An Evaluation of the Hydrocarbon Trap as a Potential Source of Organic Compounds Detected by the SAM Instrument on the Curiosity Rover
Résumé
The Sample Analysis at Mars (SAM) instrument suite aboard the Curiosity rover catalogued a suite of organic compounds using pyrolysis gas chromatography-mass spectrometry (Py-GC-MS) and evolved gas analyses (EGA) of aeolian and subsurface drill fines at Gale Crater, Mars. The sources of these organic compounds are also being investigated through laboratory experiments in which Mars analogue mixtures are analyzed by Py-GC-MS and EGA under SAM-like conditions. In this study we examined the hydrocarbon trap, a component of the SAM Py-GC-MS system, as a potential source of some of the compounds detected on Mars. Analogue mixtures consisting of olivine sand and varying amounts of Cl-containing salts and magnetite were analyzed with Py-GC-MS operated under SAM-like conditions (i.e. trap materials and pyrolysis and GC temperature programs were the same as SAM analyses). Carbon dioxide, benzene and toluene were identified in all experiments and chlorobenzenes (CBs) and HCl were identified in the experiments with Cl salts. As the concentration of Cl salts increased the abundance of CBs, HCl and CO2 increased whereas the abundance of benzene and toluene remained stable. In order to confirm that these compounds originate from the trap we analyzed the samples while bypassing the trap. Benzene was detected, although at much lower concentrations, and toluene could not be definitively identified. Traces of CB were also detected when Cl salts were analyzed without the trap, again at lower concentrations. Additionally, traces of phthalic acid were detected in the Cl salt samples; this compound readily forms CBs in the presence of HCl (Miller et al., 2013). From this we can conclude that traces of benzene, toluene, and CBs can be degradation products originating from the hydrocarbon trap. Understanding how these compounds form and their relationship to the concentration of different mineral decomposition products helps us interpret the significance of all the organic compounds identified in SAM GC-MS data.