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  • SRC User Searches for a Biological Origin—
    or a lack thereof—of Rock Varnish

    by John Morgan, Science Writer, UW Madison Synchrotron Radiation Center

    Rock varnish under magnification

    As any visitor to SRC knows, the facility rests amid lush farm fields of southern Wisconsin. In fact, it's been said that the topsoil in these parts are some of the most fertile in the entire world. A mid-summer day on this bluff above Lake Kegonsa is literally alive with thousands upon thousands of living things and complex interconnected webs of life.

    Imagine then a place in the world that may display some similar topography—rolling hills and long flat stretches-but imagine this place devoid of almost all life whatsoever. Imagine, Mars on Earth. This is the Atacama Desert in South America and this is one of the places SRC user Kim Kuhlman of the Planetary Science Institute goes to collect some of the rock samples she studies.

    Rock varnish under magnification

    Kuhlman is studying rock varnish, the microscopic film that covers rocks. What's unknown about rock varnish, particularly that which is found in super-arid areas such as the Atacama, is whether microbes contribute to the growth of varnish or not. While it indeed grows, it happens at such a slow rate that it could just as easily be inorganic mineral deposition as microbial deposition.

    "It grows an average of a nanometer a year. Your fingernail grows a nanometer a second," notes Kuhlman.

    As with almost any unsolved science problem, there is a divide between camps of thought with regard to whether rock varnish is organic or inorganic. Kuhlman's research at SRC sought to better uncover this issue by examining the type of manganese found in samples collected in similarly arid areas of Cima, CA and Parker, AZ.

    Kuhlman explains, "Many investigators believe that manganese exists in rock varnish mainly as birnessite, and that varnish has at least a partial biological origin because of the relative abundance of manganese. We have investigated the oxidation state of manganese in rock varnishes."

    At SRC Kuhlman teamed up with staff scientist Mike Albrecht and utilized SPHINX (Spectromicroscope for PHotoelectron Imaging of Nanostructures using X-rays), a PEEM II instrument installed on the High Energy Resolution Monochromator (HERMON) beamline. What they found was not only rather surprising, but also rather convincingly leans toward these rock varnishes not being biological at all.

    "L-edge spectra were obtained for a variety of different Mn oxides minerals of different oxidation states, including a synthetic birnessite. Comparison of the L-edge spectra collected from the two rock varnishes from the Mojave Desert with these reference spectra demonstrates that the manganese phase in the varnish is MnO rather than birnessite," explains Kuhlman.

    While this finding could make some waves in the scientific community, Kuhlman herself notes that she hopes that this represents an opportunity to cultivate the field rather than create a divide. In fact, she was able to present her findings at the recent Astrobiology Science Conference in California

    The underlying goal is to better understand these harsh environments and the type of life they might support. The reason this research falls under the auspices of astrobiology is a direct result of wanting to better understand places like Mars. The best way to do that, outside of getting Mars rock samples back to Earth, is to look at the most similar areas here on Earth.

    And while many have argued and perhaps hoped for a sign that life exists in these places, Kuhlman concludes that "rock varnish may be less organic than people think".

    For more about this topic, please reference:

    Elaine Bryant, Monika Kress. Astrobiology. April 1, 2008, 8(2): 427-430. doi:10.1089/ast.2008.1252.