The dwarf planet Pluto is proving to be a much more interesting object than we expected before the New Horizons probe finally reached it last year after a decade in space. Not only is the topography unique and the atmosphere cloudy, but Pluto may even have a liquid ocean. A new analysis of Pluto suggests that the subsurface ocean that gave rise to some of its geological anomalies could still be sloshing around down there.
The new work on Pluto comes from Noah Hammond and Marc Parmentier of Brown University and Amy Barr of the Planetary Science Institute. The early examination of Pluto’s surface from the data returned by New Horizons showed long, deep faults running hundreds of miles. These long canyons appeared to form as Pluto’s crust expanded, which fits with a subsurface ocean expanding as it freezes. The question of whether or not it was already completely frozen had yet to be answered. According to the new study, maybe it hasn’t.
A new model of Pluto was used in this study, devised by Hammond. It takes into account what we now know about Pluto’s size and density, and suggests that water freezing below Pluto’s surface would have formed a compacted crystal structure known as ice II. This is a rare type of ice that forms under extreme pressure. Instead of the loose hexagonal crystals you get with normal ice, ice II consists of small rhombohedral crystals (like cubes but made from rhombi instead of squares). As the ice transitioned to this form, it wouldn’t have made Pluto larger. It actually would have caused the planet to shrink.
According to the researchers, there’s no evidence on the surface that Pluto has shrunk. That means that ice II hasn’t formed as the new model predicts. Thus, the subsurface ocean has not completely frozen, assuming the model is accurate. Ice II would only have formed in Pluto if its outer shell were at least 160 miles thick so as to place sufficient pressure on the ice. Hammond’s model suggests that shell is closer to 190 miles thick, more than enough to produce ice II if the ocean had indeed frozen. It could be that the internal structure of Pluto is warmer than we thought, thanks to tidal stress or radioactive decay.
If the analysis is accurate, it could open up new frontiers of exploration on distant objects. Pluto and other Kuiper Belt objects could be harboring underground oceans that need investigation.