Volcanoes expel streams of snow. Surface ice shows signs of tectonic activity. Underground ocean of water apparently creates a baby magnetic field, surprising scientists. Approved by Congress 20 years ago, Galileo was launched aboard the space shuttle Atlantis on Oct. 18, 1989. Photo taken by Galileo spacecraft shows large impact crater on Ganymede, one of Jupiter's moons.
Ancient craters mark Jupiter's moon Ganymede.
Crater at bottom is 12 miles wide. May not be reproduced or retransmitted without permission.