Ganymede (moon)

The largest satellite of Jupiter and of the entire solar system, slightly bigger even than Mercury (but with only half its mass); it was discovered by Galileo and Simon Marius in 1610. Ganymede appears to have a differentiated internal structure, with a small molten iron or iron-sulfur core surrounded by a rocky silicate mantle with an icy shell on top. Its surface is a roughly equal blend of ancient, densely cratered dark terrain, and younger, lighter regions that are marked with a network of grooves and ridges. This network is evidently tectonic in nature, though, unlike on Earth, there are no signs of recent tectonic activity. Evidence for a very thin oxygen atmosphere on Ganymede, like that of Europa, has been found by the Hubble Space Telescope. Similar ridge and groove terrain is seen on Enceladus, Miranda, and Ariel. The dark regions are similar to the surface of Callisto. Extensive cratering is seen on both types of terrain, with a density that points to a surface age of 3 to 3.5 billion years, similar to that of the Moon. Craters both overlay and are cross-cut by the groove systems showing that the grooves are quite ancient, too. Relatively young craters with rays of ejecta are also visible. However, these craters are quite flat, lacking the ring mountains and central depressions common to craters on the Moon and Mercury. This is due to the plastic nature of Ganymede's icy crust, which can flow over millions of years and thereby smooth out any surface features. Ganymede has its own magnetosphere, embedded within Jupiter's huge one, that is probably generated in a similar way to Earth's, as a result of motion of conducting material in the interior. Galileo Regio is a prominent, dark, oval region, about 3,200 km across, centered approximately at latitude +36° and longitude 138°. Gilgamesh is the largest preserved impact basin on Ganymede, with a rim 580 km in diameter surrounding a roughly 300-km-wide smooth central depression. An ejecta blanket and secondary crater chains blasted out from the basin are visible out to approximately 500 km from the basin center.