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A mysterious shiny coating found on rocks in many of Earth's arid
environments could reveal whether there was once life on Mars,
according to new research.
The research, published in the July edition of the journal Geology,
reveals that the dark coating known as desert varnish creates a record
of life around it, by binding traces of DNA, amino acids and other
organic compounds to desert rocks. Samples of Martian desert varnish
could therefore show whether there has been life on Mars at any stage
over the last 4.5 billion years.
The researchers hope that these results will encourage any future
Mars Sample Return mission to add desert varnish to its Martian
The source of the varnish, which looks like it has been painted onto
the rocks, has intrigued scientists since the mid nineteenth century,
including Darwin, who was so fascinated that he asked the geochemist
Berzelius to investigate it. It was previously suggested that its dark
colour was the result of the presence of the mineral manganese oxide,
and that any traces of life found within the varnish came from
biological processes caused by microbes in this mineral.
However, the new research used a battery of techniques, including
high resolution electron microscopy, to show that any traces of life in
the varnish do not come from microbes in manganese oxide. The research
reveals that the most important mineral in the varnish is silica, which
means that biological processes are not significant in the varnish's
formation. On desert rock surfaces, silica is dissolved from other
minerals and then gels together to form a glaze, trapping organic
traces from its surroundings.
Dr Randall Perry, lead author of the research from the Department of
Earth Science and Engineering at Imperial College London, explained
that as life is not involved in desert varnish formation, the varnish
can act as an indicator of whether life was present or absent in the
Dr Perry said: "If silica exists in varnish-like coatings in Martian
deserts or caves, then it may entomb ancient microbes or chemical
signatures of previous life there, too. Desert varnish forms over tens
of thousands of years and the deepest, oldest layers in the varnish may
have formed in very different conditions to the shallowest, youngest
"These lustrous chroniclers of the local surroundings can provide a
window back in time. Martian desert varnish would contain a fascinating
chronology of the Martian setting," he added.
The research was carried out by researchers at Imperial College and
the Universities of Auckland (NZ); Wisconsin-Parkside and Washington
(US); and Nottingham Trent (UK).
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