Dynamic surficial changes and changes in the position of the firn line and the areal extent of Hofsjo« kull ice cap, Iceland, were studied through analysis of a time series (1973^98) of synthetic-aperture radar (SAR) and Landsat data. A digital elevation model of Hofsjo« kull, which was constructed using SAR interferometry, was used to plot the SAR backscatter coefficient (³) vs elevation and air temperature along transects across the ice cap. Seasonal and daily ³ patterns are caused by freezing or thawing of the ice-cap surface, and abrupt changes in ³ are noted when the air temperature ranges from $^5³ to 0³C. Late-summer 1997 ³ (SAR) and reflectance (Landsat) boundaries agree and appear to be coincident with the firn line and a SAR ³ boundary that can be seen in the January 1998 SAR image. In January 1994 through 1998, the elevation of this ³ boundary on the ice cap was quite stable, ranging from 1000 to 1300 m, while the equilibrium-line altitude, as measured on the ground, varied considerably. Thus the equilibrium line may be obscured by firn from previous years. Techniques are established to measure long-term changes in the elevation of the firn line and changes in the position of the ice margin.

[1] Hydrologic models increasingly require knowledge of the amount of snow cover within a pixel in order to provide accurate estimates of snow covered area. Present methods for remote sensing of subpixel snow cover require knowledge of the spectral reflectance properties of the snow as well as the background material, making these methods difficult to apply globally. Similar problems were encountered in global remote sensing of aerosol particles over varying land terrain. Since both aerosol and snow are dark at 2.1 mm, we suggest a method for sub-pixel snow mapping based on experience with remote sensing of aerosols. Here the pixel reflectance at 2.1 mm is used to estimate the reflectance of the non-snow regions in the pixel at 0.66 mm. The difference between the total pixel brightness at 0.66 mm and the derived brightness of the same pixel without the snow is used to estimate the sub-pixel snow cover with an error usually < ±0.05.

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Abstract. On Earth, the temperature regime is such that water is generally fairly close 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 covering 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 and it is important in controlling the abundance of atmospheric carbon dioxide (CO2) and water vapor. Passive microwave radiometry has been used to derive snow extent and snow depth on Earth, where scattering by snow (H20) crystals is the dominant effect on the microwave radiation emanating from the ground and emerging from the snowpack. Microwave remote sensing may also prove to be useful for assessing the coverage and thickness of the frozen H20 and CO2 on Mars, but more exact information is needed on how both H20 crystals and frozen CO2 crystals scatter and absorb passive microwave radiation. In this study, CO2 crystals have been produced in a laboratory cold chamber with temperature conditions similar to those found on the polar caps of Mars, and detailed three-dimensional images of their size and shape have been made with a low-temperature scanning electron microscope. Unlike the much larger H20 snow crystals found on Earth, which typically range in size between 0.1 mm and 1.0 mm (radius), CO2 crystals are differently shaped and considerably smaller. Bipyramid crystals (base to base four-sided pyramids) are commonly observed, some as small as 1.0 [tm. A discrete dipole model was employed to calculate the passive microwave radiation scattered and absorbed by crystals of various sizes and shapes. Modeling results indicate that the shape of the crystal, whether for frozen CO2 or H20, is of little consequence in affecting extinction efficiency. However, owing to their smaller size, frozen CO2 crystals are more emissive than the H20 crystals in the 37 GHz region of the microwave spectrum. For the larger sizes of the modeled crystals, scattering dominates over absorption since the particles approach the size of the wavelength. The scattering values are 2 orders of magnitude larger than absorption for the 900 [tm size snow particles. For CO2 crystals of 3.0 [tm in size, absorption is 7 orders of magnitude greater than scattering.

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