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.

arXiv comment6 pages, 7 figures. Appeared for publication in A&A Volume 561, January 201

References in corpus (4)

• Herschel Space Observatory - An ESA facility for far-infrared and submillimetre astronomy
• The Photodetector Array Camera and Spectrometer (PACS) on the Herschel Space Observatory
• Submillimeter Observations of Titan: Global Measures of Stratospheric Temperature, CO, HCN, HC3N, and the Isotopic Ratios 12C/13C and 14N/15N
• Preparation for the Solar system observations with Herschel: Simulation of Jupiter observations with PACS

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• Spatial Variations in Titan's Atmospheric Temperature: ALMA and Cassini Comparisons from 2012 to 2015
• Herschel map of Saturn's stratospheric water, delivered by the plumes of Enceladus
• ${}^{14}$N/${}^{15}$N isotopic ratio in CH${}_3$CN of Titan's atmosphere measured with ALMA
• Ethane in Titan's Stratosphere from Cassini CIRS Far- and Mid-Infrared Spectra
• Ground-based HCN submillimetre measurements in Titan's atmosphere: an intercomparison with Herschel observations
• The Atmosphere of Titan in Late Northern Summer from JWST and Keck Observations