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 equat orial region of Mars in eight spectral bands from 315 to 550 nm. Previ ous analysis of the data mostly concentrated on extracting the optical properties of the dust particles in the atmosphere of Mars and the pr operties of the surface (Moroz et al., Planet. Space Sci. 41, 569, 199 3). Some features of the KRFM photometric profiles, which appear to be of atmospheric origin, have however, remained unexplained. The presen t 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 li mb was much higher than the mean of the morning limb profiles, but dec reased later to the mean level as the line of sight moved further from the limb. Explaining these variations with a model of a homogeneous c onstant silicate haze does not seem possible. A model including a time evolving, near surface layer of silicate particles 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 x 10(-5) g cm(-2), and the sublimation rate of these ice mantles a fter sunrise is of the order of 10(-15) g mu m(-2) s(-1). The ice haze contribution to the total optical depth decreases from about 0.05 at dawn 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 dependenc e on the properties of the clouds is discussed. The size distribution of the cloud particles seems of secondary importance. Their assumed sh ape, however, is critical. For spherical particles, the optical depth of the cloud is deduced to be 0.05 and the water content within the cl oud to be 0.54 x 10(-5) g cm(-2). For particles with slightly irregula r shape and rough surface both of these estimates are three times high er. (3) The photometric profiles of the Tharsis Ridge where the KRFM t races passed over the Martian volcanoes exhibit an increase in brightn ess in all wavelengths, but stronger in shorter wavelengths. The spect ral dependence of the measured brightening is analysed to extract the properties of the clouds around Asis Mons and Pavonis Mons. The partic les of these clouds are likely to be smaller than 1 mu m, and the ice column density is estimated to be in the range of 2.5 x 10(-5) - 4.1 x 10(-6) g cm(-2). Copyright (C) 1996 Elsevier Science Ltd.