infocus #11 September 2008 Solving the mystery of desert varnish with microscopy

Despite centuries of scientific effort, the origin of desert varnish is still shrouded in controversy. Most investigators have looked to biological causes where microbes create varnish and preferentially concentrate manganese relative to the local soils and rocks. Finally, microscopy appears to be solving the riddle of desert varnish.

DOI: 10.22443/rms.inf.1.36

In areas such as Death Valley California whole mountains shimmer as light is reflected from widespread coatings of black opalescent desert varnish (Figure 1). Similar desert varnishes have been found on all continents, in locations such as the Gobi (Figure 2), Sonoran, Mojave, Namibian (Figure 3), Victorian and Atacama Deserts. These dark, lustrous coatings have attracted the interest of scientists for centuries. In 1852, the German naturalist and explorer Alexander Humboldt observed desert
varnish on a transatlantic expedition and questioned how this enigmatic feature may have formed. His contemporary, Charles Darwin also engaged in the search for explanations for this unusual rock coating and, in 1871, attempted to satisfy his interest by performing chemical analyses. To date many other noteworthy scientists have examined desert varnish and have commented on its bulk chemistry, the arid conditions in which it forms in and the concentration of manganese that makes it opaque and causes it to be black.

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Authors

Randall S. Perry

Randall Perry is a visiting academic at Oxford University and a Research Scientist at the Planetary Science Institute, Tucson, Arizona. He holds a Royal Society Fellowship and is a Senior Research Investigator at Imperial College, London. Dr Perry learned to use a SEM while at the University of Washington as a student in the late seventies. Many hours were spent as the solo user of an SEM before its value was discovered by other staff and students.

Mark Sephton

Mark A. Sephton is a Professor of Organic Geochemistry and Meteoritics at Imperial College London and an Associate Scientist at the Natural History Museum. Professor Sephton’s body of work in developing techniques for the analysis of organic matter in extraterrestrial samples was recognised in 2006 when he was honoured by the International Astronomical Union. Asteroid 7552 “Sephton” now orbits at 2.6 AU from the Sun. The award reflects over 15 years of successful analytical innovation address gaps in our knowledge and technical capability when dealing with challenging extraterrestrial samples.