New pictures of Pluto’s surface from Nasa’s New Horizon spacecraft reveal that Pluto is more than just a snow-filled planet. It sends two major clues that suggest the possibility of a liquid sea under its icy crust.

What New Horizons showed was the surface of Pluto is made from three different types of ices – water, nitrogen, and methane. It has high mountains and a huge heart-shaped plain. The planet also has giant faults that are hundreds of kilometers long and as deep as 4 kilometers.

It was those tectonic features that got scientists thinking that a subsurface ocean was a real possibility for Pluto.

A thermal evolution model for Pluto was updated with all the data from the New Horizon spacecraft and studied by a team led by Noah Hammond, a Ph.D. student at the Brown University in the US.

Scientists found that if Pluto’s ocean had frozen, it would have caused the entire planet to shrink. However, 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, researchers said.

Scientists think that Pluto must have heat-producing radioactive elements within its rocky core that melted parts of the planet’s ice shell. Over time in the frigid Kuiper belt, that melted portion would eventually start to refreeze. Ice is less dense than water, so when it freezes, it expands.

If Pluto had an ocean that was frozen or in the process of freezing, extensional tectonics on the surface would result, and that is what New Horizons saw.

The thermal evolution 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,” she added.

The study was published in the journal Geophysical Research Letters.