Saturn's moon Dione harbours subsurface ocean

A subsurface ocean tens of kilometres deep lies within Saturn's moon Dione, offering a long-lived habitable zone for microbial life, according to new data from the Cassini mission.
Two other moons of Saturn, Titan and Enceladus, are already known to hide global oceans beneath their icy crusts, but the new study suggests an ocean exists on Dione as well.
Researchers from the Royal Observatory of Belgium show gravity data from recent Cassini flybys can be explained if Dione's crust floats on an ocean located 100 kilometres below the surface.
The ocean is several tens of kilometres deep and surrounds a large rocky core, researchers said.

Seen from within, Dione is very similar to its smaller but more famous neighbour Enceladus, whose south polar region spurts huge jets of water vapour into space.
Dione seems to be quiet now, but its broken surface bears witness of a more tumultuous past.

Researchers modelled the icy shells of Enceladus and Dione
as global icebergs immersed in water, where each surface ice peak is supported by a large underwater keel.
Scientists have used this approach in the past but previous results have predicted a very thick crust for Enceladus and no ocean at all for Dione.
"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, lead author of the study.

"More stress would break the crust down to pieces," Beuthe said.
According to the new study, Enceladus' ocean is much closer to the surface, especially near the south pole where geysers erupt through a few kilometres of crust.
These findings agree well with the discovery last year by Cassini that Enceladus undergoes large back-and-forth oscillations, called libration, during its orbit.
Enceladus' libration would be much smaller if its crust was thicker. As for Dione, the new study finds it harbours a deep ocean between its crust and core.
"Like Enceladus, Dione librates but below the detection level of Cassini. A future orbiter hopping around Saturn's moons could test this prediction," said Antony Trinh, co-author of the study.

Dione's ocean has probably survived for the whole history of the moon, and thus offers a long-lived habitable zone for microbial life.
"The contact between the ocean and the rocky core is crucial," said Attilio Rivoldini, co-author of the study.
"Rock-water interactions provide key nutrients and a source of energy, both being essential ingredients for life," said Rivoldini.
The study was published in the journal Geophysical Research.