What scientists would really like to find are rocks that can only be formed by the action of water. This might then provide a good indication of where future Mars lander missions might be sent to look for martian fossils. A prime candidate for such a landing site appeared to be the region spotted by Mariner 9 in the 1970s, known as 'White Rock'. Sitting at the bottom of a crater, White Rock is a patch of terrain, about twelve by fifteen kilometres, that stands out brightly from the surrounding ground. It looks, from the spacecraft pictures, reminiscent of rock deposits called evaporites that form at the bottom of dried-up salt lakes on Earth.
Evaporites are essentially pure salt.
Evaporation of salty water is the only known way for evaporites to form. Probably not, Ruff says. This instrument is able to identify the composition of the surface material, and it seems to show that White Rock is not, after all, so different from the surrounding terrain. The bright feature seems to be a wind-blown deposit like a huge sand dune, and its brightness is due to a difference in texture, rather than composition, from the plain on which it sits. Ruff points out that White Rock was widely regarded as representative of several similar sites seen in spacecraft images of Mars, which were also suspected of being evaporite-type features.