Opacity of the Martian atmosphere measured by the Imager for Mars Pathfinder

The Imager for Mars Pathfinder (IMP) has obtained 1733 images of the Sun during its 83‐sol mission starting on July 4, 1997, which have been used to determine the atmospheric opacity at wavelengths of 450, 670, 883, and 989 nm. Solar images were obtained hourly between 0700 and 1700 hours, allowing detection of changes within a sol as well as from one sol to another. Atmospheric opacities were similar to those determined by the Viking landers: τ = 0.4–0.5 in all filters early in the mission (near L s 145) increasing to τ = 0.6 late in the mission (near L s 185). Opacity was variable in the mornings, especially in the blue filter; less variability was seen in the afternoons and in the longer wavelength filters. We conclude that submicron water ice particles with opacities as large as 0.14 in blue light are the cause of the diurnal variations. Orbital observations of the limb have been modeled by water ice detached hazes above a dust layer. Using this model, detached hazes of small particles provide the extra limb brightening that is observed by images in the UV taken by the Hubble Space Telescope. Thus the presence of about one‐half optical depth of dust during the interval between Viking and Pathfinder is consistent with UV observations, suggesting that Viking‐ and Pathfinder‐like dust conditions are typical for northern subtropical latitudes.