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). Photochemical models, partially validated by Huygens surface measurements measurements and Cassini spacecraft measurements as well as Earth-based observations, allow us to make a good guess on the composition of Titan’s surface. Simple organic molecules like acetylene, hydrogen cyanide, acetonitrile, etc. in their <i>solid form</i> 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. It is highly likely that crystalline polymorphs of some of these molecules 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 intermolecular interactions and for prediction of mechanical and chemical properties – such as the ability to support deep valleys, high canyon walls, and resist erosion.<br/>We will present new data on the crystal structure and physical properties of two molecules 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 temperature is unexplored. Our new data indicates a possible new structure for HCN at low temperature, and several new phases for butadiene. We will also discuss the implications of these results for Titan’s geology and evolution.

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

ACS Spring Meeting 2021

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