Sub-surface oceans in the Solar System may be far more common than we’ve realized. We’ve grown used to contemplating water under the ice of Europa, but similar oceans may well exist on Ganymede and Callisto, and there are signs of a possible ocean beneath Titan, not to mention the unusual activity we continue to observe on Enceladus. Where liquid water in cold objects seems least likely is in the Kuiper Belt, but Guillaume Robuchon and Francis Nimmo (University of California at Santa Cruz) have been making the case for an ocean inside distant Pluto, based on their models of thermal evolution and the behavior of the ice shell.
As this article in Astrobiology Magazine points out (and thanks to my friend Antonio Tavani for the pointer to this one), Pluto’s outer surface is a thin shell of nitrogen ice covering a shell of water ice. With New Horizons inbound to Pluto/Charon for an April, 2015 encounter, the researchers have been working out what surface features might help us make the call on whether such an ocean exists. One possibility is an equatorial bulge left over from the earlier days of Pluto’s formation, when it would have been spinning more rapidly. Such a bulge would be perhaps 10 kilometers high if it exists and New Horizons should be able to see it. A bulge would indicate no ocean beneath, as movement of the liquid interior over time would have reduced the protrusion.
But tensional stresses as the shell was stretched when temperatures changed over the course of Pluto’s lifetime would imply water beneath, a different kind of feature than we would expect from a solid layer below. The good news is that while New Horizons is a flyby mission, it will be able to map the entire sunlit surface of Pluto beginning in the three months before closest approach — we should get highest resolution (62 meters per pixel) when New Horizons closes to 12,500 kilometers, and ridges, valleys and possible geyser features should be discernible.
Image: Three Hubble images of Pluto. When New Horizons switches on its cameras three months before closest approach, even its most distant images of Pluto will be ten times more detailed than these. Credit: NASA, ESA, and M. Buie (Southwest Research Institute).
As to what would keep water liquid at an average 40 AU from the Sun:
The main source of energy likely stems from the rocky interior, where isotopes undergo radioactive decay. Among these elements, the researchers found potassium to be key – enough potassium in Pluto’s core would result in melted ice above it.
And signs look good – the amount of potassium needed would be about a tenth of that found in meteorites from the early solar system.
“I think there is a good chance that Pluto has enough potassium to maintain an ocean,” Nimmo said.
Robuchon and Nimmo calculate that a planet-wide ocean here would have an average depth of 165 kilometers beneath a crust of about the same thickness. Finding an ocean on Pluto would make the case for other Kuiper Belt oceans quite strong, especially on larger objects like Eris. A case for astrobiology in this extreme environment seems remote indeed, but the presence of an ocean here would remind us that the Kuiper Belt, which may contain a thousand dwarf planets or more, is likely to deal us surprises at every turn. And we can rejoice at the presence of New Horizons’ Long-Range Reconnaissance Imager (LORRI), which according to principal investigator Alan Stern, would be able to see individual buildings if flown over the Earth at New Horizons’ closest approach altitude. We are in for an incredible view in just a few years.