On Mars, Looks Can Be Deceiving

Reliable biosignatures need more than a physical resemblance to be convincing.

Hot springs (arrows) with discharge channels at El Tatio, Chile, where the intriguing silica structures were found.

In a presentation at this week’s American Geophysical Union meeting in San Francisco, Steven Ruff and Jack Farmer from Arizona State University reported on a putative biosignature—a sign of past or present life—on Mars. They suggest that outcrops of Martian silica discovered almost a decade ago resemble hot spring deposits from a geyser field in Chile, which are most likely biological in origin. Their results also appear in the journal Nature Communications.

Deposits of silica were first seen on Mars in 2007 by the Spirit rover, adjacent to a feature called “Home Plate,” located within Gusev crater. The silica had previously been described as nodular masses with a rubbly appearance. The structures are quite hard, which also explains why they resist deformation by the rover wheels.

Ruff and Farmer have found a place on Earth that they believe closely resembles what Spirit saw at Home Plate. In a geyser field called El Tatio in northern Chile, they discovered nodular and finger-like silica structures that have very similar spectra to the ones obtained by the Mars rover. The structures at El Tatio are thought to have been created by cyanobacteria. And based on the spectral match, the researchers suggest that the mineral halite (common sodium chloride salt) is present at Home Plate, which would allow any putative microorganisms to scavenge water directly from the Red Planet’s thin atmosphere—a strategy employed by microbes in very dry deserts on Earth.

There is, however, another way to explain the silica at Home Plate that doesn’t involve life.  Acid from steam vents may have condensed on already existing rocks, leaching them and leaving behind only the hard, non-leachable silica deposits. In that case, halite would not be present, because any water in the condensed steam would have dissolved the salt away long ago.

Unfortunately, identifying a specific mineral, in this case halite, based only on matching spectra, is notoriously unreliable. And physical resemblance to biogenic structures on Earth is not proof in itself that a particular structure on Mars is biological in origin. So this intriguing discovery should be added to a long list of findings regarding possible Martian life that can only be verified (or dismissed) by sending another rover to that site, with more sophisticated instruments. Let’s do that.

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