  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 (C$_4$H$_2$)
condenses below 100 km where it can be exposed to ultraviolet radiation. We
studied experimentally the photochemistry of diacetylene ice (C$_4$H$_2$) to
evaluate its potential role in the lower altitude photochemistry of Titan's
atmospheric ices. Methods. C$_4$H$_2$ ice films were irradiated with
near-ultraviolet (near-UV) photons ( {\lambda} > 300 nm) with different UV
sources to assess the impact of the wavelengths of photons on the
photochemistry of C$_4$H$_2$. The evolution of the ice's composition was
monitored using spectroscopic techniques. 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
C$_4$N$_2$ ice) that we investigated previously. Several chemical processes
occurred during the photolysis: the hydrogenation of C$_4$H$_2$ to form other
C$_4$ hydrocarbons (vinylacetylene C$_4$H$_4$ to butane C$_4$H$_{10}$); the
formation of larger and highly polymerizable hydrocarbons, such as triacetylene
(C$_6$H$_2$); and the formation of an organic polymer that is stable at room
temperature. The nondetection of diacetylene ice in Titan's atmosphere or
surface could be rationalized based on our experimental results that C$_4$H$_2$
is photochemically highly reactive in the solid phase when exposed to near-UV
radiation that reaches Titan's lower altitudes and surface. C$_4$H$_2$ 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.
