Scientists studying data sent back to Earth from NASA's Galileo spacecraft have concluded that Jupiter's largest moon, Ganymede, may possess a huge salt-water ocean beneath its crusty surface. Galileo data on two other Jovian moons, Europa and Callisto, have already indicated that they probably have subsurface water, a key building block for life.
Now, Ganymede - which is larger than Mercury or Pluto - also looks likely to be concealing a thick layer of melted, salty water beneath its icy crust, researchers told a meeting of the American Geophysical Union in San Francisco on Saturday.
Margaret Kivelson of the University of California-Los Angeles said that magnetic readings taken by the Galileo craft during close approaches in May 2000 and earlier were "highly suggestive" that a salty, liquid ocean existed there.
"It would need to be something more electrically conductive than solid ice," she said, adding that a melted layer of water several kilometres thick, beginning within 200 km of Ganymede's surface would fit the data if it were about as salty as Earth's oceans.
Other scientists studying readings from an infrared spectrometer to identify surface materials on Ganymede said portions of the moon appear to have types of salt minerals that would have been left behind by exposure of salty water near or on the surface. "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 or through cracks," said Thomas McCord of the University of Hawaii, Honolulu.
That hypothesis is also bolstered by new, high-resolution images of Ganymede sent back by Galileo, which hint that water or slushy ice may have surfaced through the fractured crust to create smooth areas in between separated areas of crust.
Dr Dave Stevenson of the California Institute of Technology said natural radioactivity in Ganymede's rocky interior should provide enough heating to maintain a stable layer of liquid water between two layers of ice, about 150 to 200 km below the surface.
"I would have been surprised if Ganymede had not had an ocean, but the issue of whether it's there is different than the issue of whether you can expect to see it clearly in the data," Dr Stevenson said.