Astronomy - Does Dione Have a Subsurface Ocean?

A new analysis suggests that a subsurface ocean might lie deep beneath the crust of Saturn’s moon Dione.

We often think of water as rare, but the outer solar system is full of it. Granted, it’s usually frozen, serving as the “rock” of worlds like Titan and Pluto. But planetary scientists have found evidence of subsurface oceans lurking beneath the crusts of several solar system bodies.

This composite of Saturn's moon Dione combines nine images from the Cassini spacecraft. On the moon's lower limb lies the large, multi-ringed impact basin Evander, which is about 350 kilometers (220 miles) wide. Cassini data suggest that Dione might have a subsurface ocean, but it's still unclear if that's truly the case.
NASA / JPL-Caltech / Space Science Institute

The most convincing case is Enceladus, a little icy moon of Saturn known for the salty geysers that spew from so-called tiger stripes near its south pole. Studies by multiple teams confirm that the moon has at least a regional, and possibly a global, subsurface ocean. But the crust of Saturn’s largest moon, Titan, also likely floats on a liquid water mantle, and Jupiter’s moons Europa — which shows signs of leaking water — Ganymede, and Callisto might all also have such layers.

A trio of scientists from the Royal Observatory of Belgium now argue that Saturn’s icy moon Dione should join the club.

At about 1,100 km across, Dione is twice as large as Enceladus. It’s also one of the densest of Saturn’s icy moons, having an overall density of 1.5 g/cm3 — half again higher than water. It’s thought to have a rock/ice ratio of more than 50% by mass — quite a bit for one of these satellites.

Back in March at the Lunar and Planetary Science Conference, Doug Hemingway (University of California, Berkeley) and others on the Cassini radio science team presented a preliminary analysis suggesting that Dione might have a subsurface ocean. They based this conclusion on the moon’s overall topography and how strongly it pulls on the Cassini spacecraft, which has been touring the Saturnian system since 2004 and made three close, carefully tracked flybys of the moon between 2011 and 2015.

This tracking reveals even the slightest change in the spacecraft’s velocity, which enables scientists to model the moon’s gravitational field.  For example, Dione’s mass seems to be concentrated toward its center, implying that it has differentiated into a dense, rocky core and a lower-density, ice-dominated mantle.

Dione rotates in locked synchrony with its orbit around Saturn, always pointing the same face at the ringed planet. Thus Saturn’s tidal pull always stretches Dione along the same axis, morphing it into an ellipsoid with the moon’s long axis pointing toward the planet. The ellipsoid’s dimensions depend on the moon’s interior. Cassini images show that the moon’s shape is about 300 meters (1,000 ft) off from what it should be — not a big difference, but enough to catch the scientists’ attention.

This little bit of “excess topography” has potentially big implications for Dione’s interior. If the moon is solid ice all the way down to its rocky core, then that extra bump will contain more mass than its surroundings (because there’s more ice there). When Cassini flies by, that extra mass will pull on the craft more than the rest of the landscape does.

Dione, shown here hovering over Saturn's rings, is one of the ringed giant's icy moons. This near-infrared image from Cassini was taken at a wavelength band centered on 728 nanometers, which is absorbed by methane. Dark areas seen here on Saturn are thus regions with thicker clouds, where light had to travel through more methane on its way into and back out of the atmosphere.
NASA / JPL-Caltech / Space Science Institute

But the spacecraft data don’t show that behavior: this feature hardly makes a difference. That means that something is canceling out its effect. A reasonable solution is that the ice-bump is floating like an iceberg in something denser (liquid water), and its root — which is less dense than the liquid water around it — essentially acts like negative mass, nullifying the effect the feature would otherwise have on the spacecraft. This equilibrium is called isostasy.

Given Cassini’s gravity and topographic measurements, Hemingway’s team concluded Dione wasn’t solid all the way down and might have an internal ocean.

Using this preliminary work, Mikael Beuthe and colleagues have now done a more detailed analysis of the gravity data to determine the lay of Dione’s interior land, as it were. The Belgian team suggests that the best fit to the data is a crust about 100 km thick, overlying a global ocean 35 to 95 km deep. The team also analyzed similar flyby data for the neighboring moon Enceladus and confirmed that a global subsurface ocean best fits that moon’s gravity and shape, in keeping with other recent work.

It’s a good case for a hidden sea on Dione, but not a slam dunk, cautions Hemingway. There may be other ways of explaining the Dione data, which are more uncertain than those for Enceladus. For example, Enceladus also wobbles, or librates, significantly in its orbit around Saturn, which wouldn’t happen if it were solid throughout and “all but proves the ocean is there,” says William McKinnon (Washington University in St. Louis), whose team is one of several that has studied the moon.

Beuthe’s team predicts Dione does librate, but not enough to show up in Cassini images. The paper appears inGeophysical Research Letters.