Searching for heavy hydrocarbons on Titan using infrared astronomy

Saturn’s moon Titan is a ‘hydrocarbon heaven’. Its mildly reducing atmosphere, composed mostly of nitrogen (N₂, ~95-97%), also possesses a significant amount of methane (CH₄, ~5-2%). With a near-absence of atmospheric oxidizers, photochemistry acts on the methane and nitrogen to break up the simpler molecules, and results in a dizzying array of more complex organic chemicals: mostly hydrocarbons (C_xH_y) and nitriles (C_xH_yCN). To date, around two dozen molecules have been definitively identified using astronomical spectroscopy, while the presence of hundreds more has been inferred from photochemical models and low-resolution mass spectroscopy of atmospheric gases from Cassini/Huygens.

Further characterization of Titan’s atmospheric chemicals is important for multiple reasons: (i) investigating how pre-biotic chemistry occurs across different environments, to build up more complex species from simpler ingredients (e.g. Miller-Urey type synthesis); (ii) deciphering the origin and history of Titan’s atmosphere; (iii) understanding the feedback between Titan’s atmospheric chemistry, dynamics and weather; (iv) putting solar system bodies in the context of the emerging gallery of exoplanets, and building out the wider parameter space of possible planetary atmospheres.

To this end, we are undertaking astronomical searches for larger molecules on Titan than those known at present, focusing on undetected C₃ and C₄ hydrocarbons (Fig. 1). This will help to expand our knowledge of the atmospheric inventory and provide valuable constraints on photochemical models. The transition to C₃ species and beyond is a critical juncture in organic chemistry. This marks the point where structural isomerism becomes possible, resulting in multiple species with the same chemical formula but different structures. Therefore, astronomical measurements become especially important for C₃ species and beyond, as unit resolution mass spectroscopy encounters significant ambiguities.

We report on current progress in the search for C₃ and C₄ hydrocarbons using the high-resolution TEXES spectrometer on the NASA 3m IRTF telescope on Maunakea, HI, and the implications for knowledge of Titan’s chemistry. These results will provide key context for science planning and a data analysis for NASA’s upcoming Dragonfly mission to Titan, as well as for possible visits by an planned ESA L4 mission to the Saturn system.

Figure 1: Hydrocarbon chemical work for Titan’s atmosphere. Most small molecules (C₁ to C₂) are well studied and characterized, while C₃ molecules are partially known, and C₄ and larger species remain mostly undetected.