The Mars rovers have sent back a lot of photos that are examined very carefully by scientists at NASA. Recently those scientists announced that they believe they see desert varnish on Martian rocks. This has spurred much interest in how varnish forms, especially because some geologists think microbes are involved. If there is desert varnish on Mars, that might indicate there is [or was] microbial life on that planet.

Rock varnish is still mysterious, but we know some things about it. Sometimes it is so thin it is more like a stain. In other instances it is thick enough to be called a crust, even though it seldom is thicker than half a millimeter. That’s about 2/100th of an inch. Archaeologists studying rock art refer to is as “patina”.

Mineralogically, desert varnish is about 70% clay. The remainder is mostly oxides of iron and manganese. If the proportion of iron is higher, the color ranges from brown to reddish. More manganese gives the varnish a black color and often makes it shiny. Iron, of course, is common in rocks and soils all around the world. Manganese is fairly common at very low levels in most soils and sea water. It plays a role in the metabolism of many organisms, including humans, although high concentrations can be toxic.

Originally it was assumed that desert varnish was formed by minerals seeping out of the rock face onto the surface. Then it was noticed that in many cases the rocks where it formed had no manganese or clay in them. Then, it was proposed that all of the constituents of varnish could be blown on the wind as dust, and given a little moisture, they might adhere to the stone.

Electron microscope studies made of desert varnish in 1978 showed that the crust was layered: bands of manganese oxide alternated with layers of clay and iron. This reminded some geologists of stromatolites, although at a very small scale. Stromatolites are layered structures left by [usually] cyanobacteria that grew on the floor of shallow seas beginning more than 3 billion years ago. They were built by photosynthesizing bacteria clustered within a mat of slime, which caught sand and mud. As sunlight was obscured by the accumulation of debris, the cells divided and created a new layer. The result over time was a layered mound. Stromatolites are still forming in some few places around the world. They incorporate manganese in their cells to protect themselves from the sun’s UV rays.

A few microbes have been found growing on the surface of desert varnish, and fossil bacteria have been found “entombed” within it. But whether they were involved in the formation of the varnish is still unclear. They may have simply taken refuge in the coating to survive desiccation and harsh sunlight. All life needs some moisture, of course. And one thing pointing to the possible role of microbes in the deposition of varnish is that dark crusts are more common where water pours over rock faces. Notice in the photo of Mummy House Ruins in northern AZ, that the black streaks appear where water would run down the slightly under slung cliffs. Likewise, varnish forms on the sides of pebbles on the ground in deserts where moisture accumulates from dew.

The Ft. Stanton Cave near Ruidoso, NM provides more hints of the microbial origin of desert varnish. This cave is the third longest in New Mexico and is most famous for the Snowy River Passage, where more than five miles of the cave floor is composed of a sparkling white calcite deposit. On the walls above the calcite pavement are layers of iron/manganese oxides bound in clay. These layers are covered with manganese-using bacteria. Because the cave is damp the coating seems to grow much faster than it would in open air. Researchers from UNM and the NM Institute of Mining and Technology are currently studying these cave deposits. But the cave deposits bring up one of the big questions about rock varnish. Manganese appears in varnish up to 50 times the concentration in local soil and water. It is reasonable that microbes using it in their metabolism would accumulate and concentrate it, but where do they get it? In the cave environment it can’t just blow in the wind as it might do at the surface.

Desert varnish allowed prehistoric artists to record their visions for centuries. Petroglyphs are designs scratched through the varnish to expose the contrasting color of the underlying rock. Where the micro-stromatolites were found in desert varnish, archaeologists hoped it would be possible to read the layers much like we read tree rings, thus dating the rock art. Some efforts have been made, but I’m not very impressed with the results so far. We can do some relative dating by comparing the repatination of petroglyphs. For example, one of my favorites on the Sand Island Panel in Utah is a small figure that has been so re-covered by desert varnish that you can hardly see it. This tells us that it must be very old, certainly much older than the figures near it that have not been repatinated. A variation on the technique in making art with desert varnish is seen in Peru’s Nazca Lines. These lines and figures [geoglyphs] were made by a prehistoric culture between 200 BCE and 700 CE by removing pebbles and boulders covered with dark varnish to expose the light colored soil of the high desert plain beneath them.

Fuel cans abandoned by British soldiers during WWII were recently found in the eastern Sahara Desert. A well developed layer of desert varnish was found on the upwind side of the cans. This is the only instance I’ve heard of that involves varnish on anything other than rocks.

The verdict on whether microbes are responsible for rock varnish is still pending. The possibility of varnish on Mars has spurred a lot of research, but it may be decades before science has a definite answer. When I began researching this topic, I had no opinion about it, but now I find the evidence in favor of microbes to be fairly convincing. However, whether there is rock varnish on Mars or not remains to be seen. The next generation of rovers, expected to land on Mars in 2012, will be equipped to detect manganese in the coating on rocks.