Methane Frosts Atop the Equatorial Mountains of Pluto

This image, captured by the NASA New Horizons spacecraft, shows a chain of mountains capped with methane-rich frost in Pluto’s otherwise dark Cthulu Regio. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

In 2015, the Long-Range Reconnaissance Imager (LORRI) instrument onboard the New Horizons spacecraft revealed bright, methane-rich frosts atop 3.5-kilometer-high mountains in the Cthulu region of Pluto. These methane-rich frosts possess an uncanny resemblance to the snow-covered mountains of the Alps on Earth, but their origin on Pluto has remained enigmatic.

On Earth, the temperatures of both the atmosphere and rocky surfaces decrease with altitude. As warm, moist air is transported upslope by wind, the air cools and condenses into clouds towards the summit of a mountain, precipitating snow onto the mountain top. However, on Pluto, observations and previous climate modeling have shown that atmospheric temperatures increase with altitude, opposite to that of Earth. Moreover, Pluto’s atmosphere is too thin to affect surface temperatures, meaning the water-ice mountains of Pluto can be colder than the atmosphere at high altitudes and suggesting that a different process forms Pluto’s frost-capped mountain ranges. This led Tanguy Bertrand from the NASA Ames Research Center and colleagues to produce a new climate model of Pluto to investigate how these frosts formed and why they are similar in appearance to the snow-capped mountains of Earth. Their numerical modelling shows that the equatorial regions of Cthulu are cold enough to condense methane directly onto the surface (without snowing) to form thin frosts micrometers thick. While these methane frosts can sublime during the daytime, in the high-altitude terrains, the rate of night-time methane condensation exceeds the rate of day-time sublimation allowing the methane-rich frost caps to thicken. Their atmospheric circulation model shows that the plutonian process of forming frost-capped mountains is unique to the dwarf planet as it is controlled by the increase in methane abundance with altitude rather than the change in temperature with altitude that drives the formation of snow-capped mountains on Earth. READ MORE