New upper limits for hydrogen halides on Saturn derived from Cassini-CIRS data

Far infrared spectra (10-600 cm(-1)) from Cassini's Composite InfraRed Spectrometer (CIRS) were used to determine improved upper limits of hydrogen halides HF, HCl, HBr, and HI in Saturn's atmosphere. Three observations, comprising a total of 3088 spectra, gave 3 sigma upper limits on HF, HCI, HBr, and HI volume mole fractions of 8.0 x 10(-12), 6.7 x 10(-11), 1.3 x 10(-10), and 1.4 x 10(-9), respectively, at the 500 mbar pressure level. These upper limits confirm sub-solar abundances of halide species for HF, HCI, and HBr in Saturn's upper atmosphere-consistent with predictions from thermochemical models and influx of material from meteoroids. Our upper limit for HCI is 16 times lower than the tentative detection at 1.1 x 10(-9) reported by Weisstein and Serabyn [Weisstein, E.W., Serabyn, E., 1996. Icarus 123, 23-36]. These observations are not sensitive to the deep halide abundance, which is expected to be enriched relative to the solar composition. (c) 2006 Elsevier Inc. All rights reserved.