Scientists have collected more and more evidence in recent years that a liquid ocean of water probably exists under the icy surface of Pluto—a wild prospect for a dwarf planet that orbits the sun nearly 40 times farther away than Earth does, with temperatures dropping below -380 degrees Fahrenheit. Those circumstances raise a fierce question: how could a subsurface ocean on Pluto, should it really exist, possibly stay unfrozen?
Just like a thermos is designed to insulate warm coffee from the cold elements of the outside world, so too does Pluto retain its own insulation tricks to keep its global ocean nice and toasty (relatively speaking). In a study published in Nature Geoscience on Monday, a team of planetary scientists demonstrate that a layer of gas likely exists just beneath the icy shell of the dwarf planet’s surface, capable of conferring a warming insulation to the ocean and keeping temperatures high enough so things stay liquid. The findings help reconcile a number of contradictory mysteries, giving scientists possibly their best explanation yet for what’s brewing beneath the surface of one of the solar system’s most exotic celestial bodies. We don’t have direct observations of the supposed subsurface ocean, but there are two main lines of evidence that support its existence. First, theoretical calculations show the radioactive decay of rocks inside Pluto would produce enough heat to sustain an underground ocean. Second, there are geological features consistent with subsurface oceans, like fractures created from ocean refreezes.
One of Pluto’s most evocative regions is called Sputnik Planitia, a big basin near the equator and the home to the western lobe of Pluto’s famous pale “heart” surface formation. Basins don’t form near equators unless they are areas of high density. Since Sputnik Planitia is a hole, the mass must be located underground; and since ice is less dense than water, it would make sense for a deep pool of liquid to reside underneath a thin sheet of ice. So if there’s an underground reservoir of water somewhere on Pluto, Sputnik Planitia is where you want to look.
That’s precisely what Shunichi Kamata was thinking—if scientists could first solve a contradiction. A researcher based at Hokkaido University of Japan and the lead author of the new study, Kamata says that an ocean would be a sign that Pluto’s interior is warm, but if the interior is warm, the planet’s icy shell would be soft enough to deform easily, creating enough ice flow where the shell could morph into a uniform global shape. Sputnik Planitia’s existence defied this expectation, and Kamata and his team wanted to know why.
