At first, figuring out how pebble-sized rocks organize themselves in evenly-spaced patterns in sand seemed simple and even intuitive. But once Andrew Leier…started observing, he discovered that the most commonly held notions did not apply. And even more surprising, was that his findings revealed answers to NASA’s questions about sediment transport and surface processes on Mars.
Leier first studied loose pebbles and rocks, also known as clasts, when he was looking at sand dunes in Wyoming and noticed that the clasts seemed to spread away from each other in an almost organized fashion. It turns out, NASA was examining similar patterns on the sandy surface of Mars.
NASA proposed that wind was moving these rocks around. But Leier, who co-authored the study with Jon Pelletier at the University of Arizona and James Steidtmann at the University of Wyoming, says that would be impossible. They also discovered that rather than being pushed backward by the breeze, clasts actually tend to move into the direction of prevailing winds…
Instead, the loose sand around clasts is removed by the wind, causing scour-pits to form in front of larger clasts. Eventually, the rocks fall forward (or laterally) into the scours and then, the process repeats.
“What I find most interesting about this is that something as seemingly mundane as the distribution of rocks on a sandy, wind-blown surface can actually be used to tell us a lot about how wind-related processes operate on a place as familiar as the Earth and as alien as Mars,” says Leier. “It’s chaotic and simple at the same time.”
Leier’s article is published in the January 2009 edition of Geology. I have to thank him for solving a puzzle that’s bemused me on many of my walks on the Caja del Rio mesa across the valley from my home. Not that the solution will impede ghost and alien hunters.
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