ARTICLE IN PRESS Planetary and Space Science 55 (2007) 358–369 www.elsevier.com/locate/pss Methane and related trace species on Mars: Origin, loss, implications for life, and habitability Sushil K. Atreyaa,Ã, Paul R. Mahaffyb, Ah-San Wonga aDepartment of Atmospheric, Oceanic, and Space Sciences, Space Research Building, University of Michigan, Ann Arbor, MI 48109-2143, USA bGoddard Space Flight Center, Greenbelt, MD 20771, USA Accepted 8 February 2006 Available online 22 August 2006 Abstract One of the most puzzling aspects of Mars is that organics have not yet been found on the surface. The simplest of organic molecules, methane, was detected in the Martian atmosphere for the first time in 2003. The existence and behavior of methane on Mars is of great significance, as methane is a potential biomarker. In this paper we review our current understanding of possible sources and sinks of methane on Mars. We also investigate the role of other trace species in the maintenance and removal of methane from the atmosphere, as well as of other organic material from the surface. In particular, we examine the exogenous, hydrogeochemical—especially serpentinization—and biological sources, for supplying methane to Mars. We suggest that comets and meteorites are the least likely, whereas low-temperature serpentinization is the most plausible of all candidates to explain the methane observations. Nevertheless, it is premature to rule out the role of biology in producing methane on Mars, in view of available data. It is important to note that the loss of methane to surface must also be factored into any ‘‘source’’ scenarios for methane. Ordinary heterogeneous loss process to surface tends to be very slow. On the other hand, a reactive surface could potentially accelerate the destruction of methane. If correct, it would imply that a larger source of methane is present than currently estimated on the basis of photochemical loss alone. A reactive surface can also explain why no organic material has ever been detected on the Martian surface. The surface could become reactive if some oxidizer were present. We suggest that vast quantities of a powerful oxidant, hydrogen peroxide, can be produced in electrochemistry triggered by electrostatic fields generated in the Martian dust devils and dust storms, and in normal saltation process close to the surface. Finally, current observations are inadequate to prove or disprove the existence of life on Mars, now or in the past. The question of extraterrestrial life is a fundamental one, and it should be addressed meticulously on future missions to Mars. Measurements planned on the Mars Science Laboratory (MSL), especially carbon isotopes and chirality, will go a long way in meeting this goal. A brief overview of the MSL Mission and measurements relevant to the question of life and habitability of Mars is also presented in this paper. r 2006 Elsevier Ltd. All rights reserved. Keywords: Mars; Methane; Life; Habitability; Oxidants; Hydrogen peroxide; Electrochemistry; Triboelectricity; Dust devils; Serpentinization; Mars Science Laboratory; Sample Analysis at Mars 1. Introduction the Gemini telescope data imply substantially greater (4250 ppbv), localized amounts (Mumma et al., 2004), Trace quantities of methane gas have been detected by which is puzzling. The PFS and IRTF/Gemini data also the Planetary Fourier Spectrometer (PFS) on Mars Express indicate that methane is variable over the planet. This too (Formisano et al., 2004). The Fourier Transform Spectro- is puzzling, considering the relatively long lifetime of CH4 meter (FTS) at Canada-French-Hawaii Telescope (CFHT) on Mars (300–600 years, Wong et al., 2003; Krasnopolsky yields similar (10 ppbv) global abundance (Krasnopolsky et al., 2004). It is tantalizing to surmise that widely et al., 2004). However, a high-resolution spectrograph dispersed microbial colonies—extinct or extant—may be (CSHELL) at the Infrared Telescope Facility (IRTF) and responsible for the existence and non-uniform distribution of methane on Mars. After all, between 90% and 95% of ÃCorresponding author. Tel.: +1 734 936 0489; fax: +1 734 936 0503. methane on Earth has a biological origin, either from living E-mail address: [email protected] (S.K. Atreya). things now, organic waste, or fossilized matter, as 0032-0633/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2006.02.005 ARTICLE IN PRESS S.K. Atreya et al. / Planetary and Space Science 55 (2007) 358–369 359 illustrated in Fig. 1. For Mars, the biological source could scrub not only the methane gas from the atmosphere, viewpoint may be bolstered by the mineralogical evidence but may also destroy or transform (Benner et al., 2000; of past (liquid) water at the Mars Exploration Rover McDonald et al., 1998) any organic material that is present (MER) sites, especially Meridiani Plenum and the non- in the soil. Electrochemistry triggered by the Martian uniformly distributed subsurface ice reported by the Mars aeolian processes—dust devils, storms, and normal salta- Odyssey researchers (Feldman et al., 2002). tion—is potentially a large source of an oxidant, hydrogen A microbial source could indeed be one possible scenario peroxide (Atreya et al., 2006; Delory et al., 2006). In this for the existence and non-uniform distribution of methane paper we will summarize the current state and discuss on Mars. However, it is by no means the only one (Atreya missing links in the above story, especially the isotope data et al., 2004; Atreya and Wong, 2004; Atreya, 2005). In fact, (e.g., 12C/13C), organics, oxidants, sulfur and halogen methane production through serpentinization is a common species, and more. We will also discuss briefly the tests occurrence on Earth, and the interior of Mars poses no required for determining the existence of life on Mars, unusual challenge (Atreya et al., 2004; Atreya and Wong, together with the measurements planned by the Sample 2004; Atreya, 2005). An effective surface sink could also be Analysis at Mars (SAM) Suite on the 2009 Mars Science responsible for non-uniform CH4. Oxidizers in the surface Laboratory (MSL) Mission. wetlands, 2. Methane sources and sinks marshes enteric 21% volcanic fermentation <0.2% Methane on Mars could be produced from any one of 20% the four potential sources: CH4 hydrates 1% 1. Internal, such as volcanoes. oceans, lakes termites, 2. Exogenous, such as meteorites, comets, or interplane- 3% other biogenic tary dust particles. 15% other non-biogenic 3. Internal, such as a hydrogeochemical process involving 6% serpentinization. biomass burning 4. Biological. natural gas, 8% coal mines rice paddies We illustrate these sources in Fig. 2. 14% 12% In order to understand how realistic the source is, Fig. 1. Methane sources on Earth. Values shown in the figure were it is important to know how methane is destroyed, as it adapted largely from Paul and Clark (1996). places a constraint on the required source strength. Fig. 2. Potential sources and sinks of methane on Mars, as discussed in this paper. ARTICLE IN PRESS 360 S.K. Atreya et al. / Planetary and Space Science 55 (2007) 358–369 The conventional loss mechanism for methane on Mars is Table 1 photolysis by the solar ultraviolet light in the upper atmosphere and oxidation closer to the surface (Fig. 2). Constituent Mixing ratio This results in a photochemical lifetime (e-folding time) of (a) Composition of the Martian atmosphere a methane between 300 and 600 years (Wong et al., 2003; CO2 0.9532 À2a Formisano et al., 2004), depending upon the amount of N2 2.7 Â 10 40Ar 1.6 Â 10À2a water vapor, solar flux, and ozone. Assuming a CH4 À3a 10 O2 1.3 Â 10 photochemical lifetime of 2 Â 10 s near the surface, we CO 7 Â 10À4a 5 À2 À1 À4b estimate that a flux of 1 Â 10 molecules cm s is H2O2Â 10 À8c required to explain a CH4 mixing ratio of 10 ppbv on H2O2 4 Â 10 À8d O3 (1–80) Â 10 Mars. This implies a disk averaged source strength of À8e À1 CH4 (170.5) Â 10 4gs , which is equivalent to approximately À7 f 5 À1 À1 CH2O p5 Â 10 (tentative) 1.26 Â 10 kg yr or 126 metric ton yr . This is relati- o3 Â 10À9g vely small compared to the source strength of 36+38Ar 5.3 Â 10À6a 515 Â 106 metric ton yrÀ1 needed to maintain approxi- Ne 2.5 Â 10À6a He (1.170.4) Â 10À6h mately 1700 ppbv of methane in the Earth’s atmosphere À7a (although the destruction mechanisms on Earth are similar Kr 3 Â 10 Xe 8 Â 10À8a to Mars, the methane lifetime is shorter, 10 years, due to À5i H2 (1.570.5) Â 10 greater solar flux and the rate of oxidation on Earth). (b) Global upper limits to possible minor constituents À9j C2H2 2 Â 10 2.1. Volcanic source À7j C2H4 5 Â 10 À7j C2H6 4 Â 10 À7j Volcanoes are not a large source of methane on Earth. N2O1Â 10 À8j NO2 1 Â 10 Less than 0.2% of all methane on Earth comes from À9j NH3 5 Â 10 fumarolic sources (Fig. 1). A part of that could ultimately À7j PH3 1 Â 10 be from the fossilized matter also. A more important À8k SO2 3 Â 10 consideration is that in the terrestrial fumarolic gases, OCS 1 Â 10À8j sulfur dioxide is between 100 and 1000 times more HCl 2 Â 10À9l À7j abundant (e.g. Walker, 1977). If the composition of the H2S1Â 10 volcanic gases on Mars were similar to that on Earth, and Ratio Earth Mars if the observed 10 ppbv of methane on Mars were of volcanic origin, then one should expect on average (c) Isotope ratiom D/H 1.56 Â 10À4 (974) Â 10À4 1–10 ppmv of SO2.