Pluto Ocean
Photos taken by NASA's New Horizons spacecraft revealed these tectonic features on Pluto, which indicate global expansion on the dwarf planet due to a slowly freezing subsurface ocean. NASA/JHUAPL/SwRI

NASA's historic New Horizons flyby of Pluto on July 14, 2015, lasted only for a brief moment, but that was more than enough time to retrieve data that will take scientists decades to unpack. With each photo or data release, Pluto brings more new surprises as a dwarf planet that defies expectations. Underneath its icy shell could be a liquid ocean, according to research published Tuesday.

Researchers also considered the possibility that Pluto may have had a liquid ocean beneath the shell that froze over time. Researchers used geological clues to determine what's happening on the dwarf planet. If data from New Horizons had revealed a contraction of Pluto, that would mean the ocean froze millions of years ago. It turns out that Pluto is actually expanding, which means the subsurface ocean likely is not completely frozen.

"What New Horizons showed was that there are extensional tectonic features which indicate that Pluto underwent a period of global expansion. A subsurface ocean that was slowly freezing over would cause this kind of expansion," lead author Noah Hammond, a Ph.D student from Brown University, said in a statement. The research was published in the journal Geophysical Research Letters.

Long grooves on the surface of Pluto indicate a period of expansion for the dwarf planet. Researchers ruled out other factors, such as the gravitational tug of Pluto's moon Charon, that could explain these tectonic features before settling on a theory of a subsurface ocean that's slowly freezing.

An updated thermal model using New Horizon data for Pluto's density and diameter revealed a freezing world with high pressure. That's important because that pressure would lead to ice becoming ice II — a highly ordered crystalline form of ice — which would lead to contraction on Pluto if the researchers determined it had an ice shell with a thickness of at least 260 kilometers (around 161 miles). The researchers found Pluto has an icy shell that's at least 300 kilometers thick, but there's no indication that ice II has formed, according to Hammond.