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A new study led by the University of Southampton and research institutes in France has uncovered the mystery of how mini sand dunes form on beaches and in deserts.

While the formation of large desert dunes is well understood, scientists haven't been able to use the same theory to explain how smaller-scale dunes鈥攖he kind you might walk through on a beach holiday, emerge.

The findings, in the journal PNAS, not only reveal how these so-called "proto dunes" occur on Earth, but could hold clues to how they form on Mars and other planets.

"These are the kind of smaller scale sand bedforms that people would see forming before their eyes on the beach before the wind stops or the waves wash them away," says Professor Jo Nield from the University of Southampton, who led the study.

"The theory of how the large, wavy dunes you might picture in the Sahara Desert form assumes you have near limitless amounts of soft, dry sand which is picked up and deposited by the wind. But this doesn't account for how these small dunes take shape on moist surfaces like a beach or in hard gravelly areas."

Proto dunes have been difficult to study in detail because they are small (up to a few centimeters in height), move and grow rapidly (from nothing to six centimeters within half an hour) and can disappear as quickly as they emerge.

The international research team, with team members from Southampton, Paris, Oxford, Loughborough, Illinois and Denver, were able to capture how these small dunes form for the first time using high-resolution laser scanning in the Namibian desert.

They found that sand moving on harder, more consolidated surfaces bounces higher and is transported more by the wind. Once it lands on a softer, rippled surface, the sand accumulates.

Prof Nield said, "On these surfaces, the sand doesn't just roll across the land, it jumps up to a meter or so and so there is a gradual transition when grains will feel the change from a consolidated to rippled surface.

"Once bumps start to form, this influences wind patterns, adding further sand and helping the dune to grow, as happens in larger dunes."

This new theory, coupled with the high-resolution data captured, has been developed by co-authors in Paris to create a computer model of the dynamics at play. Excitingly, the model can accurately reproduce what researchers have observed in their field studies in arid conditions such as Namibia but also in moist conditions in Colorado and Norfolk.

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The model also allows the team to tweak different parameters, such as the amount of sand and wind, to understand different scenarios.

Prof Philippe Claudin, a co-author on the paper from the French National Center for Scientific Research (CNRS), said, "The model can replicate almost perfectly what we see in our field data. Interestingly, we see similar patterns in arid areas with gravel and coastal areas where there's moisture.

"Using the model, we can see that if there are really strong winds, the dunes will get bigger and bigger, whereas if there is not much sand coming in, the proto-dune will erode and disappear."

These proto-dunes aren't unique to Earth. The research team are now looking at how mini dunes form on Mars.

"We are really excited to see how what we've learned on Earth could be applied to Mars and to understand similarities and differences between proto-dunes on the two planets," says Prof Nield.