Experimental investigations of diacetylene ice photochemistry in Titan’s atmospheric conditions

¹ Université Paris Cité and Univ Paris Est Creteil, CNRS, LISA, 75013 Paris, France
e-mail: benjamin.fleury@lisa.ipsl.fr
² Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
³ Aix-Marseille Université, CNRS, PIIM, UMR 7345, 13013 Marseille, France
e-mail: isabelle.couturier@univ-amu.fr

Received: 17 November 2023
Accepted: 31 January 2024

Abstract

Context. A large fraction of the organic species produced photochemically in the atmosphere of Titan can condense to form ice particles in the stratosphere and in the troposphere. According to various studies, diacetylene (C₄H₂) condenses below 100 km where it can be exposed to ultraviolet radiation.

Aims. We studied experimentally the photochemistry of diacetylene ice (C₄H₂) to evaluate its potential role in the lower altitude photochemistry of Titan’s atmospheric ices.

Methods. C₄H₂ ice films were irradiated with near-ultraviolet (near-UV) photons (λ > 300 nm) with different UV sources to assess the impact of the wavelengths of photons on the photochemistry of C₄H₂. The evolution of the ice’s composition was monitored using spectroscopic techniques.

Results. Our results reveal that diacetylene ice is reactive through singlet-triplet absorption, similar to the photochemistry of other organic ices of Titan (such as dicyanoacetylene C4N2 ice) that we investigated previously. Several chemical processes occurred during the photolysis: the hydrogenation of C₄H₂ to form other C₄ hydrocarbons (vinylacetylene C₄H₄ to butane C₄H₁₀); the formation of larger and highly polymerizable hydrocarbons, such as triacetylene (C₆H₂); and the formation of an organic polymer that is stable at room temperature.

Conclusions. The nondetection of diacetylene ice in Titan’s atmosphere or surface could be rationalized based on our experimental results that C₄H₂ is photochemically highly reactive in the solid phase when exposed to near-UV radiation that reaches Titan’s lower altitudes and surface. C₄H₂ may be one of the key molecules promoting the chemistry in the ices and aerosols of Titan’s haze layers, especially in the case of co-condensation with other organic volatiles, with which it could initiate more complex solid-phase chemistry.