Laboratory Experiments Identify Meteoritic Organic Precursors for Chlorinated Compounds Detected by the Sample Analysis at Mars (SAM) Instrument at Gale Crater

Abstract

The Viking missions landed two spacecraft on Mars each equipped with a pyrolysis‐gas chromatograph‐mass spectrometer system capable of detecting signs of life. Although working as intended, detections of chloromethane and dichloromethane were initially attributed to terrestrial contamination. Decades later, the Phoenix Mars Lander identified perchlorate salts in the Martian regolith that are known to catalyze the oxidation and chlorination reactions of organics. Subsequent analyses by the Sample Analysis at Mars (SAM) instrument suite aboard the Curiosity rover have since confirmed a variety of syngenetic chlorinated hydrocarbons in Gale crater samples. However, the nature of the parent organic matter (exogenous, abiotic endogenous, or biological) remains unresolved. Here we evaluate the Murchison (CM2) carbonaceous chondrite as a proxy for organic matter delivered to the surface of Mars by meteoritic and interplanetary dust particle infall over geologic timescales. We characterize the generation of chlorinated hydrocarbons during the pyrolysis of Murchison in the presence of 1 and 5 wt.% calcium perchlorate. Experiments produced a suite of C ₁ ‐C ₇ chlorinated hydrocarbons dominated by aromatic species, including compounds detected by both Viking and SAM. Increasing the perchlorate content from 1 to 5 wt.% did not amplify the relative abundances or diversity of chlorinated products but rather preferentially oxidized parent organics. Results suggest that organics detected at Gale crater represent a fraction of molecules liberated from a macromolecular source that is compositionally consistent with meteoritic organic matter. Unambiguously resolving the origin and diversity of Martian organic matter may ultimately require the coordinated laboratory analysis of samples returned to Earth.