Experimental study of the sublimation of ice through an unconsolidated clay layer: Implications for the stability of ice on Mars and the possible diurnal variations in atmospheric water

We have studied the sublimation of ice and water vapor transport through various thicknesses of clay (<63 mu m grain size). We experimentally demonstrate that both adsorption and diffusion strongly affect the transport of water, and that the processes of diffusion and adsorption can be separately quantified once the system comes to a steady state. At shallow depths of clay, water vapor transport is determined by diffusion through both the atmosphere and the clay layer, whereas at greater depth the rate of sublimation of the ice is governed only by diffusion through the clay. Using two different models, we determine the diffusion coefficient for water vapor through unconsolidated clay layer to be 1.08 +/- 0.04 x 10(-4) and 1.29 +/- 0.06 x 10(-4) m(2) s(-1). We also determined the adsorption isotherms for the clay layer, which follow the Langmuir theory at low water vapor pressure (< 100 Pa, where a monolayer of water molecules forms on the surface of the clay) and the BET theory at higher pressure (where multiple water layers form). From our analysis of both types of isotherms we determined the adsorption constants to be alpha = 4.9 +/- 1.0 x 10(-2) Pa-1 and c = 30 +/- 10, respectively, and specific surface areas of 1.10 +/- 0.2 x 10(5) and 9.0 +/- 0.7 x 10(4) m(2) kg(-1), respectively. Finally, we report a theoretical kinetic model for the simultaneous diffusion and adsorption from which we determine adsorption kinetic constants according to the Langmuir theory of k(a) = 2.5 +/- 0.5 x 10(-4) s(-1) and k(d) = 8.7 +/- 3.6 x 10(-5) s(-1). If the martian regolith possesses diffusive properties similar to those of the unconsolidated montmorillonite soil we investigated here, it would not represent a significant barrier to the sublimation of subsurface ice. However, at the low subsurface temperatures of high latitude (180 K on average), ice could survive from the last glaciation period (about 300 to 400,000 years ago). Higher subsurface temperatures in the equatorial regions would prevent long-timescale survival of ice in the shallow subsurface. In agreement with previous work, we show that adsorption of water by a clay regolith could provide a significant reservoir of subsurface water and it might account for the purported diurnal cycle in the water content of the atmosphere. (C) 2008 Elsevier Inc. All rights reserved.