Scientists have discovered lumps beneath the icy surface of Jupiter's largest moon, Ganymede. These irregular masses may be rock formations, supported by Ganymede's icy shell for billions of years, according to a report in the Aug. 13 issue of the journal Science.

In this image, the lumps are seen in red, indicating differences in gravity detected in data from NASA's Galileo probe.

Ganymede has a metallic core and a surrounding sphere of rock. Around that is a shell some 497 miles (800 kilometers) thick made mostly of ice with perhaps some rock mixed in. Variations in this amount of rock in the shell may be the source of these possible rock formations.

"The anomalies could be large concentrations of rock at or underneath the ice surface. They could also be in a layer of mixed ice and rock below the surface with variations in the amount of rock," said John Anderson of NASA's Jet Propulsion Laboratory. "If there is a liquid water ocean inside Ganymede's outer ice layer there might be variations in its depth with piles of rock at the ocean bottom. There could be topographic variations in a hidden rocky surface underlying a deep outer icy shell. There are many possibilities, and we need to do more studies."

The trajectory path of Galileo’s second Ganymede flyby on Sept. 6, 1996, is shown in green. The probe was purposely flown into Jupiter to end its mission last year.

-- SPACE.com Staff

Credit: NASA/JPL