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 C12/C13=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.