The Acetylene-Ammonia Co-crystal on Titan

Titan, the largest moon of Saturn, likely supports a rich organic mineralogy that we are only beginning to understand. Photochemistry in the upper atmosphere generates a complex array of organic molecules from the simple precursors N2 and CH4. These organics continue to react and combine, forming aerosol layers and ultimately depositing on the surface. Organics are transported via pluvial (rain) and fluvial (rivers/flooding) processes into the methane-based hydrocarbon lakes, where evaporation of volatile liquids can create evaporite deposits of remnant dissolved molecules. Within such deposits, chemical and physical processes may be occurring even at low temperatures. We have demonstrated in previous work that benzene and ethane rapidly form a unique and stable co-crystal at Titan surface temperatures (90–95 K), akin to a salt on Earth, where the weak van der Waals interactions in the benzene-ethane co-crystal are analogous to the ionic bonds in a salt. Here, we report the formation of a second co-crystal between acetylene and ammonia, which forms even more quickly and is stable through anticipated conditions of Titan “rain” events of liquid methane, ethane, and propane. Such co-crystals represent an exciting new class of possible minerals for Titan’s surface and may be responsible for processes such as selective sequestration and storage of species as well as having new properties for construction and erosive resistance of geological materials.