Pluto is covered by snow-capped mountains like on Earth, but not for the same reasons

CNRS Press Release
© NASA/JHUAPL/SwRI
In 2015 the New Horizons spacecraft discovered spectacular mountains on Pluto covered by bright deposits, strikingly resembling snow-capped mountain chains seen on Earth. Such a landscape had never been observed elsewhere in the Solar System. Could Pluto’s atmosphere behave like Earth’s?
New research conducted by scientists from the Laboratoire de Météorologie Dynamique (LMD-IPSL / CNRS/Sorbonne Université/École polytechnique/ENS Paris), Institut de Planétologie et Astrophysique de Grenoble(CNRS /Université Grenoble Alpes) [1], NASA Ames Research Center and Lowell Observatory (USA) show that the “snow blanket” on Pluto’s mountains is made of methane ice. Methane is a trace gas on Pluto, as water vapor is on Earth, so initially it seemed logical that these high-altitude frosts could form like on the Earth.




On our planet, atmospheric temperatures decrease with altitude, mostly because of the cooling induced by the expansion of the air in upward motions. As a consequence, surface temperatures also decrease with altitude because the cold, dense atmosphere cools the surface. Under such conditions, as moist wind approaches a mountain, it rises upslope and cools, with water condensing to form snow on the top of mountains. But on Pluto, the opposite occurs: The thin atmosphere is warmed by the Sun and actually gets warmer as altitude increases. The surface temperature remains uniformly cold. At its contact the air is cooled and flows downslope.

At left, the “Cthulhu” region near Pluto’s equator, at right the Alps on Earth. Two identical landscapes created by highly different processes.
© NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
© Thomas Pesquet/ESA

To understand how the same landscape could be produced under different conditions, the researchers used a 3D model of Pluto’s climate, developed at the Laboratoire de Météorologie Dynamique, simulating its atmosphere and surface over time. They found that, because of its particular dynamics, Pluto’s atmosphere has more gaseous methane at its warmer, higher altitudes, allowing for that gas to saturate and freeze directly on the mountain peaks tall enough to reach the enriched zone. At lower altitudes, the concentration of gaseous methane is lower, and it cannot condense.

It is remarkable to see that two very similar landscapes on Earth and Pluto can be created by two very dissimilar processes. The plutonian process could occur on Triton (a moon of Neptune with a thin atmosphere like Pluto) or other large transneptunian objects (Eris, Makemake, etc.). On Pluto, it also helps elucidate the formation process of the steep, narrow ridges of the so-called bladed terrain in the region of Tartarus Dorsa.

On Earth snow condenses at altitude because air dilates during ascending movements, and thus cools (at the rate of 1°C approximately every 100 m). On Pluto, methane ice forms on the peaks of mountains when they are high enough to reach upper atmospheric levels, which are hotter and rich in methane. © Tanguy Bertrand et al.

Source

Tanguy Bertrand, François Forget, Bernard Schmitt, Oliver White and Will Grundy. Equatorial mountains on Pluto are covered by methane frosts resulting from a unique atmospheric process. Nature Communications, 13 October 2020. DOI:10.1038/s41467-020-18845-3

Local scientific contact

Bernard Schmitt, IPAG / OSUG

This press release was published by CNRS.

Updated on 14 October 2020

[1] This lab is member of the Observatoire des sciences de l’Univers de Grenoble