There's long been speculation that Pluto might have a liquid ocean sloshing around under its icy crust, but now, a recent study has suggested that this might be true.
According to the Brown University analysis, which used a thermal evolution model for Pluto updated with data from New Horizons, if Pluto's ocean had frozen into oblivion millions or billions of years ago, it would have caused the entire planet to shrink. But there are no signs of a global contraction to be found on Pluto's surface. On the contrary, New Horizons showed signs that Pluto has been expanding.
"Thanks to the incredible data returned by New Horizons, we were able to observe tectonic features on Pluto's surface, update our thermal evolution model with new data and infer that Pluto most likely has a subsurface ocean today," said lead author Noah Hammond.
The model includes updated data from New Horizons on Pluto's diameter and density, key parameters in understanding the dynamics in Pluto's interior. The model showed that because of the low temperatures and high pressure within Pluto, an ocean that had completely frozen over would quickly convert from the normal ice we all know to a different phase called ice II. Ice II has a more compact crystalline structure than standard ice, so an ocean frozen to ice II would occupy a smaller volume and lead to a global contraction on Pluto, rather than an expansion.
"We don't see the things on the surface we'd expect if there had been a global contraction," Hammond said. "So we conclude that ice II has not formed, and therefore that the ocean hasn't completely frozen."
However, the researchers say there's good reason to believe that the ice shell is more than 260 kilometers. Their updated model suggests that Pluto's ice shell is actually closer to 300 or more kilometers thick. In addition, the nitrogen and methane ices that New Horizons found on the surface bolster the case for a thick ice shell.
"Those exotic ices are actually good insulators," Hammond said. "They may be helping Pluto from losing more of its heat to space."
Taken together, the new model bolsters the case for an ocean environment in the furthest reaches of the solar system.
The study appears in Geophysical Research Letters.