Ganymede (moon)
Image of Ganymede's anti-Jovian hemisphere taken by the Galileo orbiter (contrast is enhanced). Lighter surfaces, such as in recent impacts, grooved terrain and the whitish north polar cap at upper right, are enriched in water ice.
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| Discovery | |||||||||
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| Discovered by | Galileo Galilei | ||||||||
| Discovery date | January 7, 1610 | ||||||||
| Designations | |||||||||
| Jupiter III | |||||||||
| Adjectives | Ganymedian, Ganymedean | ||||||||
| Orbital characteristics | |||||||||
| Periapsis | 1069200 km | ||||||||
| Apoapsis | 1071600 km | ||||||||
| 1070400 km | |||||||||
| Eccentricity | 0.0013 | ||||||||
| 7.15455296 d | |||||||||
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Average orbital speed
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10.880 km/s | ||||||||
| Inclination | 2.214° (to the ecliptic) 0.20° (to Jupiter's equator) |
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| Satellite of | Jupiter | ||||||||
| Physical characteristics | |||||||||
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Mean radius
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2634.1±0.3 km (0.413 Earths) | ||||||||
| 8.72×107 km2 (0.171 Earths) | |||||||||
| Volume | 7.6×1010 km3 (0.0704 Earths) | ||||||||
| Mass | 1.4819×1023 kg (0.025 Earths) | ||||||||
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Mean density
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1.936 g/cm3 | ||||||||
| 1.428 m/s2 (0.146 g) | |||||||||
| 0.3105±0.0028 (estimate) | |||||||||
| 2.741 km/s | |||||||||
| synchronous | |||||||||
| 0–0.33° | |||||||||
| Albedo | 0.43±0.02 | ||||||||
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| 4.61 (opposition) 4.38 (in 1951) |
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| Atmosphere | |||||||||
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Surface pressure
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Trace | ||||||||
| Composition by volume | Oxygen | ||||||||
Ganymede /ˈɡænᵻmiːd/ (Jupiter III) is the largest and most massive moon of Jupiter and in the Solar System. The ninth largest object in the Solar System, it is the largest without a substantial atmosphere. It has a diameter of 5,268 km (3,273 mi) and is 8% larger than the planet Mercury, although 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. It is the third of the Galilean moons, the first group of objects discovered orbiting another planet, and the seventh satellite outward from Jupiter, 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 ice. 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 the satellite. 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.
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