Latitudinal variation of Saturn photochemistry deduced from spatially-resolved ultraviolet spectra

Abstract We obtained spatially-resolved ultraviolet spectra of Saturn in 1994 with the Faint Object Spectrometer and Goddard High Resolution Spectrograph of the Hubble Space Telescope. We observed four areas on the planet at 15° N, 33° S, 41° S, and 52° S, with a field-of-view of less than 2 × 2 arcsec 2 , compared to the 16-arcsec planet diameter. The wavelength range, 1550–2300 A, encompasses absorption from major hydrocarbons (C 2 H 6 , C 2 H 4 , C 2 H 2 , CH 3 C 2 H, C 4 H 2 ) and water. We find global hydrocarbon abundances and a C 2 H 2 vertical distribution compatible with infrared observations, in contrast with previous analyses of ultraviolet spectra. The stratospheric haze opacity decreases from polar region to the equator. Saturn mid-latitudes are photochemically distinct from the rest of the planet. At 33° S, the spectrum requires either (1) a distinctly different C 2 H 2 vertical distribution or (2) a locally enhanced water abundance. At 41° S, the hydrocarbon abundance exhibits a local minimum, within a global trend of increasing abundance from equator to pole. This global trend may result from an increased abundance of short-lived hydrocarbons such as C 4 H 2 . Photochemical models predict a depletion of hydrocarbon molecules in the presence of stratospheric water [Moses et al., 2000. Icarus 143, 166–202]. These results are consistent with a localized influx of water, in the form of high charge to mass ratio particles, flowing into Saturn's atmosphere at latitudes magnetically linked to the rings.