Carbon dioxide crystals: An examination of their size, shape, and scattering properties at 37 GHz and comparisons with water ice (snow) measurements

On Earth, the temperature regime is such that water is generally fairly clo se to its freezing point, and thus relatively small differences in climate affect how much snow and ice are present and whether or not the snow coveri ng will be seasonal or last from one year to the next. On Mars, as on Earth , the presence of ice also plays a role in large-scale climate processes an d it is important in controlling the abundance of atmospheric carbon dioxid e (CO2) and water vapor. Passive microwave radiometry has been used to deri ve snow extent and snow depth on Earth, where scattering by snow (H2O) crys tals is the dominant effect on the microwave radiation emanating from the g round and emerging from the snowpack. Microwave remote sensing may also pro ve to be useful for assessing the coverage and thickness of the frozen H2O and CO2 on Mars, but more exact information is needed on how both H2O cryst als and frozen CO2 crystals scatter and absorb passive microwave radiation. In this study, CO2 crystals have been produced in a laboratory cold chambe r with temperature conditions similar to those found on the polar caps of M ars, and detailed three-dimensional images of their size and shape have bee n made with a low-temperature scanning electron microscope. Unlike the much larger H2O snow crystals found on Earth, which typically range in size bet ween 0.1 mm and 1.0 mm (radius), CO2 crystals are differently shaped and co nsiderably smaller. Bipyramid crystals (base to base four-sided pyramids) a re commonly observed, some as small as 1.0 mu m. A discrete dipole model wa s employed to calculate the passive microwave radiation scattered and absor bed by crystals of various sizes and shapes. Modeling results indicate that the shape of the crystal, whether for frozen CO2 or H2O, is of-little cons equence in affecting extinction efficiency. However, owing to their smaller size, frozen CO2 crystals are more emissive than the H2O crystals in the 3 7 GHz region of the microwave spectrum. For the larger sizes of the modeled crystals, scattering dominates over absorption since the particles approac h the size of the wavelength. The scattering values are 2 orders of magnitu de larger than absorption for the 900 mu m size snow particles. For CO2 cry stals of 3.0 mu m in size,absorption is 7 orders of magnitude greater than scattering.