Ganymede is the 7th and largest of jupiter's known satellites.
The satellite was discover in 1610 by galileo and Marius.

Ganymede is the largest satellite in the solar system. It's larger in diameter then Mercury and much larger then 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/sulfer 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.

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

 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).