Benzoic Acid as the Preferred Precursor for the Chlorobenzene Detected on Mars: Insights from the Unique Cumberland Analog Investigation

The Cumberland drill sample from the Sheepbed mudstone in Gale Crater, Mars, revealed the first evidence of an indigenous Martian organic molecule, chlorobenzene, with the Sample Analysis at Mars (SAM) instrument on Curiosity. We created in the laboratory a mineralogical analog of the Cumberland sample (CBA) to aid in the understanding of the precursor organic molecule(s) that led to the detection of chlorobenzene. The CBA was analyzed by visible/near-infrared spectrometry, and the results are in accordance with Mastcam multispectral and the Chemical Camera passive analyses of Cumberland on Mars, demonstrating that the CBA is a relevant analog. CBA aliquots were spiked with 0.5 wt. % of benzoic acid and 1-2 wt. % of magnesium perchlorate and were run in SAM Testbed (TB). The TB evolved gas analysis (EGA) showed similarities with the Cumberland EGA on Mars in terms of the major volatiles H 2 O, CO 2 , and O 2. The TB gas chromatography mass spectrometry displayed the presence of chlorobenzene at 23-28 pmol and dichlorobenzene. CBA aliquots were also analyzed in the laboratory with SAM-like EGA and the results on the laboratory setup confirmed the generation of chlorobenzene by a reaction between the benzoic acid and the magnesium perchlorates. The case for benzoic acid as a potential precursor for the chlorobenzene detected in the Martian regolith is strengthened with this new supporting laboratory data from the CBA. The quantification of chlorobenzene in the TB led to prediction of organic precursor abundance on Mars of hundreds, if not thousands, of parts per millions by weight.