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

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 (lambda > 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.

Automated summary

A massive noxious stratospheric cloud has been detected on Titan's south pole since 2015, with an experimental simulation showing the presence of nitriles formed through photo-processing. These results are crucial for the upcoming Dragonfly mission on Titan, shedding light on the potential organic compounds present in the stratospheric cloud.