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 tak en with the Hubble Space Telescope Faint Object Spectrograph (FOS) in May 1 992 and June 1993 have been used to derive the altitude profiles of NH3 in the vicinity of the tropopause. For a given pressure level, it is shown tha t 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 x 10(-8) at 250 mbar, whereas it is four times greater in the nearby South Equatorial Belt at the same pressure level. Our findin gs agree with those of C. A. Griffith, B. Bezard, T. Owen, and D. Gautier ( 1992, Icarus 98, 82-93), who find NH3 to be depleted over the GRS with resp ect to the South Tropical Zone at the time of the Voyager encounters. Varia tions of the ammonia mixing ratio in the northern and southern hemispheres are found to be nonnonotonic in latitude, indicating local dynamical effect s. The observed latitudinal variation of the altitude profile of NH3 is lik ely to be caused by variations in the vertical eddy mixing (K), which compe tes with the photolysis and condensation losses of NH3. We also find this v ertical mixing to be nonmonotonic with latitude. Instead, it shows high var iability depending on the region and feature observed, consistent with the values of eddy mixing derived by L.-M. Lara, B. Bezard, C. A. Giffith, J. H . Lacy, and T. Owen (1998, Icarus 131, 317-333). Values at 13 degrees N, 18 degrees N, at the equator, and over the Great Red Spot region range betwee n 3 and 10 x 10(3) cm(2) s(-1) at 250 mbar, while weaker mixing occurs at 2 0 degrees S, 33 degrees S, and 7 degrees N (<1 x 10(3) cm(2) s(-1) at 250 m bar). Typically, the eddy mixing profiles have a ledge-like vertical struct ure, where a large mixing coefficient is required below the photolysis regi on (p greater than or equal to 300 mbar), followed by a drop to a minimum a s we proceed to lower pressures, and finally an increase again with decreas ing pressure. In this paper, we also update our previous results on NH3 and K following the recalibration of the FOS data using the most recent recomm endations of the Space Telescope Science Institute. (C) 1999 Academic Press .