Does Saturn’s moon Dione also have a subsurface ocean?

The cratered surface of Dione, as seen by Cassini. Did (or does) an ocean lurk beneath the surface? Credit: NASA / JPL / Space Science Institute
The cratered surface of Dione, as seen by Cassini. Did (or does) an ocean lurk beneath the surface? Credit: NASA / JPL / Space Science Institute

The outer solar system was once thought to be not much more than a frozen wasteland, at least in terms of the many moons orbiting the gas and ice giant planets. But with the intriguing discoveries made by robotic probes such as Voyager, Galileo and Cassini, we now know differently.

At least three such moons – Europa, Enceladus and Titan – are thought to have liquid water beneath their surfaces, and a lot of it. Such environments, similar to the ice-covered waters at Earth’s poles, could be habitable, at least for microorganisms, maybe even something more. Scientists now wonder if some of the other ice-covered moons out there could have subsurface oceans or seas as well. New findings suggest that indeed, Saturn’s moon Dione may also join this club of waterworlds; it may have been quite active in the past and perhaps still is.

The evidence comes from the Cassini spacecraft, which is still busy orbiting Saturn and studying its moons, including Enceladus and Titan.

On the surface, Dione looks rather boring, similar to the Moon or Mercury, basically gray and covered in craters. An 800 kilometre (500 mile) long mountain, however, called Janiculum Dorsa, provides clues to activity deep down. Interestingly, the crust bends underneath this mountain. “The bending of the crust under Janiculum Dorsa suggests the icy crust was warm, and the best way to get that heat is if Dione had a subsurface ocean when the ridge formed,” said Noah Hammond, lead author of a new paper on the findings.

According to Bonnie Buratti of NASA’s Jet Propulsion Laboratory in Pasadena, “A picture is emerging that suggests Dione could be a fossil of the wondrous activity Cassini discovered spraying from Saturn’s geyser moon Enceladus or perhaps a weaker copycat Enceladus. There may turn out to be many more active worlds with water out there than we previously thought.”

Dione may also be still geologically active, although probably to a lesser degree than say Enceladus, where huge geysers of water vapour, ice and organics regularly spray out into space from huge fractures. Cassini’s spectrometer has detected faint streams of particles coming from the moon. Some images taken also show possible evidence of a current layer of liquid or slush just beneath the surface. There are also old, and apparently inactive, fractures similar to those on Enceladus.

While Earth would seem to be the only place in the solar system with current liquid water on its surface, we have also learned that subsurface oceans may actually be fairly common on smaller icy moons, which are heated internally by the enormous gravitational pull of their giant parent planets. Who would have thought?

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