A new study led by Armando Azua-Bustos from the Center of Astrobiology in Madrid, Spain, suggests a promising place to search for Martian life that hadn’t previously gotten much attention from astrobiologists. The team did research in the hyperarid core of the Atacama desert in Chile, which is similar to Mars in regard to its mineralogy and geology. During their field work they detected a subsurface layer containing lots of smectite, a clay mineral that can hold a substantial amount of water.
The layer was found slightly more than a foot below the desert surface. Further investigation showed that the relative humidity in the smectite-rich layer was 78 percent. That’s well above what is needed for microorganisms to survive in this harsh desert, although it’s far from easy. In fact, Azua-Bustos confirmed that the smectite layer was inhabited by about 10,000 microbial cells per gram of sediment. The team identified at least 30 different salt-loving species of bacteria and archaea that were not only able to survive, but actively metabolize in this very dry setting.
Microbes at that depth are protected from harsh ultraviolet irradiation and the extreme temperature shifts that occur above ground. Considering the amount of water available in the smectite, this layer should now be considered a microhabitat in an otherwise very unforgiving environment.
And it suggests another place to look for life on Mars. Similar clay minerals exist on the Red Planet. A smectite-rich mudstone was discovered in Gale Crater by the Curiosity rover and is also expected to be present at Jezero Crater, the planned landing site of NASA’s Perseverance rover, and Oxia Planum, the planned landing site of the Rosalind Franklin rover due to launch on the European ExoMars mission in 2022. That rover carries a drill that can penetrate two meters below the surface, which should be deep enough.
Unfortunately, neither rover carries instruments to search for active life. But they might be able to identify chemical signatures indicative of life (biomarkers). The authors of the paper point specifically to lipid molecules, which can be preserved in desert climates for millions, if not billions of years. Organic molecules are frequently associated with clay minerals. Curiosity in fact discovered sulfur-rich organic compounds at Gale Crater, whose suitability as a biomarker has been discussed.
So, even if there is no longer active life in the smectite-rich layers on Mars (or there is, but we can’t detect it), we might find the remnants of life that existed many millions of years ago. And, of course, Perseverance’s job is to gather samples of soil to be returned to Earth for closer analysis. Would it be asking too much to include some smectite in those samples?