Pluto may harbour a vast liquid ocean lying deep beneath the icy dwarf planet's frozen heart, according to an analysis of the features unveiled by NASA's New Horizons spacecraft.
The idea that Pluto has a subsurface ocean is not new, but the study provides the most detailed investigation yet of its likely role in the evolution of key features such as the vast, low-lying plain known as Sputnik Planitia (formerly Sputnik Planum).
Sputnik Planitia, which forms one side of the famous heart-shaped feature seen in the first New Horizons images, is suspiciously well aligned with Pluto's tidal axis.
The likelihood that this is just a coincidence is only five per cent, so the alignment suggests that extra mass in that location interacted with tidal forces between Pluto and its moon Charon to reorient Pluto, putting Sputnik Planitia directly opposite the side facing Charon.
However, a deep basin seems unlikely to provide the extra mass needed to cause that kind of reorientation.
"It's a big, elliptical hole in the ground, so the extra weight must be hiding somewhere beneath the surface. And an ocean is a natural way to get that," said Francis Nimmo, professor at University of California, Santa Cruz in the US.
James Keane from the University of Arizona, also argues for reorientation and points to fractures on Pluto as evidence that this happened.
Like other large basins in the solar system, Sputnik Planitia was most likely created by the impact of a giant meteorite, which would have blasted away a huge amount of Pluto's icy crust.
With a subsurface ocean, the response to this would be an upwelling of water pushing up against the thinned and weakened crust of ice.
At equilibrium, because water is denser than ice, that would still leave a fairly deep basin with a thin crust of ice over the upwelled mass of water.
Nimmo and his colleagues also considered whether the extra mass could be provided by just a deep crater filled with nitrogen ice, with no upwelling of a subsurface ocean.
However, their calculations showed this would require an implausibly deep layer of nitrogen, over 40 kilometres thick.
They found that a nitrogen layer about seven kilometres thick above a subsurface ocean provides enough mass to create a "positive gravity anomaly" consistent with the observations.
"We tried to think of other ways to get a positive gravity anomaly, and none of them look as likely as a subsurface ocean," Nimmo said.
As for the subsurface ocean, Nimmo suspects it is mostly water with some kind of antifreeze in it, probably ammonia.
The slow refreezing of the ocean would put stress on the icy shell, causing fractures consistent with features seen in the New Horizons images.
There are other large objects in the Kuiper belt that are similar to Pluto in size and density, and Nimmo said they probably also have subsurface oceans.
The findings were published in the journal Nature.
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