03/27/2010 05:12 am ET Updated May 25, 2011

Jupiter Moon Melted By Asteroid, Comet Bombardment

The remaining moon, Ganymede, was a bit of a mystery. Like Callisto, it appears to be a mixture of rock and water with a big difference: on Ganymede, the water appears to have melted, allowing the rock to sink to the core, and then froze up again.

Two scientists from the Southwest Research Institute now think they know why: Ganymede was hit so hard by asteroids and comets during the solar system's early history that it underwent runaway melting.

Read more on Ars Technica