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.
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.
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.