October 18, 2017

Study suggests Saturn’s moon Dione has underground ocean

This view from NASA's Cassini spacecraft looks toward Saturn's icy moon Dione, with giant Saturn and its rings in the background, just prior to the mission's final close approach to the moon on Aug. 17, 2015. Credit: NASA/JPL-Caltech/Space Science Institute
This view from NASA’s Cassini spacecraft looks toward Saturn’s icy moon Dione, with giant Saturn and its rings in the background, just prior to the mission’s final close approach to the moon on Aug. 17, 2015. Credit: NASA/JPL-Caltech/Space Science Institute

Scientists crunching data from NASA’s Cassini spacecraft believe Saturn’s moon Dione has a liquid ocean buried 60 miles (100 kilometers) under a thick icy shell, potentially adding to a growing list of ocean worlds in the frigid outer solar system.

Using gravity measurements from Cassini’s close flybys of Dione, researchers modeled the outer structures of Dione and Saturn’s more famous moon Enceladus — already known to harbor a subsurface ocean — to determine whether any liquid lurks below the moon’s icy crust.

Scientists published their results in Geophysical Research Letters.

They started the study by modeling the icy shells of Enceladus and Dione as “global icebergs” floating in water, according to a press release from the Royal Observatory of Belgium, the home institution for the paper’s authors.

The results of the investigation “show gravity data from recent Cassini flybys can be explained if Dione’s crust floats on an ocean located 100 kilometers (60 miles) below the surface,” the Royal Observatory of Belgium said in a press release. “The ocean is several tens of kilometers deep and surrounds a large rocky core.”

“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.”

The adjustment produced different results than previous studies using the same approach and agreed with the discovery last year that Enceladus has a global underground ocean, not just localized pockets of water.

Earlier research using the same modeling technique indicated Enceladus’ ocean was buried beneath a thick layer of ice, and provided no evidence at all of an ocean on Dione, scientists said.

But Enceladus has plumes of water vapor erupting from its south pole, apparently breaking through a relatively thin section of ice just a few miles thick, raising a cloud over suggestions that its ocean is very deep. Last year, researchers tracking the wobbling motion of Enceladus concluded a hidden reservoir of liquid water stretches all the way around the moon.

The magnitude of the wobble, or libration, eliminates any possibility Enceladus’ icy crust is frozen to the rocky core believed to lie underneath, scientists said

Applying the same adjusted modeling at Dione — a moon with a diameter of 700 miles (about 1,100 kilometers) — allowed scientists to conclude that it also has a global ocean, but much deeper than the one at Enceladus, and more difficult to measure and sample directly.

“Like Enceladus, Dione librates but below the detection level of Cassini,” said Antony Trinh, co-author of the new study. “A future orbiter hopping around Saturn’s moons could test this prediction.”

The Cassini spacecraft has not detected any geysers at Dione, but fractures in its icy crust could be scars left over from a more active period.

Astrobiologists place Enceladus and Jupiter’s moon Europa, which also holds a global underground ocean and likely has intermittent plume eruptions, near the top of the list of places in the solar system that could sustain microbial life.

“The contact between the ocean and the rocky core is crucial,” said Attilio Rivoldini, co-author of the Dione study. “Rock-water interactions provide key nutrients and a source of energy, both being essential ingredients for life.”

The ocean worlds are getting more attention from space scientists in charge of planning future robotic missions. NASA is working on a spacecraft to fly by Europa dozens of times and a lander to set down on Europa’s frozen surface in the 2020s, and Enceladus and Saturn’s largest moon Titan — with seas of liquid hydrocarbons — have been added to the list of potential destinations for a new space probe NASA intends to select in a competition in the next few years.

Jupiter’s moon Ganymede also has an underground ocean, and NASA’s New Horizons mission found some signs that an ocean could lie beneath the icy plains of Pluto. The moons of Uranus and Neptune may also have oceans, but they require closer observations to confirm.

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