How frosty ice forms on Pluto’s mountains

Frosty deposits of methane ice on the mountaintops of Pluto form by a process very different from that by which snow falls on our planet, scientists say.

It’s an important reminder that things we see on other worlds aren’t necessarily created the same way as those on Earth, says Tanguy Bertrand, a planetary scientist from NASA’s Ames Research Centre, first author of a paper in the journal Nature Communications.

The snows, now known to be composed of methane, appear in images taken by NASA’s New Horizons spacecraft on its 2015 flyby of Pluto, where – among other places – they capped a chain of mountains several hundred kilometres long, just south of Pluto’s equator.

These mountains towered a stunning 2500 to 3500 metres high – the equivalent, given Pluto’s small diameter, of a 13,000 to 19,000-metre-high mountain range on Earth.

“It’s the first time we see snowcapped mountains outside of Earth,” Bertrand says.

On Earth, snow falls on the upper slopes of mountains because moisture-rich air rising from the lowlands cools as it rises. “Water vapour condenses, and we have snow,” Bertrand says. Not so on Pluto. “On Pluto the atmosphere is warmer at altitude.”

Not that Pluto’s atmosphere is all that hot. At an elevation of 20 kilometres or so, Bertrand says, it’s about 100 degrees Kelvin (minus 173 Celsius). But the surface is a lot colder: a mere 40 degrees K, or 40 degrees C above absolute zero.

That’s enough to make a big difference in the amount of methane vapour the atmosphere can hold. And when that warmer, methane-rich upper air encounters Pluto’s skyscraper mountains, it cools and flows downslope, not upslope.

The result is that the methane still deposits on the tops of the mountains, but it comes from above, rather than below.

“It is remarkable that we have two similar landscapes created by different processes,” Bertrand says.

Not that these frozen methane deposits actually come as snow. In Pluto’s thin atmosphere, only 1/100,000th as dense as Earth’s, that is impossible. Instead, they come as frost – and not a lot of it. The deposits studied by Bertrand’s team are probably only a millimetre thick.

All told, he says, the new finding is an important reminder that studying other worlds is useful for honing our understanding of how atmospheres and climates work.

“They are natural laboratories we can use to explore the diversity of possible climates. This gives us perspective on our own climate: what [about it] is unique.”

Such studies will also be important as we strive to learn more about exoplanets (worlds circling other stars), he adds.

“We are often surprised when we look outside of Earth that what we see on Earth is not the general standard. We have to explore more and more to find what really drives planetary climate, in general.”

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