9 S ep 2 00 9 Titan ’ s Prolific Propane : The Cassini CIRS Perspective

Although propane gas (C3H8) was first detected in the stratosphere of Titan by the 14 Voyager IRIS infrared spectrometer in 1980, obtaining an accurate measurement of its 15 abundance has proved difficult. All existing measurements have been made by modeling 16 the ν26 band at 748 cm : however different analyzes over time have yielded quite different 17 results, and it also suffers from confusion with the strong nearby ν5 band of acetylene. 18 In this paper we select large spectral averages of data from the Cassini Composite In19 frared Spectrometer (CIRS) obtained in limb-viewing mode at low latitudes (30S–30N), 20 greatly increasing the path length and hence signal-to-noise ratio for optically thin trace 21 species such as propane. By modeling and subtracting the emissions of other gas species, 22 we demonstrate that at least six infrared bands of propane are detected by CIRS, in23 cluding two not previously identified in Titan spectra. Using a new line list for the range 24 1300–1400 cm, along with an existing GEISA list, we retrieve propane abundances from 25 two bands at 748 and 1376 cm. At 748 cm we retrieve 4.2±0.5×10 (1-σ error) at 2 26 mbar, in good agreement with previous studies, although lack of hotbands in the present 27 spectral atlas remains a problem. We also determine 5.7± 0.8× 10 at 2 mbar from the 28 1376 cm band a value that is probably affected by systematic errors including contin29 uum gradients due to haze and also an imperfect model of the ν6 band of ethane. This 30 study clearly shows for the first time the ubiquity of propane’s emission bands across the 31 thermal infrared spectrum of Titan, and points to an urgent need for further laboratory 32 spectroscopy work, both to provide the line positions and intensities needed to model 33 these bands, and also to further characterize haze spectral opacity. The present lack of 34 accurate modeling capability for propane is an impediment not only for the measurement 35 of propane itself, but also for the search for the emissions of new molecules in many spec36 tral regions. 37