Properties of dust in the Martian atmosphere from the Imager on Mars Pathfinder

The Imager for Mars Pathfinder (IMP) returned sequences of images of the Ma rtian sky characterizing the size distribution, optical constants, and natu re of the aerosols suspended in the atmosphere of Mars. These sequences wer e executed when the solar elevation angle was approximately 15 degrees and consisted of images near the elevation of the Sun, spanning a range in azim uth from about 4 degrees to 180 degrees from the Sun. 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 s ky with multiple scattering models. Results include the following. (1) The geometric cross-section-weighted mean particle radius is 1.6 +/- 0.15 mu m almost independent of the assumed width (variance) of the size distribution . (2) The imaginary refractive index shows a steep increase with wavelength from 670 nm to shorter wavelengths, and a shallow increase toward longer w avelengths, consistent with the reflection spectrum observed by IMP for Mar tian soil. (3) For each assumed variance, two parameters governing the slop e and curvature of the portion of the phase function due to internally tran smitted light are found uniquely as functions of wavelength. (4) The varian ce of the gamma size distribution is difficult to constrain from these obse rvations alone. The shape of the single scattering phase functions derived from the IMP observations is compared to laboratory measurements of powder samples. One sample of irregular particles has a single scattering phase fu nction quite similar to that derived for Mars. Overall, the results for the mean cross-section-weighted size and imaginary refractive index as a funct ion of wavelength are in remarkably good agreement with the revised analysi s by Pollack ct al. [1995] of the observations made by the Viking lander 20 years earlier.