Molecular solids on Titan: New insights into hydrogen cyanide and butadiene

Abstract

The Cassini-Huygens mission has revealed a wide variety of Earth-like landforms on Titan's surface: plains, mountains, dunes, lakes, seas and rivers. Titan's surface appears to be constructed from org. mols., rather than rocks and minerals that make up Earth's surface. At a surface temp. of ~92 K, the non-covalent interactions are sufficiently strong to enable stable interactions among these org. mols., which form an entirely new class of cryogenic org. minerals (naturally occurring compds. with a specific compn.). Photochem. models, partially validated by Huygens surface measurements, allow us to make a good guess on the compn. of Titan's surface. Simple org. mols. like acetylene, hydrogen cyanide, acetonitrile, etc. in their solid form are expected to be important constituents of the surface. However, many of their crystal structures and properties in solid state, at Titan relevant temp., are ambiguous. It is highly likely that cryst. polymorphs of some of these mols. are yet to be discovered. The crystal structure of a solid material is one of its most fundamental properties, and is necessary for understanding of intermol. interactions and for prediction of mech. and chem. properties - such as the ability to support deep valleys, high canyon walls, and resist erosion. We will present new data on the crystal structure and phys. properties of two mols. thought to be present in significant quantities on Titan's surface: hydrogen cyanide and butadiene. We have used Raman spectroscopy and cryogenic powder X-ray diffraction to better understand the phase behavior and structure of these materials under Titan conditions. While hydrogen cyanide is known to undergo a phase transition at ~170 K, there are currently no published crystal structures available for butadiene, and its behavior at low temp. is unexplored. Our new data indicates a new structure for HCN at low temp., and several new phases for butadiene. We will also discuss the implications of these results for Titan's geol. and evolution.