<title>Rock varnish as a habitat for extant life on Mars</title>

Abstract

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 -1 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 arid 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.

Biogeological Analysis of Desert Varnish Using Portable Raman Spectrometers

“…Indeed, martian rocks exhibiting manganese-rich coatings have been recently identified by using data returned by the ChemCam instrument on the Curiosity rover (Lanza et al, 2014). Desert varnishes have been of particular interest to astrobiologists because on Earth they are host to extant organisms that survive in extreme habitats (Mancinelli and White, 1996;DiGregorio, 2002).…”

Diversity of rock varnish bacterial communities from Black Canyon, New Mexico

“…Varnish is ubiquitous, forming on exposed rock surfaces of diverse lithology in almost every type of terrestrial weathering environment, (e.g., Antarctica [Dorn et al, 1992a], Norway [Whalley et al, 1990], and Hawaii [Dorn et al, 1992b]), but is particularly abundant in arid and semiarid regions. Rock varnishes have attracted considerable research interest as a potential Quaternary dating tool for rock surfaces [Liu, 2003]; as archeological features and artifacts [e.g., VandenDolder, 1992]; as indicators of paleoclimatic change [e.g., Dorn, 1994;Liu andBroecker, 2000, 2007]; as environmental monitors because of the great scavenging abilities of Mn oxides for certain heavy metals [Dorn, 1991;Fleisher et al, 1999;Wayne et al, 2006]; because of the likely role of microbes in their formation [e.g., Perry and Adams, 1978;Krumbein and Jens, 1981;Nagy et al, 1991;Gorbushina, 2007]; as analogous environs for the search for life on other planets [DiGregorio, 2002;Gorbushina et al, 2002;Allen et al, 2004;Edwards, 2004]; and to assist in interpretations of remote sensing studies of varnished rock surfaces on Earth and Mars [e.g., Israel et al, 1997;Kraft and Greeley, 2000].…”

Mars Primordial Crust: Unique Sites for Investigating Proto-biologic Properties

“…Living systems may well have evolved in the first few hundred million years and be preserved on Mars in ancient sediment layers such as those found at the Rover landing sites (Squyres et al 2004). In such sediments, chemical signatures of early biota (Figure 3) or of compounds from 'pre-living systems' might be preserved as organo mineral (Perry et al 2004) in silica rock coatings (DiGregorio 2002;Perry and Lynne 2006; or silica (Figure 3) from ancient hot-springs (Des Marais and Walter 1999;DiGregorio 2002;Farmer 2000;Gorbushina et al 1997;Lynne and Campbell 2004;Lynne et al 2006). The preservation and identification of these primordial systems is potentially unique to Mars where exposed rocks that are as old as the Solar System remain.…”