Ganymede ("GAN uh meed") is the seventh and largest of Jupiter's known satellites. Ganymede is the third of the Galilean moons.

　　orbit: 1,070,000 km from Jupiter

　　diameter: 5262 km

　　mass: 1.48e23 kg

　　Ganymede was a Trojan boy of great beauty whom Zeus carried away to be cup bearer to the gods.

　　Discovered by Galileo and Marius in 1610.

　　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 Pluto¹.

　　Before the Galileo encounters with Ganymede it was thought that Ganymede and Callisto were composed of a rocky core surrounded by a large mantle² 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 differentiated³ into a three layer structure: a small molten iron or iron/sulfur core surrounded by a rocky silicate⁴ 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 terrain⁵: very old, highly cratered⁷ dark regions (left), and somewhat younger (but still ancient) lighter⁸ regions marked with an extensive array of grooves¹⁰ and ridges¹² (right). 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).

　　Evidence for a tenuous¹³ 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 ridge¹¹ and groove⁹ terrain is seen on Enceladus, Miranda and Ariel. The dark regions are similar to the surface of Callisto.

　　Extensive cratering¹⁴ is seen on both types of terrain. The density¹⁵ of cratering indicates an age of 3 to 3.5 billion years, similar to the Moon. Craters¹⁶ both overlay and are cross cut by the groove systems indicating the the grooves are quite ancient, too. Relatively¹⁷ young craters with rays of ejecta are also visible (left).

　　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 geologic¹⁸ time and thereby¹⁹ soften²⁰ the relief. Ancient craters whose relief has disappeared leaving only a "ghost" of a crater⁶ are known as palimpsests (right).

　　Galileo's first flyby of Ganymede discovered that Ganymede has its own magnetosphere field embedded²¹ 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.