The Titan 14N/15N and 12C/13C isotopic ratios in HCN from Cassini/CIRS

We report the detection of (HCN)-C-13 and (HCN)-N-15 in mid-infrared spectra recorded by the Composite Infrared Spectrometer (CIRS) aboard Cassim, along with the determination of the C-12/C-13 and N-14/N-15 isotopic ratios. We analyzed two sets of limb spectra recorded near 13-15 degrees S (Tb flyby) and 83 degrees N (T4 flyby) at 0.5 cm(-1) resolution. The spectral range 1210-1310 cm(-1) was used to retrieve the temperature profile in the range 145-490 km at 13 degrees S and 165-300 km at 83 degrees N. These two temperature profiles were then incorporated in the atmospheric model to retrieve the abundance profile of (HCN)-C-12-N-14, (HCN)-C-13 and (HCN)-N-15 from their bands at 713, 706 and 711 cm(-1), respectively. The HCN abundance profile was retrieved in the range 90-460 km at 15 degrees S and 165-305 km at 83 degrees N. There is no evidence for vertical variations of the isotopic ratios. Constraining the isotopic abundance profiles to be proportional to the HCN one, we find C-12/C-13 = 89(-18)(+22) at 15 degrees S, and 68(-12)(+16) at 83 degrees N, two values that are statistically consistent. A combination of these results yields a C-12/C-13 value equal to 75 +/- 12. This global result, as well as the 15 degrees S one, envelop the value in Titan's methane (82.3 +/- 1) [Niemann, H.B., and 17 colleagues, 2005. Nature 438, 779-784] measured at 10 degrees S and is slightly lower than the terrestrial inorganic standard value (89). The N-14/N-15 isotopic ratio is found equal to 56(-13)(+16) at 15 degrees S and 56(-9)(+10) at 83 degrees N. Combining the two values yields N-14/N-15 = 56 +/- 8, which corresponds to an enrichment in N-15 of about 4.9 compared with the terrestrial ratio. These results agree with the values obtained from previous ground-based millimeter observations [Hidayat, T., Marten, A., Bezard, B., Gautier, D., Owen, T., Matthews, H.E., Paubert, G., 1997. Icarus 126, 170-182; Marten, A., Hidayat, T., Biraud, Y., Moreno, R., 2002. Icarus 158, 532-544]. The N-15/N-14 ratio found in HCN is similar to 3 times higher than in N-2 [Niemann, H.B., and 17 colleagues, 2005. Nature 438, 779-784], which implies a large fractionation process in the HCN photochemistry. (C) 2007 Elsevier Inc. All rights reserved.