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E. Wegryn, M. G. Tomasko, L. Doose, M. T. Lemmon, P. H. Smith (U. Arizona)
The Imager for Mars Pathfinder (IMP) instrument returned sequences of images of the Martian sky capable of characterizing the size distribution, optical constants, and nature of the aerosols suspended in the atmosphere of Mars during its operation on the Martian surface. These sequences were executed on many mornings and evenings when the solar elevation angle was approximately 15° and consisted of a series of images near the elevation of the sun and spanning a range in azimuth from about 4° to 180° from the sun. Sets of images were obtained at four wavelengths from 444 to 965 nm. From one sequence of observations, results are shown from a comparison of absolute photometry of the Martian sky with multiple scattering models which use the Pollack and Cuzzi (1980) algorithm for single scattering from nonspherical particles. Results include the following. 1) For frames obtained near the sun, it is necessary to adjust the precise location of the frames relative to the sun by comparing the shapes of the isophotes in the data to model calculations due to small uncertainties in the pointing of the camera. 2) The geometric cross section weighted mean particle radius is 1.6±0.15 µm independent of the assumed width (variance) of the size distribution. 3) For each particle size, the imaginary refractive index as a function of wavelength is found as a function of the width assumed for the size distribution. The shape of the imaginary refractive index with wavelength shows a steep increase from 670 nm to shorter wavelengths, and a shallow increase toward longer wavelengths, consistent with the reflection spectrum observed by IMP for Martian soil. 4) For each particle size and variance, two parameters governing the slope and curvature of the portion of the phase function due to internally transmitted light are found uniquely as functions of wavelength. 5) The variance of the gamma size distribution used is difficult to constrain, with good fits possible from less than 0.2 to 0.5 or more. A real refractive index of 1.5 was assumed throughout our calculations. Overall, the results are in remarkably good argreement with those obtained by Pollack et al. (J.G.R. 100, 5235, 1995) for the observations made by Viking 20 years earlier. This suggests that stable mechanisms exist to control the size of the Martian dust particles suspended in the atmosphere.