Survivability of 1-chloronapthalene during simulated early diagenesis - Implications for chlorinated hydrocarbon detection on Mars

Plain Language Summary All missions to Mars which have attempted to detect organic molecules have detected simple chlorohydrocarbons, the source of which has yet to be firmly established. This study assessed the likelihood of these chlorinated molecules being indigenous to the sedimentary units in which they were detected or if they were chlorinated during analysis. The survivability of 1-chloronapthalene was examined via hydrous pyrolysis experiments and its dechlorination kinetics were determined. The results of these experiments were used to model the survivability of this simple chlorohydrocarbon under Mars-relevant diagenetic conditions using the Sheepbed mudstone unit as a case study. It was found that 1-chloronapthalene was rapidly dechlorinated under Noachian conditions, and thus, the detected Martian chlorohydrocarbons are unlikely to be ancient and probably formed within the rover's sample handling chain during analysis. The search for past or present life on Mars is centered on the detection of organic molecules. Most attempts to detect organic molecules on Mars have only found simple chlorinated compounds. The source of these chlorinated compounds has not been firmly established. It has been hypothesized that chlorinated organic compounds could form on the Martian surface via reactions between chlorine-bearing salts and organic compounds delivered by meteorites. This study examined the ability of simple chlorinated organic molecules to survive the high pressures and temperatures associated with burial over geological time scales. Our aim was to calculate whether chlorinated molecules detected on Mars could be ancient and preserved in the rock units, or if they formed during the analysis process, as other studies have assumed. It was found that, on a warmer ancient Mars when the sediments the chlorinated molecules were detected in were deposited, the increased surface temperatures would have promoted the loss of chlorine relatively rapidly, and the intact chlorinated organic molecules would not have survived the burial and exhumation processes to the present day. The chlorinated organics detected on Mars are therefore likely to have been formed recently, most probably through reactions with chlorine-bearing salts when heated during analysis.