The red deserts of Mars have revealed a surprise: an altogether new type of sand dune.

Today a team of scientists led by Mathieu Lapotre, a planetary geologist at Caltech, announced a new type of geological formation in the Martian sands. Lapotre's team discovered waves of windswept sand that are totally unlike the two features commonly seen on deserts on Earth—vast dunes and tiny ripples. The new sandy features are described today in the journal Science.

They found these new wind-blown waves by combining aerial photographs snapped by Martian orbiters and pictures taken by the plucky rovers rolling across the Martian terrain. They've been named wind-drag ripples, and they coexist alongside the more typical dunes and ripples. You can think of them as middle children of the Martian deserts, which form on the backs of bigger dunes and are themselves coated with sandy ripples. These new wind-drag ripples are carved by the hyper-low density of the Martian winds. Earth's thicker atmosphere explains why we don't have them here.

The new dunes are quite oddly shaped. For one, they don't form in straight lines like sand ripples. "Instead, they usually have pretty sinuous crest-lines," says Lapotre, "they're also very asymmetrical. On one side they gently slope to the point of their crest, but have a much steeper slope on the other side," he says. That makes them look like a bit like ocean waves that are about to crest and crash.

These weird dunes went undetected until now because of their size. At most, from crest to crest these wind-drag ripples reach roughly 15 feet across as they undulate over the backs of larger dunes. Viewing them from spacecraft like the Mars Reconnaissance Orbiter, scientists assumed they were just larger versions of the small inch-sized ripples you can see cresting on dunes across Earth, from the Sahara to California's Death Valley.

At first it made sense. Mars has less gravity and a thinner atmosphere. Hypothetically, sand particles could be flung farther when whipped up by the wind, creating larger ripples. But the Curiosity rover's recent foray into Mars' Bagnold dune field disproved this idea.

The rover snapped images showing that the new wind-drag ripples actually coexist alongside the tiny, normal ripples—the type you find on Earth. If Mars' low gravity and thin atmosphere were behind the large ripples, then why would you still find smaller ones? This discovery forced geologists back to the drawing board. Today, Lapotre can safely explain what the heck is going on.

Curiosity rover self-portrait at the Namib Dune of the Bagnold Dune Field, Gale crater, Mars. NASA

To understand what forms wind-drag ripples (and why they can exist on Mars but not Earth), it helps to understand what causes ripples and dunes in the first place. Both are sculpted by the steady flow of wind. While large dunes come in a variety of different shapes, from straight crested mounds to more Tatooinian crescent cliffs, they're all formed by roughly the same process. Wind blows sand into sloping piles, which are held up by grains that've settled in the wind-sheltered regions beyond the dune's face.

Ripples, on the other hand, are formed at smaller scales. They're made as gusts of wind temporarily hurl grains of sand into the air, which crash back down like miniature meteoroids. An endless volley of these tiny impacts carves out ripples. The size and distance between ripples can be affected by wind speed, atmospheric pressure, sand size, and gravity—basically anything that constrains how long a grain of sand will stay aloft before it crashes back down.

Lapotre says that wind-drag ripples aren't caused by the slow shaping that accumulates as dunes, nor by the the aerial acrobatics that create small ripples. Rather, these mid-sized waves are forged as the winds of Mars' thin atmosphere slowly roll and pull grains of sand in short slogs. According to Lapotre, you can think of Mars' super-thin air as a strangely viscous fluid that is always tugging and dragging at the planet's grains of sand. These dunes don't form on Earth because in our planet's thick atmosphere, the wind forces that create tiny ripples overpower those that could create wind-drag ripples.

Lapotre and his colleagues have already found a use for this new dune discovery. Dunes of all sizes can leave their fingerprint as they form sedimentary rock after eons. Looking at a 3.7-billion-year-old rock cliffside on Mars—called the Burns formation—Lapotre's team has found evidence that their wind-drag ripples were there. Lapotre says this is evidence that way back then, 3.7 billion years ago, Mars still had a thin wispy atmosphere, the very type needed to make these odd dunes.

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