LOOKING like a coat of paint, the dark, glossy 鈥渄esert varnish鈥� that coats rocks in arid climates may hold valuable clues about past environments 鈥� and even early life 鈥� on Mars as well as Earth.
Desert varnish has fascinated observers since Darwin, who asked the geochemist J枚ns Jakob Berzelius to investigate it. It has long been thought to have formed biologically, but Randall Perry of Imperial College London now reports that it forms when dissolved silica coats rock surfaces with a hard glaze of opal (Geology, vol 34, p 537).
鈥淭he silica can entomb microbes or whatever lands on the surface, much like flies in amber,鈥� says Perry, who used electron microscopy to examine samples of the glaze. It forms over tens of thousands of years, chemically bonding with DNA and other organic compounds left on the rock surface.
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Similar silica coatings might exist on Mars, either as desert varnish or in hot spring or cave deposits. If so, Perry says, they would 鈥渃ontain a fascinating chronology of the Martian setting鈥�, which might conceivably hold traces of life.
