Experimental Characterization of the Pyridine:Acetylene Co-crystal and Implications for Titan's Surface

TL;DR

This study demonstrates that pyridine:acetylene co-crystals are stable at Titan-like temperatures and conditions, suggesting they could form and persist on Titan's surface, providing insights into its geologic and chemical processes.

Contribution

The paper provides the first experimental evidence of pyridine:acetylene co-crystal stability under Titan-relevant conditions, including temperature, atmosphere, and liquid ethane exposure.

Findings

Co-crystal forms rapidly at 150 K via liquid-solid interactions.

Stable from 90 K to 180 K under N2 atmosphere.

Detectable after ethane exposure, indicating potential persistence on Titan.

Abstract

Titan, Saturn's largest moon, has a plethora of organic compounds in the atmosphere and on the surface that interact with each other. Cryominerals such as co-crystals may influence the geologic processes and chemical composition of Titan's surface, which in turn informs our understanding of how Titan may have evolved, how the surface is continuing to change, as well as the extent of Titan's habitability. Previous work has shown that a pyridine:acetylene (1:1) co-crystal forms under specific temperatures and experimental conditions; however, this has not yet been demonstrated under Titan-relevant conditions. Our work here demonstrates that the pyridine:acetylene co-crystal is stable from 90 K, Titan's average surface temperature, up to 180 K under an atmosphere of N2. In particular, the co-crystal forms via liquid-solid interactions within minutes upon mixing of the constituents at 150 K, as evidenced by distinct, new Raman bands and band shifts. XRD results indicate moderate anisotropic thermal expansion (about 0.5% - 1.1%) along the three principal axes between 90-150 K. Additionally, the co-crystal is detectable after being exposed to liquid ethane, implying stability in a residual ethane "wetting" scenario on Titan. These results suggest that the pyridine:acetylene co-crystal could form in specific geologic contexts on Titan that allow for warm environments in which liquid pyridine could persist, and as such, this cryomineral may preserve evidence of impact, cryovolcanism, or subsurface transport in surface materials.