Dunes on Pluto made of frozen methane grains

Thin, weak atmosphere makes find surprising

CAPE CANAVERAL, Fla. — Scientists have discovered dunes on Pluto made of tiny frozen grains of methane.

The pale gray and white ridges were revealed by NASA’s New Horizons spacecraft during its 2015 flyby. A British-led team announced the findings Thursday in the journal Science.

Researchers said the dunes appear to be made mostly of icy specks of methane the size of sand, with some frozen nitrogen likely mixed in. Thought to be relatively recent, the parallel rows of dunes are located in Pluto’s heart-shaped region at the base of mountains as tall as the Alps and formed from giant blocks of ice with frosty methane snowcaps. These plains in the left lobe of Pluto’s “heart” are known as Sputnik Planitia.

Scientists were surprised to find dunes given Pluto’s thin, weak atmosphere. They suggest nitrogen ice coating the surface of Sputnik Planitia transformed into gas that lifted methane particles into the air. Pluto’s gentle winds then carried and deposited the grains.

Dunes already have been found on Mars, Venus, Saturn’s moon, Titan, and even a comet. But Pluto’s are the only ones known to consist of methane.

“Pretty much nowhere else we know of is cold enough!” the study’s lead author, Matt Telfer of Plymouth University in England, said via email Thursday.

He noted there are dunes on the scorching surface of Venus under a dense atmosphere and out in the distant reaches of the solar system at minus 400 degrees Fahrenheit under a thin atmosphere.

“These are not just balls of ice far out in space,” he said, adding that frozen worlds on the fringes of our solar system, like the dwarf planet Pluto, might have been active early on.

Researchers liken the dunes to those at White Sands, New Mexico, or California’s Death Valley.

“It’s a little bit lower density than sand we’re used to holding on the Earth,” Brigham Young University’s Jani Radebaugh, a co-author, explained by phone. “So it would feel lighter in your hand, but it would still be granular and would kind of flow off of your hand, and your feet would kind of crunch them as you’re walking along. It would just kind of feel a lot like you’re on another sand dune on the Earth.”

The team has yet to determine the height of the dunes; Telfer guesses they’re at least tens of yards tall.

“Much work is left to do to understand dunes on Pluto,” Cornell University’s Alexander Hayes wrote in a companion article. He was not involved in the study.

The Associated Press A July 2015 image made by the New Horizons spacecraft shows dunes, small ripples at bottom right, on Pluto’s Sputnik Planitia ice plain.