Mariner 7 ultraviolet spectrometer experiment: Photometric function and roughness of Mars' polar cap surface

Abstract

The Mariner 7 ultraviolet spectrometer observed the south polar cap of Mars. The near ultraviolet polar spectrum shows the predominance of surface reflection over atmospheric scattering. The intensity of the reflected radiation, I, decreased steeply for a change in solar elevation of only about 10°. Neither a haze layer nor ground inhomogeneity is considered a likely explanation for this observation. An unusual photometric function of the surface is more probable. An analysis of intensity as a function of the incidence and emission angles ( i and ϵ) was made, assuming a photometric function of the form I ∼ cos k i cos k−1 ϵ (Minnaert's law) for the surface. Eight model atmospheres, based on information previously known for Mars or from the Mariner 6 and 7 missions, were used to correct for scattering and attenuation effects. The angular inversion revealed a rather high value of about 3 for k. This result is interpreted to mean that the surface reflection is strongly specular as from an icy or glazed surface. Applying the principle of least variation of the k value with wavelength, it was learned that the atmosphere over the polar region was not as turbid as that over the deserts; and that the ozone, discovered from spectral inversion of the same data, is either low in the atmosphere or trapped in the polar cap. An hypothesis is advanced to explain the observed phenomena and associated polar features.