As scientists search for life on Mars, they should take a close look at rock varnish, according to a paper in the current issue of the "Journal of Geophysical Research."

The paper describes how a research team led by Kimberly R. Kuhlman, of the Tucson-based Planetary Science Institute, found bacteria associated with rock varnish in an area where the surrounding soils were essentially devoid of life. The study suggests that rock varnish could provide a niche habitat for microbial life on Mars and in other extraterrestrial environments devoid of liquid water.

Rock varnish is an extremely slow-growing coating that forms on the surfaces of rocks in arid and semiarid climates. In Southwestern deserts, it often appears as a tough, dark stain on light-colored canyon walls. Ancient petroglyphs are often found etched into rock varnishes.

Kuhlman's team analyzed samples of rock varnish collected from the Yungay region of Chile's Atacama Desert, which is the closest analog to Martian environments found on Earth.

The bacteria apparently get most if not all of their moisture from fog, said Kuhlman, who lives in Madison, Wis.

The bacteria also are aerobic. So if Martian forms exist, they would have adapted to survive their planet's low-oxygen atmosphere, she added.

Rock varnish, which consists of clay glued together with iron and manganese oxides, forms very slowly and is very thin. It adds only 1 to 40 nanometers in thickness per year, and tends to be no more than 500 millimeters thick, regardless of age.

Similar rock coatings may exist on Mars because photos returned by every Martian lander show 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," Kuhlman observed. "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.

Whether the bacteria help create the varnish that protects them isn't known. Some believe bacteria are involved in its formation, while others think it's abiotic. Actually, both scenarios could be true, Kuhlman said. Rock varnish could consist of layers formed by entirely different processes, depending on the prevailing environmental conditions at the time.

Since many bacteria cannot be cultivated in the lab, Kuhlman's team used culture-independent methods to identify many of the species found in the Atacama varnish. They looked for adenosine triphosphate, a molecule that provides energy for cells that is found in all living things on Earth, and they also identified DNA from 32 species. In addition, they were able to produce live cultures of other bacteria.

Many species were related to bacteria found in the air or water, 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 discover exactly where the bacteria live. No one knows if they are found on the surface, in the middle, at the bottom or between the varnish and the rock.

Similarly, scientists don't yet know if the bacteria are simply using the varnish for sunscreen or if they exist as a community within the varnish.

The ultra-thin varnish coatings have made it difficult to answer these questions, but Kuhlman hopes to secure research grants to pursue these problems and to give planetary scientists a better understanding of how to pursue the search for Martian bacteria.

Those working with Kuhlman on the Atacama rock-varnish project include Parth Venkat, of the Planetary Science Institute; Myron T. La Duck, of the California Institute of Technology; Gregory M. Kuhlman, of the University of Wisconsin; and Christopher P. McKay, of the NASA Ames Research Center.