A COUPLED SUBSURFACE-BOUNDARY LAYER MODEL OF WATER ON MARS

We have written a one-dimensional numerical model of the exchange of H 2O between the atmosphere and subsurface of Mars through the planetary boundary layer (PBL). Our goal is to explore the mechanisms of H2O ex change and to elucidate the role played by the regolith in the local H 2O budget. The atmospheric model includes effects of Coriolis, pressur e gradient, and frictional forces for momentum: radiation, sensible he at flux, and advection for heat. The model differs from Flasar and Goo dy by use of appropriate Viking-based physical constants and inclusion of the radiative effects of atmospheric dust. The pressure gradient f orce is specified or computed from a simple slope model. The subsurfac e model accounts for conduction of heat and diffusion of H2O through a porous adsorbing medium in response to diurnal forcing. The model is initialized with depth-independent H2O concentrations (2 kg m-3) in th e regolith and a dry atmosphere. The model terminates when the atmosph eric H2O column abundance stabilizes to 0.1% per sol. Results suggest that in most cases, the flux through the Martian surface reverses twic e in the course of each sol. In the midmorning, the regolith begins to release H2O to the atmosphere and continues to do so until midafterno on, when it once more becomes a sink. ft remains an H2O sink throughou t the Martian night. In the early morning and late afternoon, while th e atmosphere is convective, the atmosphere supplies H2O to the ground al a rapid rate, occasionally resulting in strong pulses of H2O into t he ground. The model also predicts that for typical conditions, perhap s 15-20 sols are required tor the regolith to supply an initially dry atmosphere with its equilibrium load The effects of surface albedo, th ermal inertia, solar declination, atmospheric optical depth, and regol ith pore structure are explored. Increased albedo cools the regolith, so less H2O appears in the atmospheric column above a bright surface. The friction velocity is higher above a dark surface, so there is more diurnal H2O exchange: relative humidities are much higher above a bri ght surface. Thermal inertia I affects the propagation of energy throu gh a periodically heated homogeneous surface. Our results suggest that higher thermal inertia forces more H2O into the atmosphere because th e regolith is warmer at depth. Surface stresses are higher above a low I surface, but there is less diurnal exchange because the atmosphere is dry. The latitude experiment predicts that the total diurnal insola tion is more important to the adsorptively controlled H2O column abund ance than the peak daytime surface temperature. Fogs and high relative humidity will be far more prevalent in the winter hemisphere. The dus t opacity of the atmosphere plays a very significant role; the PBL hei ght, column abundances, relative humidity. and surface stresses all in crease very strongly as the optical depth approaches zero. The dust op acity of the atmosphere must be considered in subsequent PBL models.