Evaluation of the Tenax trap in the Sample Analysis at Mars instrument suite on the Curiosity rover as a potential hydrocarbon source for chlorinated organics detected in Gale Crater

Abstract

The Sample Analysis at Mars (SAM) instrument suite aboard Curiosity has detected chlorinated organic compounds in Martian sediment samples. The chlorine in these molecules is thought to derive from oxychlorine salts in Martian sediments, but the carbon source remains under investigation. To constrain possible carbon sources, we investigated how the composition and concentration of oxychlorine phases in solid samples affect organic molecules released from the Tenax traps on board SAM. We created Mars analogue soils by spiking olivine sand with calcium perchlorate, magnesium perchlorate, or ferric iron chloride and analyzed the volatiles generated during pyrolysis-gas chromatography-mass spectrometry using commercial instruments operated under SAM-like conditions, with and without a Tenax trap. Benzoic acid, phthalic anhydride, high molecular weight aromatics, and chlorobenzenes are produced from the trap in response to volatiles released during Cl salt pyrolysis. Changes in composition or concentration of oxychlorine phases between samples could thus potentially produce an increase in chlorobenzene, as observed between samples from Rocknest and Cumberland. However, in our experiments benzoic acid, phthalic anhydride, and chlorobenzenes increase in proportion with the amount of HCl sent to the trap, while in Cumberland samples the chlorobenzene increase showed no corresponding increase in HCl. Based on our experiments, the Tenax trap is a possible source of the traces of chlorobenzene observed at Rocknest, John Klein, and Confidence Hills. The order-of-magnitude higher chlorobenzene abundances observed at Cumberland cannot be attributed to the Tenax trap. Furthermore, we found no evidence of significant trap degradation after hundreds of experiments with Cl salt-containing analogue soils.

Identification of Chlorobenzene in the Viking Gas Chromatograph‐Mass Spectrometer Data Sets: Reanalysis of Viking Mission Data Consistent With Aromatic Organic Compounds on Mars

“…Another source of instrument contaminants are materials derived from the instrument itself, either through bleeding of the GC column or from sticky organic compounds in the walls of the MS system, which have also been observed in the SAM GCMS system (Freissinet et al, ; Miller et al, ). For example, the detection by the SAM GCMS of some aromatic hydrocarbon fragments, including low levels of chlorobenzene in blanks, has been attributed to the degradation of Tenax‐TA [poly (2,6‐diphenyl‐ p ‐phenylene oxide] used in hydrocarbon and injection traps upstream from the GC columns.…”

“…For example, the detection by the SAM GCMS of some aromatic hydrocarbon fragments, including low levels of chlorobenzene in blanks, has been attributed to the degradation of Tenax‐TA [poly (2,6‐diphenyl‐ p ‐phenylene oxide] used in hydrocarbon and injection traps upstream from the GC columns. Tenax‐TA degradation was likely caused by HCl released from the decomposition of martian perchlorate in the GC columns (Freissinet et al, ; Miller et al, ), an issue that would have been unknown to the Viking team.…”

“…In this scenario, organic compounds would be released from the mineral matrix in a sample at certain temperatures and react with perchlorate, or its degradation products, forming chlorobenzene prior to detection by the MS. Navarro‐González et al () showed that chloromethane and dichloromethane were produced this way, but they were not focused on looking for chlorobenzene. However, chlorobenzene is known to be produced in this way as well (Miller et al, ). Indeed, the chlorobenzene signal in VL‐2 runs was observed between 350°C and 500°C, consistent with observations by the SAM instrument of chlorobenzene between 250°C and 450°C.…”

Comment on “Evaluation of the Tenax Trap in the Sample Analysis at Mars Instrument Suite on the Curiosity Rover as a Potential Hydrocarbon Source for Chlorinated Organics Detected in Gale Crater” by Miller et al. (2015)

“…Glavin et al () have showed that reactions of the pyrolysis products of oxychlorines, such as Cl 2 or HCl, with Tenax TA may result in the production of chlorinated organic compounds, including chlorobenzene. Miller et al () used a Mars analog soil, olivine sand spiked with calcium perchlorate, magnesium perchlorate, or Fe (III) chloride to analyze organic compounds produced during pyrolysis‐GC‐mass spectrometer (py‐GC‐MS) experiments. Some of the experiments using calcium perchlorates were performed both with and without a Tenax TA trap between the pyrolysis unit and the mass spectrometer in order to better understand the origin of chlorinated organic compounds produced in the Tenax TA trap, including chlorobenzene.…”

“…(a) Structure of the polymer Tenax TA. (b) Mass spectra (Miller et al, ) and tentative structure of A1 and (c) the same for A2. Here compounds A1 and A2 are tentatively identified as monomers of Tenax TA.…”