Infrared Search for Vinyl Cyanide in Cassini/CIRS Polar Winter Observations

Titan’s atmosphere is a dense and active chemical reactor forming complex organic and nitrile species relevant to prebiotic chemistry. Molecular nitrogen (N₂) and methane (CH₄) undergo photolysis and subsequently react to form more complex molecules that then continue this photochemical process creating a vast chemical network leading to the formation of organic hazes that give Titan its characteristic glow. Nitriles in Titan’s atmosphere, specifically, are of astrobiological interest as they make up the necessary precursors to more complex molecules, such as amino acids, necessary for the formation of life.  One nitrile species, vinyl cyanide (C₂H₃CN), previously detected in ALMA (Atacama Large Millimeter Array, Palmer et al. 2018) observations of Titan’s atmosphere bears significant astrobiological relevance.  Stevenson et al. (2015) reported that vinyl cyanide had the capability to form self-assembled structures that resembled cell membranes in oxygen poor environments such as Titan. Furthermore, Mayer and Nixon (2025) recently proposed a mechanical mechanism for the formation of vesicles through precipitation induced spray droplets from the surface of Titan’s methane lakes when a thin monolayer of amphiphiles such as vinyl cyanide are present. Here, we present the search for vinyl cyanide in infrared observations of Titan’s south polar limb from Cassini’s Composite InfraRed Spectrometer (CIRS). We make use of a newly obtained pseudo line list from a high-resolution measurement of the vinyl cyanide mid-infrared spectrum. Using this new spectroscopic information, we search for the  vibrational mode, centered at 682 cm^-1, in CIRS observations from Cassini’s T110 flyby of Titan’s south polar limb (89 S) during the southern polar winter in March of 2015. Vinatier et al. (2018) used these observations previously to detect the infrared spectral signature of benzene ice at 680 cm^-1; however, at that time, the infrared spectrum of vinyl cyanide was not well characterized. Even following the inclusion of benzene ice into the spectrum, there is still a significant residual remaining in the CIRS spectrum near 682 cm^-1, indicating a missing gas in the radiative transfer model of these observations. With this detection, we can also show how vinyl cyanide is enriched at Titan’s winter pole and assess the astrobiological relevance of this key nitrile species.

References:

Palmer, M. Y. et al. ALMA detection and astrobiological potential of vinyl cyanide on Titan. Sci. Adv. 3, e1700022 (2017).

Stevenson, J., Lunine, J. & Clancy, P. Membrane alternatives in worlds without oxygen: Creation of an azotosome. Sci. Adv. 1, e1400067 (2015).

Mayer, C. & Nixon, C. A. A proposed mechanism for the formation of protocell-like structures on Titan. Int. J. Astrobiol. 24, e7 (2025).

Vinatier, S. et al. Study of Titan’s fall southern stratospheric polar cloud composition with Cassini/CIRS: Detection of benzene ice. Icarus310, 89–104 (2018).

Acknowledgement:

Portions of this research were performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration and California Institute of Technology.