Back in the days when I was reading Poul Anderson’s The Snows of Ganymede and thought of the moons of Jupiter as icy wastelands, I never would have dreamed there could be an ocean below their surfaces. But now we have oceans proliferating. Ganymede’s may contain more water than all Earth’s oceans, while Callisto is also in the mix, and we’ve known about Europa for some time now. At Saturn, the case for an ocean inside Titan seems strong, while Enceladus continues to spark mission proposals to study its frequent geysers.
If you’re a Centauri Dreams regular, you know that we’ve talked about Pluto’s oceanic possibilities for some time, now strengthened in new work from Brandon Johnson (Brown University). Johnson and colleagues have modeled an ocean layer on Pluto more than 100 kilometers thick, with a salt content more or less like that of the Dead Sea on Earth. Johnson focused on Sputnik Planum, the 900-kilometer basin that comprises part of the heart-shaped feature we all learned to recognize during last summer’s New Horizons flyby.
The work is helpful because unlike previous studies, it puts constraints on the depth of the ocean and offers information about its composition. Sputnik Planum, as it turns out, sits exactly on the tidal axis linking tidally-locked Charon with Pluto, suggesting that the area has a positive mass anomaly; i.e., there is more mass here than the average for Pluto’s crust.
Thus Charon’s gravity would eventually pull this higher-mass area into the alignment we see. A large impact, like the one that created Sputnik Planum, would eventually be filled with liquid water, and in Johnson’s scenario, nitrogen ice deposited later would tip the scales, creating the positive mass anomaly. “This scenario, says Johnson, “requires a liquid ocean.” And of the various thicknesses of the water layers modeled, 100 kilometers works out best at creating the features we see at Sputnik Planum.
Usefully, the team’s computer models showed that the thickness of the ocean layer could be tied directly to the production of the mass anomaly, which also turns out to be sensitive to the salinity of the water, affecting the water’s density. We can tell a lot from this impact of an object 200 kilometers across or larger and its effects upon an ocean with a salinity of about 30 percent.
“What this tells us is that if Sputnik Planum is indeed a positive mass anomaly —and it appears as though it is — this ocean layer of at least 100 kilometers has to be there,” Johnson says. “It’s pretty amazing to me that you have this body so far out in the solar system that still may have liquid water.”
The Case for Dione
Now we have word of yet another possible ocean, this one on Saturn’s moon Dione. Mikael Beuthe (Royal Observatory of Belgium) and team have been modeling the icy shells of Enceladus and Dione, using gravity data from Cassini flybys. The models study the shells of both moons as global icebergs immersed in water — here surface peaks are held up by a large mass under the water. The approach has not previously proven effective in modeling Enceladus, for it produces a thick crust for the moon that is inconsistent with our observations.
But isostasy, the equilibrium in parts of the Earth’s crust, can be similarly modeled, as blocks floating on the underlying mantle, which rises and falls as material is added or removed. What the researchers have done is to add a new wrinkle:
“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,” says Beuthe. “More stress would break the crust down to pieces.”
This revised model works well at producing an ocean for Enceladus under a thinner crust than previous models, with water close enough to the surface to produce the famous geysers at the south pole. The model is also consistent with the libration of Enceladus in its orbit, oscillations that would be smaller if the moon had a thicker crust. Under the same model, Dione is revealed to have a much thicker crust covering a deep ocean between the crust and the moon’s core.
The Cassini data thus become consistent with an ocean about 100 kilometers below the surface of Dione, an ocean perhaps tens of kilometers deep surrounding a large, rocky core. We may be able to test this prediction with future spacecraft in the Saturn system, which may be able to measure the libration of Dione, an oscillation the researchers believe would be well below the detection capabilities of Cassini, but a useful marker as evidence of the subsurface ocean.
Image: Dione with Enceladus in the background. This image was taken by the Cassini spacecraft on 8 September 2015. Credit: NASA/JPL-Caltech/Space Science Institute.
And what of other outer system objects? From the paper:
Beyond Enceladus and Dione, our new take on isostasy is applicable to large icy satellites
with global oceans, such as Europa [Nimmo et al., 2007b], Titan [Nimmo and Bills,2010], and particularly Ganymede whose gravity and shape will be measured by the JUICE mission. According to Park et al. , gravity and shape data from the Dawn mission suggest isostasy on Ceres, but the case is far from clear because compensation does not occur for all gravity components. Finally, isostasy plays a crucial role in understanding the long-wavelength gravity and shape as well as estimating the crust thickness of the planets Mars [Wieczorek and Zuber , 2004], Venus [James et al., 2013], and Mercury [Perry et al., 2015]. Thanks to the simultaneous availability of gravity/shape and libration data, Enceladus’s case constitutes the first validation of planetary-scale isostasy.
The paper is Beuthe et al., “Enceladus’ and Dione’s floating ice shells supported by minimum stress isostasy,” published online by Geophysical Research Letters 28 September 2016 (abstract / preprint). The paper on Pluto’s ocean is Johnson et al., “Formation of the Sputnik Planum basin and the thickness of Pluto’s subsurface ocean,” Geophysical Research Letters 19 September 2016 (abstract).