The contribution of atmospheric aerosols to the Martian opposition effect

Abstract

The Mie theory is used to compute the integrated scattering intensities for submicron aerosol particles with various indices of refraction and several size distributions in an effort to determine if the presence of atmospheric aerosols can account for the Martian opposition effect, as observed by O'Leary and by Koval' in 1967. This nonlinear surge in brightness, as the planet approaches a phase angle of 0°, is reported to be much more pronounced in the ultraviolet than in the infrared. The calculations show that neither substances having a refractive index n between 1.20 and 1.50, which include ice, water, and solid CO 2, nor highly absorbing materials, such as limonite, can produce the opposition effect. On the other hand, aerosols having n > 1.50 with little or no absorption, such as meteoric particles or suspended surface dust composed of semitransparent minerals, do exhibit a definite increase in reflectivity at small phase angles. By introducing an assumed surface function, which is added to the contribution by aerosols with n = 1.75, a model is obtained which compares reasonably well with the observations. Similar fits were obtained for n = 1.55 and 1.65 and could probably be obtained for any real index between 1.55 and 1.75. This range includes most semitransparent minerals, thus making them good candidates for producing the opposition effect. This study demonstrates that the presence of a small amount of atmospheric aerosols, with the proper index of refraction, could provide the observed increased opposition effect for Mars in the ultraviolet, where the albedo is very low, but at the same time make a negligible contribution in the infrared, where the surface albedo is high.