Detection of Organic Carbon in Mars-Analog Paleosols With Thermal and Evolved Gas Analysis

Ancient, buried soils, or paleosols, may have been preserved in the geological record on Mars, and are considered high-priority targets for biosignature investigation. Studies of paleosols on Earth that are similar in composition to putative martian paleosols can provide a reference frame for constraining their organic preservation potential on Mars. However, terrestrial paleosols typically preserve only trace amounts of organic carbon, and determining what carbon is original is complicated by diagenesis and additions of modern carbon. The objectives of this study were (a) to determine whether organic carbon in Mars-analog paleosols can be detected with thermal and evolved gas analysis, and (b) constrain the age of organic carbon using radiocarbon (C-14) dating. Oligocene (33 Ma) paleosols from Oregon were examined with an instrument similar to the Sample Analysis at Mars Evolved Gas Analysis instrument onboard the Mars Science Laboratory Curiosity rover. Trace amounts of organic carbon and fragments of organic molecules were observed in all samples. Total organic carbon (TOC) ranged from 0.002 to 0.032 +/- 0.006 wt. %. The near-surface horizons of paleosols had significantly higher TOC relative to subsurface layers. Radiocarbon dating of four samples revealed an organic carbon age of similar to 6,200-14,500 years before present and a fraction modern ranging from 0.16 to 0.46. Modeled abundances of modern carbon in bulk samples ranged from 0.41%-3.1% +/- 0.11%, which were consistent with additions of small amounts of modern organic carbon. This work demonstrates that martian paleosols are a potential high priority location for in-situ biosignature investigation.

Automated summary

Ancient buried soils on Mars may contain preserved organic carbon, making them important targets for investigating potential signs of life. This study examines paleosols from Oregon that are similar to martian soils and finds trace amounts of organic carbon and organic molecules. The age of the organic carbon is estimated using radiocarbon dating to be around 6,200-14,500 years old. These findings suggest that martian paleosols could be a priority location for studying possible biosignatures.