Ice hazes and clouds in the Martian atmosphere as derived from the Phobos/KRFM data

During the 1989 Phobos mission, the KRFM instrument observed the equatorial region of Mars in eight spectral bands from 315 to 550 nm. Previous analysis of the data mostly concentrated on extracting the optical properties of the dust particles in the atmosphere of Mars and the properties of the surface (Moroz et al., Planet. Space Sci. 41, 569, 1993). Some features of the KRFM photometric profiles, which appear to be of atmospheric origin, have however, remained unexplained. The present work considers three cases that exhibit such features and infers the effects to be caused by various types of water ice in the atmosphere. (1) For two sessions of observations, the intensity in UV near the limb was much higher than the mean of the morning limb profiles, but decreased later to the mean level as the line of sight moved further from the limb. Explaining these variations with a model of a homogeneous constant silicate haze does not seem possible. A model including a time evolving, near surface layer of silicate partieles covered with water ice provides a better fit to the data. The inferred column density of water vapour that condensed overnight on the dust particles is about 0.5 × 10−5 gcm−2, and the sublimation rate of these ice mantles after sunrise is of the order of 10−15 g μm−2 s−3. The ice haze contribution to the total optical depth decreases from about 0.05 at dawn to zero within about an hour. (2) A brightness increase of another type, observed in the afternoon UV profiles of the Valles Marineris is shown to be consistent with the presence of water ice clouds. The dependence on the properties of the clouds is discussed. The size distribution of the cloud particles seems of secondary importance. Their assumed shape, however, is critical. For spherical particles, the optical depth of the cloud is deduced to be 0.05 and the water content within the cloud to be 0.54 × 10−5 g cm−2. For particles with slightly irregular shape and rough surface both of these estimates are three times higher. (3) The photometric profiles of the Tharsis Ridge where the KRFM traces passed over the Martian volcanoes exhibit an increase in brightness in all wavelengths, but stronger in shorter wavelengths. The spectral dependence of the measured brightening is analysed to extract the properties of the clouds around Arsia Mons and Pavonis Mons. The particles of these clouds are likely to be smaller than 1 μm, and the ice column density is estimated to be in the range of 2.5 × 10−6–4.1 × 10−6 g cm−2.