Pluto Is Losing Less Atmosphere Than We Thought

March 17, 2016 | by Alfredo Carpineti

Photo credit: Plutonian landscapes in twilight, under a hazy sky. NASA/JHU APL/SwRI

In less than a year, Pluto has gone from a minor object far away from the Sun to a key piece in our understanding of how the Solar System formed, as evidence by the publication of five new papers in the journal Science this week.

Here, we take a look at two of those papers, examining Pluto’s atmosphere and the space environment surrounding the dwarf planet, respectively. You can check out our other story for the other three papers.

During NASA’s New Horizons flyby last July, the probe was able to analyze the atmosphere of Pluto in detail. The lower part of the atmosphere is consistent with previous ground-based observations, but the upper atmosphere is significantly different to that of previous models.

According to the first paper, the outer layers of the atmosphere seem to have a significantly lower temperature than expected.

“We have a model for the atmospheric variance with altitude and it seems that temperature profile has to be a lot colder in the upper atmosphere,” lead author Dr. Randall Gladstone told IFLScience. “This has a big implication about the composition of the upper atmosphere.”

Because it’s so cold, the abundance of nitrogen falls off more steeply with altitude than the methane does. Normally they would separate at high altitude, but on Pluto, that happens a lot lower than expected.

“It gets much colder than we thought it was. And it lowers how much nitrogen can escape from Pluto, it’s much less than we thought and that affects the space environment,” Dr. Gladstone added.

“We thought that maybe up to a kilometer worth of nitrogen ice had escaped from Pluto over the age of the Solar System. If it’s anything like the current escape rate that couldn’t be true, it would be a few centimeters at most.”

Backlit by the sun, Pluto’s atmosphere is visible against the dark planet. NASA/JHUAPL/SwRI

The second paper looking at the space environment, also published in Science, has some unexpected findings, showing that the region surrounding Pluto is not like any other in the Solar System.

Astronomers expected to find a large outgassing (release of gas) from Pluto's atmosphere, and that would have had a large interaction with the solar wind. But the team discovered that the interaction was significantly more modest than expected.

Speaking to IFLScience, Professor Fran Bagenal, lead author of the second paper, said: “The space environment is quite different from what we thought. Pluto is quite an unusual obstacle to the solar wind, and we saw 10 times less particle intensity around Pluto [than predicted].”

Another important discovery is the amount of dust in the surrounding region. Scientists thought that meteorite impacts on Pluto and its moons might have filled the system with debris, but the data suggests that this is not the case.

Although the papers have shone a light on many aspects of Pluto, mysteries still remain about it. For example, the New Horizons team is not sure if the small outgassing of Pluto's atmosphere is a long-term or a short-term trend.  

“It’s a big 'if' the current escape rate that we are measuring is a good long-term average,” said Dr. Gladstone. “We are not really sure what’s cooling it off, we thought it was hydrogen cyanide but there’s contradictory evidence from ground observations. We are still trying to figure out.”

While Pluto’s atmosphere remains a more complex system than expected, some questions have been answers. Pluto has layers of haze moving in its skies, and the paper suggests gravity waves as the culprit for its formation. Gravity waves (not to be confused with gravitational waves) are waves generated by the different buoyancy of layers, like sea waves or clouds being shaped when they move over mountains.

“Pluto has mountains too and the wind blowing over those mountains could form gravity waves, and concentrate particles of haze into layers,” concluded Dr. Gladstone. “With reasonable assumption it seems to fit all the reasonable data so far.”

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