Seeking Ancient Microbial Biosignatures With PIXL on Mars 2020

Identifying ancient microbial biosignatures in the geologic record requires an assemblage of evidence, and almost never results from a single "smoking gun". When a potential biosignature such as a microfossil, organic deposit or stromatolite is found, sources of evidence for testing the biogenic hypothesis include the inherent properties of the feature itself, the assemblage characteristics of multiple candidate features, the geologic context of the features, and the relationship between changes in the features and changes in the geologic context. Given these diverse sources of evidence, multiple analytical techniques necessarily come into play. Some of the most powerful techniques emerging for ancient biosignature investigations are those that detailed, spatially correlated observations of fine texture, microstructure and composition (mineralogy, chemistry, organics etc). MicroX-ray fluorescence is a significant new method for in situ compositional analysis at sub-millimeter scales. Micro-XRF scans a focused X-ray beam across the surface of a rock to create a map of elemental composition at mm-cm scales, comprising individual spot measurements of 100 microns diameter. A micro-XRF instrument, PIXL (Planetary Instrument for X-ray Lithochemistry) was selected for flight aboard the Mars 2020 rover. We have used PIXL to analyze to diverse geological samples including ancient stromatolites, reduction spots, ooids, altered basalts, concretions and shales, and found that PIXL element maps are not only a highly effective way to quickly ascertain the compositional variability of the sample, but also that unexpected features typically emerge that can be critical for interpreting sedimentary, diagenetic and biological processes. For example, in two different stromatolite cases, elemental distribution revealed sedimentary and diagenetic textures that constrained the processes of stromatolite formation and implied biological influence. These textures were invisible in reflected light and too large to be observed at microscope scales. Fine scale elemental analysis by PIXL will be an effective way of searching for, and characterizing, potential ancient biosignatures on Mars.