Evidence of ancient microbial life in Martian soil will not be easy to find, and some of it may have been destroyed by the flow of liquid acid, according to research published in Scientific Reports.
The idea of life on Mars has tantalized astronomers for hundreds of years. Although no concrete proof of extraterrestrial life has ever been confirmed on the Red Planet, signs that it may have been capable of supporting microorganisms has been steadily mounting as scientists uncover evidence that it once held water on its surface.
But not all liquids are good for Martian life, explained Alberto Fairén, first author of the paper and a visiting scientist at Cornell University. "When clays are exposed to acidic fluids, the layers collapse and the organic matter can't be preserved. They are destroyed. Our results in this paper explain why searching for organic compounds on Mars is so sorely difficult."
Fairén and his colleagues devised a chemistry experiment to mimic Martian soil conditions. They slipped samples of glycine, an amino acid involved in the production of proteins, into clay minerals. Next, they treated the soil-like mixture with radiation, as well as with acidic and alkaline fluids, to see how it might affect the glycine in a closed chamber pumped with the same concentration of gases found on Mars.
Ultraviolet rays and hydrochloric acid destroyed the glycine, turning it into a lumpy gel. The team reckons the same thing probably occurred on Mars too. Acid may have degraded biosignatures like amino acids, making it difficult for rovers to find. "We know that acidic fluids have flowed on the surface of Mars in the past, altering the clays and its capacity to protect organics," Fairén added.
Perseverance, the latest Martian rover to be deployed from NASA, is currently on its way towards the Red Planet. Launched in July, it is equipped to drill into the Martian surface to collect soil samples to return to Earth. The team suggests avoiding regions that might have been flooded with acid.
"Being successful in the search for life in clays on Mars will depend on the characteristics of the fluids that have been interacting with those clays in the past: our paper shows that alkaline fluids will not hamper the capability of clays to host organics," Fairén told El Reg.
"Perseverance got it right from the beginning: the rover is going to Jezero crater, one of the few spots on Mars where carbonates are abundant. And carbonates are evidence that acidic fluids have not been pervasive since their deposition, because carbonates dissolve readily in acids. So, Jezero is a sweet spot for the search for life on Mars."
Perseverance is expected to reach Mars in February. It's the heaviest trundlebot yet, and hosts a range of instruments including some 23 cameras, multiple types of spectrometers, a laser zapper, and two microphones. ®