Part of the wonder of seeing new worlds is the radical difference from the planet you know. But if you know a little bit about the processes that shape our Earth, it’s also enthralling to see those same processes play out under alien conditions. It’s a marriage of exotic and familiar, like an instantly recognizable melody appearing in a style of music you’re hearing for the first time.

One familiar process is the formation of dunes. Large, repeating ridges of wind-blown sand can form in the desert, but they can also form as small ripples can on sandy stream bottoms or beaches. Wherever you have solid particles in a moving medium, dune-like landforms are possible. And we have seen plenty of them on Mars, on Titan, and even on comet 67P, despite its lack of a substantial atmosphere. In a new paper led by Plymouth University’s Matt Telfer, researchers working on the images from the New Horizons probe add another weirdo to the list of dune-bearing worlds—the dwarf planet Pluto.

Solid methane

Obviously, Pluto looks a bit different from the sand sea of the Sahara. Hanging out around -230°C, its surface is mainly covered with solid forms of substances we know as gases, like nitrogen, carbon monoxide, and methane. With an atmosphere that is 100,000 times thinner than ours, it’s hard to imagine winds pushing much of anything.

But images of the edge of Pluto’s vaguely heart-shaped plain, in an area bordering a mountain range of what is probably water-ice “bedrock,” the researchers were struck by a surface pattern that sure looks like dunes. Rather than noodle-like dunes that stretch out parallel to the wind, these look like the type of dune that forms perpendicular to the wind where plenty of sand is available. That’s supported by the presence of a few dark streaks that point perpendicular to the dunes—the kind of thing that appears in the sheltered wake of some tall object that blocks the wind.

To work out how plausible all this is, the researchers started with the particles. Either nitrogen or methane ice could make small particles in those conditions, although methane is a better candidate since it has greater hardness and lower density. Methane has been seen accumulating like snowpack on mountaintops on Pluto, likely during its long winter season. If that’s also true for the peaks of the al-Idrisi Montes next door, plenty of methane “snow” could be drifting down to the plain.

For a rough estimate of the size of these methane grains, the researchers applied a formula relating their size to the spacing of the dune ridges and wind speed. That puts the grain diameter at 0.2 to 0.3 millimeters—the size of an average sand grain on Earth.

Sublime

Next, the researchers modeled the physics of blowing those grains around. They found that Pluto’s winds—which would be strongest coming down the slopes of the neighboring mountains—should definitely be able to move methane grains of that size around. There’s a catch, though. The winds seem too weak to pluck grains off the ground and start them moving.

However, there’s another, stranger process that could make this happen. What atmosphere Pluto does have is mainly sustained by afternoon sublimation of surface ice—molecules occasionally jumping from the solid phase directly to gas when warmed in the sunlight. This sublimation provides upward force as it rises through the pile of particles at the surface. We’ve seen this process loft particles in Mars’ polar regions, for example. The physics seem to work on Pluto, too, and particles lofted this way would be free to blow in Pluto’s meager wind.

So, calling these ridges “dunes” seems fair. Examining the ground they rest on shows that the dunes are relatively young rather than ancient relics—and might even be currently active. Pluto’s heart-shaped plain (which is called Sputnik Planitia) sports an unusual polygonal pattern a little like the surface of gently boiling water. That is hypothesized to be the result of slow convection in the thick layer of nitrogen and other ices over a few hundred thousand years. The dunes must be much more recent than the action of that convection, or they would be churned apart by it.

This means that a trip to Pluto could include hiking shifting dunes while gritty grains of solid methane blow around your legs—also, freezing to death. Still, it’s a wild thought.

Science, 2018. DOI: 10.1126/science.aao2975 (About DOIs).