Almost three years have passed since New Horizons performed its historic flyby of Pluto, but we might just be getting started when it comes to the discoveries it will bring. New analysis of images collected by the space probe of the dwarf planet's surface has confirmed the presence of dunes. Despite being an unlikely characteristic of an icy world with little wind, scientists also believe they have an explanation of how they formed.
Jani Radebaugh is a geology professor at Brigham Young University and was one of the many scientists to pore over the data and imagery sent back to Earth by New Horizons after its flyby. One of the major revelations of those early observations of the dwarf planet was the presence of a huge icy basin that came to be known as Sputnik Planitia. But there was something else that piqued Radebaugh's interest.
"I was looking at the top corner of the glacier, right next to the mountains, and I could see long, straight, regular ridges," she says. "And as I looked really closely, I thought, those are dunes. There's no question in my mind those are dunes."
But this challenged conventional wisdom in a couple of ways. Generally speaking, dunes are sculpted by winds and Pluto's atmosphere, 1,000 times thinner than ours, was thought to be too weak to generate strong enough gusts for this purpose. The other issue was the material itself. With temperatures of around -230° C (-382° F) and the surface primarily shrouded in frozen gases like nitrogen, carbon monoxide and methane, what could these fine, sand-sized particles be made of?
Further investigation by an international team of geographers, physicists and planetary scientists has now filled in the blanks. The researchers carried out spatial analysis of nearby wind streaks and the dunes themselves, along with spectral and numerical modeling.
This led them to conclude that the particles are most likely the result of sublimation, where solid nitrogen is converted into gas and releases sand-sized grains of methane in the process. The winds that do blow across Pluto, at speeds of 29 to 40 km/h (18 to 25 mph), then carry those particles into the space where the Sputnik Planitia ends and the adjacent mountain range begins.
"On Earth, you need a certain strength of wind to release sand particles into the air, but winds that are 20 percent weaker are then sufficient to maintain transport," says Dr Eric Parteli, Lecturer in Computational Geosciences at the University of Cologne, and study co-author. "The considerably lower gravity of Pluto, and the extremely low atmospheric pressure, means the winds needed to maintain sediment transport can be 50 times lower. The temperature gradients in the granular ice layer, caused by solar radiation, also play an important role in the onset of the saltation process. Put together, we have found that these combined processes can form dunes under normal, everyday wind conditions on Pluto."
The scientists estimate that these dunes formed sometime in the last 500,000 years, due to their undisturbed form and the historically convective glacial ice beneath it. They now plan to carry on investigating the dunes through computer simulations, which will in turn further enlighten them about how Pluto's winds shaped its geography.
"We knew that every solar system body with an atmosphere and a solid rocky surface has dunes on it, but we didn't know what we'd find on Pluto," says Matt Telfer, lecturer in physical geography at the University of Plymouth and lead author of the paper. "It turns out that even though there is so little atmosphere, and the surface temperature is around minus 230 degrees Celsius we still get dunes forming. … It is another piece of the jigsaw in making sense of this diverse and remote body."
The research was published in the journal Science.
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