Galileo Image of Ganymede
(June 1996)
Ganymede Fact Sheet
www.jpl.nasa.gov/galileo/ganymede/

The largest satellite of any planet in the solar system is Ganymede, one of Jupiter's moons. Measuring 3273 miles (5268 km) across, it is by volume a bit larger than Mercury, about half the size of Mars, and about three and a half times the size of our own moon.

Ganymede has a metallic core that supports a magnetic field, an ice and silicate mantle, and a thick crust that is mostly ice. Its very thin oxygen atmosphere is produced by high-energy particles that break up surface water molecules.

More about Ganymede, including pictures: http://www.jpl.nasa.gov/galileo/ganymede/

More about Ganymede's atmosphere (and aurorae): http://galileo.ivv.nasa.gov/hst7.html

Source: The Learning Kingdom's Cool Fact of the Day (http://www.tlk-lists.com/join/) for August 14, 1998