Photograph by  Cassini Imaging TeamSSIJPLESANASA
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On Saturn’s moon Enceladus, a global, underground ocean vents through features called “tiger stripes” at its south pole (left, in this false-color image). Such alien oceans are prime candidates in the search for life beyond Earth.

Photograph by  Cassini Imaging TeamSSIJPLESANASA
New Science in Brief

Vast Ocean Underlies Ice on Saturn's Moon Enceladus

The discovery of a global ocean beneath its icy rind makes Enceladus an even better potential extraterrestrial incubator than previously thought.

A global ocean lurks inside Enceladus, Saturn’s saltwater-spitting moon—and that’s good news for alien hunters seeking an A-list extraterrestrial petri dish.

After sorting through more than seven years of data gathered by NASA’s Cassini spacecraft, which the team plotted by hand, scientists have found that Enceladus wobbles ever so slightly as it orbits Saturn. While slight, the wobble is too large for a moon with a completely solid interior: It’s best explained by a free-floating crust that’s in contact with liquid all around, the team reported September 11 in Icarus.

"If the surface and core were rigidly connected, the core would provide so much dead weight that the wobble would be far smaller than we observe it to be," said the SETI Institute’s Matthew Tiscareno in a statement. "There must be a global layer of liquid separating the surface from the core."

The study is solid work, says Bill McKinnon of the Washington University in St. Louis. Earlier work based on a set of gravity data taken by Cassini backs up the finding, he says, and suggests that a global ocean is the easiest way to explain some of Enceladus’ features.

Global Ocean in Saturn's Moon Enceladus
A new analysis of more than seven years of Cassini data suggests the presence of a global, liquid ocean tucked between Enceladus’ icy crust and rocky core.

On the Must-See List

With geysers spewing saltwater and organic molecules into space, Enceladus has been at (or near) the top of every astrobiologist’s wish list for a visit since Cassini first spotted the plumes in 2005.

“We’re looking for liquid water oceans that are rich in chemistry and which might have been around for a long time,” says astrobiologist Kevin Hand of the Jet Propulsion Laboratory.

Yet at Enceladus, evidence that a long-lived reservoir powered the plumes was sparse. Early theories pointed toward a small regional sea, perhaps created by an impact, that could be too young for life to have evolved.

But a global ocean could be stable and long-lived enough for alien microbes to gain a foothold. “A global ocean is harder to explain as a short-lived event,” Hand says. “That’s good news for habitability.”

Still a Mysterious World

Enceladus now joins a smattering of famous worlds with icy rinds encapsulating ocean layers, including Europa and Ganymede, which orbit Jupiter. There, Jupiter’s enormous gravity and the tugging and jostling of its other large satellites help keep the moons warm enough to maintain fluid innards.

At Enceladus, it’s not as clear how thick the ocean is, how that heat is generated, or why the crust at its south pole (and only its south pole) is thin enough for geysers to punch through.

“Is the seafloor active in the south but not in the north?" asks Hand. "The ice shell is hiding some secrets about the ocean and geophysics below.”

We may get more answers from the Cassini spacecraft, which will continue to study the Saturnian system through 2017, before plunging into the giant ringed planet. Its next closeup of Enceladus will be in October, and its last flyby of the 500-kilometer-wide world will be in December.

As spacecraft fly farther into space and sidle up to small, icy worlds, it’s becoming increasingly clear that we don’t have a good grasp of how these bodies work—from soft, pockmarked Ceres in the asteroid belt, to the relentlessly intriguing dwarf planet Pluto, on the fringe of the solar system. Maybe, if we’re lucky, coming decades will see a fleet of spacecraft exploring these and other alien worlds, uncovering their secrets and continuing to send otherworldly postcards home.

Follow Nadia Drake on Twitter and on her blog at National Geographic's Phenomena.