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 organic molecules, rather than rocks and minerals that make up Earth’s surface. At a surface temperature of ~92 K, the non-covalent interactions are sufficiently strong to enable stable interactions among these organic molecules, which form an entirely new class of cryogenic organic minerals (naturally occurring compounds with a specific composition). Simple organic molecules 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 temperature, are ambiguous. In addition, many of these materials have order/disorder transitions with significant volume changes that may have implications for geological processes.

We will present new data on the crystal structures and physical properties of two molecules thought to be present in significant quantities on Titan’s surface: hydrogen cyanide (HCN) and butadiene (C₄H₆). 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 temperature is unexplored. Our data indicate a new structure for HCN at low temperature, and we will also report a structure for crystalline butadiene. Finally, we will discuss the implications of these results for Titan’s geology and evolution.