'Dune Spiders' on Mars

Researchers using the UA's HiRISE camera onboard NASA's Mars Reconnaissance Orbiter see seasonal changes on far-northern Martian sand dunes caused by warming of a winter blanket of frozen carbon dioxide.

As spring advances in Mars' northern polar region and the surface of carbon dioxide ice begins to crack, trapped gas gushes out, spilling dark sand from the dunes below onto the light frozen ground.

Earth has no naturally frozen carbon dioxide, though pieces of manufactured carbon-dioxide ice, called "dry ice," sublime directly from solid to gas on Earth, just as the vast blankets of dry ice do on Mars.

A driving factor in the springtime changes where seasonal coverings of dry ice form on Mars is that thawing occurs at the underside of the ice sheet. There, it is in contact with dark ground being warmed by early-spring sunshine through translucent ice. The trapped gas builds up pressure and breaks out in various ways.

Transient grooves form on dunes when gas trapped under the ice blanket finds an escape point and whooshes out, carrying out sand with it. The expelled sand forms dark fans or streaks on top of the ice layer at first, but this evidence disappears with the seasonal ice, and summer winds erase most of the grooves in the dunes before the next winter. The grooves are smaller features than the gullies that earlier research linked to carbon-dioxide sublimation on steeper dune slopes. Similar activity has been documented and explained previously where seasonal sheets of frozen carbon dioxide form and thaw near Mars' south pole. Details of the different northern seasonal changes are newly reported in a set of three papers for the journal Icarus.

To learn more, view Mars: Dry Ice & Dunes. The findings reinforce growing appreciation that Mars today, however different from its former self, is still a dynamic world, and however similar to Earth in some respects, displays some quite unearthly processes.

"It's an amazingly dynamic process," said Candice Hansen of the Planetary Science Institute in Tucson. She is lead author of the first of the three new reports.

"We had this old paradigm that all the action on Mars was billions of years ago. Thanks to the ability to monitor changes with the Mars Reconnaissance Orbiter, one of the new paradigms is that Mars has many active processes today."

With three Martian years (six Earth years) of data in hand from the Mars Reconnaissance Orbiter's HiRISE camera, the researchers report on the sequence and variety of seasonal changes. The spring changes include outbursts of gas carrying sand, polygonal cracking of the winter ice blanketing the dunes, sandfalls down the faces of the dunes, and dark fans of sand propelled out onto the ice.

"It is a challenge to catch when and how those changes happen, they are so fast," said Ganna Portyankina of the University of Bern in Switzerland, lead author of the second report. "That's why only now we start to see the bigger picture that both hemispheres actually tell us similar stories."

The process of outrushing gas that carves grooves into the northern dunes resembles the process creating spider-shaped features in far southern Mars, as seen in the image below, but the spiders have not been seen in the north.

The seasonal dry-ice sheets overlie different types of terrain in the two hemispheres. In the south, diverse terrains include the flat, erodible ground where the spiders form, but in the north, a broad band of sand dunes encircles the permanent north polar ice cap. Another difference is in brightening on parts of the ice-covered dunes. This brightening in the north results from the presence of water-ice frost, while in the south, similar brightening is caused by fresh carbon dioxide.

The third paper of the Icarus set, by Antoine Pommerol of the University of Bern and co-authors, reports distribution of the water frost using the Compact Reconnaissance Imaging Spectrometer for Mars. The light water frost is blown around by spring winds.

These "spider"-like features form when the sun warms the surface in Mars' south pole region, causing carbon dioxide trapped underneath a dry ice layer to gush out and blow dark sand onto the frozen surface (below). (Photos: NASA/JPL/UA)

What the gushing sand geysers may look like. (Artist rendering by Ron Miller/JPL/ASU)