Kivelson's team used Galileo's magnetometer to take the measurements. The same team in August announced compelling evidence of a liquid ocean less than five miles below the surface of the sister moon Europa. Frozen water is plentiful in the solar system. But water in liquid form is considered a crucial ingredient for life to develop and therefore a key signpost in the search for extraterrestrial life. With the new evidence, Ganymede, the largest moon in the solar system, joins the thin ranks of worlds showing signs of liquid water below the surface, including Europa, the geologically dead Jovian moon Callisto and the planet Mars.
Earth has the only confirmed bodies of water in liquid form anywhere in the cosmos.
The new views hint at how the water or slushy ice may have surfaced through the fractured crust, creating geological patterns similar to some found on Europa. It was named for a boy in Greek mythology whose beauty prompted Jupiter, king of the gods, to dispatch an eagle to bring him to Olympus. The effects are tricky enough to measure at Europa, but even more so at Ganymede, Kivelson said. Magnetic field and gravity data had indicated earlier that Ganymede's metallic core is surrounded by a shell of rock and an outer layer of ice. And scientists have speculated that the fractures that tear through the surface are connected to a liquid layer below.