While flying by Pluto in the year 2015, NASA's New Horizons spacecraft caught a glimpse of a landscape that seemed all too familiar. The famous dwarf planet revealed icy mountain tops that looked very similar to the ones found on Earth.
These frosty tips aren't merely uncanny — they have not been observed on any other world.
Since then, scientists have sought to discover how snow could form on Pluto's mountain tops, and whether it was through a process similar to that on Earth.
Now, an international team of scientists led by the French National Centre for Scientific Research may have discovered the atmospheric process behind Pluto's snowcapped mountains. It is starkly different than anything we may have expected from what we know about Earth's own weather systems.
The new findings are detailed in a study published Tuesday in the journal Nature.
Tanguy Bertrand, a postdoctoral fellow at NASA's Ames Research Center, and lead author of the new study, recalls seeing the images of Pluto's mountain tops and wondering how this snow could have formed, and whether it formed the same way it does on Earth's mountain tops.
"We wanted to know if Pluto’s atmosphere behaves the same as Earth’s atmosphere," Bertrand tells Inverse.
The team first looked at the New Horizon's images to uncover what the snow on Pluto is actually made of. Unlike the water that makes up Earth's snow, that found on Pluto is made of methane, a type of greenhouse gas which, on this icy planetary body, acts like water vapor does on Earth.
The researchers then simulated the atmospheric cycle that drives ice formation on Pluto using climate models similar to ones used to forecast weather on Earth.
"When we used this model, we discovered that the conditions on Pluto were completely different than the ones on Earth," Bertrand says.
On Earth, the lower part of the atmosphere is warm and wet, so water vapor forms in this region. The warm air rises from the bottom up, expanding and cooling on its way up. That leads to more moisture in the air.
The water vapor freezes on top of Earth's mountains, resulting in snowcaps.
On Pluto, the process is completely opposite.
Pluto's atmosphere is warmer at higher altitudes due to solar radiation. And the methane vapor is more abundant at higher altitudes. The methane only condenses to form snow on mountains that are high enough to reach this level of the atmosphere.
This climate mechanism may be unique to Pluto, and has never been observed anywhere else in the universe.
"Pluto is very interesting because there are many landscapes that remind us of Earth, sometimes much more than any other planet in the Solar System," Bertrand says. "Despite the fact that they have similar landscapes, there’s still plenty of dynamical processes in space that we don’t know about."
Now that they have solved the mystery of Pluto's snowcapped mountains, the scientists are turning their attention to other mysteries of the dwarf planet's atmosphere and geology. One enigma is what causes Pluto's steep, narrow ridges of ice, which resemble massive knife blades.
"Now that we know how methane forms snow on Pluto's mountain tops, maybe we are going to explore the formation of these dunes next," Bertrand says.
Abstract: Pluto is covered by numerous deposits of methane, either diluted in nitrogen or as methane-rich ice. Within the dark equatorial region of Cthulhu, bright frost containing methane is observed coating crater rims and walls as well as mountain tops, providing spectacular resemblance to terrestrial snow-capped mountain chains. However, the origin of these deposits remained enigmatic. Here we report that they are composed of methane-rich ice. We use high-resolution numerical simulations of Pluto’s climate to show that the processes forming them are likely to be completely different to those forming high-altitude snowpack on Earth. The methane deposits may not result from adiabatic cooling in upwardly moving air like on our planet, but from a circulation-induced enrichment of gaseous methane a few kilometres above Pluto’s plains that favours methane condensation at mountain summits. This process could have shaped other methane reservoirs on Pluto and help explain the appearance of the bladed terrain of Tartarus Dorsa.