H2o-silicate Microphysics in Ascending Volcanic Plumes on Mars

probable poor cohesion. The upper surface might be slightly indurated [2]. The MER rover may have the opportunity to examine this deposit, which occurs near the center of the Gusev landing ellipse. The deposition of other airfall units in the Sinus Meridiani region coincides with increased erosional activity there, suggesting a role for airfall deposits in the delivery or mobilization of volatiles [3,4,5]. Microphysical interactions determine how water behaves in the plume and, through release of latent heat, effect plume energetics and sedimentation [6]. The surface properties of ash are altered by condensation, which increases agglomeration and speeds precipitation. Although other workers have investigated the interaction of H2O and silicates in rising plumes, both on Earth and Mars [6,7,8], they have not considered the possible interaction of H2O and silicates via adsorption; no vapor is removed from the plume until saturation is reached. To examine the importance of the silicate-H2O interaction, we have developed a 5-component model of an ascending volcanic plume, based on the model of Glaze and Baloga [8]. We have expanded their model by allowing H2O to exist as vapor, unfrozen water (adsorbate or bulk liquid), and ice. We use the scheme shown in Figure 2 to determine the distribution of H2O among phases. Unfrozen water exists in all fields. If the plume is in the vapor field, the total H2O is partitioned between the adsorbed and vapor phases such that the adsorption isotherm is satisfied. Because we do not know the adsorptive response of primary martian ash, we assume here the basalt adsorption isotherm reported by Fanale and Cannon [9]. If the plume is in the liquid phase, all remaining vapor is assumed to condense to liquid, and the presence of liquid sets the vapor pressure in the plume element. When the plume reaches the freezing point, most of the remaining water is frozen, although a small amount of unfrozen water always remains, in equilibrium with the temperature and vapor pressure over ice.