The tops of Pluto's mountains and the rims of its craters have bright coatings, which stand out in contrast to the rest of the dark equatorial region dubbed Cthulhu. To Earth visitors, these would look like snow-capped mountains, and planetary scientists initially wondered if they might be formed through a similar process, with methane substituting for water in temperatures well below -200ºC (-328ºF). However, a new study proposes the formation of these frost deposits occurs in a manner never seen on Earth or any other world we are familiar with, revealing a lot about Pluto's atmosphere in the process.
When the New Horizons spacecraft first sent back images of Pluto, the bright peaks that stood out against Cthulhu's dark features drew attention. However, there was initial uncertainty as to whether these were formed predominantly from methane ice or a methane-nitrogen mix, let alone how they got there.
In Nature Communications, a team led by Dr Tanguy Bertrand of NASA's Ames Research Center report these deposits are primarily methane, with only small nitrogen-rich areas. They argue that to understand them we need to abandon our Earth-based assumptions.
“On Earth, atmospheric temperatures decrease with altitude, mostly because of adiabatic [constant heat content] cooling and warming in upward and downward air motions, respectively,” the paper notes. The higher wind goes up a mountain, the cooler surface temperatures get because the atmosphere cools the ground. When water-bearing winds run into mountains, they flow up the side and cool, causing the water to condense into deposits of frost or snow, which persist in the low temperatures.
Pluto's atmosphere is very different. For the first few kilometers above the surface, temperatures increase because methane in the atmosphere captures what little sunlight Pluto receives. The surface meanwhile is so cold it draws some of the modest heat the lower atmosphere has out. With such a thin atmosphere, however, this isn't enough to substantially warm the surface.
When wind hits Pluto's mountains, the air piles up and becomes denser, causing it to flow downhill, which rules out methane condensation in a manner analogous to water on Earth. Instead, the paper proposes the methane-covered tips are a product of atmospheric-stratification, with circulation cells driving methane disproportionately upwards.
Some atmospheric methane condenses during Pluto's nights even in the equatorial zones, particularly in winter, the authors believe. At lower altitudes, the methane turns back to gas in the daytime, but it persists higher up the slopes because the more-methane rich air at heights above 4 kilometers (2.5 miles) deposits enough overnight that some survives. Once a little methane builds up, it reflects more light than areas around it, resembling snow on Earth in this way, if nothing else. That has a cooling effect, which causes extra methane to condense, creating a reinforcement cycle.
The authors suspect, but cannot yet confirm, a similar process produced the enormous methane deposits in the area known as Tartarus Dorsa.