An insulating layer of gas beneath Pluto's icy exterior may be protecting a subsurface ocean from freezing, scientists claim.
The team from the Hokkaido University in Japan conducted computer simulations covering a timescale of 4.6 billion years, when the solar system began to form.
In July 2015, NASA's New Horizons spacecraft flew through Pluto's system, providing the first-ever close-up images of this distant dwarf planet and its moons.
The images showed Pluto's unexpected topography, including a white-colored ellipsoidal basin named Sputnik Planitia, located near the equator.
Due to its location and topography, scientists believe a subsurface ocean exists beneath the ice shell which is thinned at Sputnik Planitia.
However, these observations are contradictory to the age of the dwarf planet because the ocean should have frozen a long time ago and the inner surface of the ice shell facing the ocean should have also been flattened.
Researchers at Tokushima University in Japan and the University of California, Santa Cruz in the US, considered what could keep the subsurface ocean warm while keeping the ice shell's inner surface frozen and uneven on Pluto.
The team hypothesised that an "insulating layer" of gas hydrates exists beneath the icy surface of Sputnik Planitia. Gas hydrates are crystalline ice-like solids formed of gas trapped within molecular water cages.
They are highly viscous, have low thermal conductivity, and could therefore provide insulating properties.
The research, published in the journal Nature Geoscience, showed the thermal and structural evolution of Pluto's interior and the time required for a subsurface ocean to freeze and for the icy shell covering it to become uniformly thick.
They simulated two scenarios: one where an insulating layer of gas hydrates existed between the ocean and the icy shell, and one where it did not.
The simulations showed that, without a gas hydrate insulating layer, the subsurface sea would have frozen completely hundreds of millions of years ago; but with one, it hardly freezes at all.
Also, it takes about one million years for a uniformly thick ice crust to completely form over the ocean, but with a gas hydrate insulating layer, it takes more than one billion years.
The simulation's results support the possibility of a long-lived liquid ocean existing beneath the icy crust of Sputnik Planitia.
The most likely gas within the hypothesised insulating layer is methane originating from Pluto's rocky core.
This theory, in which methane is trapped as a gas hydrate, is consistent with the unusual composition of Pluto's atmosphere -- methane-poor and nitrogen-rich.
Similar gas hydrate insulating layers could be maintaining long-lived subsurface oceans in other relatively large but minimally heated icy moons and distant celestial objects, the researchers conclude.
"This could mean there are more oceans in the universe than previously thought, making the existence of extraterrestrial life more plausible," said Shunichi Kamata of Hokkaido University who led the team.