Laboratory experiments in support of the Dragonfly space mission : Simulation of the photochemical aging process of benzene-containing clouds in Titan's stratosphere

Context: After more than thirteen years of exploration, Cassini-Huygens space mission provided a large amount of data about the atmosphere of Saturn’s largest moon, Titan. It is the only satellite in the solar system to house such a diversified chemistry triggered by the dissociation of N₂and CH₄under the action of different sources of energy (electrons, ions, solar photons, …)¹that reach the highest atmospheric layer. It results in the formation of complex carbon and nitrogen-based molecules. At lower altitudes, depending on temperature profile and saturation vapor pressures variations, these same compounds condense, thereby forming icy clouds in the stratosphere. In particular, between 2013 and 2017, two distinct benzene-containing clouds have been identified by the Cassini Composite Infrared Spectrometer for the first time during the mission, at the south pole at high stratospheric altitudes that are crossed by long-UV solar photons (λ>230nm). For the highest cloud located below 300km, the spectral signature of icy benzene is mixed with the ones of other molecules unassigned yet². The second cloud detected around 250km of altitude³, comes from a more complex process consisting in the simultaneous condensation of benzene with hydrogen cyanide. Thereafter in the mission, a significant warming-up in the stratosphere was reported, contributing to the sublimation of these same ices photo-processed.

 

Aim: Laboratory experiments have demonstrated that stratospheric ices evolve photo-chemically under long-UV solar photons and contribute to the formation of polymeric materials and volatile photo-products that will subsequently sediment at the surface. This work has been realized in the context of the preparation of the future Dragonfly space mission dedicated to analyze the organic layer that recovers the surface of Titan. We have chosen to simulate experimentally the photochemical aging process undergone by these benzene-containing icy clouds to characterize the chemical composition of the polymers photo-produced - to determine if their spectroscopic signature can match the one of the stratospheric aerosols layer observed by VIMS instrument - as well as the nature of the volatile photo-products released during the warming-up of the stratosphere. To do so, we irradiated pure benzene (C₆H₆) and hydrogen cyanide (HCN) ices, first isolated and then condensed simultaneously, with a high-pressure vapor mercury lamp (λ>230nm) - energetic conditions similar to Titan’s stratospheric ones - in a high vacuum chamber. This experimental set-up⁴ is designed to characterize the solid phase via in situ FT-IR spectroscopy and the volatile photo-products by a GC-MS instrument.

 

References :

1. Waite, J. H. et al. Science 316, 870–875 (2007).
2. Vinatier, S. et al. Icarus (2017) doi:10.1016/j.icarus.2017.12.040.
3. Anderson, C. et al. in vol. 49 304.10 (2017).
4. Abou Mrad, N., Duvernay, F., Theulé, P., Chiavassa, T. & Danger, G. Anal. Chem. 86, 8391–8399 (2014).