Dark microbiome and extremely low organics in Atacama fossil delta unveil Mars life detection limits

Identifying unequivocal signs of life on Mars is one of the most important objectives for sending missions to the red planet. Here we report Red Stone, a 163-100 My alluvial fan-fan delta that formed under arid conditions in the Atacama Desert, rich in hematite and mudstones containing clays such as vermiculite and smectites, and therefore geologically analogous to Mars. We show that Red Stone samples display an important number of microorganisms with an unusual high rate of phylogenetic indeterminacy, what we refer to as dark microbiome, and a mix of biosignatures from extant and ancient microorganisms that can be barely detected with state-of-the-art laboratory equipment. Our analyses by testbed instruments that are on or will be sent to Mars unveil that although the mineralogy of Red Stone matches that detected by ground-based instruments on the red planet, similarly low levels of organics will be hard, if not impossible to detect in Martian rocks depending on the instrument and technique used. Our results stress the importance in returning samples to Earth for conclusively addressing whether life ever existed on Mars. Unique microorganisms of a fossil river delta in the Atacama Desert unveil the current limits of life detection on Mars.

Automated summary

By studying Red Stone samples formed in the Atacama Desert, scientists have discovered a unique group of microorganisms called "dark microbiome" and identified biosignatures associated with extant and ancient microorganisms. Comparing with minerals on Mars, they found that the organics on the red planet are extremely low and may be difficult to detect with current instruments. Therefore, bringing samples back to Earth for further analysis is crucial in determining the existence of life on Mars.