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Astrochemistry

Methane ice dunes spotted on Pluto

Nitrogen sublimation and gentle breezes may form the hills

by Sam Lemonick
May 31, 2018 | APPEARED IN VOLUME 96, ISSUE 23

NASA’s New Horizons spacecraft flew by Pluto in 2015, snapping photographs and collecting data that revealed the dwarf planet’s vast icy plains, towering mountains, and thin atmosphere. Closer inspection has possibly revealed a surprise: dunes (Science 2018, DOI: 10.1126/science.aao2975).

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Credit: NASA/JHUAPL/SwRI
NASA's New Horizons spacecraft snapped this photograph of Pluto in 2015.

Matt W. Telfer of the University of Plymouth and his colleagues make the case that dunes of methane ice grains stretch across several thousand square kilometers in the northwest lobe of a heart-shaped plain of nitrogen ice called Sputnik Planitia. If the researchers are correct, Pluto would join a small club, including Earth, Mars, and Saturn’s moon Titan, known to feature dunes.

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Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
Plains on Pluto's surface (bottom right) may be home to methane ice dunes. Photograph is about 80 km across.

Telfer says that spotting the possible dunes—in photos taken when New Horizons was 12,500 km above Pluto—was relatively easy; explaining how they formed was harder. Spectroscopic data from the spacecraft suggested that the dunes are predominantly methane. The atmospheric pressure on the dwarf planet is just 0.001% that of Earth’s, and the strongest winds barely top 10 meters per second, a speed that would make large tree branches sway on our planet. Through computational studies, the researchers show that such light winds could carry methane ice grains once they were aloft but lack the oomph to lift the particles off the ground. Instead, the scientists propose the grains are lifted up when nitrogen ice sublimates under the sun’s glare in mountainous glaciers.

Ralph D. Lorenz, an expert on extraterrestrial dunes at the Johns Hopkins University Applied Physics Laboratory, says the group makes a reasonable but not definitive case for dunes on Pluto, adding that it will be hard to confirm the idea because there are no planned follow-up missions to the dwarf planet.

Telfer says his group is now working to better explain how sublimating nitrogen might lift methane ice grains.

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