Pluto: Another Abode for Life?

Icy worlds might be more habitable than previously thought.

Sputnik Planitia on Pluto, as seen by New Horizons. (NASA/JHUAPL/SwRI)
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In a new paper published in the journal Nature, Shunichi Kamata from Hokkaido University in Japan and colleagues highlight the potential of Pluto to harbor liquid water in the form of a subsurface ocean.

This dwarf planet in the frozen outer regions of the Solar System is not usually an object of interest to astrobiologists. However, scientists haven’t been so sure about that since the New Horizons mission flew past Pluto in 2015 and imaged Sputnik Planitia, a Texas-size, relatively young region consisting mostly of apparently thin ice near the planet’s equator.

Incorporating the new data from the New Horizons mission, Kamata’s team ran computer simulations with two different scenarios: one with a layer of methane clathrates between Pluto’s ice shell and the suspected water ocean beneath, and one without such a layer. Methane clathrates are known to occur on the bottom of Earth’s oceans, where cold temperatures and high pressures prevail. They are also thought to be common on icy bodies in the outer Solar System. Methane is trapped in the crystal structure of water, causing the layer to act as a good insulator that traps interior heat.

The simulations showed that without the insulating layer, it would take just one million years for a uniformly thick ice crust to form over Pluto’s subsurface ocean. But if methane clathrates are present, it would take a billion years! Moreover, the underlying ocean would probably never freeze over completely, given the expected heat from tidal forces exerted by Pluto’s large moon Charon, as well as occasional hits by large asteroids and comets.

If much of Pluto’s methane is trapped within the ice, it would also explain why the dwarf planet’s atmosphere is so rich in nitrogen, but poor in methane. The layer of methane clathrates would work much like an ice cover over Earth’s lakes in winter, which allows fish to survive in liquid water.

The results are relevant not only to possible liquid water and perhaps even life on Pluto, but also to other icy bodies and so-called “rogue” planets that wander through space without orbiting any sun. Whether life exists at Pluto or any other icy world is, of course, highly speculative, as liquid water doesn’t necessarily guarantee life. However, this kind of insulating mechanism would certainly enhance a planet’s potential habitability. And given that there are many other icy worlds in our Solar System alone, and that by some estimates there is at least one rogue planet for every star in the sky, the finding by Kamata and colleagues could be very significant.

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