[meteorite-list] Rock Varnish: A Promising Habitat for Martian Bacteria

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 17 Dec 2008 10:01:14 -0800 (PST)
Message-ID: <200812171801.KAA15639_at_zagami.jpl.nasa.gov>

http://www.psi.edu/press/

Rock Varnish: A Promising Habitat for Martian Bacteria
Planetary Science Institute
December 16, 2008

Dec. 16, 2008 - 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.

Kuhlmann'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 from 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.
Received on Wed 17 Dec 2008 01:01:14 PM PST