BY DR EMILY BALDWIN
Posted: 22 December, 2008
Naturally occurring rock varnish could offer suitable habitats for bacterial life on Mars, report scientists in this month’s issue of the Journal of Geophysical Research.
Rock varnish, which consists of clay cemented together with iron and manganese oxides, is an extremely slow growing coating that forms on the surfaces of rocks in arid and semi-arid climates, such as deserts. It adds between just 1 and 40 nanometers in thickness per year, and tends to be no more than 500 millimetres thick, regardless of age. On the Earth it often appears as a dark stain on light-coloured canyon walls.
Ancient petroglyphs are often found etched into rock varnish, revealing a lighter colour rock below the dark stain of the varnish.
In order to characterise the habitability potential of rocks lined with this varnish, a research team led by Kimberly Kuhlman of the Planetary Science Institute in Arizona analysed samples collected from the Yungay region of Chile's Atacama Desert, which is also one of the closest analogues to Martian environments found on Earth. They found bacteria associated with rock varnish in an area where the surrounding soils were essentially devoid of life, suggesting that rock varnish could provide a niche habitat for microbial life in extraterrestrial environments devoid of liquid water, such as on Mars.
The hardy bacteria apparently acquire the majority of their moisture from fog. They are also aerobic, which means they rely on oxygen for energy, so if Martian equivalents exist they must have adapted to survive their planet's low-oxygen atmosphere. Scientists believe that occurrences of rock varnish could be prolific across the Red Planet since photos returned by every Mars lander and rover reveal what looks like rock varnish coating the rocky surfaces.
However, Kuhlman cautions that these coatings might not actually be rock varnish. "A number of different coatings, like silica, can masquerade as rock varnish," she says. "So you can't really identify it for sure until you crack it open and look at a cross section under the microscope." If it is rock varnish, it could provide bacteria with the same benefit it does on Earth — protection form ultraviolet radiation.
Martian rovers have examined rocks that exhibit a varnish-like coating, but its genesis is currently unknown. Image: NASA/JPL-Caltech.
The bacteria’s role in creating the varnish is also an unknown, with some believing that it is directly involved, and others believing that it is abiotic — that is, non-living. Or, it could consist of layers formed by entirely different processes, depending on the prevailing environmental conditions at the time. Many of the bacteria identified in Kuhlman’s collection were related to bacteria found in water or air, suggesting that their ancestors may have been carried into the area during wetter periods and then evolved in the varnish niche as conditions changed. A similar scenario might have played out on Mars, with varnish bacteria surviving from the planet's wetter eras.
Now Kuhlman would like to determine exactly where the bacteria live: on the surface of the varnish or touching the rock, or somewhere in between, and what exactly they are doing there. The ultra-thin coatings have made it difficult to answer these questions, but future work on understanding bacteria living in terrestrial varnishes could give planetary scientists a better understanding of how to pursue the search for Martian bacteria.
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