If there is a subsurface sea on Pluto, why hasn't it frozen solid? According to computer simulations devised by researchers from Japan's Hokkaido University and elsewhere, it may be that the hypothetical Plutonian sea may remain liquid thanks to an insulating gas layer keeping in the heat.

When NASA's New Horizons robotic probe flew by Pluto in July 2015, it changed many of our ideas about the dwarf planet as well as other celestial bodies in the cold outer reaches of the solar system. One fascinating discovery was that the nature of the equatorial, Texas-sized, white, ellipsoidal basin called Sputnik Planitia is such that is may have a slushy ocean located beneath its icy surface.

It's an intriguing idea, but it raises the question of how could it exist on a world so far from the Sun like Pluto, which doesn't have a molten core and isn't in the vicinity of a giant planet that could heat it through tidal forces. According to the math, such an ocean should have frozen solid billions of years ago, and if there ever was a sea beneath Sputnik Planitia, then it should also have frozen and the ice cap above it collapsed flat on top of it.

However, the new study that includes scientists from the Tokyo Institute of Technology, Tokushima University, Osaka University, Kobe University, and the University of California, Santa Cruz, suggests that the presence of a layer of gas hydrates between the cap and the slushy ocean could keep the Plutonian sea liquid.

Gas hydrates are a special form of water where ice forms crystalline cages that trap in gases like methane. Such hydrates are highly viscous, and have low thermal conductivity. In other words, they keep in heat very well. Hydrates are very common on Earth, where there are 6.4 trillion tonnes of methane hydrates in the deep ocean floor.

What the new study found was that computer simulations covering a timescale of 4.6 billion years, or about the time since the formation of the solar system, showed that without a gas hydrate layer the interior of Pluto would have frozen solid hundreds of millions of years ago, but with it, the subsurface sea would stay liquid. In addition, the gas hydrates would keep the sea free of a uniformly thick ice cap for one billion years, instead of just one million years without a gas hydrate insulating layer.

According to the team, the simulations suggest that the methane for the hydrates came from the rocky core of Pluto and may be part of the reason why the Plutonian atmosphere is poor in methane and rich in nitrogen. In addition, these sorts of capped subsurface seas may be very common.

"This could mean there are more oceans in the universe than previously thought, making the existence of extraterrestrial life more plausible," says team leader Shunichi Kamata of Hokkaido University.

The research was published in Nature Geoscience.

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