Dunes Of Methane Grains Discovered On Pluto’s Icy Surface

The Baré mountains and the icecap with tiny dune shapes visible on it. Modeling has shown these dunes are probably made from methane grains.  NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

It's been almost three years since New Horizons sped past Pluto, but we are still just starting to process what it saw. This includes an explanation for dunes whose shape resembles those of the Sahara, but are probably made from frozen particles of fart gas.

Among Pluto's surprisingly complex surface features were 357 pale ridges on the plain known as Sputnik Planitia. The ridges run parallel with the AIM mountain range and Viking Terra to Planitia's west, but fan out to the east. They cover an area about 75 kilometers (45 miles) across.

Planetary scientists were puzzled as to what could have produced these ridges. They were shaped like objects sculpted by moving air and located on an ice cap near mountains, where winds would be strongest. On Earth, or even Mars, this would make sense, but Pluto's atmospheric pressure is 100,000 times less than that of Earth, and almost a thousand times less than Mars. How could something so thin move solids?

A paper in Science shows even the exceptionally weak forces such an atmosphere can apply are sufficient to carry particles of methane ice size of grains of sand. Nitrogen ice particles, although most common on Pluto, are probably too soft, so the harder grains of methane ice are suspected. Further evidence comes from the presence of nearby dark streaks on the surface, also shaped as if by the wind.

Sputnik Planitia (A), close-ups of the dunes (B and D), the dark windblown patches (E) and the dunes with the mountains that shape the wind that then shapes them. Telfer et al/Science

Dunes are a common feature of the Solar System, being found on Venus, Mars, and Titan, as well as Earth, but all have distinct atmospheres. Dune-like shapes have been seen on comet 67P/Churyumov-Gerasimenko. Comets have no permanent atmosphere, but the gasses escaping as the comet's icy surfaces are exposed to sunlight provide a wind that can shape the landscape in similar ways.

Pluto, despite recent evidence it may be a billion comets pretending to be a planet, is a different matter, yet the authors' modeling shows it's possible. Grain sizes 0.21-0.31 millimeters (0.005-0.008 inches) are key.

“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,” explained Dr Eric Parteli of the University of Cologne in a statement

Pluto's winds are thought to reach 30-40 km/h (20-35 mph). Even at Pluto's distance sunlight can cause surface ices to turn to gas, carrying grains of ice with them in an updraft for the winds to act on.

“Despite being 30 times further away from the sun as the Earth, it turns out Pluto still has Earth-like characteristics,” said De Jani Radebaugh of Bringham Young University.

The glacial ice of Sputnik Planitia is relatively new, so it is thought the dunes much have formed in the last 500,000 years.

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