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- Ridge features on Pluto's surface have been revealed to be dunes of solid methane ice grains
- Researchers thought dunes would be unlikely on Pluto, but it's more like Earth than they thought
(CNN)Nearly three years after NASA's New Horizons spacecraft flew past Pluto, we're still learning about the dwarf planet.
A study published Thursday in the journal Science reveals that the dwarf planet has dunes. But these aren't like sand dunes on Earth; Pluto's dunes are made from solid methane ice grains.
Within our solar system, dunes are more rare than you might think. They exist on Earth, Mars, Venus, Saturn's moon Titan and Comet 67P.
"When we first saw the New Horizons images, we thought instantly that these were dunes but it was really surprising because we know there is not much of an atmosphere," Jani Radebaugh, study co-author and associate professor in geological sciences at Brigham Young University, said in a statement. "However despite being 30 times further away from the sun as the Earth, it turns out Pluto still has Earth-like characteristics. We have been focusing on what's close to us, but there's a wealth of information in the distant reaches of the solar system too."
An international team of planetary scientists, physicists and geographers looked at detailed photos of Pluto's surface, captured by New Horizons, that offered a bird's eye view of the dwarf planet's active surface. It is composed of nitrogen, carbon dioxide, water and methane, and it's diverse in landforms, including plains, trenches and mountain ranges.
They saw 357 regularly spaced pale ridges and six darker wind streaks across 46 miles of the Sputnik Planitia, a sprawling ice plain that makes up one of Pluto's largest surface features. The scientists noticed that the ridges ran parallel with a major mountain range, that seemed to shift and spread out. They also change locally and consistently. This indicated that wind was creating the ridges.
But with the surface frozen, the researchers wondered what it could be made of.
Combining an analysis of wind streak and dune-like features with spectral and numerical modeling, the scientists determined what might be the underlying architect of dunes on Pluto.
Wind could create the dunes out of fine particles once they're airborne. And since Pluto's surface pressure is 100,000 times lower than Earth's, those grains don't need as much of a lift.
They discovered that was possible if there was sublimation at the surface, when a solid is turned directly into a gas. In Pluto's case, the sun heats the icy surface enough that gas can be released into the atmosphere. This also lifts the methane particles, which mostly likely come from the nearby mountains. But the researchers haven't ruled out that these could be made of nitrogen ice.
Pluto's winds can reach 18 to 24 miles per hour, creating and sculpting dunes.
"On Earth, you need a certain strength of wind to release sand particles into the air, but winds that are 20% weaker are then sufficient to maintain transport," Eric Parteli, study co-author and lecturer in computational geosciences at the University of Cologne, said in a statement.
"The considerably lower gravity of Pluto, and the extremely low atmospheric pressure, means the winds needed to maintain sediment transport can be a hundred 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 [movement of particles over an uneven surface]. Put together, we have found that these combined processes can form dunes under normal, everyday wind conditions on Pluto."
These dunes are largely undisturbed on Pluto's icy crust, which suggests that they were formed within the past 500,000 years -- or even more recently. Pluto presents a bit of an age conundrum this way.
Instead of craters, the dwarf planet has polygonal shapes and features that indicate the surface is geologically active and young. The surface itself is only about 500,000 years old, although the dwarf planet itself is about 4.5 billion years old. This activity is most likely caused by a thermal, convective overturning of the ice. This helps date when the dunes formed, as well.
"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," Matt Telfer, lead study author and lecturer in physical geography at the University of Plymouth, said in a statement. "It turns out that even though there is so little atmosphere, and the surface temperature is around -230 degrees Celsius, we still get dunes forming. The New Horizons data has given us a new level of detail, but we had to work hard to explain how it was possible to get the supply of sediment, a non-cohesive surface and wind you need for dunes.
"It is another piece of the jigsaw in making sense of this diverse and remote body, and gives us a more fundamental understanding of the geological processes which are influencing it."
