Constraints on the NH3 and PH3 distributions in the great red spot

Spatially resolved IUE observations of the Great Red Spot and the South Tropical Zone in the wavelength region of the NH₃ predissociation bands between 1900 and 2200 Å show slightly stronger absorption in the Great Red Spot than in the South Tropical Zone. Neglecting stratopheric haze, vertically inhomogeneous Rayleigh scattering radiative transfer models find an enhanced [NH₃]/[H₂] mixing ratio at the 80- to 125-mbar pressure level in the Great Red Spot of a factor of 3 to 10 with respect to the South Tropical Zone. Upper limits on the mixing ratio of PH₃ and the eddy diffusion coefficient above the Great Red Spot are considerably lower than earlier predictions.