Pluto’s surface may have dunes of methane ice
A new study of Pluto surface images captured by NASA’s New Horizons spacecraft suggests the dwarf planet has dunes likely composed of methane ice. The features were seen by scientists three years ago when images of the region, including mountains adjacent to Sputnik Planitia, were returned by the spacecraft, but were surprised by them because Pluto’s atmosphere is so thin.
Dunes are created when winds transport tiny surface particles and sweep them into a particular shape. In the Solar System, they have been seen only on Earth, Venus, Mars, Titan, and Comet 67P/Churyumov-Gerasimenko.
Now, an international team of researchers composed of planetary scientists, physicists, and geographers have analyzed the photos and maps of Pluto’s surface returned by New Horizons and combined these with a computer model to conclude the dwarf planet has dunes made of solid methane ice grains.
Pluto’s diverse surface features a variety of land forms, including mountain ranges, trenches, and plains. Its composition includes nitrogen, methane, carbon dioxide, and water.
In their analysis, the researchers found 357 pale ridges on Sputnik Planitia—an icy plain that constitutes the left side of Pluto’s heart feature—all regularly spaced, as well as dark wind streaks running over 46 miles (74 kilometers). The fact that the ridges run parallel to a mountain range that is spread out and subject to consistent local changes is evidence that these features are created by winds.
On Pluto, winds are produced when heat from the Sun causes surface ices to sublimate and turn directly into gases. Given the dwarf planet’s low gravity and surface pressure, sublimated nitrogen gas is sufficient to sweep methane particles from adjacent mountains upward, creating the dunes, the researchers found. Winds on Pluto can be as high as 18 to 24 mph (29-39 kph), sufficient for dune sculpting.
“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,” said Eric Partell of the University of Cologne in Germany, who took part in the study. “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.”
As appropriate for Sputnik Planitia’s young surface, the dunes do not show any sign of having been disturbed, indicating they likely formed within the last 500,000 years. The lack of craters and presence of polygonal shapes in this region indicate the surface is young and undergoing geological activity likely caused by thermal convection of surface ices.
“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,” said study leader Matt Telfer of the University of Plymouth in the UK. “It turns out that even though there is so little atmosphere, and the surface is around minus 230 degrees Celsius (minus 382 degrees Fahrenheit), 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.”
While methane is the most likely suspect for the dunes’ composition, nitrogen ice is another possibility. Higher resolution images are needed to confirm these features are definitely dunes. Some scientists are already exploring a possible return to Pluto with an orbiter, a lander, or both.
Findings of this study have been published in the journal Science.
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.