Herschel/PACS spectroscopy of trace gases of the stratosphere of Titan

Abstract: Aims: We investigate the composition of Titan's stratosphere from new medium-resolution far-infrared observations performed with the full range of Herschel's Photodetector Array Camera and Spectrometer (PACS) (51-220 $\mu$m at a resolution $\lambda$/$\Delta \lambda$ ranging from 950 to 5500 depending on wavelength and grating order). Methods: Using PACS, we obtained the spectral emission of several features of the Titan's stratosphere. We used a line-by-line radiative transfer code and the least-squares fitting technique to infer the abundances of the trace constituents. Results: Numerous spectral features attributable to CH$_4$, CO, HCN, and H$_2$O are present. From the flux density spectrum measured and by a detailed comparison with synthetic spectra, we constrain the stratospheric abundance of CH4, which is assumed to be constant with altitude, to be 1.29 $\pm$ 0.03%. Similarly, we constrain the abundance of CO to be 50 $\pm$ 2 ppm, and the HCN vertical distribution consistent with an increase from 40 ppb at $\sim$100 km to 4 ppm at $\sim$200 km, which is an altitude region where the HCN signatures are sensitive. Measurements of three H$_2$O rotational lines confirm the H$_2$O distribution profile recently obtained with Herschel. Furthermore, we determine the isotopic ratios $^{12}$C/$^{13}$C in CO and HCN to be 124 $\pm$ 58, and 66 $\pm$ 35, respectively. Comparisons between our results and the values derived with other instruments show that our results are consistent with the vertical distributions and isotopic ratios in previous studies, except for the HCN distribution obtained with Cassini/CIRS, which does not agree with the PACS lines at the 1-sigma confidence interval.