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We've all seen it, so what is it? Here's a neat article from New Scientist that I just read.
Martian sheen: Life on the rocks
New Scientist Magazine
12 February 2010 by Barry E. DiGregorio
WHEN NASA's Viking landers touched down on Mars, they were looking for signs of life. Instead, all their cameras showed was a dry, dusty - and entirely barren - landscape.
Or so it seemed. But what the 1976 Viking mission, and every subsequent one, saw was a scene littered with rocks coated with a dark, highly reflective sheen. That coating looks a lot like a substance known on Earth as "rock varnish", found in arid regions similar to those on Mars. The latest evidence hints that rock varnish is formed by bacteria. Could there be microbes on Mars making such material too?
Rock varnish has long been something of a mystery. It is typically just 1 to 2 micrometres thick, but can take a thousand years or more to grow, making it very hard to discover whether biological or purely chemical processes are responsible. If it is biological, though, the race will be on to discover whether the same thing has happened on Mars - and whether microbes still live there today.
If you go to Death Valley in California, you can find rock varnish covering entire desert pavements. Also known as desert varnish, it forms in many places around the globe, and despite its glacial growth rates, can cover vast areas. The smooth, high sheen, dark brown-to-black coating is mainly made up of clay particles, which bind the iron and manganese oxides that give the coating its mirror-like reflectivity. In the Khumbu region of Nepal, not far from Mount Everest, it has turned the boulders black. Halfway around the world, it enabled ancient peoples to create the Nazca Lines in the Peruvian desert. These giant, elaborate images - some over 200 metres across and created over 1000 years ago - were made by simply removing rows of varnished stones to exposing the lighter stones or soil beneath.
George Merrill coined the phrase desert varnish in 1898, while working for the US Geological Survey (USGS). No one really studied it, though, until 1954, when Charles Hunt showed that the veneer forms on many different rock types - meaning that it wasn't simply a chemical production from a certain kind of rock and prompting the first questions about where it might come from (Science, vol 120, p 183). Hunt went on to find rock varnish in humid regions, tropical rainforests and at high altitudes in the Alps and the Rocky mountains.
Theories on how rock varnish forms weren't long in coming - and, initially at least, biology didn't get a look-in. In 1958 Celeste Engel of the USGS and Robert Sharp from the California Institute of Technology explained it as a chemical weathering phenomenon similar to iron oxide stains - red/orange coatings arising when iron particles from the air collect on the surface of rocks and bind together when made wet by dew (Geological Society of America Bulletin, vol 69, p 487).
It made sense to think that rock varnish had a chemical origin, since many similar-looking coatings were already known to form chemically. Silica glaze, for example, is one of the most common coatings and forms when silicic acid carried in dust and dew condenses onto rock surfaces.
Everything changed, though, when people saw the internal structure of rock varnish. Electron microscopic images taken by Randal Perry and John Adams at the University of Washington in Seattle in 1978 revealed an intricate layer-cake pattern, with black strips of manganese oxides alternating with orange layers of clay and iron (Nature, vol 276, p 489). No other rock coating combines this microlayered mixture of clays and metal oxides.
The implications here were enormous. This microstructure looked strikingly similar to that of fossil stromatolites - layered rock-like structures formed by ancient microbes as they collected sediments from seawater to build themselves a home. Though they still grow today in some isolated spots, stromatolites were one of the first life forms on Earth, dominating the fossil record from 3.5 billion years ago until about 600 million years ago.
That meant rock varnish could have a biological origin, and a flurry of investigations ensued to find out which microbes were responsible. Backing up the idea was the fact that microbes developed the ability to make a manganese oxide coat early in their evolution, to protect themselves from the harsh UV rays of the young sun.
Manganese proved pivotal three years later, for Ronald Dorn at Arizona State University in Tempe and Theodore Oberlander of the University of California, Berkeley. They found what looked like the fossilised remains of a few budding bacteria within the manganese oxide layer. Manganese concentration peaked around them, suggesting these bacteria were involved in producing it.
Dorn and Oberlander then managed to isolate two manganese-depositing microbes, Metallogenium and Pedomicrobium, from the surface of varnish samples collected in California's Mojave desert. When they added these to sterilised chips of rock in test tubes, they were able to grow a thin manganese varnish in about six months. The findings were published under the title "Microbial Origin of Desert Varnish" (Science, vol 213, p 1245).
That proved to be premature. The microstromatolite texture of natural rock varnish was absent in the lab-grown version, and anyway it formed way too quickly. It was around then that natural rock varnish was discovered to grow as slowly as 1 or 2 micrometres every thousand years.
Although Dorn conceded that his experiments were not conclusive proof, he believed the manganese layering had to be microbial and stuck by his theory. Two key questions remained. Why does rock varnish contain so few fossilised microbes, and how could they take so long to concentrate manganese but leave no trace of their existence?
David Krinsley might have an explanation: over thousands of years, chemical changes within the deposit could have destroyed any bacterial remains, he argues. A sedimentologist from the University of Oregon in Eugene, Krinsley has studied dozens of rock varnish samples and in every one he has seen a scattering of fossilised bacteria. But it's still not proof that they made the varnish, he admits.
Dorn is not surprised there are so few bacteria in rock varnish, considering the time it takes to form. "My hypothesis is that very rare bacterial forms concentrate the manganese and iron," he says. He also believes the slow rate rules out chemical theories of rock varnish formation because silica glaze and other types of chemical rock coating grow in relatively rapid annual cycles.
But the real answer to the rock varnish mystery could come from a remarkable cave in New Mexico. The floor of the Fort Stanton cave is made of a sparkling white calcite "river" formed by hundreds of years of flooding - but it is the dark coatings covering the cave walls that interest rock varnish researchers.
Most of it is just simple manganese oxide minerals, but on their most recent trip to the cave, Mike Spilde of the University of New Mexico in Albuquerque and Penny Boston at New Mexico Institute of Mining and Technology in Socorro found coatings that seem to fulfil all the definitions of rock varnish - iron and manganese oxides bound together by clays, in the characteristic microstromatolite layers. The coatings were covered with bacteria known to deposit manganese. "These deposits are biological in origin," Spilde concluded when he presented the findings at the Geological Society of America meeting in Portland, Oregon, last October.
Further confirmation is needed, says Dorn, but the cave deposits "sure look like rock varnish". One difference is that the cave varnish seems to form far more rapidly: the coating is already starting to grow back in areas where Spilde's team had removed deposits a few years earlier. The cave is damp, so perhaps this helps the coating grow more rapidly and explains the incredibly slow growth rate of rock varnish in desert conditions, Spilde suggests.
Where does all this leave the search for life on Mars? If Earth is anything to go by, there are only three possible explanations for the shiny rocks on Mars - rock varnish, silica glaze or a simple polishing of the rocks themselves by wind-blown sand.
The latter is the easiest to discount. Martian rocks are entirely cloaked in their shiny coating, whereas natural sandblasting would polish only the windward face. To confirm matters, infrared images from Mars rovers have proved that the shiny surface is an extra coating rather than part of the rocks themselves.
Silica glaze seems unlikely too. NASA's Mars exploration rovers Spirit and Opportunity can detect silica, and in 2007, Spirit dug into the soil and found a large deposit of it, providing strong evidence that liquid water once flowed on the planet's surface. The rovers have never detected silica glaze on the rocks they have analysed, however.
Which leaves rock varnish. All the raw ingredients are known to exist on Mars - and, given the harsh UV rays that continually bombard the planet, under a protective coating might be exactly where you would expect to find evidence of life. Mars is free of many of the processes that erode rock varnish on Earth, from rain to lichens, so it might harbour evidence of ancient life millions of years old.
The uncertainty over rock varnish's origins - and its complex structure and chemical make-up, which make it difficult to definitively detect - mean that no instrument has ever been designed specifically to search for rock varnish on Mars. It should be possible to identify some components, though.
For example, the thermal emission spectrometer that enabled Spirit to detect silica should theoretically be able to detect manganese oxides too. The mineral has never been spotted, but that might be because it is present in such small quantities compared with the underlying rock, says Steve Ruff at Arizona State University, who runs the TES instruments on the rovers. That means any signal is too low for the instrument to detect.
Both rovers are also fitted with an alpha proton X-ray spectrometer, which fires alpha particles and X-rays at rock surfaces to detect which chemical elements are present. These instruments have never detected the elevated manganese levels that would be expected in rock varnish - but again the signal could be obscured by the elements in the underlying rock. "We can't say for certain if the rock coating is manganese-enriched or not," says Harry McSween, a geologist on the Mars rover projects.
Future missions will be better equipped for the hunt. The next rover to land on the Red Planet will be NASA's Mars Science Laboratory, due to arrive in 2012. MSL can detect rock varnish, says Roger Wiens, the Los Alamos National Laboratory scientist who will run a new instrument on MSL called a laser-induced breakdown spectrometer. This will fire laser pulses at the rock coatings, and the wavelengths of light emitted as the coating atomises will tell Wiens what elements are present.
NASA is also working with the European Space Agency on the ExoMars programme, which will send two rovers in 2018, in part to hunt for evidence of life on rock surfaces. A subsequent ESA mission, Mars Sample Return - pencilled in for 2020 - might finally get the definitive answer, as for the first time the mission will bring Mars samples back to Earth.
If the cave varnish and the Mars varnish turn out to be the same as rock varnish, then Mars Sample Return might actually be bringing Martians to Earth.
Barry E. DiGregorio is a science writer and a research associate at the Cardiff Centre for Astrobiology, UK
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Thanks Dawn. 