Scientists hazard a guess on Pluto’s impossible ocean

With temperatures topping out at -223C, Pluto should not have the ocean scientists know it does. So why hasn’t it frozen?

A view of Pluto, made from images by the New Horizons spacecraft in July 2015. Picture: NASA/JHUAPL/SwRI

Pluto should not have an ocean. At 40 times further out in the solar system than the Earth, the sun is just a bright star and “summer” is marked by temperatures topping out at -223C.

Yet beneath its surface there is almost certainly liquid water. But why hasn’t it frozen?

Scientists have suggested that the reason it has been kept so well-insulated is thanks to a ­kilometres-thick foamy mix of ice and gas. They suggest that the same mechanism could protect oceans, and perhaps life, in otherwise inhospitable planets throughout the universe.

When the New Horizons probe passed Pluto in 2015, it flew over a 1000km-wide impact crater on the “tidal axis” where Pluto’s largest moon, Charon, tugs on the planet.

It seemed quite a coincidence that a comet hit exactly on the tidal axis. NASA’s leading theory is that it didn’t. Instead, the impact weakened the crust, allowing an ocean to move closer to the ­surface.

This caused a “mass anomaly” that led to the dwarf planet being tugged into a new alignment.

But if there is an ocean there, how has it stayed liquid? It could be that it contains ammonia as “antifreeze”, but it would have to do so in volumes that seem ­implausible.

Other scientists suggest that radioactive decay in the core could keep heating it, but the quantities needed to achieve this do not fit with our understanding of the solar system.

The latest research, published in the journal Nature Geoscience, proposes a different solution. Riding on the top of the ocean, but beneath the crust, is a layer of “gas hydrates” — gas molecules trapped in a latticework of ice, ­creating a highly insulating barrier between the warm water and the cold of space beyond.

According to calculations by the scientists from Japan, such a layer would form naturally if dissolved concentrations of gas such as methane were high enough.

It would also allow oceans to persist for hundreds of millions of years after the formation of the planets, despite the freezing conditions. One reason the theory is interesting is because of what it might mean for the rest of the ­universe.

Life on Earth began in the oceans, and astrobiologists think that life on other planets will probably require liquid water too. Because of this, they restrict the search for extraterrestrial life to the “Goldilocks zone”, where ­liquid water can exist.

This latest research suggests that the zone may be far wider than was suspected.

“This could mean there are more oceans in the universe than thought, making the existence of extraterrestrial life more plausible,” the researchers said.

The Times