Photochemistry of benzene (C6H6) hydrogen cyanide (HCN) co-condensed ices part 1: A source of solid-state production of volatile nitrile compounds in Titan's stratosphere

Abstract

Since 2015, during northern spring, a massive “noxious” stratospheric cloud has been detected at 250 km of altitude at the south pole of Titan, which persisted until the last targeted Cassini's flybys in July 2016. Its chemical composition seems to be the result of a co-condensation process of benzene and hydrogen cyanide in a 4:1 mixing ratio. Since, its altitude of detection allows to undergo long-UV solar radiations known to trigger photochemical aging processes, we experimentally simulated its evolution under stratospheric-like radiation conditions. The ice photo-processing (λ > 200 nm) led to the detection of nitriles by infrared spectroscopy such as acetonitrile (CH3CN), benzonitrile (C6H5CN) as well as a partial identification of propionitrile (CH3CH2CN) and acrylonitrile (C2H3CN). Gas chromatography coupled to mass spectrometry (GC–MS) analyses of the volatile fraction released in the gas phase during the warming of the photo-processed ice have been confirmed by previous assignments and provided the detection of several other hydrocarbons ranging from C3 to C8. These experimental results are of prime interest in the context of the future Dragonfly mission. They provide a list of nitrile derivatives that can be produced from this stratospheric cloud. Indeed, they may contribute at the end to the organic layer that recovers Titan's surface and would be analyzed by the mass spectrometer (Dra-MS) of this space mission.