Life on Saturn's moon? How a mountain gave clues to a subsurface sea.

Dione, one of Saturn's many moons, hasn't attracted a lot of attention before now. It has faded into the near-anonymity of the dozens of other icy satellites orbiting gas giants Jupiter and Saturn. (At last count, Jupiter's up to 66 moons and Saturn has 62.)

But behind its "bland cueball" exterior, Dione might be hiding some secrets of her own – like an ocean anywhere from 5 to 30 miles deep, trapped beneath a frozen surface, according to an article recently published in Icarus.

"The presence of a subsurface ocean at Dione would boost the astrobiological potential of this once-boring iceball," writes Jia-Rui Cook in a NASA press release. Other moons thought to have subsurface seas – like Jupiter's Europa, Ganymede, and Callisto; Neptune's Triton; and Saturn's Titan and Enceladus – are some of the most geologically active worlds in our solar system, she notes. Their activity and hints of liquid water make them fascinating targets for scientist looking for life.

"Maybe one day we'll go to these moons, explore the subsurface oceans, and vacation one day with our jet skis," jokes Noah Hammond, the lead author on the recent study. But until then, he cautions, "evidence for past subsurface oceans on Dione and Rhea [another of Saturn's moons] is tantalizing but far from conclusive."

Dione's liquid-water ocean would be down deep, if it's there. Hammond's team has calculated that Dione has a brittle, frozen-solid outer layer about 2 miles thick (3.5 +/- 1 km), sitting on a thicker layer – probably 30 to 60 miles thick (50-100 km) – "that behaves like a fluid but isn't a liquid," he explains. "It flows over geological time scales," like glass or Earth's own mantle layer. Below that is the liquid ocean, anywhere from 6 to 30 miles (10-50 km) thick, and then a rocky core.

How a mountain revealed a hidden sea

Counter-intuitive though it seems, the secret to this subsurface ocean was in the mountains.

"I noticed that there was an extremely large ridge called Janiculum Dorsa that nobody had really looked at before," says Hammond, who did the research while at SETI Institute and is now at Brown University..

Janiculum Dorsa, an ancient, long, skinny mountain range, reaches about 500 miles long and stands between half a mile and a mile high – more or less the same size and shape as the Blue Ridge Mountains in the southeastern U.S.

Hammond noticed that Dione's surface bends a bit the around the edge of the mountains. "There's a depression on each side of the ridge" he noted. "This looks promising."

A crust capable of bending – flexure, in science-speak – suggests a lot more flexibility than a cold, hard cueball should be capable of.

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