New Constraints on Titan’s Stratospheric n-Butane Abundance

Abstract

Curiously, n-butane has yet to be detected at Titan, though it is predicted to be present in a wide range of abundances that span over 2.5 orders of magnitude. We have searched infrared spectroscopic observations of Titan for signals from n-butane (n-C4H10) in Titan’s stratosphere. Three sets of Cassini Composite Infrared Spectrometer Focal Plane 4 (1050–1500 cm−1) observations were selected for modeling, having been collected from different flybys and pointing latitudes. We modeled the observations with the Nonlinear Optimal Estimator for MultivariatE Spectral AnalySIS radiative transfer tool. Temperature profiles were retrieved for each of the data sets by modeling the ν 4 emission from methane near 1305 cm−1. Then, incorporating the temperature profiles, we retrieved abundances of all of Titan’s known trace gases that are active in this spectral region, reliably reproducing the observations. We then systematically tested a set of models with varying abundances of n-butane, investigating how the addition of this gas affected the fits. We did this for several different photochemically predicted abundance profiles from the literature, as well as for a constant-with-altitude profile. Ultimately, though we did not produce any firm detection of n-butane, we derived new upper limits on its abundance specific to the use of each profile and to multiple different ranges of stratospheric altitudes. These results will tightly constrain the C4 chemistry of future photochemical modeling of Titan’s atmosphere and also motivate the continued search for n-butane and its isomer, isobutane.

Material Properties of Organic Liquids, Ices, and Hazes on Titan

“…We also choose to include dicyanoacetylene (C 4 N 2 ), which has been detected in the solid phase (Khanna et al 1987;Anderson et al 2016), although only an upper limit has been established for its gas phase (Samuelson et al 1997;Jolly et al 2015). Some organic species are predicted by photochemical models to form in Titan's atmosphere, but they have either not been detected (e.g., 1,3-butadiene; Willacy et al 2016;Vuitton et al 2019) or only upper limits of their abundances are established (e.g., butane: Steffens et al 2022; pyridine and pyrimidine: Nixon et al 2020). In this work, we choose not to include these molecules.…”