Organic carbon concentrations in 3.5-billion-year-old lacustrine mudstones of Mars

Abstract

The Sample Analysis at Mars instrument stepped combustion experiment on a Yellowknife Bay mudstone at Gale crater, Mars revealed the presence of organic carbon of Martian and meteoritic origins. The combustion experiment was designed to access refractory organic carbon in Mars surface sediments by heating samples in the presence of oxygen to combust carbon to CO₂. Four steps were performed, two at low temperatures (less than ∼550 °C) and two at high temperatures (up to ∼870 °C). More than 950 μg C/g was released at low temperatures (with an isotopic composition of δ¹³C = +1.5 ± 3.8‰) representing a minimum of 431 μg C/g indigenous organic and inorganic Martian carbon components. Above 550 °C, 273 ± 30 μg C/g was evolved as CO₂ and CO (with estimated δ¹³C = -32.9‰ to -10.1‰ for organic carbon). The source of high temperature organic carbon cannot be definitively confirmed by isotopic composition, which is consistent with macromolecular organic carbon of igneous origin, meteoritic infall, or diagenetically altered biomass, or a combination of these. If from allochthonous deposition, organic carbon could have supported both prebiotic organic chemistry and heterotrophic metabolism at Gale crater, Mars, at ∼3.5 Ga.

Keywords: Mars; astrobiology; carbon isotopes.

Fig. 1.

(A) Landing ellipse of the Mars Science Laboratory Curiosity Rover at Gale Crater, Mars. (B) Rover Traverse from 2012 to 2022. Red dot shows the location of the Cumberland sample in the Yellowknife Bay formation. (C) Rover scale view of the Sheepbed mudstone member of the Yellowknife Bay formation, with locations of John Klein and Cumberland drill holes. Image Credit: NASA/JPL-Caltech/ASU, NASA/JPL-Caltech/Univ. of Arizona and Scott Rowland, NASA JPL-Caltech/MSSS.

Fig. 2.

TLS and QMS carbon abundances and TLS δ¹³C data. Bulk δ¹³C for steps 3 and 4 combined is −3.6 ± 3.1‰.

Fig. 3.

Carbon isotopic composition of carbon reservoirs on Mars. Our combustion data overlap with both igneous refractory carbon, meteoritic organics, and carbonate carbon. Atmospheric data from (29); Meteoritic EOM from (25, 26); Meteoritic IOM data from (34); Carbonate SNC range from (48); ALH84001 data from (7, 50); Igneous refractory carbon from (1); Gale Crater data from (13) Box and whiskers show the median, upper and lower quartiles, and maximum and minimum values, with outlier values shown as black dots.