Enceladus’ ocean goes global

Forget about a measly southern sea. A global ocean of liquid water lurks beneath the ice of Saturn’s moon Enceladus, a new study suggests.

“It’s a very exciting result and moves us to the next level,” says William McKinnon, a planetary scientist at Washington University in St. Louis. “We can stop talking about whether the ocean is global or regional.”

The ocean on Enceladus announced itself by giving the moon a little extra twist. As Enceladus orbits Saturn, it subtly shimmies about its axis. Images from the Cassini spacecraft show that the moon sways a bit too far for the icy surface to be clinging to the rocky core. The ice instead probably floats upon a liquid layer, planetary scientist Peter Thomas and colleagues report online September 11 in Icarus.

Enceladus has been dropping hints about its ocean ever since Cassini saw water geysers blasting through cracks in the ice in 2005. More recent gravity data pointed toward a large reservoir under the south pole (SN: 5/3/14, p. 11) but could not establish whether that water extended to the rest of the moon.

Researchers suspect that several moons of Jupiter and Saturn harbor subterranean seas. “Before we started exploring things with spacecraft, the notion that there are several oceans under the outer planets’ satellites would have been regarded as nuts,” says Thomas, of Cornell University.

How deep the Enceladus ocean goes is a mystery, as is how long it has been there. “The ocean in some form has probably been around for a long time but might not have been the same ocean,” McKinnon says. The ice shell probably waxes and wanes in the gravitational tug-of-war between Enceladus and Saturn that heats the moon. Figuring out how and when the ocean formed depends on bringing together knowledge about not just Enceladus but also the interior of Saturn and how all its other moons influence one another.

“It gets very complicated,” Thomas says. “That’s part of the fun and part of the difficulty.”