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Title: CARBON DIOXIDE CRYSTALS: AN EXAMINATION OF THEIR SIZE, SHAPE AND SCATTERINGPROPERTIES AT 37 GHZ AND COMPARISONS WITH WATER ICE (SNOW) MEASUREMENTS

Author
item FOSTER, JAMES - NASA
item CHANG, A - NASA
item HALL, D - NASA
item WERGIN, WILLIAM
item ERBE, ERIC
item BARTON, J - NASA

Submitted to: Journal of Geophysical Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/23/1998
Publication Date: N/A
Citation: N/A

Interpretive Summary: One problem in space exploration is that little is known about the types and amounts of snow that exist on Mars. On Earth, when the temperature is below the freezing point of water, precipitation occurs as snow or ice, which may accumulate and cover our planet. On Mars, where the temperature is much colder, snow can result from frozen water as well as frozen carbon dioxide. On Earth, satellites measure how microwaves that are emitted by our planet are affected by snow. This helps us to estimate how much snow has accumulated. If we could determine how snow formed from carbon dioxide affects the microwaves we could use this information to determine how much snow on Mars comes from frozen water and how much comes from frozen carbon dioxide. In this collaborative study with researchers from NASA, snow was made from carbon dioxide and photographed in a special type of microscope called a scanning electron microscope. Although snow crystals from water are generally six-sided, those from carbon dioxide were generally eight-sided and only about one-tenth the size of snow crystals from water. A model called a discrete dipole model was used to determine how microwaves were affected by the carbon dioxide crystals. This data is being used by scientists to improve the design of future instruments that will be aboard satellites to determine how much snow is present on Mars and if the snow originated from water or from carbon dioxide

Technical Abstract: On Earth, water is generally close to its freezing point. Therefore, small differences in temperature affect the amount of snow and ice that cover the planet at any one time. On Mars, climate processes also control the solid/gaseous status of carbon dioxide (CO2) and water (H2O) vapor. Passive microwave radiometry is used to estimate snow accumulation on Earth. Scattering by snow crystals affects the microwave radiation emanating from the ground. Microwave remote sensing may also be useful for assessing the thickness of the frozen H2O and CO2 on Mars, but information is needed about how H20 and CO2 crystals alter microwave radiation. In this study, CO2 crystals were produced in a laboratory and images of their sizes and shapes were recorded in a low-temperature SEM. The CO2 crystals were differently shaped, considerably smaller than H2O crystals, and consisted of bipyramid crystals as small as 1.0 æm. A discrete dipole model was employed to calculate the passive microwave radiation scattered and absorbed by the crystals. Modeling indicated that the shape of the crystals had little affect on extinction efficiency. However, the small size of CO2 crystals was more emissive than the H2O crystals in the 37 GHz region of the microwave spectrum. For the larger crystal sizes, the scattering values were several orders of magnitude larger than absorption.