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

Abstract

We report the detection of H 13CN and HC 15N in mid-infrared spectra recorded by the Composite Infrared Spectrometer (CIRS) aboard Cassini, along with the determination of the 12C/ 13C and 14N/ 15N isotopic ratios. We analyzed two sets of limb spectra recorded near 13–15° S (Tb flyby) and 83° 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° S and 165–300 km at 83° N. These two temperature profiles were then incorporated in the atmospheric model to retrieve the abundance profile of H 12C 14N, H 13CN and HC 15N 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° S and 165–305 km at 83° 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° S, and 68 −12 +16 at 83° N, two values that are statistically consistent. A combination of these results yields a 12C/ 13C value equal to 75 ± 12 . This global result, as well as the 15° 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° S and is slightly lower than the terrestrial inorganic standard value (89). The 14N/ 15N isotopic ratio is found equal to 56 −13 +16 at 15° S and 56 −9 +10 at 83° N. Combining the two values yields 14N/ 15N = 56 ± 8, which corresponds to an enrichment in 15N 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., Bézard, 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 15N/ 14N ratio found in HCN is ∼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.