Ganymede (moon)

Ganymede^[p] is the largest of the 95 known moons of the planet Jupiter. It is also the largest moon in the Solar System. Ganymede is larger in diameter than the planet Mercury, but has only about half of its mass as Ganymede is much less dense. Ganymede is part of a group called the Galilean Satellites. These also include Io, Europa and Callisto.

Ganymede

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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 orbit radius	1,070,400 km (0.007155 AU)
Eccentricity	0.002[1]
Orbital period	7.15455296 d (0.019588 a)
Average 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
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 by volume	oxygen

Galileo Galilei discovered this moon in 1610.^[4] Simon Marius suggested the name "Ganymede" soon after. In Greek mythology, Ganymede was Zeus' cup-bearer.^[5] This name and the names of the other Galilean satellites were not favoured for a long time, and were not put into common use until the mid-20th century. Instead, it is simply referred to by its Roman numeral designation (a system that was 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.

Structure

Ganymede is composed of roughly equal amounts of silicate rock and water ice.^[7] Its body has an iron-rich, liquid core, and an internal ocean that may contain more water than all of Earth's oceans together.^{[8][9][10][11][12]} Its surface has dark regions, with impact craters dated to four billion years ago. This covers about a third of the satellite. Lighter regions, crosscut by grooves and ridges and only slightly less ancient, cover the rest. The cause of the light terrain's disrupted geology is not fully known, but was likely the result of tectonic activity caused by tidal heating.^[13] Ganymede is also the only known moon to have a magnetic field of its own.^[14]

Notes

^{[p] ^} The name "Ganymede" is said as /Gan-ee-Meed/.