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Weird sand ripples on Mars tell tales about Red Planet’s past and present climate

Sand structure on Mars
Two sizes of ripples can be seen in this view of a Martian sand dune. The larger ripples are roughly 10 feet apart, and unlike any type seen in earthly sand fields. The smaller ripples, superimposed on the larger ripples, are similar to those seen on Earth. (Credit: NASA / JPL-Caltech / MSSS)

The sands of Mars move in mysterious ways – including one way that’s not seen on Earth’s surface, but only on the sandy bottom of bodies of water. And the scientists behind NASA’s Curiosity rover mission say those weird medium-sized ripples can reveal how Mars’ atmosphere has changed, or not, over the course of billions of years.

The alien ripples are the focus of a research paper published today by the journal Science.

“Earth and Mars both have big sand dunes and small sand ripples, but on Mars, there’s something in between that we don’t have on Earth,” Caltech researcher Mathieu Lapotre said in a NASA news release. Lapotre, who works with the Curiosity mission’s science team, is the lead author of the Science report.

The report is based on a close-up examination of the Bagnold Dunes, a stretch of Martian sand that Curiosity passed through as it made its way toward the foothills of 3-mile-high Mount Sharp (a.k.a. Aeolis Mons).

The big dunes, which typically span more than 100 yards, looked familiar. Earth’s winds can blow sand into similarly huge piles that are shaped by avalances on their downward slides. Curiosity also saw small ripples, arranged in rows that were typically less than a foot apart. On Earth, such impact ripples are created when windblown sand collides with other sand grains along the ground.

But the midsize ripples, measuring about 10 feet across, posed a puzzle. The dynamics of Earth’s winds don’t create ripples that size. The closest analog turned out to be the sand ripples that form underwater.

That was the key to solving the puzzle.

“The size of these ripples is related to the density of the fluid moving the grains, and that fluid is the Martian atmosphere,” Lapotre said.

Lapotre and his colleagues say the midsize ripples are unique to the Martian surface due to the Red Planet’s thin atmosphere. And that’s not all.

“We think Mars had a thicker atmosphere in the past that might have formed smaller wind-drag ripples or even have prevented their formation altogether,” Lapotre said. “Thus, the size of preserved wind-drag ripples, where found in Martian sandstones, may have recorded the thinning of the atmosphere.”.

Researchers checked the patterns of wind-drag ripples preserved in sandstone formations that they estimated were more than 3 billion years old. They found that those ancient ripples were at best only slightly smaller, suggesting that the atmosphere was about as thin back then as it is today. That finding is consistent with the prevailing view that Mars lost most of its atmosphere relatively early in its 4.5 billion-year history.

“Finding evidence of these ripples in the rocks tells us that Mars has had a thin atmosphere for a long time,” study co-author Ryan Ewing of Texas A&M said in a news release.

Curiosity is continuing its Mount Sharp trek, almost two years after its touchdown on Mars. As the rover makes its way up the mountain, it’s tracing billions of years of geological history preserved in the layers of rock – and keeping an eye out for more clues about Mars’ ancient climate change.

Lapotre and Ewing are among 22 authors of the Science paper, titled “Large Wind Ripples on Mars: A Record of Atmospheric Evolution.” Western Washington University’s Melissa Rice is also a co-author.

Source: Geekwire Space

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