Phosphine Mixing Ratios and Eddy Mixing Coefficients in the Troposphere of Saturn

S. G. Edgington, S. K. Atreya (U. Michigan), L. M. Trafton (U. Texas), J. J. Caldwell (York U.), R. F. Beebe, A. A. Simon (NMSU), R. A. West (JPL), C. Barnet (GSFC)

The reflectance of Saturn has been examined at various latitudes ( tex2html_wrap_inline21 S, tex2html_wrap_inline21 N, tex2html_wrap_inline25 N, tex2html_wrap_inline27 N, tex2html_wrap_inline29 N, tex2html_wrap_inline31 N, tex2html_wrap_inline33 N, tex2html_wrap_inline35 N, tex2html_wrap_inline37 N) in the ultraviolet. The data were obtained with the Hubble Space Telescope Faint Object Spectrograph (FOS) in October 1992 and covered a wavelength range of 180-230 nm. The observations have an angular resolution of 1". The absorption bands of acetylene are clearly visible in the spectra along with several broad features longward of 210 nm produced by Raman scattering of solar photons. The slope between roughly 190-210 nm can be reproduced using phosphine as a continuum absorber. No ammonia absorption bands are present in the spectra. Phosphine alone is unable to fit the spectra at wavelengths longer than 210 nm and some other continuum absorber must be invoked. With the use of a photochemical model, which combines the hydrocarbon, ammonia, and phosphine chemical cycles, and a multiple scattering radiative transfer model, we self-consistently determine the altitude profile of phosphine needed to fit the measured albedos from 190 to 210 nm. At tex2html_wrap_inline25 N, the phosphine mixing ratio is found to be 1.4x10 tex2html_wrap_inline41 at 91 mbar dropping to 2.9x10 tex2html_wrap_inline43 at 43 mbar due to photolysis. Due to problems concerning grating scattered light in the FOS, we are only able to estimate the acetylene mixing ratio to be approximately 2.0x10 tex2html_wrap_inline45 . Using the height distribution of phosphine, we determine the eddy mixing coefficient in the upper troposphere. The eddy mixing coefficient is found to be sensitive to the phosphine mixing ratio at the lower boundary of our model located at 1.4 bar. For a phosphine mixing ratio of 2.0x10 tex2html_wrap_inline47 , the eddy mixing is determined to be 2.5x10 tex2html_wrap_inline49 cm tex2html_wrap_inline51 s tex2html_wrap_inline53 at tex2html_wrap_inline25 N dropping to 3.4x10 tex2html_wrap_inline57 cm tex2html_wrap_inline51 s tex2html_wrap_inline53 at tex2html_wrap_inline33 N. Within the range of expected values of the phosphine mixing ratio in the troposphere, the eddy mixing coefficient is found to be inversely proportional to this mixing ratio.