Benzene-Ethane Co-Crystals on the Surface of Titan

Tuesday, 16 December 2014
Robert P Hodyss1, Tuan Vu1, Morgan L Cable1, Helen Elizabeth Maynard-Casely2, Michael J Malaska1 and Patricia M Beauchamp1, (1)Jet Propulsion Laboratory, Pasadena, CA, United States, (2)Bragg Institute, Australian Nuclear Science and Technology Organization, Kirawee DC NSW, Australia
Benzene is found at high abundance in Titan’s atmosphere and is a likely constituent of evaporite deposits formed around the hydrocarbon lakes. This work aims to understand the composition and nature of the surface evaporites by focusing on the interaction between benzene and ethane, a principal component of the lake fluids. We have discovered a new benzene-ethane co-crystalline structure which forms under Titan-like conditions (90-150 K and 1 bar), resulting in recrystallization of the benzene lattice that can be detected via micro-Raman spectroscopy. Evidence for ethane incorporation includes two new distinctive ethane features at 2873 and 1455 cm-1 and marked red shifts of the benzene peaks in the Raman spectra. Vibrational analysis reveals a C-Hπ interaction between the aromatic ring of benzene and the hydrogen atoms of ethane through a monodentate contact. The kinetics of co-crystal formation is also determined, giving a relatively mild activation energy of 10.2 kJ/mol. It is shown that the formation process would reach completion in ~18 hours, and that benzene precipitates selectively as the co-crystal from a mixture of liquid ethane and methane. Synchrotron powder X-ray diffraction data confirms the crystalline nature of the new material. These results imply that benzene and similar organics may act as potential hydrocarbon reservoirs due to this incorporation mechanism. These novel structures represent a new class of materials for Titan’s surface that may influence evaporite characteristics, such as particle size and infrared spectral properties.