Take a close look at the geometric pattern above. What appears to be a swath of reptilian scales or a complex cellular structure isn't biological. In fact, what you’re seeing isn't even located on earth. Those polygons are sand dunes found in a Martian crater about 20 miles across and spotted from above by NASA’s Mars Reconnaissance Orbiter
earlier this year. As the globe celebrates World Space Week
—with a focus on Mars—this pocked terrain offers a glimpse of what awaits explorers and their dune buggies, perhaps some day.
“These dune patterns are fairly common on Mars,” said Nathan Bridges, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory. “But they’re restricted to within craters and other low-lying terrain,” such as basins and canyons. The theory is that, in these recessed landscapes, winds blowing from multiple directions can push sand around in various ways. For instance, during the day, air heated by the sun rises up towards the crater's rim, possibly sweeping sand along with it, says Scot Rafkin, a planetary scientist at the Southwest Research Institute in Boulder, Colorado. At night, air cools and moves in the opposite direction, away from the crater's edge—potentially blowing sand, too. What's more, geological features within craters—such as mountains—add turbulence, causing gusts to collide and swirl around. Swept back and forth by the vagaries of wind and confined to a crater, sand can pile into intersecting ridges oriented in many different directions, resulting in polygonal shapes like those in the picture above.
The sand that forms these dunes is comparable in size to what you’d find in a sand box, says Bridges—each grain is about a tenth of a millimeter in diameter. But unlike sand on earth, which is mostly quartz, Martian sand is primarily dark, iron-rich basalt
—the same rock that makes up Hawaiian volcanic lava floes.
So far, nobody has found such large fields of polygonal dunes on our planet. “We just don’t have a lot of craters,” says Bridges. The closest pattern to these Martian dunes that we see on earth are star dunes
, he says. They can be found in deserts all around the world, including the Mojave Desert in the U.S. and parts of the African Sahara.
Related Science Friday Link