Methane plume observations on Mars: Difference between revisions
→Carbon 12 / 13 ratios
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==Carbon 12 / 13 ratios==
One way Trace Gas Orbiter might help to distinguish between biogenic and abiogenic sources of methane might be to measure the carbon-12 to carbon-13 ratio, which is expressed as a percentage relative to a reference standard {{Wikipedia|δ13C}}. Methanogens produce a gas which is much richer in the lighter carbon-12 than the products of serpentization.<ref name=Baucom/>
Here carbon 12 is the light stable isotope of carbon which gets taken up preferentially by biological processes through {{Wikipedia|Kinetic fractionation|kinetic fractionation}}. The energy costs are lower if the carbon in the organism uses the lighter isotope. Carbon 13 is also stable but not so much favoured by biology. (Techy note, this is not to be confused with carbon 14 dating - carbon 14 is radioactive and unstable. Carbon 12 and 13 are both stable and don't decay at all.)
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Plants have values of from -10% or so down to -30% or less, clustering at around -13% and -28%<ref>O'Leary, Marion H. [http://sethnewsome.org/sethnewsome/EE_files/O%27Leary%201988.pdf "Carbon isotopes in photosynthesis."] Bioscience 38, no. 5 (1988): 328-336.</ref>. Algae have a similar range of values, from higher than -10% down to -30% or less. Coal and marine petroleum typically has values around -25%, terrestrial petroleum around -30%, and land plants average around -25%, but with a fair bit of variation around those figures. See figure 1 in this article.Park, Roderic, and Samuel Epstein. [https://authors.library.caltech.edu/62406/1/4259733.pdf "Metabolic fractionation of C13 & C12 in plants."] Plant Physiology 36, no. 2 (1961): 133.
==Challenges for interpreting carbon 12 /13 ratios from TGO ==
The carbon 13 can be depleted by abiotic proceses. It can also be depleted in methane produced by heating organics (thermogenic methane) and meanwhile, sometimes methane produced by life is not depleted in carbon 13. It also depends on the isotopic composition of its precursor. Magmatic carbon, for instance, might be depleted in carbon 13. Also processes that alter the methane such as oxidation by hydrogen peroxide can deplete the carbon 12 turning a possible microbial signature into one that looks abiotic. Meanwhile diffusion through permeable rock can increase the carbon 12 levels to mimic a biogenic signature - this could be the dominant process if the methane has to pass through an almost impenetrable "cap" on its way to the surface.<ref name=nomadmethane>[https://meetingorganizer.copernicus.org/EPSC2018/EPSC2018-211-3.pdf NOMAD on ExoMars Trace Gas Orbiter: status and preliminary results] Ann C. Vandaele., Jose-Juan Lopez-Moreno, GiancarloBellucci, Manish R. Patel, FrankDaerden, IanR. Thomas,Eddy Neefs, BojanRistic, Sophie Berkenbosch, Bram Beeckman, Roland Clairquin, Claudio Queirolo and the NOMAD Team, EPSC Abstracts,Vol. 12, EPSC2018-211-3, 2018, European Planetary Science Congress 2018</ref>
[[Image:NOMAD methane|thumb|"Figure 1: Ranges of terrestrial δ13C-CH4 and C1/C2 values for the three classes of methane origin (microbial, thermogenic, abiotic), their possible extension to Mars (thin arrows) and po-tential post-genetic modifications (dashed ar-rows). "<ref name=nomadmethane/>]]
The NOMAD instrument on Mars can measure the concentrations of other gases such as ethane. This could help as microbial gases typically have ethane concentrations of a thousandth or less.
So high concentrations of ethane could suggest abiotic origins. However the ratios could be different for Martian microbes, and it could be due to ancient organics degraded by temperature. Meanwhile there are also processes that can deplete the ethane on its way to the surface, through molecular fractionation and through oxidizing of the gas once it reaches the atmosphere.
TGO can localize the measurements to regions of Mars, so if the gas has higher concentration in plumes, locally then the geology may also help with interpretation.
They conclude:
{{quote|In conclusion, there will be a considerable degree of uncertainty regarding the origin of any 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 add insight into gas origins, but in future missions, direct gas detection in the Martian sub-soil, coupled with a better knowledge of subsurface geology (type of rocks, permeability, tem-peratures) should reduce the interpretative uncertainties.}}
== See also==
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