Abstract
Aims. The aim of this study is to measure the vertical distribution of HCN on Titan’s stratosphere using ground-based submillimetre observations acquired quasi-simultaneously with the Herschel ones. This allows us to perform a consistency check between space and ground-based observations and to build a reference mean HCN vertical profile in Titan’s stratosphere. Methods. Using APEX and IRAM 30-m, we obtained the spectral emission of HCN (4-3) and (3-2) lines. Observations were reduced with GILDAS-CLASS. We applied a line-by-line radiative transfer code to calculate the synthetic spectra of HCN, and a retrieval algorithm based on optimal estimation to retrieve the temperature and HCN vertical distributions. We used the standard deviation-based metric to quantify the dispersion between the ground-based and Herschel HCN profiles and the mean one. Results. Our derived HCN abundance profiles are consistent with an increase from 40 ppb at ~100 km to 4 ppm at ~200 km, which is an altitude region where the HCN signatures are sensitive. We also demonstrate that the retrieved HCN distribution is sensitive to the data information and is restricted to Titan’s stratosphere. The HCN obtained from APEX data is less accurate than the one from IRAM data because of the poorer data quality, and covers a narrower altitude range. Comparisons between our results and the values from Herschel show similar abundance distributions, with maximum differences of 2.5 ppm ranging between 100 and 300 km in the vertical range. These comparisons also allow us to inter-validate both data sets and indicate reliable and consistent measurements. The inferred abundances are also consistent with the vertical distribution in previous observational studies, with the profiles from ALMA, Cassini/CIRS, and SMA (the latest ones below ~230 km). Our HCN profile is also comparable to photochemical models by Krasnopolsky (2014) and Vuitton et al. (2019) below 230 km and consistent with that of Loison et al. (2015) above 250 km. However, it appears to show large differences with respect to the estimates by Loison et al. (2015), Dobrijevic & Loison (2018), and Lora et al. (2018) below 170 km, and by Dobrijevic & Loison (2018) and Lora et al. (2018) above 400 km, although they are similar in shape. We conclude that these particular photochemical models need improvement.
Highlights
The atmosphere of Titan, one of the moons of Saturn, is cold, dense, and nitrogen (N2)-dominated, and exhibits a great diversity of molecules and a complex atmospheric chemistry
We carried out complementary Atacama Pathfinder Experiment (APEX) and Institut de Radioastronomie Millimetrique (IRAM) 30-m Hydrogen cyanide (HCN) (4-3) and (3-2) line observations, respectively, in Titan’s atmosphere around the times of Herschel/Photodetector Array Camera and Spectrometer (PACS) and Spectral and Photometric REceiver (SPIRE) observations, and measured the HCN abundance using a retrieval algorithm based on optimal estimation
The quality and coverage of these data are sufficient for us to make a precise determination of the HCN abundance in the atmosphere of Titan at altitudes of 100-150 km and 80-180 km from APEX and IRAM data, respectively
Summary
The atmosphere of Titan, one of the moons of Saturn, is cold, dense, and nitrogen (N2)-dominated, and exhibits a great diversity of molecules and a complex atmospheric chemistry. HCN is generated photochemically in Titan’s atmosphere from reactions of hydrocarbon radicals with atomic nitrogen HCN composition in Titan’s stratosphere has been investigated based on a limited number of high-resolution submillimetre observations performed on June 23 and December 15, 2010, with the Herschel Space Observatory (Pilbratt et al 2010) using the Photodetector Array Camera and Spectrometer (PACS) (Poglitsch et al 2010), and on July 16, 2010, using the Spectral and Photometric REceiver (SPIRE) (Griffin et al 2010), within the framework of the guaranteed time key programme "Water and related chemistry in the Solar System" (HssO) (Hartogh et al 2011). The accuracy of the measurements is assessed through comparisons with previous, correlative results from Herschel and the literature, and we present a mean HCN profile obtained from our ground-based observations and the Herschel ones. Fiducial reference HCN abundances for atmospheric studies of Titan-like exoplanets are needed, and studies assessing whether or not these data sets are suitable for such purposes are essential
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