Eigenbrode, et al. (and 21 co-authors), report that establishing the presence and state of organic matter, including its possible biosignatures, in Martian materials has been an elusive quest, despite limited reports of the existence of organic matter on Mars. They report the in situ detection of organic matter preserved in lacustrine mudstones at the base of the ~3.5-billion-year-old Murray formation at Pahrump Hills, Gale Crater, by the Sample Analysis at Mars instrument suite onboard the Curiosity rover. Diverse pyrolysis products, including thiophenic, aromatic, and aliphatic compounds released at high temperatures (500° to 820°C), were directly detected by evolved gas analysis. Thiophenes were also observed by gas chromatography–mass spectrometry. Their presence suggests that sulfurization aided organic matter preservation.
At least 50 nanomoles of organic carbon persists, probably as macromolecules containing 5% carbon as organic sulfur molecules.organic matter preservation is central to understanding biological potential on Mars through time. Whether it holds a record of ancient life, is the food for extant life, or has existed in the absence of life, organic
matter in martian materials holds chemical clues to planetary conditions and processes. Prior reports of organic matter indigenous to Martian sediments include 150 to 300 parts per billion (ppb) of chlorobenzene, with lesser amounts of C2 to C4 dichloroalkanes, detected in Sheepbed mudstone upon heating to <400°C by the Sample Analysis at Mars (SAM) instrument suite of the Mars Science Laboratory (MSL) mission. Martian organic carbon may have also contributed to CO and CO2 pyrolysis and combustion products evolved below 700°C from various sediments by SAM. Chloromethanes detected in Viking’s pyrolysis gas chromatography–mass spectrometry (GC-MS) analysis of martian regolith have been reinterpreted as having a martian origin.
Additionally, indigenous organic matter in martian meteorites is related to a deep igneous domain excavated by impact events. Although these reports indicate the presence of martian organic matter, they do not constrain our understanding of ancient organic matter in sediments. Exploration of the lowermost exposed sedimentary rocks at the base of Aeolis Mons in Gale Crater by the Curiosity rover has led to the discovery of a finely laminated mudstone succession, the Murray formation, that is interpreted to record deposition in a long-lived ancient circumneutral to alkaline lake fed by a fluviodeltaic sedimentary system. Mudstones are composed of basaltic minerals mixed with phyllosilicate, sulfate, iron oxide, and x-ray amorphous components. The ~3.5-billion-year-old Gale lake environment(s) are expected to have been ideal settings for concentrating and preserving organic matter.
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