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G. B. Hansen (HIGP/SOEST, Univ. of Hawaii)
Remote sensing results from the Mars Global Surveyor orbiter will be reported in a continuing project to analyze the surface composition, infrared emissivity, and solar albedo of the seasonal polar caps of Mars, with the goal of producing a detailed model of the heat balance for each polar cap. Thermal infrared spectra, and infrared and solar bolometric radiances, from the thermal emission spectrometer (TES) are the primary tools in this study, although measurements from the Mars Orbiter laser altimeter (MOLA) and the Mars Orbiter camera will be used as necessary to constrain the short-wave components of the energy balance. A study has already been completed (Hansen, 1999, JGR, in press) which shows how the combined use of TES and MOLA data can lead to new insights into Mars' polar radiative properties. Data from some or all of the assessment orbits and science phasing orbits are already, or will soon become, available for this analysis, covering seasonal ranges (LS) between 183 - 218, 278 - 317, and 335 - 33 (encompassing spring through early fall in the south and fall through early spring in the north). The optical properties of the seasonal polar caps are primarily influenced by the most abundant constituent, CO2 ice, and the modulation of its emissivity and albedo by the variation of its particle size, and/or of dust and water-ice mixtures. Preliminary conclusions show that the northern cap in polar night is predominately large-grained CO2 (r>1mm) with a significant but small (<0.1 regions of bright, low-emissivity fine-grained CO2 (r<1mm). These low-emissivity regions have highly variable amounts of dust (including nearly pure CO2 snow) and preferentially occur on high topographic slopes. The fringe of the northern cap appears to include significant amounts of segregated, fine-grained water frost over the first several hundred kilometers.