Martian sand dunes boast unearthly qualities

Two sizes of wind-sculpted ripples on Mars sand dune: the larger ripples are roughly three meters apart and of a type not seen on Earth. 

No sign yet of giant sandworms, but NASA's Curiosity Mars rover has sent back close-up images that show sand ripples on the Red Planet's dunes are of a type not found in Earth's deserts. The information has provided new clues on the history of the planet's atmosphere and provides another piece of the puzzle regarding the planet's surface and how it's changed over time.

The bedforms, or ripples, that run horizontally down the face of sand dunes are caused by wind-borne sand grains colliding with surface sand. These "impact ripples" are found only on the windward side of the dune, and not on the leeward face, which is typically steeper due to sand avalanches. Due to their size, measuring anywhere from 100 meters (328 ft) to a kilometer (0.6 mi), the dunes on both planets are considered true dunes. On Earth's dunes, the impact ripples are typically just under a 30 cm (1 ft) apart and sometimes lead to a corrugated surface.

On Mars, scientists have known for years through orbital images that the ripples on the planet's dunes are about 3 m (10 ft) apart. But as the Curiosity rover got an up close look of the "Namib Dune" in the Bagnold Dune Field on the northwestern flank of Mount Sharp, it discovered a subset of mid-size ripples the same size and shape as impact ripples found on Earth, but they were located between the larger ripples and exhibited unique characteristics.

"These large Martian wind ripples have crest-to-crest spacings ranging between one and five meters (3.3 to 16.4 ft), and they do not look like bedforms one would find in sand in Earth's deserts," Mathieu Lapotre, a graduate student at California Institute of Technology and member of the research team for the Curiosity mission, told Gizmag. "Their crestlines are sinuous, contrasting with the straighter crests of the small-scale ripples. Furthermore, the large ripples have asymmetric profiles, with steep downwind faces. On those steep downwind faces, we observed small avalanche deposits. Altogether, the large wind ripples on Mars closely resemble current ripples that one would find on Earth's riverbeds."

The conclusion reached by researchers is that Mars' mid-size ripples are created by sand particles being dragged along the surface by the wind, in the same way water drags sand to form ripples on Earth.

"On Mars, the very low density of the Martian atmosphere actually enhances the relative efficiency of drag; such that both types of bedforms appear to form by the action of winds on a sandy surface," said Lapotre. "We thus call the intermediate-scale bedforms 'wind-drag ripples,' by analogy to the fluid-drag ripples formed underwater on Earth, and by opposition to the impact-ripples found in Earth's sandy deserts."

It's believed Mars had a thicker atmosphere in the past that may have formed smaller wind-drag ripples, or prevented their formation altogether. Thus, wind-drag ripples found preserved in Martian sandstone may have recorded the thinning of the planet's atmosphere. Curiosity and NASA's Opportunity rover both investigated sandstone sites that showed preserved wind-drag ripples more than three billion years old, which were similar in size to ripples on currently active dunes. This lends further credence to the belief that most of Mars original atmosphere was lost early in its history.