Understanding the Origin of Methane on Mars Through Isotopic and Molecular Data from Nomad (Exomars): Will There Be More Answers Or Questions? G

Scientific Workshop: “From Mars Express to ExoMars” 27–28 February 2018, ESAC Madrid, Spain

UNDERSTANDING THE ORIGIN OF METHANE ON MARS THROUGH ISOTOPIC AND MOLECULAR DATA FROM NOMAD (EXOMARS): WILL THERE BE MORE ANSWERS OR QUESTIONS? G. Etiope 1Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy

Introduction: The NOMAD spectrometer suite In addition, post-genetic alterations, such as on ExoMars Trace Gas Orbiter (TGO) can oxidation, can greatly modify the original CH4 measure the stable C isotope composition isotopic composition. On Mars, CH4 oxidation, 13 12 ( C/ C) of methane (CH4) and the concentra- especially by hydrogen peroxide in the regolith, 13 tion of ethane (C2H6) in the atmosphere of Mars. can increase the δ C value, transforming an On Earth, these parameters are known to pro- apparent “microbial” signature into an “abiotic” vide useful, although not decisive, information one (Fig.1). Isotopic fractionation during diffu- on the origin of methane, biotic (microbial or sion in low permeability rocks can instead lead thermogenic) vs. abiotic. Such information is to 12C-enrichment in the released gas. Although therefore expected from ExoMars. In particular, advection is the dominant mechanism of gas 12 it has been considered that C-enriched CH4 migration to the surface (seepage; [7]), diffusion and high CH4/C2H6 ratios on Mars may indicate steps may take place through less permeable, the existence of microbial activity, while 13C- sealing rocks met by the gas on its way to the enriched CH4 should be attributed to abiotic (in- surface. organic) gas [1,2]. But things are not exactly like that. The interpretation of isotopic and molecu- Methane/ethane ratio lar gas data is not straightforward: not on Earth, Terrestrial microbes produce CH4 and, in spe- and even less on Mars. Here, I briefly discuss cial conditions, trace amounts of C2H6. High the main interpretative problems and ambigui- C1/C2 values (>1000) are often suggestive of a ties that we will certainly face in the upcoming dominantly microbial gas. Nevertheless, ther- atmospheric measurements of Exomars. mogenic gas, produced by thermal breakdown of organic matter at relatively high temperatures Isotopic CH4 composition (over-mature source rocks), can have C1/C2 Today we know that methanes of different ori- values, similar to those of microbial gas [4]. And 13 12 13 gins may have similar C/ C (or δ C) values: abiotic gas has a wide range of C1/C2 ratios, for example, abiotic methane is not necessarily overlapping microbial and thermogenic ranges 13C-enriched and may have isotopic composi- (Fig. 1). Laboratory experiments have shown tion similar to biotic gas (Fig. 1) [3-4]. Vice that the lower the temperature of the inorganic versa, microbes in special environments can (Fischer-Tropsch Type) reaction, the lower the 12 13 produce relatively C-depleted CH4, with δ C energy for polymerization of CH4 molecules to values between -30 and -40‰, resembling form C2+ hydrocarbons [3]. Overall, the C1/C2 thermogenic gas [4]. The CH4 isotopic composi- ratio is not a reliable indicator of gas origin. tion is basically controlled by the isotopic com- Also, we must consider the possibility that Mar- position of its precursor (a carbonate or CO2, tian microbes, if they exist, could produce more considering only inorganic compounds), which ethane, relative to methane, compared to ter- on Mars may be quite variable. If CH4 derives restrial microbes. 13 from atmospheric fractionated CO2, with δ C ~ In addition, chemical and physical post-genetic +46‰ [5], it will be likely very 13C-enriched, with processes may change the original molecular 13 positive δ C-CH4 values, regardless its genetic composition of the gas. During gas migration in mechanism. If the precursor CO2 is atmospheric rocks, the C1/C2 ratio may increase because of unfractionated or has a magmatic origin similar molecular fractionation or segregation, a sort of to CO2 observed in Zagami meteorites (with filtration during advection in permeable, frac- 13 tured rocks [4]. Also, once gas has reached the δ C from -20 to 0‰; [6]), then CH4 may have δ13C values similar to those observed on Earth. atmosphere, C2H6 is more rapidly oxidised than 13 CH4, resulting in a further increase of the C1/C2 Using CO2 with δ C of -40‰, laboratory experiments of Sabatier reaction produced abiotic ratio. 13 CH with δ C as low as -140‰, resembling mi- 4 The problem of analysing methane in the crobial gas [3]. Temperature and degree of re- atmosphere action completeness influence the isotopic On Earth, we know that interpretation of the ori- composition of produced CH4. gin of CH4 produced in the subsurface strictly depends on the type of system where gas is

Workshop organized by ESA-ESAC & IAA-CSIC, with the support of the ESAC Faculty and the EU H2020 project UPWARDS Scientific Workshop: “From Mars Express to ExoMars” 27–28 February 2018, ESAC Madrid, Spain sampled and analysed (atmosphere, surface or add insight into gas origins, but in future mis- subsurface water, soil, rocks). The atmosphere, sions, direct gas detection in the Martian sub- in particular, is not the best place to study the soil, coupled with a better knowledge of subsur- origin of methane released from the ground. As face geology (type of rocks, permeability, tem- described above, a series of post-genetic altera- peratures) should reduce the interpretative un- tions may occur during seepage and, on Mars, certainties. significant isotopic and molecular fractionations are expected in the strongly oxidising atmosphere. Accordingly, the isotopic composition of CH4 and the CH4/C2H6 concentration observable by NOMAD can be totally different from those of the original gas produced in the subsurface. This problem can be mitigated by inte- grating the NOMAD observations with geological analyses of potential gas emission sites, whose location can be estimated by atmospheric circulation modelling [8]. In future mis- sions, it would be desirable that gas be analysed directly in the soil or in the subsurface [7].

Conclusion The isotopic composition of methane and the methane/ethane ratio observable by NOMAD in the Martian atmosphere may have a wide range of values; each value could reflect a number of possible genetic mechanisms (microbial, thermogenic and abiotic). In addition, the isotopic and compositional features observable in the 13 Martian atmosphere will likely be different from Figure 1: Ranges of terrestrial δ C-CH4 and those of the gas originally produced in the sub- C1/C2 values for the three classes of methane surface, and thus may not represent the genetic origin (microbial, thermogenic, abiotic), their mechanism. However, the following general possible extension to Mars (thin arrows) and potential post-genetic modifications (dashed ar- statements can be made: rows).

(1) the 13C/12C ratio of CH could be similar to 4 References: that observed on Earth only if it derives from unfractionated or magmatic CO2, or a C precursor 13 [1] Allen M., Sherwood Lollar B., Runnegar B., with negative δ C values. Oehler D.Z., Lyons J.R., Manning C.E., Sum- mers M.E., EOS, 87, 433-448, 2006. (2) If both methane and ethane are detected by [2] Atreya, S.K., Mahaffy, P.R., Wong, A.-S., NOMAD, then the gas could be abiotic, assum- Planet. Space Sci. 55, 358–369, 2007. ing that there are no ethane-producing microbes [3] Etiope G., Ionescu A., Geofluids, 15, 438- on Mars, and that there is no ancient organic 452, 2015. matter in deep sedimentary rocks that could be [4] Etiope G., Natural Gas. Encycl. of Geo- degraded by temperature. chemistry, Earth Sciences Series, Springer, 1- 5, 2017. (3) if ethane is not detected, then we may hy- [5] Webster C.R., et al., Science, 341, 260-263, pothesize that the gas is: (a) microbial, (b) 2013. abiotic or thermogenic, both molecularly frac- [6] Niles, P.B., Boynton, W.V., Hoffman, J.H., tionated, (c) thermogenic from overmature or- Ming, D.W., Hamara, D., Science 329, 1334- ganic-rich source rocks, or (d) abiotic generated 1337, 2010. at very low T. [7] Oehler D., Etiope G., Astrobiology,

http://dx.doi.org/10.1089/ast.2017.1657, 2017. In conclusion, there will be a considerable de- [8] Viscardy, S., Daerden, F., and Neary, L., gree of uncertainty regarding the origin of any Geoph. Res. Lett., 43, 1868-1875, 2016. methane detected by NOMAD. Interpreting methane-ethane data will not be easy, and probably there will be more questions than answers. Atmospheric and geological analysis can

Workshop organized by ESA-ESAC & IAA-CSIC, with the support of the ESAC Faculty and the EU H2020 project UPWARDS