Earlier studies on Pluto suggest the possibility that the dwarf planet may have a subsurface ocean if not currently, at least at some point in its history.
Noah Hammond from Brown University and colleagues, however, have found evidence that suggests the icy world is hiding a present-day liquid ocean beneath its crust.
In a new study published in the journal Geophysical Research Letters, the researchers developed a computer model called "thermal evolution model" to simulate Pluto's evolution.
The researchers then incorporated the data collected by NASA's New Horizons spacecraft, which visited the dwarf planet in July 2015 and took photos of the alien world's icy surface.
The model showed that if Pluto once had an ocean that dried up millions of years ago, it would have a unique phase of ice with reduced volume, which means that the dwarf planet would have shrunk.
Because of the icy world's low temperatures and high pressure, a completely frozen ocean would have immediately been converted to a different phase of ice known as ice II.
Compared with standard ice, Ice II is characterized by a more compact crystalline structure, so an ocean that was frozen to ice II would take up a smaller volume that could cause a global shrinking of the dwarf planet.
Hammond and colleagues, however, have not found signs that indicate Pluto contracted. Instead, they found that the dwarf planet went through a period of global expansion. Based on this, the researchers think there is a high possibility the dwarf planet presently hosts a subsurface ocean.
"One mechanism that could drive extensional tectonic activity is global surface expansion due to the partial freezing of an ocean," the researchers wrote in their study.
"Since there is no evidence for recent compressional tectonic features, we argue that ice II has not formed and that Pluto's ocean has likely survived to present day."
Ice II only forms if the thickness of Pluto's ice shell is at least 260 kilometers (162 miles). If the shell is thinner, the ocean could freeze sans forming ice II, but the researcher's computer model suggests that Pluto's ice shell is closer to at least 300 kilometers (186 miles) in thickness.
The presence of nitrogen and methane ices on the dwarf planet's surface, as shown by images captured by Horizon's during its flyby, also strengthens the case of Pluto having a thick ice shell. Hammond explained that those ices are good insulators that help prevent Pluto from losing its heat to space.
