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Researchers looking for evidence of life on Mars should be taking a closer look at a shiny coating that forms on rocks in the desert, according to scientists at London University's Imperial College (IC).

The team is now calling for these shiny rocks to be added to the "Martian shopping list" on any sample-return mission to the red planet.

Exactly how the coating forms has been a puzzle for many years. Charles Darwin even asked the geochemist Berzelius to investigate it. Now, the researchers at IC have been able to establish that the main component of the varnish is silica. In their paper, they describe how silica is dissolved from other minerals on desert rock surfaces and then gels together to form a glaze, trapping organic traces from its surroundings.

This indicates that life is not integral to its formation, even if evidence for life is trapped within its layers.

Dr Randall Perry, lead author of the research from the Department of Earth Science and Engineering at Imperial College London, commented: "If silica exists in varnish-like coatings in Martian deserts or caves, then it may entomb ancient microbes or chemical signatures of previous life there, too. Desert varnish forms over tens of thousands of years and the deepest, oldest layers in the varnish may have formed in very different conditions to the shallowest, youngest layer."

Martian desert varnish would, he added, contain "a fascinating chronology of the Martian setting". ®

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