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Orbit: 1,070,000 km from Jupiter
Orbital period: 172 hours
Gravity: 1.428 m/s²
Diameter: 5262
Mass: 1.48e23 kg

Ganymede is the largest satellite in the  solar system.       It is larger in diameter than Mercury but only       about half its mass.  Ganymede is  much larger than  Pluto.

Before the Galileo encounters with  Ganymede it was thought that Ganymede and Callisto were composed of a rocky core surrounded       by a large mantle of water or  water ice with an ice surface (and that        Titan and Triton      were similar).       Preliminary  indications from the Galileo data now suggest that Callisto has a uniform  composition while Ganymede is differentiated into a three layer structure: a  small molten iron or iron/sulfur core surrounded by a rocky silicate mantle with  a icy shell on top. In fact, Ganymede may be similar to Io with an additional  outer layer of ice.

surface photo of Ganmede

Ganymede’s surface is a roughly equal mix of two types of  terrain: very        old, highly       cratered dark  regions (above),       and somewhat younger (but still ancient) lighter        regions marked with an extensive array of grooves and ridges       (below). Their       origin is clearly of a tectonic nature, but the details are  unknown.       In this respect, Ganymede may be more similar to the Earth    than either Venus or Mars   (though there is no evidence of recent tectonic activity).

surface photo of Ganymede

Evidence for  a tenuous oxygen atmosphere on Ganymede, very similar to the  one found on       Europa, has been found recently by        HST  (note that this is definitely  NOT evidence of life).

Similar ridge and groove terrain is seen on  Enceladus, Miranda and Ariel. The dark regions are similar to  the  surface of Callisto.

Ganymede surface

Extensive cratering is seen on both types of terrain.       The  density of cratering indicates an age of 3 to 3.5 billion years,       similar  to the Moon. Craters both overlay       and are cross  cut by the groove systems indicating that the grooves are       quite ancient,  too.  Relatively young craters with rays of ejecta are also visible (above).

Unlike the Moon, however, the craters are quite flat, lacking the       ring  mountains and central depressions common to craters on the Moon    and Mercury. This is probably due to the       relatively  weak nature of Ganymede’s icy crust which can flow over       geologic time and  thereby soften the relief. Ancient craters whose relief has disappeared leaving  only a “ghost” of a crater are known as palimpsests (below).

Ganymede surface

Galileo’s first flyby of Ganymede       discovered that Ganymede has its own      magnetosphere field embedded inside  Jupiter’s huge one. This is probably generated in a similar fashion to the  Earth’s: as a result of motion of conducting material in the interior.




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