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Experimental Studies of HCN Ices with Direct Relevance to Titan’s Stratospheric Ice Clouds

Presentation #402.01 in the session Titan: Cooking with Gas.

Published onOct 20, 2022
Experimental Studies of HCN Ices with Direct Relevance to Titan’s Stratospheric Ice Clouds

HCN is the most abundant nitrile gas in Titan’s stratosphere and has been observed as the main constituent contained in numerous observed clouds. This includes two stratospheric co-condensed ice clouds observed by Cassini’s Composite InfraRed Spectrometer (CIRS; e.g., Anderson et al. 2018b), as well as Titan’s north polar hood (e.g., Samuelson and Mayo, 1991) and the Imaging Science Subsystem (ISS)-discovered south polar cloud near 300 km (West et al. 2013; 2016), both of which were confirmed to contain HCN by the Visible and Infrared Mapping Spectrometer (VIMS; Clark et al. 2010, de Kok et al. 2014, Le Mouélic et al. 2018). While it remains to be seen if the VIMS-observed clouds are purely composed of HCN or are instead comprised of a more chemically-complex ice mixture, the CIRS-observations have always revealed HCN ice to be contained within a co-condensed ice mixture, and never observed as a pure ice. Irrespective of whether Titan’s HCN-containing stratospheric clouds are comprised of pure or mixed HCN ice, it is essential that we understand the behavior of pure HCN ice when formed in a manner that mimics its formation in Titan’s stratosphere; otherwise, its chemical, optical, and physical behaviors, especially when part of a chemically-complex mixture, will never be fully understood. For example, previous studies that formed HCN ice analogs by the annealing process, which is a common experimental approach, is contradictory to the formation mechanism of ice clouds in Titan’s stratosphere. Thus, new experiments are vitally important that form the ice analogs by direct vapor deposition at a given [Titan stratospheric-relevant] temperature. Here we present a detailed experimental study on HCN ice analogs, in which we performed thin ice film transmission spectroscopy across the near- to far-IR (50 – 8000 cm-1; 0.85 – 200 μm) spectral region at temperatures between 30 and 105 K, using the SPECtroscopy of Titan-Related ice AnaLogs (SPECTRAL) Chamber (Anderson et al. 2018a), along with Raman spectroscopy and X-Ray diffraction of HCN ice as well. We also review the HCN ice experiments in the literature and compare/contrast the differences between these studies with the present work.

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