Mountains on Pluto Capped with Methane Ice

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?

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New research conducted by an international team of scientists, including researchers at NASA Ames Research Center, Laboratoire de Météorologie Dynamique and Institut de Planétologie et Astrophysique de Grenoble in France, and Lowell Observatory in Flagstaff (USA), has revealed 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 this discovery raised the question of whether the formation of these high-altitude methane frosts follows the same process that forms snowpack on terrestrial mountain summits.

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 dwarf planet's atmosphere actually gets warmer as altitude increases, the surface temperature remains the same, and winds that travel down mountain slopes dominate.

To understand how the same landscape could be produced with different materials and under different conditions, the researchers used a 3D model of Pluto's climate, developed at the Laboratoire de Météorologie Dynamique in France, simulating its atmosphere and surface over time. They found that Pluto's atmosphere has more gaseous methane at its warmer, higher altitudes, allowing for that gas to saturate, condense and then freeze directly on the mountain peaks (without any formation of cloud). At lower altitudes, the concentration of gaseous methane is lower, and thus there is no condensation and no methane frost.

It is particularly remarkable to see that two very similar landscapes on Earth and Pluto can be created by two very dissimilar processes. The Plutonian process is, to date, thought to be unique in the Solar System, but it could occur on Triton (a satellite of Neptune with a thin nitrogen atmosphere like Pluto) or other large and volatile-rich transneptunian objects. On Pluto, it also helps explain why methane ice is largely found at high altitude (e.g. on mountains and crater rims) and elucidates the formation process of the steep, narrow ridges of the so-called bladed terrain in the equatorial region of Tartarus Dorsa. 

Two similar landscape created by very different climatic processes on Pluto (in the Cthulhu region near the equator) and on the Earth (European Alps)

On the Earth water ice snow condenses at higher elevations because the air is cooled by the expansion of the air in upward motions. On Pluto methane ice forms on mountain summits because they stand out in warm atmospheric layers that are more methane rich than at lower elevations.   

Bertrand

NASA Post-Doc , NASA Ames Research Center

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