Abstract
Abstract Benzene ice contributes to an emission feature detected by the Cassini Composite InfraRed Spectrometer (CIRS) near 682 cm−1 in Titan’s late southern fall polar stratosphere. It is also one of the dominant components of the CIRS-observed High-Altitude South Polar ice cloud observed in Titan’s mid stratosphere during late southern fall. Titan’s stratosphere exhibits significant seasonal changes with temperatures that spatially vary with seasons. A quantitative analysis of the chemical composition of infrared emission spectra of Titan’s stratospheric ice clouds relies on consistent and detailed laboratory transmittance spectra obtained at numerous temperatures. However, there is a substantial lack of experimental data on the spectroscopic and optical properties of benzene ice and its temperature dependence, especially at Titan-relevant stratospheric conditions. We have therefore analyzed in laboratory the spectral characteristics and evolution of benzene ice’s vibrational modes at deposition temperatures ranging from 15 to 130 K, from the far- to mid-IR spectral region (50–8000 cm−1). We have determined the amorphous-to-crystalline phase transition of benzene ice and identified that a complete crystallization is achieved for deposition temperatures between 120 and 130 K. We have also measured the real and imaginary parts of the ice complex refractive index of benzene ice from 15 to 130 K. Our experimental results significantly extend the current state of knowledge on the deposition temperature dependence of benzene ice over a broad infrared spectral range, and provide useful new data for the analysis and interpretation of Titan-observed spectra.
Highlights
Benzene (C6H6), the simplest aromatic hydrocarbon, is a molecule that has raised great interests in the astrophysical community for almost four decades
Laboratory astrophysical investigations have mostly focused on performing vibrational spectroscopy of ion, electron or UV irradiated C6H6 gas and C6H6 ice to provide data on the spectral properties of the irradiated C6H6 materials, compare them with spectra obtained from astronomical observations, or study photo-processed benzene ices to understand the fate of benzene ices in Titan's stratosphere and help understanding the formation of aerosols analogs observed in Saturn moon’s stratosphere (Mouzay et al 2021)
All the original data files of the optical constants as well as the absorbance spectra obtained in this study for C6H6 ice at the different deposition temperatures will be made available on the NASA website https://science.gsfc.nasa.gov/691/spicelab
Summary
Benzene (C6H6), the simplest aromatic hydrocarbon, is a molecule that has raised great interests in the astrophysical community for almost four decades. The formation and evolution of benzene in planetary environments or other Solar System objects, represents a fundamental primary stage of the PAHs production and other subsequent relevant chemical and prebiotic processes (like soot formation) In this context, several works related to benzene have been devoted to better understand the physico-chemical processes of irradiated C6H6, in its gaseous and solid phases, and the derived products, by acquiring highresolution astronomical spectra, carrying out detailed laboratory studies or developing theoretical modelling (Allamandola et al 1989 and references therein; Callahan et al 2013; Materese et al 2015; Mouzay et al 2021). Direct vapor deposition studies (as opposed to annealing experiments) are almost nonexistent and are crucial to identify the chemical composition of Titan’s CIRS-observed stratospheric ice clouds
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