Infrared Search for N-Butane and Trans-2-Butene in Titan's Atmosphere

Abstract

This dissertation constitutes a study of the possible presence of trace hydrocarbons, namely n-butane (n-C4H10) and trans-2-butene (trans-2-C4H8) in Titan’s atmosphere. These molecules, both of which bear specific connections of interest to Titan’s astrobiological potential, are predicted by photochemical models to occur at detectable abundances within Titan’s atmosphere. In spite of this, neither has been detected to date, by any means. For this work, both of these molecules were characterized in the laboratory by obtaining a comprehensive set of highresolution, infrared cross section measurements at cold temperatures appropriate for Titan. In the case of n-butane, a pseudoline list (i.e. an emipirical model of a spectral line list) which can be directly integrated into existing radiative transfer codes, was also derived. Using the NEMESIS planetary retrieval code, these results were implemented in the modeling of observations of Titan coming from Cassini CIRS and NASA IRTF in search of emissions of these molecules. The NASA IRTF observations were designed and calibrated to be optimized toward investigating Titan’s n-butane and trans-2-butene abundances. Ultimately, this study claims no explicit detection of either molecule. Instead, a comprehensive set of upper limits on the abundances of n-butane in Titan’s atmosphere is attained, and rough upper limits are derived for the first time for trans-2-butene. The upper limits for these molecules will be crucial in further constraining photochemical models, as a major effort is currently underway to fully add Titan’s C4 chemistry to these models. Results from this work may also provide insight into the formation of Titan’s ubiquitous hazes, clouds, and may even shed light on the composition of Titan’s unique lakes and seas.