Experimental Simulation of Titan's Stratospheric Photochemistry: Benzene (C₆H₆) Ices

Abstract

Until its conclusion in September 2017, the Cassini-Huygens space mission permitted an extensive exploration of the atmosphere of Saturn's largest moon, Titan, and provided much information on its chemical composition (Coustenis et al., 2010; Hörst, 2017; Vinatier et al., 2010, and references therein). However, lots of data related to Titan's haze still remains to be analyzed, in particular the spectroscopic data which give evidence of its chemical composition complexity. The interpretation of data coming from Cassini, Voyager and ground-based observations has nevertheless provided evidence of hundreds of complex organic compounds such as hydrocarbons [ethane (C 2 H 6), acetylene (C 2 H 2), ethylene (C 2 H 4), methylacetylene (C 3 H 4), propane (C 3 H 8), propene (C 3 H 6), diacetylene (C 4 H 2)], nitriles [hydrogen cyanide (HCN), cyanoacetylene (HC 3 N), ethylcyanide (C 3 H 5 N), vinyl cyanide (C 3 H 5 N), and cyanogen (C 2 N 2)] (

Influences on Infrared Spectra of Benzene Ices for Titan, Comets, and Beyond: Annealings, Artifacts, and Isosbestic Points

“…Of more general interest is how the IR spectra of amorphous and crystalline benzene and other compounds can depend on the conditions of ice growth, post-growth thermal history, and spectral measurement. Viewed this way, the present paper links and expands on those of Mouzay et al (2021), who reported reflection-IR spectra of benzene ices after growth followed by warmings and coolings, and the study of Nna-Mvondo & Anderson (2022), who published transmission-IR spectra for ices grown at multiple temperatures, but no subsequent temperature changes for the samples.…”

“…Ishii et al (1996) also found that crystallization could be initiated by growth of an ice by vapor-phase deposition at temperatures slightly under 50 K, an observation we return to later. A third independent report of crystallization in the 50-60 K region comes from the study of Mouzay et al (2021).…”

“…These same two papers reported new results, but additional work on solid benzene remains to be done. For example, the IR-reflection arrangement adopted by Mouzay et al (2021) characterized the form of the authors' benzene ices, but the relative intensities of the spectral features shown do not match those in a transmission spectrum, and the band intensities and peak heights do not vary linearly with sample thickness or IR path length through the ice sample. Neither the ice sample's thickness nor the angle of the incident IR beam with respect to the gold-coated substrate were stated, making quantitative reproduction of the authors' work difficult.…”

“…Of particular interest here are two recent planetary-science papers presenting mid-IR spectra of solid benzene. Mouzay et al (2021) reported spectra of benzene ices obtained by condensation of benzene vapor onto a gold-coated surface at 16 K. Spectra reflected from the ice and substrate were presented at a resolution of 1 cm −1 at 16 to 130 K, with the amorphous and crystalline forms of the solid being clearly identified. In a separate study, Nna-Mvondo & Anderson (2022) condensed C 6 H 6 onto a diamond substrate at 12 temperatures from 15 to 130 K, with transmission spectra being recorded at 4 cm −1 resolution, but with no spectra shown for ices that were subsequently warmed or cooled.…”

Benzene Vapor Pressures at Titan Temperatures: First Microbalance Results

“…Some relevant benzene detections include those in meteorites, comets, the atmosphere of Titan, and the interstellar medium (Cernicharo et al 2001;Sephton 2002;Coustenis et al 2003;Schumann et al 2019). A search of the refereed astrochemical literature shows that in roughly the past year alone, at least six papers have appeared with benzene in the title (Mouzay et al 2021a(Mouzay et al , 2021b(Mouzay et al , 2021cDubois et al 2021;Nna-Mvondo & Anderson 2022;. To help understand the chemistry and physics of solid benzene in extraterrestrial environments, such as Titan's atmosphere, here we examine one of the compound's physical properties-its vapor pressure.…”