Ancient humans used it to create rock paintings durable enough to last millennia. Biologist Charles Darwin puzzled over its dark color and asked Swedish geochemist Jons Berzelius to investigate it. And generations of scientists since then have struggled to penetrate its mystery. Now researchers believe they have isolated the origin of desert varnish, a glassy coating on rocks and canyon walls. What's more, the findings suggest that the material could be an invaluable repository of past life on Earth--and perhaps even on Mars.
To crack the nature of the varnish, an international team led by micropaleontologist Randall Perry of Imperial College London used the latest analytic methods, including high-resolution electron microscopy and spectroscopy. The analysis shattered a leading varnish theory: that the substance is made by organisms that live on the rocks. If that was the case, says Perry, the varnish would consist primarily of iron or manganese, common byproducts of bacterial and algal metabolism. Instead, the team found that the varnish is made mostly of silica, which can fall from the atmosphere or leach from the rock itself. Over time, silica breaks down into a gellike form and hardens.
The team also discovered that, just like amber that entombs ancient insects, desert varnish tends to trap fingerprints of biology, such as amino acids, DNA fragments, and even microorganisms from past eras that lived on the rocks or simply became stuck to them. This process, says Perry, makes desert varnish a prime target for any Mars sample-return mission if varnished rocks are found on the Red Planet. "If silica exists in varnishlike coatings in martian deserts or caves, then it may entomb ancient microbes or chemical signatures of previous life there, too," he explains.
In addition, the varnish could provide a useful record of climate change. Perry says the fine layers of desert varnish could record minute environmental and organic changes. The slow pace of formation means that the record might span thousands or even millions of years.
Willard Moore, an isotope geochemist at the University of South Carolina, Columbia, says he's not yet convinced that organisms aren't involved in making the varnish. "You still have a large amount of iron and manganese to explain," he says. Still, if the varnish is in fact composed of silica, Moore says it likely has an abiotic origin. That's because his own studies of the substance reveal that it contains beryllium-7, a radioactive isotope born when cosmic rays collide with molecules in Earth’s atmosphere. That supports the notion that the source of the varnish fell as dust from the sky.