We can often use our knowledge of planet Earth to explain things we see on other worlds, although we may have to adjust the physics to account for a different temperature or a weak atmosphere. But planetary scientists cannot always assume that a well-known landscape feature formed in a known way.
When the New Horizons spacecraft gave us our first close-up of Pluto, alien wonders abounded. But there were also mountain peaks dusted with something bright, very much like the snow-capped peaks of the earth. On Earth, these snow caps are produced by more precipitation as the air above the mountains rises and cools, combined with the colder temperatures at higher elevations.
On Pluto, that statement may not work for several reasons. First, temperatures in general increase if you go up a few miles from Pluto’s surface because of gases absorbing solar energy. Winds also tend to blow descent because the colder surface cools the air nearby, making it denser. So what makes up the clear dust and how does it get there?
West of Pluto’s heart-shaped bright plain lies a dark-colored region called Cthulhu. Within that region, there are some mountain ranges that stand out for their clear caps. Analyzing spectral data from images of this region, the researchers say the bright regions are mostly made up of methane ice. It also occurs on north-facing rims and walls of impact craters in the region.
To see why methane ice would form at those locations, the researchers turned to a climate model of the dwarf planet. The model allowed ice of methane and nitrogen to form (or disappear by turning into gas), and the researchers focused on the conditions at the time of New Horizons’ visit. Just by entering the physics and topography, the model successfully creates methane ice on the mountains and craters of Cthulhu. In many places, some methane frosts can form at night, but disappear during the day. But a greater concentration of methane gas at higher elevations leads to a net accumulation on the mountaintops during that daily cycle.
That extra methane up there is brought there by patterns of atmospheric circulation. The behavior of nitrogen ice is important to Pluto’s atmosphere, with sublimation in the northern hemisphere and condensation in the south driving north-to-south winds over the heart-shaped plains. Methane is also lifted in that loop. The higher topography on the western edge of the plain also helps push methane higher over Cthulhu.
So ultimately the reason for the white tops is simply that methane is enriched in the air around it. And on the north-facing walls and rims of the impact craters, methane dust formation is likely related to weaker sunlight during the long Plutonian winter.
The researchers say this pattern of methane ice accumulation at higher elevations may help explain the formation of the “leaf-shaped terrain” regions on Pluto. These jagged ridges are also primarily made of methane ice. By analogy with the snow penitentes found in a few places on Earth, the terrain with blades has been thought to have to do with sublimation. But instead of sublimation deepening low spots, this study suggests a way for deposition to favor high spots. Both could happen at the same time, the researchers say — one process is known and the other unknown.
Nature Communications, 2020. DOI: 10.1038/s41467-020-18845-3 (About DOIs).