Dust devils from Mars
It looks for all the world like bare flesh inscribed with a particularly imaginative tattoo. In fact, it is an image of the surface of Mars taken by NASA’s Mars Reconnaissance Orbiter. The spidery markings are actually the trails of dust devils spinning dizzily across the Martian plains. Their passage exposes the dark rock beneath.

Mars, unlike the Moon, is a world of violent movement. When carbon dioxide is driven explosively from the polar caps by sunlight, it dramatically stirs up the ultra-thin atmosphere. Super-fast winds hoist fine dust particles into the atmosphere, not only creating the miniature tornadoes of dust devils but sometimes even global dust storms that can obscure the entire planet.

As Elton John sang, “Mars ain’t the kind of place to raise your kids.” But the Red Planet was not always this way. Being significantly smaller than the Earth, it cooled more quickly from its original molten state. There is abundant evidence that, during its first billion years, rivers ran across its surface, draining into substantial oceans. There were waterfalls and cloudbursts and rainbows.

Since life appears to have arisen on Earth the instant the planet was cool enough, it is not unlikely that the same thing happened on Mars. And, if it did, there is a good chance – because we find rocks ejected from Mars on Earth – that it came here. Want to see a Martian? Try looking in a mirror.