ESTIMATES OF ABSOLUTE FLUX AND RADIANCE FACTOR OF LOCALIZED REGIONS ON MARS IN THE 2-4 MU-M WAVELENGTH REGION

Seven spectrophotometric observations of Mars obtained at the NASA Infrared Telescope Facility during the 1986 opposition are used to estimate absolute flux and radiance factor values in the 2.0-4.15-μm wavelength region. The derived radiance factors of 0.47 and 0.31 at 2.2 and 3.45 μm, respectively, for the bright region Elysium are comparable to but higher than previously determined values of geometric albedos of Arabia (0.42 and 0.28 at 2.2 and 3.45 μm, respectively, McCord and Westphal 1971, Astrophys. J. 168, 141; McCord et al. 1971, Icarus 14, 245-251). These discrepancies appear to be due to differences in illumination and viewing geometries. The derived radiance factor of 0.26 at 2.2 μm for a region south of Syrtis Major is brighter than the previously derived geometric albedo for the dark central portion of Syrtis Major (0.14, McCord and Westphal 1971). This discrepancy likely arises from sampling different regions on the surface but, alternatively, may represent some temporal changes in the surface reflectivity of Mars and/or atmospheric dust loading. Bright/dark ratios exhibit a wavelength dependence similar to those observed during 1990 (Bell and Crisp 1991, Lunar Planet. Sci. XXII, 73-74, and EOS 72, 521) but the spectral contrast for the 1986 observations is lower than the 1990. This difference could be due to differences in the location, size of the areas sampled, and/or amount of dust suspended in the Martian atmosphere. © 1992.