Ganymede

Overview

Ganymede is the largest and most massive of the Solar System's moons. The ninth-largest object (including the Sun) of the Solar System, it has a diameter of 5,268 km, making it 26 percent larger than the planet Mercury by volume, although it is only 45 percent as massive. Outward from Jupiter, it is the seventh satellite and the third of the Galilean moons. 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. 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.

Geography

Like most known moons, Ganymede is tidally locked, with one side always facing toward the planet. With a diameter of about 5,270km and a mass of 1.48×10²⁰kg, Ganymede is the largest and most massive moon in the Solar System. It is slightly more massive than the second most massive moon, Saturn's moon Titan, and is more than twice as massive as Earth's Luna. It is larger than the planet Mercury, which has a diameter of 4,880km but is only 45 percent of Mercury's mass. Ganymede is the ninth-largest object in the solar system, but the tenth-most massive.
The Ganymedian surface is very asymmetric; the leading hemisphere is brighter than the trailing one. This is similar to Europa, but the reverse for Callisto. Beyond hemispheres Ganymede's surface is a mix of two types of terrain: very old, highly cratered, dark regions and somewhat younger, lighter regions marked with an extensive array of grooves and ridges. The dark terrain, which comprises about one-third of the surface, contains clays that could indicate the composition of the impactors from which Jovian satellites accreted.
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. The brighter, grooved terrain contains many fewer impact features, which have been only of a minor importance to its tectonic evolution. 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 grooved terrain. 
Craters both overlay and are crosscut by the groove systems, indicating that some of the grooves are quite ancient. Relatively young craters with rays of ejecta are also visible. Ganymedian craters are flatter than those 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. One significant feature on Ganymede is a dark plain named Galileo Regio, which contains a series of concentric grooves, or furrows, likely created during a period of geologic activity.

Type

Planetoid / Moon

Location under

Jupiter