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Aims: We investigate the composition of Titan's stratosphere from newmedium-resolution far-infrared observations performed with the full range ofHerschel's Photodetector Array Camera and Spectrometer (PACS) (51-220 $\mu$m ata resolution $\lambda$/$\Delta \lambda$ ranging from 950 to 5500 depending onwavelength and grating order). Methods: Using PACS, we obtained the spectralemission of several features of the Titan's stratosphere. We used aline-by-line radiative transfer code and the least-squares fitting technique toinfer the abundances of the trace constituents. Results: Numerous spectralfeatures attributable to CH$_4$, CO, HCN, and H$_2$O are present. From the fluxdensity spectrum measured and by a detailed comparison with synthetic spectra,we constrain the stratospheric abundance of CH4, which is assumed to beconstant with altitude, to be 1.29 $\pm$ 0.03%. Similarly, we constrain theabundance of CO to be 50 $\pm$ 2 ppm, and the HCN vertical distributionconsistent with an increase from 40 ppb at $\sim$100 km to 4 ppm at $\sim$200km, which is an altitude region where the HCN signatures are sensitive.Measurements of three H$_2$O rotational lines confirm the H$_2$O distributionprofile recently obtained with Herschel. Furthermore, we determine the isotopicratios $^{12}$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 otherinstruments show that our results are consistent with the verticaldistributions and isotopic ratios in previous studies, except for the HCNdistribution obtained with Cassini/CIRS, which does not agree with the PACSlines at the 1-sigma confidence interval.

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