Something weird is at work on the cold, dark plains of Pluto, where hundreds of regularly spaced ridges look a bit like an alien thumbprint pressed into extraterrestrial ice. Today, though, researchers report in the journal Science that the strange landscape is actually a dune field crafted from methane "sand."
The observation is startling, given that scientists thought Pluto's tenuous atmosphere wasn't necessarily robust enough to support the formation of dunes, which by most definitions are sculpted by the wind.
"Dunes on Pluto, or any other body, tell us there is a significant enough atmosphere to move materials around, and that there are particles to move-in this case, frozen methane sand," says study coauthor Jani Radebaugh of Brigham Young University, who studies wind-sculpted landforms throughout the solar system and on Earth.
Scientists spotted the dunes in images from NASA's New Horizons spacecraft, which visited Pluto in 2015. The pictures show methane mounds parked on the massive glacier that makes up the western half of Pluto's "heart," a region now called Sputnik Planitia. These pale, linear mounds sometimes stretch for more than 12 miles, and overall, they occupy an area that's twice the size of Utah Lake.
The winds responsible for transporting that frozen sand into features resembling earthly dunes are likely blowing onto the glacial plain from the direction of a mountain range that rises along the heart's border. These winds, which blow perpendicular to the piles of sand, have left dark streaks of material on Sputnik Planitia that allowed scientists to retrace their path.
And because the glacier the dunes adorn is made predominantly of soft nitrogen ice, scientists think the methane sand comes from snowcaps atop those nearby mountains as well, which reach more than 10,000 feet skyward.
Sublimation, perhaps at those frozen peaks or on the glacier itself, turns ice directly into gas, which lifts the methane grains into Pluto's thin (but quite voluminous) atmosphere. Once airborne, the grains are pushed around by prevailing winds, which can blow between 18 and 24 miles an hour.
It's not yet clear how tall the resulting dunes are, but scientists suspect the features are rather young-less than 500,000 years old. Radebaugh compares Pluto's dunes to the smaller, confined desert dunes of North America, like those in the Mesquite Flat area of Death Valley or the transverse dunes of the Taklamakan of western China.
It's not the first time dunes have been spotted on otherwise improbable worlds. Perhaps most surprisingly, scientists found dune-like ripples on Comet 67P/Churyumov-Gerasimenko, which the European Space Agency's Rosetta spacecraft explored from 2014 to 2016. And on Titan, Saturn's largest moon, giant dunes made of frozen hydrocarbons rise from equatorial latitudes.
"If an extremely tenuous atmosphere like that of Pluto can support the generation of bedforms from wind-driven sediment, what kind of [similar] activity might we see on places like Io (a moon of Jupiter) or Triton?" Cornell University's Alexander Hayes writes in a commentary accompanying the published observations.
Still, Pluto's dunes are just one of many discoveries that have delighted scientists studying this place in the frozen wasteland of the Kuiper belt-a world that defied expectations by proving to be more dynamic and colorful than anyone thought possible.