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Detection of C2HD and the D/H ratio on Titan |
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| Authors: | Athena Coustenis Donald E Jennings Yves Bénilan Sandrine Vinatier Gordon L Bjoraker Ronald C Carlson |
| Institution: | a Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique (LESIA), Observatoire de Paris-Meudon, 5, Place Jules Janssen, 92195 Meudon Cedex, France b NASA/Goddard Space Flight Center, Code 693, Greenbelt, MD 20771, USA c LISA, CNRS, Univ. Paris 12 and Paris 7, 94010 Créteil Cedex, France d Department of Astronomy, University of Maryland, College Park, MD 20742, USA e IACS, Catholic University of America, and NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA |
| Abstract: | We report here the first detection of mono-deuterated acetylene (acetylene-d1, C2HD) in Titan's atmosphere from the presence of two of its emission bands at 678 and 519 cm−1 as observed in CIRS spectral averages of nadir and limb observations taken between July 2004 and mid-2007. By using new laboratory spectra for this molecule, we were able to derive its abundance at different locations over Titan's disk. We find the C2HD value ( ) to be roughly constant with latitude from the South to about 45° N and then to increase slightly in the North, as is the case for C2H2. Fitting the 678 cm−1ν5 band simultaneously with the nearby C2H2 729 cm−1ν5 band, allows us to infer a D/H ratio in acetylene on Titan with an average of the modal values of 2.09±0.45×10−4 from the nadir observations, the uncertainties being mainly due to the vertical profile used for the fit of the acetylene band. Although still subject to significant uncertainty, this D/H ratio appears to be significantly larger than the one derived in methane from the CH3D band (upper limit of 1.5×10−4; Bézard, B., Nixon, C.A., Kleiner, I., Jennings, D.E., 2007. Icarus, 191, 397-400; Coustenis, A., Achterberg, R., Conrath, B., Jennings, D., Marten, A., Gautier, D., Bjoraker, G., Nixon, C., Romani, P., Carlson, R., Flasar, M., Samuelson, R.E., Teanby, N., Irwin, P., Bézard, B., Orton, G., Kunde, V., Abbas, M., Courtin, R., Fouchet, Th., Hubert, A., Lellouch, E., Mondellini, J., Taylor, F.W., Vinatier, S., 2007. Icarus 189, 35-62). From the analysis of limb data we infer D/H values of  (at 54° S),  (at 15° S),  (at 54° N) and  (at 80° N), which average to a mean value of 1.63±0.27×10−4. |
| Keywords: | Titan Atmospheres composition Infrared observations Satellites atmospheres Satellites composition |
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