Recent evidence that Ganymede, the solar system's largest moon, may hide an ocean beneath its surface is less surprising than news of water on Mars. Two other Jovian moons, Europa and Callisto, are also believed to harbor subterranean seas. But no one expected the data--being presented this week at the annual meeting of the American Geophysical Union--to suggest the presence of a slushy ocean so clearly. In fact, three different sorts of measurements--all taken by NASA's Galileo spacecraft--indicate that a thick layer of salty water lies under parts of Ganymede's icy crust.

Magnetic readings taken in May and earlier are "highly suggestive" of a salty ocean on Ganymede, says Margaret Kivelson from the University of California, Los Angeles, who is principal investigator for Galileo's magnetometer instrument. "It would need to be something more electrically conductive than solid ice," she says of what causes part of the magnetic field on the moon. The scenario that best fits the data is a melted layer of water as salty as Earth's oceans and several kilometers thick, starting about 200 kilometers down. Natural radioactivity in the moon's interior likely provides enough heat to maintain a layer of liquid water at that depth.

Data from Galileo's infrared spectrometer too shows traces of salty minerals on Ganymede's surface that would be consistent with a buried ocean. "They are similar to the hydrated salt minerals we see on Europa, possibly the result of brine making its way to the surface by eruptions of through cracks," says Thomas McCord from the University of Hawaii in Honolulu. And new photos that Galileo snapped of Ganymede from within 809 kilometers in May hint at just those sorts of cracks. "Bright broken swaths, disrupted dark plains and the astounding Arbela Sulcus suggest Ganymede may be more similar to Europa than previously believed," says Brown University's Robert Pappalardo. "It's possible that Arbela Sulcus has formed by complete separation of Ganymede's icy crust, like bands on Europa, but unusual for Ganymede." Scientists will get another look at these features when Galileo sweeps past the satellite again on December 28.