Preservation of organic traces of life under Martian conditions: Influence of the nature of the smectite in presence

Clay-rich Martian rocks are considered promising targets in the search for fossilized remains of ancient Martian life. However, the actual influence of the clay mineral compositions in preserving microbial biosignatures remains poorly understood. Here, we explore the biopreservation potential of three pure smectites typically found on Mars and containing Al in their tetrahedral sheets (i.e. a Mg-rich, a Fe-rich and a Al-rich smectite), relying on experiments run using E. coli as a biological analog to simulate hydrothermal alteration scenarios relevant to Mars. The results show that Mg-rich smectites (saponite) are more effective at preserving biomolecules from thermal degradation than Fe-rich and Al-rich smectites (nontronite and beidellite). Plus, in contrast to saponite, neither nontronite nor beidellite appears to significantly trap (and thus preserve) organic compounds within their interlayer spaces. Overall, the present study highlights that both the chemistry and the abundance of organic material in ancient Martian clay-rich rocks will depend on the compositional nature of smectites initially present.