Grain Sizes and Surface Areas of H2O Ices Used to Simulate Surfaces of Icy Moons

Mean grain sizes and specific surface areas (SSA) of ice substrates formed by vapor deposition at low temperatures are of importance in simulating external surfaces of icy moons and ice/snow-covered terrestrial environments. These microphysical parameters are crucial for quantifying the exchange of reactive trace gases between ice and snow-covered regions and their overlying atmospheres. Environmental scanning electron microscopy (ESEM) was used to obtain granule sizes and to observe the phase of ice granules, prepared on borosilicate, quartz, silicon, and metallic plates. Ices prepared at a temperature lower than 140 K appear to be amorphous, and their granule sizes are typically about sub-micron dimensions. At slightly warmer temperatures near 180 200 K, ice films are comprised of either hexagonal or cubic granules with sizes up to a few microns. When annealed to even warmer temperatures, ice granule sizes approach ~10 μm. Specific surface areas of ices were determined from BET (Brunauer, Emmett, and Teller) analysis of gas adsorption isotherms in the temperature range from 83.5 to 261 K. Specific surface areas vary from ~100 m2/g at ~83.5 K to ~0.22 m2/g at ~261 K and exhibit a positive correlation with ice mass above a particular ice mass demarcation at specified deposition temperatures. We conclude that the overall decrease in SSA is primarily due to metamorphism and sintering. BET data suggests that the transition from amorphous to cubic-crystalline form occurs at ~150 K. Possible implications to theoretical models of remote sensing using visible and near-infrared spectrophotometers are also discussed.