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Ganymede is the fourth of Jupiter's four Jovian moons. Approximately 5,200 kilometers across and with a mass of 1.5 x 10²⁶ kg, Ganymede is the largest satellite in our solar system. It has an orbital period of a little over seven Earth days. Ganymede is deeply immersed the Jovian plasma environment surrounding Jupiter, and readily interacts with it. Its low density of 1.94 gm/cm³ suggests that it is not particularly solid, and that water or ice most likely fills much of its interior structure, mimicking the icy covering visible in satellite pictures. Its core takes up 50% of its diameter, and is most likely comprised of rock. Satellite images reveal detailed tectonic features on the surface. These features include fragmented regions of dark terrain split by layers of bright grooved terrain covering surface. The dark regions are especially cratered, suggesting that they are the oldest part of Ganymede's surface. Several bright young craters can be seen on the surface, including a linear chain of craters near the center of the image, which may have resulted from the impact of fragmented comets. While Ganymede has no known atmosphere, Hubble has recently detected ozone at its surface. This ozone is produced as charged particles trapped in Jupiter's magnetic field rain down on the surface, interacting with the icy surface. This reaction produces ozone, suggesting that Ganymede may have a thin oxygen atmosphere. (Courtesy of Voyager 2 mission by NASA)
References:
http://www.jpl.nasa.gov/galileo/sepo
http://www-pdsimage.jpl.nasa.gov/cgi-bin/GCBGenCatalogPage.pl?08201996

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