What’s warming Pluto’s icy heart?

Pluto's heart-shaped Sputnik Planitia could be hiding a liquid water ocean.

Image of Pluto taken by New Horizons

Something is keeping Pluto’s icy heart from freezing over.

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A layer of gas-infused ice could be acting as a blanket allowing the dwarf planet to host a liquid ocean beneath its crust, according to a recent study.

When NASA’s New Horizons probe flew past the dwarf planet in 2015, a heart-shaped region near the world’s equator attracted the attention of planetary scientists and the public alike.

The left hand lobe of this region, now called Sputnik Planitia, drew particular interest amongst geologists because it appears to be a giant basin.

We’ve found a new, generic mechanism to maintain a subsurface ocean for a long time

“We do not expect big basins near the equator,” says Shunichi Kamata from Hokkaido University, who led the research.

“If you look at the Moon you’ll find a big basin near the south pole. If you look at Mars, what you find near the equator is not basins but huge volcanoes.”

When a basin is on the equator, it should mean that the mass around the planet’s middle is lop-sided, which makes the planet wobble as it spins.

As Pluto doesn’t wobble, it suggests that there must be something dense under the surface of Sputnik Planitia making up the mass – for instance, an ocean.

However, a world as small and far from the Sun as Pluto should have frozen right through billions of years ago.

If there is an ocean, something has to be keeping Pluto warm.

Kamata’s team wondered whether a layer of clathrate gas hydrides – ice where the water molecules act like a cage, trapping gas within it – could be acting as a blanket around the planet, keeping the inside warm.

To test the theory, the group created a computer simulation modelling how Pluto would cool over its 4.5-billion-year lifespan, both with and without such a layer.

They revealed that with the layer, the ocean barely freezes at all.

“We’ve found a new, generic mechanism to maintain a subsurface ocean for a long time,” says Kamata. “To make gas hydrates stable, they have to be pressurised – on Earth, natural gas hydrates are not stable at sea level. Only relatively large icy moons can possess a clathrate hydrate layer above the ocean.”

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It could be possible that similar layers are keeping icy moons – such as Ganymede and Titan, both of which are thought to have liquid oceans under the surface – from freezing solid too.