with a high proportion of water ice. Indications from the Galileo orbiter data suggest that Ganymede is differentiated into a three-layer structure: a silicate core, a mantle composed of mixed ice and rock, and a crust of refrozen ice. The core is predicted to be made of "Prinn-Fegley" assemblage rock, which is rock predicted to be condensed from a proto-Jovian nebula, or the anhydrous PTC assemblage. CI carbonaceous chondrite is plausible as a lower mantle constituent. The three aforementioned types of rock have densities of 2.766, 3.262, and 3.756 g/cm³ respectively.^[12]

Voyager 2 image mosaic of Ganymede's anti-Jovian hemisphere. The ancient dark area of Galileo Regio lies at the upper right. It is separated from the smaller dark region of Marius Regio to its left by the brighter and younger band of Uruk Sulcus. Fresh ice ejected from the relatively recent Osiris Crater created the bright rays at the bottom.

A sharp boundary divides the dark Nicholson Regio from the bright Harpagia Sulcus.

Surface features

The Ganymedean 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. Their origin is clearly of a tectonic nature, probably formed by the extension, stretching, and faulting of the icy crust. The dark terrain contains clays and organic materials that could indicate the composition of the impactors from which Jovian satellites accreted.^[14] Extensive cratering is seen on both types of terrain. The density of cratering indicates an age of 4 billion years for the dark terrain, similar to the highlands of the Moon, and a somewhat younger age for the bright grooved terrain (but how much younger is uncertain). Craters both overlay and are cross-cut by the groove systems indicating that some of the grooves are quite ancient. Relatively young craters with rays of ejecta are also visible.^[15] Unlike on the Moon, however, Ganymedean craters are quite flat, lacking the ring mountains and central depressions common to craters on the Moon and Mercury. This is probably due to the relatively weak nature of Ganymede's icy crust which can flow and thereby soften the relief. Ancient craters whose relief has disappeared leaving only a "ghost" of a crater are known as palimpsests. The largest feature on Ganymede is a dark plain named Galileo Regio, which contains a series of concentric grooves, or furrows, that are remnants of an ancient multi-ringed impact basin that has been mostly obliterated by subsequent geological activity.

Atmosphere

In 1972, a team of Indian, British and American astronomers working at Indonesia's Bosscha Observatory in Lembang detected a thin atmosphere around Ganymede during an occultation when Jupiter (and Ganymede) passed in front of a star.^[16] Evidence for a tenuous oxygen atmosphere on Ganymede, very similar to the one found on Europa, has since been found by the Hubble Space Telescope.^[17] Note that this is not evidence of life: it is thought that the oxygen is 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.

Magnetosphere

Enhanced-color Galileo spacecraft image of Ganymede's trailing hemisphere^[18]

The Galileo orbiter's first flyby of Ganymede discovered that Ganymede has its own magnetic field, embedded inside Jupiter's huge field. Ganymede is the only moon known to have a magnetosphere.^[17] Ganymede's intrinsic magnetic field is probably generated in a similar fashion to the Earth's: as a result of conducting material moving in the interior, likely originating in its metallic core. Ganymede also has an induced magnetic field component, indicating that the satellite contains a subsurface layer that acts as a conductor. It is thought that this conductive material is a layer of liquid water containing salt, located at about 150 km depth and sandwiched between layers of different density forms of ice. Ganymede is the most largely concentrated, solid body known in the solar system, which suggests complete global differentiation and a metallic core. The magnetic field detected around Ganymede is assumed to be caused by thermal or compositional convection in the core, if the magnetic field is the product of dynamo action or magnetoconvection. Convection within the silicate mantle is also a possibility during Ganymede's history.^[19]

Origin and evolution

Ganymede was formed four billion years ago.^[20] Ganymede likely formed by a slow accretion in Jupiter’s subnebula, which was a disk of the gas and dust that existed around Jupiter after its formation.^[21] Ganymede cooled slowly, slower than Europa or Io.^[22] Ganymede experienced a period of heavy cratering 3 to 3.5 billion years ago.^[17] Ganymede's interior was differentiating. This meant a heating to power the differentiation had occurred.^[23] Internal heating also formed grooves. From the grooves rose a rocky slush that gave the grooves their color and covered any nearby craters.^[22] This created a clash between the old and new terrain. The older terrain is dark and cratered, while the newer is brighter and less cratered. These bright regions were likely formed by water erupting from beneath the surface. The brighter regions have small fissures in them running in parallel lines.^[10] The major forces that shaped Ganymede are volcanic and tectonic.^[24] The tectonic forces could be caused by several factors, such as a change in temperature or convection in the mantle.^[25]

Possibility of life

It has been suggested that Ganymede may be amenable to life, due to experiments in which bacteria have survived intense pressure.^[26] However, scientists say that the chances of life existing on Europa are much higher than on Ganymede, because Europa's ocean is closer to the surface than Ganymede's.^[27]

See also

• List of craters on Ganymede
• Lunar and Planetary Institute
• Colonization of Ganymede
• Jupiter's moons in fiction

References

External links

• Ganymede Profile by NASA's Solar System Exploration
• Ganymede page on The Nine8 Planets
• Jupiter's Moon Ganymede on Views of the Solar System
• Ganymede Crater Database Lunar and Planetary Institute

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