Methane plume observations on Mars: Difference between revisions
→Challenges for interpreting carbon 12 /13 ratios from TGO
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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/>. However abiotic sources can sometimes have similar results, and the ratios can be modified in various ways after formation. The TGO can also measure concentrations of ethane which may help with the analysis (very low in most microbial gases), but this also is not conclusive. It's ability to localize the measurements of plumes to regions of Mars may help but it is likely any results are preliminary and to need later work to interpret them.
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.png|thumb|"
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.
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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,
== See also==
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