Properties and Behavior of the Acetonitrile-Acetylene Co-Crystal under Titan Surface Conditions

Titan, the largest satellite of Saturn, possesses a complex photochemical cycle producing a broad inventory of organic molecules in its thick atmosphere and on its surface. Two of the most common molecules in this inventory include acetylene (C2H2) and acetonitrile (CH3CN). We have previously demonstrated that certain organic molecules (such as benzene and ethane) readily form co-crystals under Titan-relevant conditions. We report here Raman spectroscopic and X-ray diffraction evidence for a co-crystal between acetonitrile and acetylene at Titan surface conditions. This molecular mineral could be relatively abundant on Titan, in particular in the large fluvially-dissected plateaux of labyrinth terrains and possibly the undifferentiated plains, which are believed to be the end-stage product of the labyrinth terrains. Co-crystals might provide a mechanism for storing energy-rich molecules such as acetylene, permitting sequestration and transport to the subsurface where these deposits could be accessed by putative life.