When NASA's Viking landers touched down on Mars, they were looking for signs of life. Instead, all their cameras showed was a dry, dusty - and entirely barren - landscape.
Or so it seemed. But what the 1976 Viking mission, and every subsequent one, saw was a scene littered with rocks coated with a dark, highly reflective sheen. That coating looks a lot like a substance known on Earth as "rock varnish", found in arid regions similar to those on Mars. The latest evidence hints that rock varnish is formed by bacteria. Could there be microbes on Mars making such material too?
Rock varnish has long been something of a mystery. It is typically just 1 to 2 micrometres thick, but can take a thousand years or more to grow, making it very hard to discover whether biological or purely chemical processes are responsible. If it is biological, though, the race will be on to discover whether the same thing has happened on Mars - and whether microbes still live there today.
If you go to Death Valley in California, you can find rock varnish covering entire desert pavements. Also known as desert varnish, it forms in many places around the globe, and despite its glacial growth rates, can cover vast areas. The smooth, high sheen, dark brown-to-black coating is mainly made up of clay particles, which bind the iron and manganese oxides that give the coating its mirror-like reflectivity. In the Khumbu region of Nepal, not far from Mount Everest, it has turned the boulders black. Halfway around the world, it enabled ancient peoples to create the Nazca Lines in the Peruvian desert. These giant, elaborate images - some over 200 metres across and created over 1000 years ago - were made by simply removing rows of varnished stones to exposing the lighter stones or soil beneath.