Thirty-five-gigahertz measurements of CO2 crystals

In order to learn more about the Martian polar caps, it is important to compare and contrast the behavior of both frozen H2O and CO2 in different parts of the electromagnetic spectrum. Because relatively little attention has been given, thus far, to observing the seasonal Martian polar caps in the thermal microwave part of the spectrum, in this experiment, a 35-GHz handheld radiometer was used to measure the microwave emission and scattering from layers of manufactured CO2 (dry ice). Compared to natural snow crystals, results for the dry ice layers exhibit lower microwave brightness temperatures for similar thicknesses, regardless of the incidence angle of the radiometer. For example, at 50° H (horizontal polarization) and with a covering 18 cm of dry ice, the brightness temperature was 76 K. When the total thickness of the dry ice was 27 cm, the brightness temperature was 86 K. The lower brightness temperatures are due to a combination of the lower physical temperature and the larger crystal sizes of the commercial CO2 crystals compared to the snow crystals. While little is known about the CO2 and water snowpacks on Mars, it is likely that the particles are in close contact with one another as is the case for ice sheets on Earth — the grains are interconnected. This would qualify as a dense media. In densely packed media, the particles do not scatter independently; rather, they interact with other particles. Dense media calculations compare very favorably with the observed values from the handheld radiometer. The calculated versus observed TBs are within 10% for each case with the exception of the 26 cm layer thickness and the 0.8 mm particle size (15%) and for the 14 cm layer thickness and the 2.0 mm particle size (16%). Thus, it appears that dense media transfer modeling (DMRT) will be useful for modeling the flow of energy emerging from frozen CO2 deposits. Because the dry ice used in this experiment was manufactured in the shape of cylindrical pellets, an effort was made to see what effect, if any, the shape of the crystal, for different particle sizes, has on microwave scattering. Results from a discrete dipole scattering (DDSCAT) model show that differently shaped crystals, having the same effective radius of a sphere, give very similar cross section/efficiencies.