Titan's prolific propane: The Cassini CIRS perspective

Although propane gas (C3 H8) was first detected in the stratosphere of Titan by the Voyager IRIS infrared spectrometer in 1980, obtaining an accurate measurement of its abundance has proved difficult. All existing measurements have been made by modeling the ν26 band at 748 cm- 1: however, different analyzes over time have yielded quite different results, and it also suffers from confusion with the strong nearby ν5 band of acetylene. In this paper we select large spectral averages of data from the Cassini Composite Infrared Spectrometer (CIRS) obtained in limb-viewing mode at low latitudes (30{ring operator}S-30{ring operator}N), greatly increasing the path length and hence signal-to-noise ratio for optically thin trace species such as propane. By modeling and subtracting the emissions of other gas species, we demonstrate that at least six infrared bands of propane are detected by CIRS, including two not previously identified in Titan spectra. Using a new linelist for the range 1300-1400cm- 1, along with an existing GEISA list, we retrieve propane abundances from two bands at 748 and 1376cm- 1 . At 748cm- 1 we retrieve 4.2 ± 0.5 × 10- 7 (1 - σ error) at 2 mbar, in good agreement with previous studies, although lack of hotbands in the present spectral atlas remains a problem. We also determine 5.7 ± 0.8 × 10- 7 at 2 mbar from the 1376cm- 1 band - a value that is probably affected by systematic errors including continuum gradients due to haze and also an imperfect model of the ν6 band of ethane. This study clearly shows for the first time the ubiquity of propane's emission bands across the thermal infrared spectrum of Titan, and points to an urgent need for further laboratory spectroscopy work, both to provide the line positions and intensities needed to model these bands, and also to further characterize haze spectral opacity. The present lack of accurate modeling capability for propane is an impediment not only for the measurement of propane itself, but also for the search for the emissions of new molecules in many spectral regions. © 2009 Elsevier Ltd