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

Far infrared spectra (10–600 cm−1) from Cassiniʹs Composite InfraRed Spectrometer (CIRS) were used to determine improved upper limits of hydrogen halides HF, HCl, HBr, and HI in Saturnʹs atmosphere. Three observations, comprising a total of 3088 spectra, gave 3σ upper limits on HF, HCl, HBr, and HI volume mole fractions of 8.0 × 10 −12 , 6.7 × 10 −11 , 1.3 × 10 −10 , and 1.4 × 10 −9 , respectively, at the 500 mbar pressure level. These upper limits confirm sub-solar abundances of halide species for HF, HCl, and HBr in Saturnʹs upper atmosphere—consistent with predictions from thermochemical models and influx of material from meteoroids. Our upper limit for HCl is 16 times lower than the tentative detection at 1.1 × 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.