Mineral analysis may reveal life on Mars

http://www.newscientistspace.com/article.ns?id=dn8534

a.. 11:11 05 January 2006
b.. NewScientist.com news service
c.. Maggie McKee
Minerals - as opposed to organic compounds - could reveal the presence of
ancient life on Mars, a new study reports. The research suggests relatively
simple experiments aboard future landers or sample-return missions to the
Red Planet could be used to test for life.

Some evidence suggests Mars was warm and wet in its first hundred million
years, raising the possibility that it could have fostered life. But
neither of the two Viking landers found organic molecules when they studied
the planet's soil in the 1970s.

Now, researchers led by Fabien Stalport of the University of Paris in
France say inorganic compounds, which tend to survive longer than their
organic counterparts, may act as "tracers of biological activity". To test
their idea, they studied the mineral calcite, which is the crystallised
form of calcium carbonate.

On Earth, calcite is formed in three ways. Living organisms create biotic
calcite - limestone, for example, is produced when biologically formed
calcite falls to the ocean floor. Alternatively, geologic processes such as
magmatism can form abiotic calcite. And a combination of processes, which
might include biological ones, can act on existing rocks to produce
diagenetic calcite.

Early degradation
The researchers took 12 terrestrial calcite samples - from sources
representing all three types of the mineral - to see if they could detect
differences among them. Using X-ray diffraction and electron scanning
microscopy to study the samples' mineralogical and chemical composition,
they found the abiotic samples were pure. But the biotic samples contained
impurities - they grew faster and sometimes substituted magnesium atoms for
calcium in their crystalline structures.

These defects "structurally weaken the calcite", the authors write in
Geophysical Research Letters. This weakening was detected when the
researchers steadily heated the samples and noted when they began to lose
mass by decomposing into gaseous carbon dioxide.

The biotic samples started degrading at a temperature 40°C cooler than the
abiotic ones. And two diagenetic samples that had been shaped by biological
processes began to decompose at a temperature close to that of the biotic
calcite.

"This result is encouraging for a possible preservation of mineral biotic
calcite structures on Mars for billions of years," the authors write. They
suggest that future Mars landers be equipped with instruments to heat and
weigh minerals to determine whether they had been formed by life.

Garbled signals
Christopher Romanek, a geochemist at the University of Georgia in Athens,
US, says such instruments should not be difficult to put on a rover - but
interpreting the results may be harder. He cautions that even if calcite is
biological in origin, geological processes can cover up, or overprint, the
biological signal.

"The older materials are, the more likely they've been overprinted by many,
many processes," he told New Scientist. "It can garble the signal so much
it's hard to tell much about it at all."

Steve Squyres, principal investigator for NASA's Mars Exploration Rovers
now exploring the Red Planet, says mineral studies "could be a very useful
clue" in understanding the history of life on Mars. "Detecting evidence of
life on Mars is probably going to be very difficult, so investigating every
reasonable possible avenue makes sense," Squyres told New Scientist.

Still, he points out that there is little evidence for significant amounts
of calcite on Mars. The authors acknowledge this, but say evidence for
carbonates has been detected in Martian dust and meteorites. They plan to
focus future studies on carbonates associated with primitive life on Earth,
such as stromatolites - ancient dome-like structures which once housed
bacteria, and other biologically formed minerals, such as silica.

Journal reference: Geophysical Research Letters (vol 32, p L23205)


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