Saturday, January 10, 2009

Why are Martian rocks spread out so evenly?

Who's been landscaping Mars with evenly spaced rocks layed out like paving stones? The old theory was extreme wind events.

But UA geoscientist Jon Pelletier calculated that the winds would have to have force ten times that of Hurricane Katrina, and even then, why should the rocks be so evenly spaced.

So, wind-tunnel tests and computer modeling by Jon and colleagues found a more realistic and pretty nifty natural process. According to an article in the new issue of Geology (vol 37, p 55), the coarse clasts on the surface "modify the air flow around them, causing erosion of the underlying substrate on the windward side and deposition on the leeward side until a threshold bed-slope condition is reached, after which the clast rolls into the windward trough.

Clasts can migrate across an erodible substrate in repeated cycles of trough formation and clast rolling" in a repetitive process so that the motion spreads the rocks out more evenly, with rocks eventually a few diameters apart. [right, Spirit Rover camera images of intercrater plains between Lahontan Crater and Columbia Hills illustrating examples of uniformly spaced clast distributions. Credit, Geology]

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