New benzene absorption cross sections in the VUV, relevance for Titan’s upper atmosphere

• Published in: Icarus (New York, N.Y. 1962) Vol. 265; pp. 95 - 109
• Main Authors: Capalbo, Fernando J., Bénilan, Yves, Fray, Nicolas, Schwell, Martin, Champion, Norbert, Es-sebbar, Et-touhami, Koskinen, Tommi T., Lehocki, Ivan, Yelle, Roger V.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01-02-2016; Elsevier
• Subjects: Absorption cross sections; Abundance; Astrophysics; Atmospheres, composition; Atmospheric and Oceanic Physics; Benzene; Capture cross sections; Earth and Planetary Astrophysics; Instrumentation and Detectors; Instrumentation and Methods for Astrophysic; Occultations; Ocean, Atmosphere; Physics; Saturn satellites; Sciences of the Universe; Spectroscopy; Titan; Titan, atmosphere; Ultraviolet; Ultraviolet observations; Upper atmosphere; Occultations; Spectroscopy; Atmospheres, composition; Ultraviolet observations; Titan, atmosphere; Atmospheres: composition; Titan: atmosphere
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2015.10.006

•We measured benzene absorption cross sections in the temperature range 215–298K.•We presented the first analysis of temperature dependency, which is small.•The cross sections were used to analyze 4 stellar occultations measured by UVIS.•We retrieved 4 density profiles of benzene in the upper atmosphere of Titan.•Our results indicate that photochemical models do not produce enough benzene.
Benzene is an important molecule in Titan’s atmosphere because it is a potential link between the gas phase and the organic solid phase. We measured photoabsorption in the ultraviolet by benzene gas at temperatures covering the range from room temperature to 215K. We derived benzene absorption cross sections and analyzed them in terms of the transitions observed. No significant variation with measurement temperature was observed. We discuss the implications of our measurements for the derivation of benzene abundance profiles in Titan’s thermosphere, by the Cassini/Ultraviolet Imaging Spectrograph (UVIS). The use of absorption cross sections at low temperature is recommended to avoid small systematic uncertainties in the profiles. We used our measurements, together with absorption cross sections from other molecules, to analyze four stellar occultations by Titan, measured by UVIS during flybys T21, T41, T41_II, and T53. We derived and compared benzene abundance profiles in Titan’s thermosphere between approximately 530 and 1000km, for different dates and geographical locations. The comparisons of our benzene profiles with each other, and with profiles from models of the upper atmosphere, point to a complex behavior that is not explained by current photochemical models.