The 12 C/ 13 C isotopic ratio in Titan hydrocarbons from Cassini/CIRS infrared spectra

doi:10.1016/j.icarus.2008.01.012

Authors

Nixon, Conor A.; Achterberg, Richard K.; Vinatier, Sandrine; Bézard, Bruno; Coustenis, Athéna; Irwin, Patrick G. J.; Teanby, Nicholas A.; de Kok, Remco; Romani, Paul N.; Jennings, Donald E.; Bjoraker, Gordon L.; Flasar, Michael + 8

Contributors

Department of Astronomy, University of Maryland; Laboratoire d'études spatiales et d'instrumentation en astrophysique (LESIA); Université Pierre et Marie Curie - Paris 6 (UPMC)-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire de Paris; Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Université Paris Diderot - Paris 7 (UPD7)-Centre National de la Recherche Scientifique (CNRS); Pôle Planétologie du LESIA; Laboratoire d'études spatiales et d'instrumentation en astrophysique = Laboratory of Space Studies and Instrumentation in Astrophysics (LESIA); Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire de Paris; Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Université Paris Cité (UPCité)-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire de Paris; Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Université Paris Cité (UPCité); Department of Atmospheric, Oceanic and Planetary Physics Oxford (AOPP); University of Oxford; NASA/Goddard Space Flight Center (NASA/GSFC) + 11

Publisher

HAL CCSD; Elsevier

Abstract

International audience ; We have analyzed infrared spectra of Titan recorded by the Cassini Composite Infrared Spectrometer (CIRS) to measure the isotopic ratio 12 C/ 13 C in each of three chemical species in Titan's stratosphere: CH 4 , C 2 H 2 and C 2 H 6 . This is the first measurement of 12 C/ 13 C in any C 2 molecule on Titan, and the first measurement of 12 CH 4 / 13 CH 4 (non-deuterated) on Titan by remote sensing. Our spectra cover five widely-spaced latitudes, 65° S to 71° N and we have searched for both latitude variability of 12 C/ 13 C within a given species, and also for differences between the 12 C/ 13 C in the three gases. For CH 4 alone, we find C12/C13=76.6±2.7 (1- sigma), essentially in agreement with the 12 CH 4 / 13 CH 4 measured by the Huygens Gas Chromatograph/Mass Spectrometer instrument (GCMS) [Niemann, H.B., and 17 colleagues, 2005. Nature 438, 779-784]: 82.3±1.0, and also with measured values in H 13 CN and 13 CH 3 D by CIRS at lower precision [Bézard, B., Nixon, C., Kleiner, I., Jennings, D., 2007. Icarus 191, 397-400; Vinatier, S., Bézard, B., Nixon, C., 2007. Icarus 191, 712-721]. For the C 2 species, we find C12/C13=84.8±3.2 in C 2 H 2 and 89.8±7.3 in C 2 H 6 , a possible trend of increasingly value with molecular mass, although these values are both compatible with the Huygens GCMS value to within error bars. There are no convincing trends in latitude. Combining all fifteen measurements, we obtain a value of C12/C13=80.8±2.0, also compatible with GCMS. Therefore, the evidence is mounting that 12 C/ 13 C is some 8% lower on Titan than on the Earth (88.9, inorganic standard), and lower than typical for the outer planets ( 88±7 [Sada, P.V., McCabe, G.H., Bjoraker, G.L., Jennings, D.E., Reuter, D.C., 1996. Astrophys. J. 472, 903-907]). There is no current model for this enrichment, and we discuss several mechanisms that may be at work.