29 September 2016

Research suggests Saturn’s moon Dione may harbor a subsurface ocean

Posted by llipuma

An image of Saturn's icy moon Dione, with giant Saturn and its rings in the background. New research suggests Dione harbors a subsurface ocean. Credit: NASA/JPL-Caltech/Space Science Institute.

An image of Saturn’s icy moon Dione, with giant Saturn and its rings in the background. New research suggests Dione harbors a subsurface ocean.
Credit: NASA/JPL-Caltech/Space Science Institute.

By The Royal Observatory of Belgium

A subsurface ocean could lie deep within Saturn’s moon Dione, according to a new study using publicly available data from the Cassini mission to Saturn.

In 2013, images from NASA’s Cassini spacecraft hinted that Dione had a subsurface ocean when the moon formed, but the new study suggests the ocean could still exist today. The study was accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union.

Dione seems to be quiet now, but planetary scientists agree its broken surface bears witness to a more tumultuous past.

Two other moons of Saturn, Titan and Enceladus, are already known to hide global oceans beneath their icy crusts. If Dione has an existing ocean, it would probably have survived for the whole history of the moon, and thus could offer a long-lived habitable zone for microbial life, according to the study’s authors.

In the new study, researchers used computer modeling techniques to show gravity data from Cassini flybys can be explained if Dione’s crust floats on an ocean located 100 kilometers (62 miles) below the surface.

Although Dione (near) and Enceladus (far) are composed of nearly the same materials, Enceladus endures a constant rain of ice grains from its south polar jets. As a result, Enceladus’ surface is much brighter, but the interiors of the two moons could look alike, according to new research. Credit: NASA/JPL-Caltech/Space Science Institute.

Although Dione (near) and Enceladus (far) are composed of nearly the same materials, Enceladus’ surface is much brighter. 
Credit: NASA/JPL-Caltech/Space Science Institute.

The authors modeled 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 and compression necessary to maintain surface landforms,” said Mikael Beuthe, a planetary scientist at the Royal Observatory of Belgium in Brussels and lead author of the new study. “More stress would break the crust down to pieces.”

According to the model, Dione’s ocean would be several tens of kilometers deep and would surround a large rocky core. This suggests the moon’s interior is very similar to its smaller but more famous neighbor Enceladus.

The new findings agree with past findings by the Cassini mission that Enceladus’ ice shell is much thinner near its south pole.

A representation of the interior of Saturn's moon Enceladus with a global liquid water ocean between its rocky core and icy crust, as revealed by the gravity data and the large libration measured by Cassini. New research suggests Dione may have a similar subsurface ocean. Credit: NASA/JPL-Caltech.

A representation of the interior of Saturn’s moon Enceladus with a global liquid water ocean between its rocky core and icy crust. 
Credit: NASA/JPL-Caltech.

Cassini discovered last year that Enceladus undergoes large back-and-forth oscillations, called libration, during its orbit. The new study suggests Dione also experiences libration.

“We find that, like Enceladus, Dione undergoes libration but below the detection level of Cassini,’’ said Antony Trinh, a planetary scientist at the Royal Observatory of Belgium and co-author of the new study. “A future orbiter hopping around Saturn’s moons could test this prediction.”

The club of “ocean worlds” — icy moons or planets with subsurface oceans — gains new members with each new mission to the outer solar system. Three ocean worlds orbit Jupiter, two and possibly more orbit Saturn and Pluto could also belong to the club, according to recent observations of the New Horizons spacecraft. The approach to modeling planetary bodies used in this study is a promising tool to study these worlds if we can measure their shape and gravity field, according to Beuthe.

“Future missions will visit Jupiter’s moons, but we should also explore Uranus’ and Neptune’s systems,” he said.

—This post originally appeared as a press release on the Royal Observatory of Belgium website.