STRUCTURE OF SATURN MESOSPHERE FROM THE 28-SGR OCCULTATIONS

We analyze an extensive data set of immersion and emersion lightcurves of the occultation of 28 Sgr by Saturn's atmosphere on 3 July 1989, T he data give profiles of number density as a function of altitude at a variety of latitudes, at pressures ranging from about 0.5 to about 20 mu bar. The atmosphere is essentially isothermal in this range, with a temperature close to 140 K for an assumed mean molecular weight of 2 .135. Owing to favorable ring geometry, an accurate radial scale is av ailable for all observations, and we confirm the substantial equatoria l bulge produced by zonal winds of similar to 450 m/s first observed i n the Voyager radio-occultation experiments. The fact that the bulge i s still present at microbar pressures suggests that the equatorial win ds persist to high altitudes. According to our radial scale, the 2.4-m u bar level, which corresponds to half-flux in the stellar occultation s, is at an equatorial radius of 60,960 km. This radial scale is in go od agreement with the Voyager radio-occultation data at mbar pressures and allows smooth interpolation of the isothermal structure between t he stellar-occultation and radio-occultation regions. We do not have s uch a smooth interpolation between our data and Voyager ultraviolet oc cultation data, unless we discard the lowest 200 km of Voyager ultravi olet data. When this is done, we obtain a complete atmospheric model f rom an equatorial radius of 61,500 km down to an equatorial radius of 60,500 km. This model gives excellent agreement between all 28 Sgr, Vo yager, and Pioneer 11 data. (C) 1997 Academic Press.