A winter’s tale (in the Kuiper belt)

Winter is coming on Pluto. And it is a winter, scientists say, unlike anything imaginable on Earth, a winter in which the dwarf planet’s entire atmosphere is expected to freeze out as frost, leaving it nearly as airless at the Moon.

Not that Pluto has ever had a thick atmosphere. Its current surface pressure is only a bit more than 0.001% that of Earth’s, and there are indications that this is the densest it ever gets.

What’s surprising, says Eliot Young, a planetary scientist at the Southwest Research Institute, Boulder, Colorado, is that Pluto’s nitrogen atmosphere is as actually as dense as it is. Since 1988, when its density was first measured by observing the way it dimmed starlight when Pluto passed in front of a star, it’s actually increased by a factor of nearly three, something that can only occur if frost has been steadily sublimating from the surface during the intervening years.

That would make sense if Pluto’s elliptical orbit was carrying it closer to the Sun. But it reached its closest approach in 1989 and has since moved 10% farther out, which, given the way solar heating works, means it is receiving only 77.8% as much energy from the Sun now, as then.

“It’s a surprise that Pluto’s atmosphere is as big as it is and has been growing for 30 years,” Young said on Monday at a virtual meeting of the American Astronomical Society’s Division for Planetary Sciences.

Now, however, it appears that this trend is on the cusp of reversing itself.

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Read more: In the Kuiper Belt, a baffling lack of small craters

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In August 2018, astronomers realised that Pluto was about to pass in front of a fairly bright star – an occasional event which, like the first one studied in 1988, allows them to probe the density of its atmosphere by watching how the starlight dims before it winks out… and how it brightens as Pluto moves out of the way.

Better yet, this was going to be observable along a broad arc running across much of the US from Texas to Virginia – making it easy for Young’s team to deploy a dozen or so portable telescopes (16- to 20-inch apertures) along that arc in the hope of observing the two-minute event from as many cloud-free locations as possible.

Luck was on their side, and they not only had clear skies, but even managed to predict the centerline of the arc so well that one telescope wound up only six kilometres from the perfect location. “We’ve never gotten that close to the centreline before,” Young says. “It wasn’t long ago where we’d have been happy to have gotten within 100 kilometres.”

When NASA’s New Horizons spacecraft flew by Pluto in 2015, it measured the surface pressure of Pluto’s atmosphere at 11.5 microbars. (One microbar is one millionth the pressure of Earth’s atmosphere).

If Pluto’s atmosphere had continued thickening at the rate seen in prior studies, Young says, his team would have expected to see a pressure of 14.4 microbars. But instead, they got 11.4 microbars, “basically the same thing New Horizons saw.”

That may not sound like the onset of winter, but it’s definitely the beginning of fall. And once Pluto’s temperature starts to drop, Young says, its atmosphere is going to freeze out very quickly, with half of it freezing out with each 1.5°C drop in surface temperature.  

The reason the big freeze has been delayed this long, he adds, is similar to the reason why beach sand continues to heat up after high noon, or why the hottest time of year isn’t the summer solstice, but later on. Even though the solar intensity is waning, heat has penetrated below ground, from where it is slow to dissipate. “It keeps the surface warm,” he says.

Studying the timing of this process as Pluto continues to move outward from the Sun (eventually to about 1.67 times the distance it was in 1989) will teach scientists a lot about how its subsurface retains heat, Young says, including such factors as how porous its materials might be.

“It’s a chance to look below the surface and see how heat is stored,” he says.