PRODUCTION OF CHLORINATED HYDROCARBONS DURING THE THERMAL DECOMPOSITION OF METAL CARBONATES AND PERCHLORATE SALTS

Introduction: The first report of the in situ detection of organic compounds of martian origin was based on the results of the Viking Carbon Assimilation experiment, also referred to as the Pyrolytic Release (PR) experiment [1]. Following the discovery of perchlorates in martian soil [2], the generation of chloromethane (CH3Cl) and dichloromethane (CH2Cl2) during sample analysis in the Viking gas chromatographmass spectrometer (GCMS) [3] was attributed to the thermally induced reaction of soil organics with perchlorate [4]. More recently, CH3Cl, CH2Cl2 and CHCl3 were detected during thermal analysis of martian soils using the Sample Analysis at Mars (SAM) instrument suite on the Curiosity rover [5]. We have reexamined the Viking GCMS and PR results to establish mechanisms that may result in the formation of chlorinated hydrocarbons during the thermal analysis of martian soils. We show, using Viking protocols, that the thermal decomposition of carbonates in the presence of metal catalysts can, in some cases, result in methane formation and, when perchlorate is present, in the formation of CH3Cl and CH2Cl2. We hypothesize that the origin of the organics detected in the Viking PR experiment was not due to the fixation of atmospheric carbon into organic carbon as presumed [1], but instead may have been due the formation and subsequent thermal decomposition of inorganic carbonates. Although terrestrial organic contamination may have provided the dominant carbon source for the production of chlorinated hydrocarbons during the Viking GCMS analyses, based on our results, it appears that a contribution from inorganic carbonates is also possible. Experimental: A custom built system was used to replicate key aspects of the Viking GCMS [3]. Thermally evolved gases were separated using a polymetaphenoxylene coated 2,6-diphenyl-para-phenylene oxide (Tenax, 2m, 60/80 mesh) GC column. In experiments that used H2 carrier gas, as was done in the Viking GCMS, a palladium/silver H2 separation tube was used to remove the carrier prior to sample product elution into the MS. Experiments were also carried out using helium as the carrier gas. A Pyroprobe 2000 with a 1500 GC interface (CDS Analytical Inc.) was used to heat samples held in a 50 micro liter quartz tube and to introduce the evolved volatiles into the GC column. Thermal analyses were performed using three different sample headspace gases: CO2, H2 (the gases used in the Viking GCMS), and He. NiCa(CO3)2 and FeCa(CO3)2 were synthesized to test nickel and iron as catalysts for the production of hydrocarbons during the thermal decomposition of carbonates [6]. Thermal analyses were performed using the mixed metal carbonates alone and with different combinations of magnetite, hematite, magnesium perchlorate and/or acetone. Results and Discussion: Acetone contamination at ppb levels was detected during the Viking lander 2 (VL2) GCMS analyses [3]. We show that, as expected, when perchlorate is combusted in the presence of acetone, CH3Cl and CH2Cl2 are produced (fig. 1). During sample analysis on Mars, the VL1 GCMS detected the presence of CH3Cl while the VL2 GCMS detected CH2Cl2. We believe that differences in sensitivity caused by the effluent divider, which split variable amounts of carrier gas from the Viking MS, can account for the fact that each of the VL1 and VL2 instruments did not detect both CH3Cl and CH2Cl2. Our results indicate that the volatilization of acetone and perchlorate produces small amounts of CH3Cl and CH2Cl2, and that this may explain the VL2 GCMS results. However, we note that although CH3Cl was observed using the VL1 GCMS, acetone was not. This