干旱区生态环境知识资源中心

Many of the rocks on the surface of Mars that have been imaged by the Viking and Mars Pathfinder Landers display dark shiny surface coatings resembling Mn-rich terrestrial rock varnish. On our planet, these thin (5 um - I mm) coatings can be the result of a combination of various weathering processes combined,with microbial precipitation of mineral oxides over a wide variety of geographical locations but most commonly in those with and and semi-arid conditions. Terrestrial Mn-rich rock varnish is produced by a wide variety of microorganisms including epilithic and edolithic cyanobacteria, bacteria and microcolonial fungi. As these microorganisms absorb trace amounts of Mn and Fe from atmospheric dust, rain and fog, they slowly precipitate "reddish" iron and "brown to black" manganese oxides as well as magnetite particles. These microbial communities then produce secretions that cement the Mn/Fe mix together with clay particles in a process involving time periods of perhaps thousands of years for a thin 5 um layer. Mn-rich rock varnish has been found to form on the surfaces of undisturbed desert fragments and even sand grains. Both Mn and Fe would serve as a UV shield for any microflora residing beneath and within the layers of varnish thus protecting against high UV irradiation, dissication, and widely varying temperature extremes. Recent research on rock varnish has led to the discovery that some microbial communities that produce dark ferromanganese varnishes also precipitate biogenic magnetite. In view recent independent evidence put forth by D. McKay and E.I. Friedmann et al for indigenous biogenic magnetite-chains in ALH 84001 along with meteorological models showing the possibility for small quantities of liquid water on the surface of Mars in combination with data obtained from the Viking LR experiment 27 years ago, recommendations are made to elucidate on whether or not the shiny dark-coatings covering some Martian rocks have been produced by living or extinct microbial communities.