Why it snows on Pluto’s mountains

In 2015, the New Horizons space probe discovered spectacular snow-covered mountains on Pluto that are strikingly similar to the mountains on Earth. Such a landscape had never before been observed anywhere else in the solar system. However, there is one major difference: On Earth, atmospheric temperatures decrease with altitude. On Pluto, however, they increase with altitude due to the sun’s radiation. So where does this ice come from?

An international team of researchers has now investigated this. The scientists first discovered that the “snow” on the mountains of Pluto is actually frozen methane, with traces of this gas in the atmosphere of Pluto, just like water vapor on Earth. Then, to understand how the same landscape could be formed under such different conditions, they used a climate model for the dwarf planet, which showed that the atmosphere of Pluto is rich in gaseous methane due to its particular dynamics at high altitudes.

As a result, only on the peaks of mountains high enough to reach this enriched zone does the air contain enough methane to condense snow from it. At lower altitudes, the air is too poor in methane to allow ice to form. This research, published in Nature Communications, may also explain why the thick methane glaciers observed elsewhere on Pluto have spectacular rugged ridges, as opposed to the shallow water glaciers of the Earth.

On earth, snow condenses at altitude because the air expands and cools down as it rises (at a rate of 1°C about every 100 m). On Pluto, methane ice forms on the peaks of mountains when they are high enough to reach the upper atmospheric levels, which are warmer and richer in methane. (Picture: Tanguy Bertrand et al.)
On the left the region “Cthulhu” near the Pluto equator, on the right the Alps on earth. Two identical landscapes created by very different processes. (Picture: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute and Thomas Pesquet/ES
One of the most interesting features of Pluto, Cthulhu, extends almost halfway around the Pluto equator, starting from the west of the large nitrogen ice sheets known as Sputnik Planum. With a length of about 3,000 kilometers and a width of 750 kilometers, Cthulhu is slightly larger than Alaska. The appearance of Cthulhu is characterized by a dark surface, which scientists attribute to the fact that it is covered by a layer of dark tholines – complex molecules that form when methane is exposed to sunlight. The geology of Cthulhu is characterized by a wide variety of landscapes, ranging from mountainous to smooth, to highly cratered and rugged. The reddish magnified color image, seen in the left section, shows a mountain range in the southeast of Cthulhu that is 420 kilometers long. The mountain range lies between craters with narrow valleys that separate its peaks. The upper slopes of the highest peaks are covered with a bright material that contrasts sharply with the dark red color of the surrounding plains. (Picture: NASA / JPL)

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BrandonQMorris
  • BrandonQMorris
  • Brandon Q. Morris is a physicist and space specialist. He has long been concerned with space issues, both professionally and privately and while he wanted to become an astronaut, he had to stay on Earth for a variety of reasons. He is particularly fascinated by the “what if” and through his books he aims to share compelling hard science fiction stories that could actually happen, and someday may happen. Morris is the author of several best-selling science fiction novels, including The Enceladus Series.

    Brandon is a proud member of the Science Fiction and Fantasy Writers of America and of the Mars Society.