With average surface temperatures around -230°C, Pluto doesn’t seem like it would resemble a beach, but at the base of its icy mountains is a field of towering dunes.
At the edge of a mountain range called Al-Idrisi Montes, a belt of ridges spans 75 kilometres atop a glacier of nitrogen ice. Matt Telfer at the University of Plymouth in the UK and his colleagues examined data from NASA’s New Horizons spacecraft to determine whether these ridges could be dunes of tiny ice particles, similar to those formed from sand on Earth.
There are several signs that they are. They are regularly spaced – set apart by about 400 metres to a kilometre – and the angles between them depend on the mountains they border. That suggests they are formed by winds flowing down the sides of the peaks.
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Proof of that wind is found in streaks of dark material among the dunes. These are caused by the wind passing over spots of dark material, and dragging it along.
Fly me to the dune
He and his team found that the dunes are most likely made of methane ice and could reach tens of metres in height. New Horizons detected an excess of methane in the area, and it’s less dense than other ices on Pluto, making it easier for the feeble wind to pick it up.
The wind may be too weak to blow the methane grains away on its own, but they might be lofted into the air when sunlight causes the surrounding nitrogen ice to sublimate. As the nitrogen turns into gas and floats away, it carries with it particles of harder-to-melt methane that the wind picks up and piles into dunes. Some of the particles may get welded together to form a firmer surface, like ice cubes left in the freezer too long.
What would a day spent on those dunes be like? “As you look up at this landscape in the dusky twilight that is full daylight on Pluto,” Telfer says, “you see the peaks of the Al-Idrisi Montes to one side, and the bright glacier of Sputnik Planitia to the other side, and you crunch your way through the fine, granular ice of the dunes.”
These dunes sit atop the edge of Sputnik Planitia’s soft ice, which shows signs of thermal convection. “It’s sitting there churning like a slow-motion pot of oatmeal,” says team member Will Grundy at Lowell Observatory in Arizona. Because that churning hasn’t buried the dunes yet, the researchers say they must have formed in the last 500,000 years.
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