Ammonia and Eddy Mixing Variations in the Upper Troposphere of Jupiter from HST Faint Object Spectrograph Observations

Ultraviolet spectra of the northern and southern hemispheres of Jupiter taken with the Hubble Space Telescope Faint Object Spectrograph (FOS) in May 1992 and June 1993 have been used to derive the altitude profiles of NH ₃ in the vicinity of the tropopause. For a given pressure level, it is shown that the vertical profile of ammonia varies with latitude and the atmospheric feature being observed. The mixing ratio of ammonia present above the Great Red Spot (GRS) is 8×10 ^-8 at 250 mbar, whereas it is four times greater in the nearby South Equatorial Belt at the same pressure level. Our findings agree with those of C. A. Griffith, B. Bézard, T. Owen, and D. Gautier (1992, Icarus98, 82-93), who find NH ₃ to be depleted over the GRS with respect to the South Tropical Zone at the time of the Voyager encounters. Variations of the ammonia mixing ratio in the northern and southern hemispheres are found to be nonmonotonic in latitude, indicating local dynamical effects. The observed latitudinal variation of the altitude profile of NH ₃ is likely to be caused by variations in the vertical eddy mixing ( K), which competes with the photolysis and condensation losses of NH ₃. We also find this vertical mixing to be nonmonotonic with latitude. Instead, it shows high variability depending on the region and feature observed, consistent with the values of eddy mixing derived by L.-M. Lara, B. Bézard, C. A. Giffith, J. H. Lacy, and T. Owen (1998, Icarus131, 317-333). Values at 13°N, 18°N, at the equator, and over the Great Red Spot region range between 3 and 10×10 ³ cm ² s ^-1 at 250 mbar, while weaker mixing occurs at 20°S, 33°S, and 7°N (<1×10 ³ cm ² s ^-1 at 250 mbar). Typically, the eddy mixing profiles have a ledge-like vertical structure, where a large mixing coefficient is required below the photolysis region ( p≥300 mbar), followed by a drop to a minimum as we proceed to lower pressures, and finally an increase again with decreasing pressure. In this paper, we also update our previous results on NH ₃ and K following the recalibration of the FOS data using the most recent recommendations of the Space Telescope Science Institute.