Insulating Shell of Gas Hydrates May Prevent Pluto’s Subsurface Ocean from Freezing

Many icy bodies in our Solar System have underground oceans. On the dwarf planet Pluto, an area called Sputnik Planitia suggests the presence of a subsurface ocean and a locally thinned ice shell. To maintain an ocean, the dwarf planet needs to retain heat inside. On the other hand, to maintain large variations in its thickness, Pluto’s ice shell needs to be cold. Using computer simulations, a team of planetary researchers in Japan shows that the presence of a thin layer of gas hydrates at the base of the ice shell can explain the long-term survival of Pluto’s underground ocean.

This Pluto mosaic was made from New Horizons LORRI images taken on July 14, 2015, from a distance of 49,700 miles (80,000 km). This view is projected from a point 1,118 miles (1,800 km) above Pluto’s equator, looking northeast over the dark, cratered Cthulhu Regio toward the bright, smooth expanse of icy plains called Sputnik Planum. Pluto’s North Pole is off the image to the left. This mosaic was produced with panchromatic images from the New Horizons LORRI camera, with color overlaid from the Ralph color mapper onboard New Horizons. Image credit: S.A. Stern et al.

“We hypothesized that an insulating layer of gas hydrates exists beneath the icy surface of Sputnik Planitia,” said Hokkaido University’s Dr. Shunichi Kamata and colleagues.

“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.”

Dr. Kamata’s team conducted computer simulations covering a timescale of 4.6 billion years.

They 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 ocean 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.

A schematic diagram of the interior structure of Pluto.The ice shell has a thin clathrate hydrate layer at its base. The temperature changes substantially across this layer immediately above the subsurface ocean, leading to a conductive shell rather than a convective shell. Nitrogen-rich ice on the surface is the bright surface of Sputnik Planitia. Image credit: Kamata et al, doi: 10.1038/s41561-019-0369-8.

“The most likely gas within the hypothesized insulating layer is methane originating from Pluto’s rocky core,” the scientists said.

“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,” they said.

“This could mean there are more oceans in the Universe than previously thought, making the existence of extraterrestrial life more plausible,” Dr. Kamata added.

The research is published in the journal Nature Geoscience.

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Shunichi Kamata et al. Pluto’s ocean is capped and insulated by gas hydrates. Nature Geoscience, published online May 20, 2019; doi: 10.1038/s41561-019-0369-8