The surface of Mars may be cold and desolate, but it is not unchanging.
New images show that avalanches of dust scour dozens of Martian sites each year.
Without the abundant water and plate tectonics that keep Earth's surface in motion, the surface of Mars is much slower to change. But in one way it is more active.
While Earth's atmosphere shields us from asteroids smaller than 30 metres across, which burn up or shatter too high above the ground to have much effect on us, Mars's atmosphere is just 1 per cent the density of Earth's. Even rocks less than a metre across make it to the ground and gouge out craters.
NASA's Mars Reconnaissance Orbiter spots about 20 new craters between 1 and 50 metres across on Mars each year - scars that were not present in earlier images. Now closer scrutiny of these images has found thousands of small avalanches near 16 of the craters.
The avalanches appear as dark streaks on the hilly terrain that surrounds the craters (a similar but more dramatic avalanche is shown in the image).
They show up only in areas where there is a lot of light-coloured dust on the ground. To form, it seems the surface's dust coating was shaken loose and slid downhill, revealing the darker rocks beneath, says a team led by Kaylan Burleigh of the University of Arizona in Tucson (Icarus, DOI: 10.1016/j.icarus.2011.10.026).
The team carried out computer simulations that showed that, surprisingly, the avalanches do not seem to be caused by meteorites hitting the ground, but by the shock wave generated by a rock's passage through the atmosphere.
This spreads across an area about a million times larger than the craters. "It was astonishing that a relatively small impact could affect a large area," says team member Jay Melosh of Purdue University in West Lafayette, Indiana.
In one case, a cluster of 20-metre-wide craters is surrounded by thousands of dust avalanches in an area 4 kilometres square. The many small avalanches give the whole area a darker hue, like a giant black eye around the craters - except for a narrow light zone shaped like a curved dagger.
That light zone is telling. As a rock tears through the atmosphere at supersonic speed it generates a shock wave, before triggering a second blast when it hits the ground.
The team's simulations show that the second shock interferes with the first, reinforcing it in some places and cancelling it out in others.
Where it is cancelled out, a narrow curved strip of relatively undisturbed ground is left behind - just like the light zone seen around the crater cluster, says Melosh.
The Martian surface may be the best place in the solar system for recording the effects of these shock waves, since fewer impactors are blocked by its atmosphere than on Earth, says Mark Boslough of Sandia National Laboratory in Albuquerque, New Mexico.
The Lynx Supersonic Wind Tunnel Model positioned In the MSFC wind tunnel
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