|Foster, James - NASA|
|Chang, A T - NASA|
|Hall, D - NASA|
Submitted to: Microscopy Society of America Proceedings
Publication Type: Proceedings
Publication Acceptance Date: May 28, 1997
Publication Date: N/A
Interpretive Summary: A renewed interest in cloud seeding offers potential for increasing the water supply for agricultural purposes in arid regions of the United States. One of the most effective materials that has been used for this purpose is dry ice (crystals of solid carbon dioxide). It is believed that the unique structure of these crystals somehow serves as a seed or nucleus around which minute water droplets aggregate to eventually form raindrops. However, a problem is that no one has ever seen the minute structure of these crystals because they evaporate at temperatures of -110 degrees F. We have used a special microscope that has a stage cooled to -320 degrees F that enables us to observe the structure of crystals of solid carbon dioxide. The crystals, which are as small as 1/200,000 of an inch, generally appear as eight-sided structures called octahedrons. The results, which illustrate the crystalline structure of carbon dioxide for the first time, will be used by scientists to learn how the crystals cause formation of rain and to identify more efficient or economical materials that can be used to produce rain in arid agricultural regions.
Technical Abstract: Earth and Mars are the only planets in our solar system having polar ice caps that expand and contract in response to changes in the seasons. On Earth, the polar caps are composed of water-ice (snow crystals). Whereas, on Mars, the extreme low temperatures at the polar cap regions result in the precipitation of CO2 ice (Martian snow) in addition to water-ice. Viking spacecraft have recorded various spectral data on this CO2 mantle and the transmission spectra for CO2 have been measured in the laboratory; however, little is known about the structural features of the crystals. Because previous studies have shown that low temperature SEM could be used to image structural variation of snow crystals and because data on the structure of CO2 crystals are needed to develop scattering and emission models of CO2 ice, a study was undertaken to image CO2 crystals. Results obtained with TEM and SEM indicate that CO2 crystals can be imaged and suggest that other gases, which precipitate at low temperatures and are of interest to interplanetary studies, may also be observed using this technique.