Ongoing geological activity on Pluto seen by NASA's New Horizons spacecraft could be caused by the partial freezing of a subsurface ocean that likely still exists today, a new paper in Geophysical Research Letters says.

Noah P. Hammond, Brown University, is lead author on the paper, "Recent Tectonic Activity on Pluto Driven by Phase Changes in the Ice Shell." Planetary Science Institute Senior Scientist Amy C. Barr is Hammond's academic graduate advisor and co-author on the paper.

"Our model shows that recent geological activity on Pluto can be driven just from phase changes in the ice - no tides or exotic materials or unusual processes are required. If Pluto's most recent tectonic episode is extensional, that means that Pluto may have an ocean at present. This lends support to the idea that oceans may be common among large Kuiper Belt objects, just as they are common among the satellites of the outer planets," said Barr, who helped formulate the numerical model and interpret the results.

"In our paper, we look at tectonic features on the surface of Pluto to understand the interior and we run thermal evolution models to help us understand how Pluto's interior may have evolved over time," Hammond said. "Our study further supports that hypothesis by showing that if the ocean froze, ice II would likely form, causing compressional tectonic features which are absent from the surface.

"The formation of ice II would cause Pluto to experience volume contraction and compressional tectonic features to form on the surface," Hammond said. "Since the tectonic features on Pluto's surface are all extensional and there is no obvious compressional features, it suggests that ice II has not formed and that therefore, Pluto's subsurface ocean has likely survived to present day."

Ice II is a phase of ice that is 25 percent more dense than the ice we are familiar with on Earth, which floats on water. Ice II forms at high pressures and low temperatures, the kind of conditions expected in Pluto's ice shell.

"We have been waiting a long time to see the surface of Pluto, and it did not disappoint," Barr said. "Many people thought that Pluto would be geologically 'dead,' that it would be covered in craters and have an ancient surface.

"Our work shows how even Pluto, at the edge of the solar system, with very little energy, can have tectonics. We are grateful to the New Horizons team for working so hard to guide the craft to Pluto and return the beautiful images that motivated our study. They have provided another piece in the puzzle of the comparative planetology of icy worlds."

"Recent Tectonic Activity on Pluto Driven by Phase Changes in the Ice Shell," Noah P. Hammond, Amy C. Barr and Edgar M. Parmentier, 2016, Geophysical Research Letters