Small Saturn moon has underground ocean, study shows

Saturn’s icy moon Enceladus, 850 million miles away from Earth in the outer solar system, seems an unlikely place for liquid water.

But the small body orbiting the giant ringed planet can be added to the growing list of places beyond Earth that have oceans — and prospects for hosting life — according to research published Thursday n the journal Science.

Gravity measurements taken by NASA's Saturn-orbiting Cassini spacecraft indicate the moon contains an underground ocean in its southern hemisphere. The ocean is believed to be at least as big as Lake Superior — almost 32,000 square miles, according to the research.

Computer models indicate the ocean is likely sandwiched between the moon's rocky core and its ice-covered surface, said planetary scientist David Stevenson of the California Institute of Technology.

The ocean likely was formed and is sustained by tidal heat from gravitational tugging by Saturn and sister moons on Enceladus.

The Cassini spacecraft previously discovered water plumes jetting out from hot spots in the South Pole of Enceladus. Analysis showed the plumes contain salts and organic molecules.

An underground ocean "provides one possible story to explain why water is gushing out of these fractures," Stevenson said in a statement.

The prospect of liquid water, particularly water that comes close enough to rock to leach out minerals, raises the likelihood that Enceladus has chemistry suitable for life, planetary scientist Jonathan Lunine with Cornell University told reporters during a conference call on Wednesday.

"The interior of Enceladus is a very attractive potential place to look for life," Lunine said.

Enceladus, which is only about 300 miles in diameter, joins Saturn's large moon Titan and Jupiter's Europa and Ganymede as places beyond Earth that are likely to contain oceans.

Only Enceladus and Europa, however, show evidence that their oceans are in contact with rock.

To get Enceladus' gravity maps, scientists had to tease out signals in Cassini's radio transmissions that changed by just a fraction of a millimeter per second.

The measurements were made as Cassini flew close by Enceladus three times from 2010 to 2012. The flybys showed Enceladus had a different gravitational grip on Cassini depending on whether the spacecraft passed over the moon's northern or southern hemisphere.

Taking into account what materials are available in the outer solar system — namely rock and ice — the scientists then set about running computer models to assess the most likely cause of Enceladus' asymmetrical gravity.

Their answer: a large ocean about 6 miles deep that is covered by 19 to 25 miles of ice.

The Cassini spacecraft, which has been studying the Saturn system for a decade, has three more passes by Enceladus before its mission ends in September 2017. No more gravity measurements are slated during those flybys, however.

Scientists are working on follow-on missions proposed for both Enceladus and Europa.

"I don't know which of the two is going to be more likely to have life. It might be both. It could be neither. I think what this discovery tells us is that we just need to be more aggressive in getting the next generation of spacecraft both to Europa and to the Saturn system once the Cassini mission is over," Lunine said.

Reuters