P067-0010
The Acetonitrile-Acetylene Co-Crystal: A New Mineral for Titan’s Surface

Tuesday, 15 December 2020
Poster
Morgan L Cable1, Tuan Hoang Vu1, Michael Malaska2, Mathieu Choukroun3, Helen Elizabeth Maynard-Casely4 and Robert P Hodyss3, (1)NASA Jet Propulsion Laboratory, Pasadena, CA, United States, (2)Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, United States, (3)Jet Propulsion Laboratory, Pasadena, CA, United States, (4)Australian Nuclear Science and Technology Organization, Kirrawee, NSW, Australia
Abstract:
Titan hosts a multitude of organic molecules and is considered a prebiotic chemical laboratory on a planetary scale. Photochemistry in the atmosphere creates a chemical cascade, dissociating N2 and CH4 and forming a variety of molecules, including acetonitrile (CH3CN) and acetylene (C2H2). We have demonstrated previously that some organic molecules readily form co-crystals in Titan-relevant conditions, including acetylene. These molecular minerals represent an exciting new class of compounds for Titan’s surface. We report here characterization of a new co-crystal between acetonitrile and acetylene that may comprise the vast plains, dunes, and labyrinth terrains of Titan.

We have studied the acetonitrile-acetylene co-crystal with Raman spectroscopy and powder X-ray diffraction. Red and blue shifts up to 9.3 cm-1 are observed in most prominent vibrational modes of both molecules. These shifts are similar in magnitude to those observed for other Titan-relevant co-crystals and clathrate hydrates. The co-crystal forms within minutes at 90 K, and is stable up to 150-170 K, suggesting that it would be stable under Titan surface and subsurface conditions.

Co-crystals may influence Titan surface material characteristics such structural hardness and resistance to erosion. Molecular dynamics simulations suggest that acetonitrile forms aggregates when exposed to liquid methane/ethane, which suggests that deposits of pure acetonitrile might form at the base of a Titan lake or river delta as these clusters grow and precipitate. Delivery of acetylene-laden liquid could allow co-crystal formation as the soluble and insoluble materials would mix intimately. The acetonitrile-acetylene co-crystal could therefore be a major component of organic geological units such as labyrinth terrains, or other locations where mixed organic molecules were deposited and later mineralized/lithified to form landscape features such as channel walls, lithified plains, or cemented dunes.

<< Previous Abstract | Next Abstract >>