PROBING VENUS CLOUD STRUCTURE WITH GALILEO NIMS

The spectral image cubes obtained by the Near-Infrared Mapping Spectro meter (NIMS) on Galileo as it flew by Venus have been analyzed to cons train the vertical structure of the clouds, the nature of the aerosol particles, and the location and particle properties of the opacity var iations responsible for high-contrast features observed in the near-in frared windows at 1.7 and 2.3 mu m. A radiative transfer program was u sed to simulate mid-latitude curves of limb darkening at 3.7 mu m. Bes t-fit models to these curves demonstrate that the upper clouds are dom inated by mode 2 particles (($) over bar r=1.0 mu m), with a contribut ion of approximate to 15% of opacity from mode 1 particles (($) over b ar r=0.3 mu m). The low-latitude upper cloud is well represented by a dual scale-height model, with a particle scale height of approximate t o 1 km from an altitude of 61-63 km, and a scale height of approximate to 6 km above this, up to the level where tau=1 at approximately 71 k m. This model also successfully simulates limb-darkening curves at 11. 5 Can from the Pioneer Venus Orbiter Infrared Radiometer. Successful s imulations of correlation plots of 1.7 vs 2.3 mu m intensities reveal that mode 3 particles (($) over bar r=3.65 mu m) represent the dominan t source of opacity in the lower and middle clouds, and that variation in total cloud opacity reflects chiefly the addition and removal of m ode 3 particles near the cloud base. We find that the full spectrum of brightnesses at 1.7 and 2.3 mu m implies that the total cloud optical depth varies from approximate to 25 to approximate to 40.