Chloromethane release from carbonaceous meteorite affords new insight into Mars lander findings

Abstract

Controversy continues as to whether chloromethane (CH3Cl) detected during pyrolysis of Martian soils by the Viking and Curiosity Mars landers is indicative of organic matter indigenous to Mars. Here we demonstrate CH3Cl release (up to 8 μg/g) during low temperature (150–400°C) pyrolysis of the carbonaceous chondrite Murchison with chloride or perchlorate as chlorine source and confirm unequivocally by stable isotope analysis the extraterrestrial origin of the methyl group (δ2H +800 to +1100‰, δ13C −19.2 to +10‰,). In the terrestrial environment CH3Cl released during pyrolysis of organic matter derives from the methoxyl pool. The methoxyl pool in Murchison is consistent both in magnitude (0.044%) and isotope signature (δ2H +1054 ± 626‰, δ13C +43.2 ± 38.8‰,) with that of the CH3Cl released on pyrolysis. Thus CH3Cl emissions recorded by Mars lander experiments may be attributed to methoxyl groups in undegraded organic matter in meteoritic debris reaching the Martian surface being converted to CH3Cl with perchlorate or chloride in Martian soil. However we cannot discount emissions arising additionally from organic matter of indigenous origin. The stable isotope signatures of CH3Cl detected on Mars could potentially be utilized to determine its origin by distinguishing between terrestrial contamination, meteoritic infall and indigenous Martian sources.

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

“…Perchlorate salts are strongly oxidizing and facilitate the combustion of organic compounds when heated to elevated (>150°C) temperatures such as those used to thermally desorb or pyrolyze organic matter from solid samples in experiments conducted by the Mars surface exploration missions Viking, Phoenix, and MSL. The pyrolysis of oxychlorine phases in the presence of organic compounds, however, also results in the formation of low quantities of chlorinated organic compounds, resulting from a competition between combustion and oxidative chlorination of the organic molecules [Navarro-Gonzalez et al, 2010;Steininger et al, 2012;Glavin et al, 2013;Keppler et al, 2014;Freissinet et al, 2015]. Glavin et al, 2013;Ming et al, 2013].…”

Protostellar and cometary detections of organohalogens

“…Its total production rate approaches 3 megatonnes per year, with the majority originating from biological processes [8]. Recent observations of Cl-bearing organic molecules, including methyl chloride, on Mars by the rover Curiosity, has challenged a straightforward connection between organohalides and biology; one proposed source of Cl-bearing organic molecules on Mars is meteoritic impacts [9,10]. This naturally raises the question of whether circumstellar and interstellar environments can produce organohalogens abioticially, and in which amount.…”

Theoretical study of the microhydration of 1‐chloro and 2‐chloro ethanol as a clue for their relative propensity toward dehalogenation

“…Enzymes that produce this dehalogenation process (known as dehalogenases and dehaloperoxidases) are also present in numerous organisms. Organohalogens seem to play an important role also in the interstellar media (ISM) as it is reported in recent papers …”