Titan's Carbon Conundrum

Details Titan's Carbon Conundrum Titan's Carbon Conundrum

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p11a1329n&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #P11A-1329

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[5405] Planetary Sciences: Solid Surface Planets / Atmospheres, [5455] Planetary Sciences: Solid Surface Planets / Origin And Evolution, [5464] Planetary Sciences: Solid Surface Planets / Remote Sensing, [6281] Planetary Sciences: Solar System Objects / Titan

Scientific paper

As recently as a year ago, a consensus was emerging that carbon-13 in Titan's methane was enriched by some ~10% over the terrestrial value (12C/13C = ~77-82 on Titan versus 89 on Earth, Niemann et al 2005, Nixon et al 2008). At the same time, several measurements of 12C/13C in ethane, the main product of methane photolysis, appeared to show no enrichment (Nixon et al 2008, Jennings et al 2009). This led to the suggestion that a steady state equilibrium was being reached, with a Kinetic Isotope Effect (KIE) in a key reaction (C2H + CH4 → C2H2 + CH3) responsible for the slight enrichment in the atmospheric reservoir relative to both the incoming flux of methane and outgoing flux of ethane (Jennings et al 2009).
This paradigm was overturned earlier this year when the Huygens GCMS team revised their measurement of 12CH4/13CH4 upwards to agree with the terrestrial value (Niemann et al, in preparation), eliminating any need for the KIE fractionation. However, this presents a new problem in the sense that the KIE effect is probably real - it is confirmed for the CH3D and 12CH4 reactions with ethynyl (Opansky and Leone 1996), so almost certainly for 13CH4-12CH4 pair as well - and so some fractionation of methane should be occurring. This is true regardless as to whether the atmospheric methane is being replenished or not - differing only in degree - provided the ethynyl abstraction reaction is the dominant path for methane loss as predicted by current models (Lavvas et al. 2008).
In this forum we will present updated measurements by the CIRS team of the 12CH4/13CH4 derived from recent high signal-to-noise Titan observations, and discuss the degree of agreement with both the earlier published ratios, and the newer revised GCMS results. We will also discuss the implications for Titan's methane evolution over geologic time including clues from the D/H ratio. We conclude by highlighting the currently open questions and avenues for future work.
Jennings, D.E. et al., J. Chem. Phys., 113(42), 11101-11106, 2009. Lavvas, P.P. et al., Plan. Space Science, 56, 27-66, 2008. Niemann, H.B. et al., Nature, 438, 779-784, 2005. Nixon, C.A. et al., Icarus, 195, 778-791, 2008. Opansky, B.J and S.R. Leone, J. Phys. Chem., 100, 4888-4892, 1996.

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