Laser-powered device could detect microbial fossils on Mars

The first life on Earth formed four billion years ago, as microbes living in pools and seas: but how would we know if the same thing had happened on Mars?

Scientists hoping to identify fossil evidence of ancient Martian microbial life have now found a way to test their hypothesis, by detecting the fossils of microbes in gypsum samples that are a close analogy to sulfate rocks on Mars. Their work has been published in the journal Frontiers in Astronomy and Space Sciences.

Water, water everywhere

Billions of years ago, the water on Mars dried up. Gypsum and other sulfates formed when pools evaporated, leaving behind minerals that precipitated out of the water — and potentially fossilising any organic life left behind. This means that if microbes such as bacteria lived there, traces of their presence could be preserved as fossils.

“Gypsum has been widely detected on the Martian surface and is known for its exceptional fossilisation potential,” said Youcef Sellam, a PhD student at the University of Bern and first author on the study. “It forms rapidly, trapping microorganisms before decomposition occurs, and preserves biological structures and chemical biosignatures.”

To identify these microbial fossils, we first need to prove we can identify similar fossils in places where we know such microbes existed — such as Mediterranean gypsum formations that developed during the Messinian Salinity Crisis.

“The Messinian Salinity Crisis occurred when the Mediterranean Sea was cut off from the Atlantic Ocean,” Sellam explained. “This led to rapid evaporation, causing the sea to become hypersaline and depositing thick layers of evaporites, including gypsum. These deposits provide an excellent terrestrial analog for Martian sulfate deposits.”

Scientists selected an instrument that could be used on a space flight: a miniature laser-powered mass spectrometer, which can analyse the chemical composition of a sample in detail as fine as a micrometre. They sampled gypsum from Sidi Boutbal quarry, in Sellam’s home country of Algeria, and analysed it using the mass spectrometer and an optical microscope, guided by criteria which can help distinguish between potential microbial fossils and natural rock formations. These include morphology which is irregular, sinuous and potentially hollow, as well as the presence of chemical elements necessary for life, carbonaceous material, and minerals like clay or dolomite which can be influenced by the presence of bacteria.

Life on Mars?

The scientists identified long, twisting fossil filaments within the Algerian gypsum, which have previously been interpreted as benthic algae or cyanobacteria, and are now thought to be sulfur-oxidising bacteria like Beggiatoa. These were embedded in gypsum, and surrounded by dolomite, clay minerals and pyrite. The presence of these minerals signals the presence of organic life, because prokaryotes — cells without a nucleus — supply elements which clay needs to form. They also facilitate dolomite formation in an acidic environment like Mars by increasing the alkalinity around them and concentrating ions in their cell envelopes. For dolomite to form within gypsum without the presence of organic life, high temperatures and pressures would be needed that would have dehydrated the gypsum, and which aren’t consistent with our knowledge of the Martian environment.

If the mass spectrometer could be integrated into future Mars rovers or landers for in-situ analysis, and identify the presence of clay and dolomite in Martian gypsum in addition to other biosignatures, this would be a key signal of fossilised life — one which could then be reinforced by analysing other chemical minerals present and by looking for similar organically formed filaments.

“While our findings strongly support the biogenicity of the fossil filament in gypsum, distinguishing true biosignatures from abiotic mineral formations remains a challenge,” Sellam noted. “An additional independent detection method would improve the confidence in life detection. Additionally, Mars has unique environmental conditions, which could affect biosignature preservation over geological periods. Further studies are needed.”

Image credit: iStock.com/Artur Plawgo