Microbial Habitability in Gale Crater: Sample Analysis at Mars (SAM) Instrument Detection of Microbial Essential Carbon and Nitrogen.

Chemical analyses of Mars soils and sediments from previous landed missions have demonstrated that Mars surface materials possessed major (e.g., P, K, Ca, Mg, S) and minor (e.g., Fe, Mn, Zn, Ni, Cl) elements essential to support microbial life. However, the detection of microbial essential organic-carbon (C) and nitrate have been more elusive until the Mars Science Laboratory (MSL) rover mission. The objective of this work is to discuss the detections of nitrate and organic-C in Gale Crater, Mars by the Sample Analysis at Mars (SAM) instrument onboard the MSL Curiosity rover. Eolian fines and drilled sedimentary rock samples were heated in the SAM oven from ~30 to 860°C where evolved gases (e.g., nitrous oxide (NO) and CO₂) were released and analyzed by SAM’s quadrupole mass spectrometer (MS). The temperatures at which NO and CO₂ were evolved were diagnostic of the phases in which they are derived. The temperatures of evolved NO was assigned to nitrate while evolved CO₂ was assigned to organic-C and carbonate. The CO₂ releases in several samples occurred below 400°C suggesting organic-C dominated in those samples. As much as 7 µmole NO₃-N/g and 200 µmole CO₂-C/g have been detected in the Gale Crater materials. These N and C levels coupled with assumed microbial biomass (9 x 10^-7 µg/cell) and C (0.5 µg C/µg cell) and N (0.14 µgN/µg cell) requirements, suggest that <1 % C and <10% N in Gale Crater materials, woud be required if available, to accomodate the biomass requirements of 1 x 10⁵ cells/g sediment. While nitrogen is the limiting nutrient, the potential exists that sufficient N and organic-C were present to support limited heterotrophic microbial populations that may have existed on ancient Mars.