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Formation of methane and (per)chlorates on Mars Svatopluk Civis, Antonín Knížek, Paul Brandon Rimmer, Martin Ferus, Petr Kubelík, Marketa Zukalova, Ladislav Kavan, and Elias Chatzitheodoridis ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.8b00104 • Publication Date (Web): 17 Dec 2018 Downloaded from http://pubs.acs.org on December 23, 2018
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ACS Earth and Space Chemistry
Formation of methane and (per)chlorates on Mars Svatopluk Civiša*, Antonín Knížeka,b, Paul B. Rimmerc,d,e, Martin Ferusa, Petr Kubelíka,f, Markéta Zukalováa, Ladislav Kavana and Elias Chatzitheodoridisg aJ.
Heyrovský Institute of Physical Chemistry, Czech Academy of Sciences Dolejškova 3, CZ18223 Prague 8, Czech Republic. bCharles University, Faculty of Sciences, Department of Physical and Macromolecular Chemistry Hlavova 8, CZ12843, Prague, Czech Republic cCavendish Astrophysics, University of Cambridge, JJ Thomson Ave, Cambridge, CB3 OHE dMRC Laboratory of Molecular Biology, Francis Crick Ave, Cambridge, CB2 0QH eDepartment of Earth Sciences, University of Cambridge, Downing St, Cambridge CB2 3EQ fInstitute of Physics, Czech Academy of Sciences, Na Slovance 1999/2, 182 00, Praha 8, Czech Republic gNational Technical University of Athens, School of Mining and Metallurgical Engineering 9 Heroon Polytechneiou str.; GR-15780 Zografou, Athens, Greece. KEYWORDS: methane on Mars, photocatalytic reduction, perchlorates, infrared spectroscopy, X-ray photoelectron spectroscopy
Abstract: Methane, perchlorates, chlorates and methylchlorides have all been detected on Mars. The origin of these species has never been adequately explained. In this paper, we irradiated mixtures of CO2, HCl and a mineral catalyst – anatase, rutile, montmorillonite and the Nakhla meteorite – with soft UV radiation for up to 3500 hours and observed the formation of perchlorates, chlorates, methylchlorides and methane in a single experiment. Additionally, the methanogenesis for anatase was observed at -196°C. Further, we propose that while methane is decomposed relatively quickly and therefore attains a steady-state concentration (0.41 ± 0.16 ppbv), the chlorinated compounds are much more stable and therefore would have accumulated throughout the Martian history. We estimate that this mechanism would be sufficient in the course of Martian history to accumulate perchlorate in the soil in 0.X wt % in 5-50 cm depth, which is in accordance with the observed perchlorate content on Mars. This predicted perchlorate gradient may be observed with the Insight rover. Further, if microbes are present on Mars, they will likely inhabit depths below the perchlorate, i.e. 5-50 cm. This chemistry likely still continues on Mars to a certain extent and any future exploration by rovers or planetary models should account for this process during their analyses.
1 Introduction Methane and perchlorates are two chemical species that are directly relevant to life on Earth. Up to 95% of methane on present day Earth is of biogenic origin1 and for this reason, it is often used as a biomarker gas for exoplanetary atmosphere detections2.
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Perchlorates, on the other hand, have bacteriocidal effects, which are enhanced by the presence of iron oxides, peroxides and UV radiation, as has been shown on the viability of Bacillus subtilis in simulated Martian conditions3. Detection of both these species on Mars4–10 has therefore direct relevance to the study of potential life on Mars. Indeed, the discovery of methane (CH4) on Mars initiated a debate concerning, among others, the possible chemical evidence of ongoing microbial metabolism on the Red planet11,12. Also, the presence of perchlorates does not ultimately rule out the presence of life on the Red Planet13, but the chances of survival in perchlorate-rich soil are significantly lower than on Earth. In this paper, we present an abiotic scenario of the creation of both species through photochemical reactions on the Martian surface.
1.1 Chlorinated compounds on Mars Several chlorine-containing species were detected on Mars, but their origin has not been explained yet. Up to this day, the detected chlorinated molecules on Mars are perchlorates, chlorates and chloroalkanes such as dichloroethane, dichloropropane, methylchlorides (CHxCly, where x,y = {1,2,3,4}) and dichlorobutane4,5. Chlorobenzene and chlorinated organics were also detected by the rovers Viking and Curiosity, but they are likely to have formed in the instrument oven during analysis. Perchloric acid is highly reactive and readily forms perchlorate salts, which accumulate on the surface and whose content in Martian soil reaches 0.5 to 1 wt%14. The presence of perchlorates has been observed by the Thermal and Evolved Gas Analyzer (TEGA) and the Microscopy, Electrochemistry and Conductivity Analyzer (MECA) on board the Phoenix lander at its landing site with concentrations about 5 times those of Cl- anions15. This detection has been recently confirmed by the Sample Analysis at Mars (SAM) instrument on board the Curiosity rover4. The most abundant perchlorate compounds are proposed to be calcium perchlorate (Ca(ClO4)2) and magnesium perchlorate (Mg(ClO4)2). The origin of perchlorates on Mars is discussed by Smith et al.16, who assumed a sequence of reactions similar to those in the Atacama desert. The authors claim that this mechanism is likely on Mars, but that the obtained concentrations are too low to explain the observed abundance16, and that heterogeneous non-gas phase processes must be further explored and taken into account. Another pathway discussed is the creation of perchlorate by UV irradiation of titanium-containing crystals in aqueous solutions of chloride17. This mechanism, however, would require the presence on liquid water on Mars. In parallel with the detection of perchlorates and chlorates, methylchlorides have also been discovered on Mars in recent years. The pyrolysis of rock samples from Rocknest4 and Sheepbed Mudstone5 by Curiosity’s SAM showed the release of CH3Cl and O2. The Viking lander also observed CH3Cl and CH2Cl2 upon heating of surface samples of the regolith18. So far, their presence has been attributed either to external contamination or to the reaction of Martian chlorine and terrestrial carbon brought in by the rover.
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ACS Earth and Space Chemistry
1.2 Methane on Mars Another intensely studied compound on Mars is methane, whose origin has also never been explained. Since 2003, Mumma et al.6 followed up by Krasnopolsky et al.7 and others8–12 published ground-based and satellite detection of methane in their breakthrough studies and discussed, among other explanations of methane sources, the possibility of the presence of deep microbial life, which utilizes hydrogen (H2) as a means of reducing carbon dioxide (CO2) to CH4. Twelve years later, the Curiosity rover searched for the presence of methane in the Gale crater at the foot of Aeolis Mons on Mars again. Webster et al.19,20 presented a thorough analysis of measurements performed by the rover over the first five years of its operation, where they confirmed the presence of methane on Mars and established a background concentration 0.41 ± 0.16 ppbv in the Crater. Methane is decomposed in the oxidized atmosphere of Mars and exhibits a limited lifetime. Photochemical models were so far unsuccessful in the explanation of the observed background concentrations of methane, as they usually predict a lifetime of ~300 years21. If no synthesis or new releases occur, methane could not be reliably detected with stable concentration at the observed levels21–23, so even today, a source of methane must be present on Mars. Apart from the scenario of biogenic production of methane, several other abiotic sources have been proposed, which include synthesis in a high-pressure and high-temperature hydrothermal fluid24, volcanic outgassing25, cometary impact26, meteorite impact, subsurface aquifers27, serpentinization of olivine28 or slow decomposition of subsurface organic matter reservoirs29. None of these explanations, however, was able to predict the background concentration observed by Curiosity20.
1.3 The mineralogy and atmosphere of Mars Current Mars resembles a desert with no life in it and a very low organic compound content. The surface is covered with regolith, a fine particulate material with high content of iron oxides. These oxides are responsible the reddish hue of the Martian surface. The regolith also contains up to 1 % of TiO2 and admixtures of clays. The upper crust is prevalently basaltic with localized sites of plagioclase, pyroxene and olivine30. Clay minerals in exposures of Noachian (> 3.7 Ga) crust as well as Noachian and Hesperian (3.7 - 3.1 Ga) clay and sulfate sediments in paleolake deposits30 indicate that the climate on the Red planet during that era was warmer and wetter. Records of Amazonian clays exist as well, but are sparser31. New analyses of deep deposits also show that primordial clays on Mars formed beneath a steam or supercritical atmosphere26. Indeed, primary and secondary hydrous alteration of rocks shows that up to 3.5 billion years ago, Mars sustained liquid water on its surface32,33. The water was gradually lost and late Hesperian and Amazonian (< 3.1 Ga) records of water activity are much less frequent30. The Martian atmosphere is dominated by 95.32 % CO2 mixed with 2.7 % nitrogen (N2), 1.6 % argon (Ar), 0.13 % oxygen (O2), 0.08 % carbon monoxide (CO) and minor amounts of water, nitrogen oxides, neon, DHO, krypton, xenon, ozone (O3) and also methane. The current atmosphere of Mars is kept in apparent equilibrium with polar caps and the regolith. 3 ACS Paragon Plus Environment
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The current overall atmospheric chemistry is assumed to be driven mainly by the UV flux and its interaction with the surface. The surface UV flux on Mars is significantly affected by the effective thickness of the atmosphere, the geographic location, O3 concentration and the swirling of fine regolith particles. The solar irradiation reaching the upper atmosphere of Mars oscillates around 600 W.m-2 (determined by the inverse square law). Of this, 1×10-4 – 1×10-2 W.m-2, in the
