A team of planetary scientists led by researchers at the California Institute of Technology (Caltech) has discovered a type of dune on Mars intermediate in size between decimeter-wavelength ripples and hundred-meter- to km-wavelength dunes, and unlike anything seen on our planet.
Two sizes of ripples are evident in this December 13, 2015, view of a top of a Martian sand dune, from Curiosity. Sand dunes and the smaller type of ripples also exist on Earth. The larger ripples — roughly 10 feet (3 m) apart — are a type not seen on our planet nor previously recognized as a distinct type on Mars. Scale bar – 2 m (79 inches). Image credit: NASA / JPL-Caltech / MSSS.
Mars and Earth both have true dunes with downwind faces shaped by sand avalanches, making them steeper than the upwind faces.
Our planet also has smaller ripples — appearing in rows typically less than a foot (less than 30 cm) apart — that are formed by wind-carried sand grains colliding with other sand grains along the ground.
“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,” said lead author Mathieu Lapotre, from Caltech.
Lapotre and co-authors analyzed images of Martian bedforms taken by NASA’s Curiosity Mars rover and Mars Reconnaissance Orbiter.
While the deposits the team analyzed were indeed formed by wind blowing over sand, they were more similar in dune shape and spacing to ripples that form under water.
Building on decades of water flume experiments on Earth, the scientists developed a scaling relationship to predict the crest-to-crest spacing between underwater ripples, ultimately showing it accurately predicted the spacing between the new Martian wind ripples.
The researchers designated these deposits ‘wind-drag ripples.’
Because Lapotre and his colleagues could also show that the size of these ripples changed with atmospheric density, they have created a way to use observations of sedimentary Martian rocks to measure global changes in the Martian atmospheric density over time.
Two sizes of wind-sculpted ripples are evident in this view of the top surface of a Martian sand dune. Image credit: NASA / JPL-Caltech / MSSS.
“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.
“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.”
“Thus, the size of preserved wind-drag ripples, where found in Martian sandstones, may have recorded the thinning of the atmosphere.”
The scientists checked ripple textures preserved in sandstone more than 3 billion years old at sites investigated by Curiosity and Opportunity rovers.
They found wind-drag ripples about the same size as modern ones on active dunes. That fits with other lines of evidence that the Red Planet lost most of its original atmosphere early in its history.
The team’s results are reported in a paper published online today by the journal Science.
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M.G.A. Lapotre et al. 2016. Large wind ripples on Mars: A record of atmospheric evolution. Science, vol. 353, no. 6294, pp. 55-58; doi: 10.1126/science.aaf3206
