Number 279 (Story #3), July 15, 1996 by Phillip F. Schewe and Ben Stein GANYMEDE IS COVERED WITH A DIRTY CRUST OF WATER ICE. The Galileo spacecraft, now patrolling Jupiter's neighborhood, recently swung past Ganymede, the largest moon in the solar system. Galileo recorded the presence of a self-generated magnetic field at the moon, and new high-resolution pictures testify to the presence of tectonic-like forces at work, giving Ganymede's surface a network of fault lines. (JPL press release, July 11, 1996.)