Strange forces on Martian rocks

The Spirit and Opportunity Mars rovers, which have recently celebrated their 5th anniversary on the red planet, seem to have discovered a strange force that is pushing rocks around the Martian surface.

If you look at the photos of the Martian surface, it is strewn with small rocks ranging in size from pebbles to cobbles, but interestingly, none of them are touching.  The rocks are all evenly spaced as if something was pushing them away from one another.  The surface of Mars is very windy, but because the atmosphere is about a hundredth as dense as it is on Earth, the wind is nowhere near powerful enough to move this size of rock.

Jon D. Pelletier from the University of Arizona has worked out what is most likely causing this rock spacing, and it is an effect that will make sense to anyone who has every tried to dam a stream on a sandy beach.

If you put a rock in the stream, you may have noticed that the sand gets eroded away where the water is moving fastest - in front of and to the sides of the rock, where the water must accelerate to get around the blockage.  If you leave the rock for long enough it will fall upstream into the hole that has been created.

Something similar appears to happen on Mars.  If you have two rocks touching, the wind will be slowed between them but faster going around the opposite sides.  This means that the wind will tend to move the sand they are sitting on from in front of the rocks, and deposit it between them.  So like the stream eventually they will roll apart.  This gives the appearance that the rocks repel one another and spread out, and no little green men are involved at all.