The base of Pluto’s crust, scientists say, may contain a layer of an exotic form of ice known as gas hydrate, insulating and preserving the dwarf planet’s underground ocean since the dawn of the solar system.
Scientists have for several years postulated that Pluto might once have had a subsurface ocean, generated by the melting of ice due to tidal heating as its giant moon Charon spiralled into its present orbit.
But once that process reached its present state, with Pluto and Charon permanently facing each other, like Earth’s moon permanently turns only one face to us, the tidal heating should have ceased, and the ocean should have frozen solid.
When NASA’s interplanetary space probe New Horizons swung by Pluto in 2015, however, it provided clues that strongly suggested that the water still exists.
Among these clues is the giant basin of Sputnik Planitia — the western lobe of Pluto’s famous “heart” – which appears to be associated with a gravity anomaly that can best be explained by an underlying ocean.
That means something has to be insulating that ocean from the subfreezing conditions of Pluto’s surface, and the best candidate for that, says Shunichi Kamata, a planetary scientist at Hokkaido University in Sapporo, Japan, is gas hydrate.
Gas hydrate is a form of ice that crystalises in cage-like shapes that can trap gases such as methane, making microscopic ice beach balls.
They generally form under high-pressure conditions, and exist on Earth near ocean-bottom natural gas seeps, from which they can be brought to the surface and set on fire in YouTube demonstrations that prove just how different they are from normal ice.
They also turn out to be good insulators, Kamata says: good enough that only a few kilometres of them at the base of Pluto’s ice crust would be enough to provide the amount of insulation needed to keep an underlying ocean from freezing for billions of years.
What’s important, he adds, is that the gas hydrate layer must cap the entire subsurface ocean, rather than just existing in localised patches.
“If it is localised, then heat can be removed from the non-capped region,” he says.
Other scientists are intrigued.
“This is really cool,” says Robert Pappalardo, a planetary scientist at NASA’s Jet Propulsion Laboratory, in Pasadena, California, US.
We’ve long known that the ice on so-called icy worlds like Pluto isn’t pure water ice, he says, but because we don’t have good estimates of the levels of other materials — he calls them “impurities” — it’s easy to make the simplifying assumption that their ice is actually pure water.
But when you start thinking about the other things that might be there, he says, it starts to open doors.
“You can’t just ignore it,” he continues. “It can really help explain phenomena that are otherwise very difficult to explain.
“I like models where you take basic phenomena that we know about and apply them to weird places, and suddenly the light bulb goes off, ‘Oh, why didn’t we think of that?’ I feel like this is one of those instances.”
William McKinnon, a planetary scientist at Washington University in St Louis in Missouri, agrees.
At present, he says, it’s not possible to prove whether gas hydrates exist on Pluto.
“They decompose at low pressures,” he says, “which is why they haven’t been directly detected so far on planetary surfaces.”
However, he adds, Pluto should have formed with “a good deal” of both methane and carbon dioxide, “both of which are ready clathrate-formers”.
(A clathrate is a compound in which molecules of one component are trapped inside the crystal structure of another.)
And if Pluto has such materials, it stands to reason that other outer solar system bodies also have them, including not only the moons of Jupiter and Saturn, but possibly other large Kuiper Belt objects, even farther out from the sun than Pluto.
“So a larger number of oceans may exist than previously thought,” Kamata says.
Pappalardo adds: “That’s where the punchline is. It comes down to the search for water in our solar system. It’s pretty impressive that there may be ways to preserve oceans over long time scales in pretty basic ways, [such as] ‘just add a little methane’.”
Could there be life in them?
Who knows? “Liquid water is important, but not the only ingredient for life,” Kamata says.
“I hope the next step is to consider if other conditions for life are met in Pluto’s ocean.”
The researchers published their findings in the journal Nature Geoscience.
Related reading: Planet or not, Pluto started out hot