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A bewildering form of dune on Mars

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Researchers have discovered a type of dune on Mars intermediate in size between tiny ripples and wavier dunes, and unlike anything seen on Earth. Because dunes can be preserved in rock over time, these mysterious sedimentary deposits may represent a way to gain insights into the evolution of Mars' atmosphere from a more hospitable realm to the harsh, dry climate observed there today. On Earth, wind and water passing over sand causes the formation of either large dunes or small ripples, collectively called bedforms. Here, using a combination of images from the Mars Reconnaissance Orbiter and the Curiosity rover, Andre Lapotre and colleagues analyzed images of bedforms on Mars. While the deposits they analyzed were indeed formed by wind blowing over sand, they say, 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 researchers 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 – formed by atmospheric conditions unlike those found on Earth – "wind-drag ripples." Because Lapotre and 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 red planet's atmospheric density over time.

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