Experimental Investigation of the Acetylene-Benzene Co-Crystal on Titan

Abstract:

Acetylene (C₂H₂) and benzene (C₆H₆) are two common molecules formed in Titan’s atmosphere and are likely components of the lake evaporites. It is known that these two molecules can form a co-crystal (e.g., [1]), a molecule with a unique structure composed of a specific molar ratio of each compound. In recent years, several other Titan-relevant co-crystals with acetylene have been discovered in the laboratory [2-4]. Thus, we sought to study the formation of the acetylene-benzene co-crystal using an experimental setup that simulates Titan surface conditions (90 K, 1.5 bar).

Using a custom-built Titan chamber at the University of Arkansas [5], we condense a 1:1 ratio of benzene and acetylene onto a sample dish (90 K) within the chamber where the compounds are sequentially deposited under a 1.5 bar N₂ atmosphere. We use FTIR spectroscopy (Nicolet 6700, 1-2.5 µm) and optical cameras to characterize spectral absorptions, band positions, band shifts, and the morphology of the sample.

Our results indicate that the co-crystal forms within minutes at ~135 K (orthorhombic phase of acetylene is stable) and is stable down to 90 K. Upon co-crystal formation, we observe new spectral bands from 1.569 to 1.598 µm (vertical lines in figure), and from 1.943 to 2.122 µm, which are neither present in pure acetylene nor pure benzene. We also observe red and blue shifts of these vibrational modes that agree with previous results of IR band shifts of the acetylene-benzene co-crystal. Drastic changes in sample morphology observed here are another indicator of co-crystal formation.

These findings can be applied to co-condensation processes in Titan’s atmosphere, as well as the ongoing effort to better characterize the composition and spectral properties of Titan’s lake evaporites. Knowledge of these spectral and optical changes will be useful for future missions such as Dragonfly, which can closely examine molecular minerals like co-crystals on Titan’s surface.

This work was funded by the NESSF Grant #80NSSC17K0603. The authors acknowledge Kendra Farnsworth and Walter Graupner for assistance in the lab.

[1] Busker et al. 2008. Angew. Chem., 47, 10094

[2] Cable et al. 2018. ACS Earth & Space Chem., 2, 366

[3] Cable et al. 2019. ACS Earth & Space Chem., 3, 2808

[4] Cable et al. 2020. ACS Earth & Space Chem., In Press

[5] Wasiak et al. 2013. Adv. Space Res., 7, 1213