Present day Pluto may have a liquid ocean sloshing around under its icy crust, according to to a new analysis of data from NASA's New Horizons spacecraft. When New Horizons buzzed by Pluto last year, it unveiled clues that the dwarf planet might have - or had at one time - a sub-surface liquid ocean. Scientists from Brown University in the US, who used a thermal evolution model for Pluto updated with data from New Horizons, found that if its 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, researchers said. On the contrary, New Horizons showed signs that Pluto has been expanding. The pictures New Horizons sent back from its close encounter with the Kuiper Belt's most famous denizen showed that Pluto was much more than a simple snowball in space. It has an exotic surface made from different types of ices - water, nitrogen and methane. It has mountains hundreds of meters high and a vast heart-shaped plain. It also has giant tectonic features - sinuous faults hundreds of kilometres long as deep as 4 kilometres. It was those tectonic features that got scientists thinking that a subsurface ocean was a real possibility for Pluto. "What New Horizons showed was that there are extensional tectonic features, which indicate that Pluto underwent a period of global expansion," said Noah Hammond, a graduate student in Brown University. "A subsurface ocean that was slowly freezing over would cause this kind of expansion," Hammond said. Scientists think that there may have been enough heat-producing radioactive elements within Pluto's rocky core to melt part 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 said. The new model bolsters the case for an ocean environment in the furthest reaches of the solar system. The study was published in the journal Geophysical Research Letters.