Experimental Models of Extraterrestrial Minerals: Propionitrile Ice

Abstract:

Titan, Saturn’s largest moon, is the only body in the solar system known to have stable bodies of liquid—lakes, rivers, and seas—that undergo dynamic processes similar to Earth’s hydrological cycle. To study the formation of potential minerals on Titan, we analyze structural information of the constituents present on Titan’s surface. Among them is propionitrile, a compound that, until January 2020, lacked a reported crystal structure. Though a structure was recently published, our calorimetric data and synchrotron powder diffraction data both indicate the presence of two additional high-temperature phases. We perform structure solution of this compound using X-ray diffraction techniques, supported by single-crystal diffraction, neutron diffraction, differential scanning calorimetry, Raman spectroscopy, and theoretical calculations. Our studies confirm the crystal structures of these two additional phases and demonstrate the occurrence of appreciable dynamics of propionitrile below its freezing point, providing compelling evidence for the existence of dynamic and, potentially, static disorder within the crystal system. This ongoing structural characterization will allow us to shed light on the physicochemical properties of propionitrile, which, in turn, will aid future investigations into potential cocrystals and minerals involving this elusive nitrile ice.