By the standards of the earth, one could hardly imagine a foreign world other than icy Pluto. It lies in the far corner of the solar system on average about 4 billion miles from the sun. The NASA's New Horizons, which flew within 7,800 miles of the dwarf planet in July 2015, gave us the best view of Pluto. There the temperatures drop to minus 380 Fahrenheit. At Plutonian noon, the sunlight is comparable to the dusk of the earth.
But perhaps the most surprising result of New Horizons' journey was not the deeper look on Pluto's curios. The spaceship also revealed Pluto's versions of known geography. Like the earth, Pluto has jagged mountains, vast plains and changing seasons. A new study published Thursday in the journal Science reveals another feature that is shared over billions of miles: Pluto has dunes.
This world, where gravity is one-sixteenth as strong as Earth's, is "so different from our own in many ways," said study author Matthew Telfer, an expert in physical geography at the University of Plymouth, UK. But the dunes, he said, show that Pluto is "really similar."
NASA released images of New Horizons shortly after the flyby, including an iconic image of a heart-shaped basin on Pluto. In the western lobe of this heart, a plain called Sputnik Planitia, Telfer, and other planetary scientists discovered something peculiar.
Where Sputnik Planitia approaches icebergs, its surface curls. For Telfer and other planetary scientists, these waves looked like windswept sand – a small puzzle, because scientists were not sure if Pluto's thin atmosphere could generate enough wind to sweep.
Brigham Young University Planetary Scientist Jani Radebaugh, author of the new study, recalled that the features of New Horizons were posted on Facebook. "They really looked like dunes," she said. No other team had any plans to investigate the formations, so she joined Telfer and the members of the New Horizons Mission.
The first reports of possible dunes – as the New Horizons scientists carefully examined them in 2015 – did not fit. "There are a lot of things out there that are vaguely dune-like," Telfer said. But in one area of the plain the evidence accumulated.
"It's an exciting discovery, I'll say for sure," said Ryan Ewing, a geologist at the University of Texas at Austin, who had nothing to do with this report. "The way they see the dunes on the surface is the same technique we used to identify dunes on Mars."
To build a dune, you need sand and wind. Quartz fragments usually provide the sand for earth dunes. Pluto's Sputnik Planitia is covered in nitrogen ice, which, according to scientists, is not stable enough to form dunes. Frozen nitrogen, Radebaugh said, has a consistency that resembles that of toothpaste.
But also on this toothpaste glacier sits solid methane. Methane ice breaks up into tiny particles. The scientists calculated that the methane grains are likely to have a diameter of one-hundredth of an inch. Keeping a handful of methane grains when you can handle the cold would be like shoveling "really fine sand," said Radebaugh.
Walking on these dunes would be "like walking on a dune on earth." Sanding, we believe, "said Telfer. (If you would feel something through the well-insulated soles of your spacesuit.)
Pluto's dunes rise to about 100 feet, about as high as the Mesquite Flat Dunes in Death Valley National Park. Streaks on the surface indicate that the wind is blowing perpendicular to the dunes.
"The paper is a very compelling case, these are dunes," which have the necessary distance and orientation, said William B. McKinnon of Washington University in St Louis, who studies the geology of the worlds in the outer solar system and not an author this report was
But the sand has to flow. A model of the Pluto climate indicates that the wind can be as fast as 20 mph. That's enough to make floating particles into a dune shape. "This is one of the windiest places on Pluto," Telfer said, "and the winds are just in the right direction."
But these winds are too gentle to drop methane grains, which are normally crowded together, off the surface. It's a bit like flying a kite on a calm day: it takes a running start to lift the kite off the ground, where a light breeze can carry it. Something else has to give the fertilizer grains this initial reinforcement.
The authors of the study suggest that a process called sublimation serves the purpose. On Earth, we know the evaporation better – when you come out of the sea and walk along the beach, the sun dries and turns liquid water into steam. At Pluto, the sun turns solids directly into gas. Telfer and his colleagues discovered pits on the edge of the plain, where they suspected of sublimating nitrogen ice. Nitrogen sublimation would be strong enough to hurl methane, Telfer said, providing Pluto's airborne components for a dune.
McKinnon said that the sublimation theory was "somewhat speculative." He said other explanations as periodic increases in Pluto's atmospheric pressure could also lead to raised dune particles.
According to Radebaugh, the decisive result is the existence of the dunes. With its dunes, Pluto joins a club that includes Earth, Mars, Venus, Saturn's moon Titan, and possibly a comet.
"We just have to turn a spacecraft into a body and look at a region we do not have." For Radebaugh, it was a whole new realm of discovery. For New Horizons, this opening will take place on New Year's Day 2019, when it crosses the boundaries of the known world to an object in the Kuiper belt called MU69 nicknamed Ultima Thule – what goes on.
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