Evolved gas analysis of Mars-analog paleosols

Abstract:

Three to four billion-year old surface environments on Mars may have once been habitable. Accordingly, ancient surface environments which appear to resemble paleosols (buried, fossil soils) on Mars have recently been named a high-priority target for biosignature investigation and Mars Sample Return. However, there has been little effort to characterize paleosols on Earth using instruments analogous to those on current and future missions to Mars, which is imperative for understanding if potential paleosols should be targeted because of their ability to preserve biosignatures. The objectives of this study were to perform the first thermal analysis of Mars-analog paleosols with robotic mission-like instrumentation, and to determine if organic carbon in these paleosols can be detected in the presence of a perchlorate salt. Early Oligocene (33 Ma) paleosols rich in dioctahedral phyllosilicates and amorphous materials were collected near the Painted Hills in eastern Oregon. Three paleosol types were analyzed with a thermal and evolved gas analyzer configured to operate similarly to the Sample Analysis at Mars Evolved Gas Analysis instrument (SAM-EGA) onboard the Mars Science Laboratory (MSL) Curiosity rover. Select paleosol samples were spiked with 1 wt. % sodium perchlorate to assess the oxidation of organic carbon in paleosols during SAM-EGA, which has been shown to hinder the detection of organic molecules on Mars because of combustion of the organics. Most samples evolved CO₂ with peaks at ~400 °C and ~700° C from the thermal decomposition of organic carbon (C) and carbonate (CaCO₃), respectively. After accounting for high-temperature (~700° C - 900° C) evolutions of CO₂ from inorganic carbonates, coevolved CO and CO₂ at ~400°C suggests that organic carbon (0.002 - 0.031 ± 0.006 wt %) is retained in paleosols which formed in well-drained and oxidizing settings. Simple-chain organic compounds evolved at temperatures below the release of O₂ from the thermal decomposition of perchlorate, which is favorable for the identification of organic molecules during SAM-EGA. This work demonstrates that surface horizons of phyllosilicate-rich paleosols should be considered high-priority targets for in-situ detection of organic carbon and nitrogen on Mars and selection of samples for return to Earth.