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

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.