Another Saturn Moon May Hide Subsurface Ocean

Cassini observations show that icy Dione is the latest small moon in the solar system possessing tantalizing clues of a liquid water ocean beneath its crust.

NASA's Cassini spacecraft has spotted many watery delights while orbiting Saturn's system. There's Enceladus' 101 geysers, spewing fountains up from the ice and giving strong evidence of an ocean below. And there's Titan, a strange, soupy, orange world that may also have an ocean somewhere under the surface.

Over the last few years, another strong candidate has emerged: Dione. It's a tiny moon whose radius is about the same distance as a drive between San Francisco and Los Angeles (about 380 miles).

In 2013, images from Cassini showed the crust bend under the mountain Janiculum Dorsa was best explained if there was an ocean underneath; magnetometer measurements also showed a faint particle stream. Now a new study suggests that there is an ocean still underneath the ice, but far down: some 60 miles below the surface.

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The authors of the new study modeled the ice shells of both moons Enceladus and Dione. While this approach has been done in the past -- showing that Dione likely had no ocean -- the authors made a change.

"As an additional principle, we assumed that the icy crust can stand only the minimum amount of tension or compression necessary to maintain surface landforms," said Mikael Beuthe, of the Royal Observatory of Belgium and lead author of the new study, in a statement. "More stress would break the crust down to pieces."

The new results suggest that Dione would have a "deep ocean" underneath the crust, but it couldn't be picked up by Cassini. This is because the moon's back-and-forth movements suggested in the study are too small for the spacecraft to detect. It will take a future spacecraft to confirm this.

For Enceladus, the study suggests the ocean is quite close to the surface; its back-and-forth oscillations (which have been seen by Cassini) would be smaller if there was a large layer of ice.

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If Dione indeed has an ocean, it would have been around for the whole moon's history -- providing ample time for life to develop, if the conditions are right.

"The contact between the ocean and the rocky core is crucial,", said Attilio Rivoldini, who is also with the Royal Observatory of Belgium. "Rock-water interactions provide key nutrients and a source of energy, both being essential ingredients for life."

A paper based on the study was recently published in Geophysical Research Letters.

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