With a diameter of 5 280 kilometers, Ganymede is not only Jupiter's largest satellite, but also the largest satellite in the entire solar system. Less than a million kilometers from the planet, it is somewhat similar to the Moon, but also has a complex network of parallel furrows and crests which seem to have been made by an enormous rake. These grooves, with a width that varies from 5 and 15 kilometers, are probably cracks in the crust filled by fluid material which then solidified. Furthermore, the furrow systems, which were formed at Ganymede's primordial beginnings, are a real mosaic, where the oldest pieces stop brusquely on the border of the more recent parts and only rarely cross through them. Another of Ganymede's particular features is a wide and very dark circular area that is so extensive that it can even be observed from the Earth with telescopes. Surrounded by very old craters, it is crossed by brilliant, parallel and slightly curved strips. Geologists believe that these might also have been formed by a large meteorite: the traces of the impact probably were wiped out by the formation of young grooved land. The very low density (slightly less than double that of water) and the presence, next to very well-defined and certainly more recent craters, of barely visible circular shapes, called "phantom" craters, is further evidence of the many periods of "settling" by Ganymede's crust. Today, it is believed that there is a liquid water mantle below the crust of this moon that surrounds a solid nucleus filled with light siliceous rocks similar to those which formed the Earth's crust. This hypothesis would seem to be confirmed by the light-colored rays, of recent ice, which extend outwards from each crater.
With a diameter of 5 280 kilometers, Ganymede is not only Jupiter's largest satellite, but also the largest satellite in the entire solar system. Less than a million kilometers from the planet, it is somewhat similar to the Moon, but also has a complex network of parallel furrows and crests which seem to have been made by an enormous rake. These grooves, with a width that varies from 5 and 15 kilometers, are probably cracks in the crust filled by fluid material which then solidified. Furthermore, the furrow systems, which were formed at Ganymede's primordial beginnings, are a real mosaic, where the oldest pieces stop brusquely on the border of the more recent parts and only rarely cross through them. Another of Ganymede's particular features is a wide and very dark circular area that is so extensive that it can even be observed from the Earth with telescopes. Surrounded by very old craters, it is crossed by brilliant, parallel and slightly curved strips. Geologists believe that these might also have been formed by a large meteorite: the traces of the impact probably were wiped out by the formation of young grooved land. The very low density (slightly less than double that of water) and the presence, next to very well-defined and certainly more recent craters, of barely visible circular shapes, called "phantom" craters, is further evidence of the many periods of "settling" by Ganymede's crust. Today, it is believed that there is a liquid water mantle below the crust of this moon that surrounds a solid nucleus filled with light siliceous rocks similar to those which formed the Earth's crust. This hypothesis would seem to be confirmed by the light-colored rays, of recent ice, which extend outwards from each crater.
