THE CONTINUUM SLOPE OF MARS - BIDIRECTIONAL REFLECTANCE INVESTIGATIONS AND APPLICATIONS TO OLYMPUS MONS

For Mars, the near-infrared continuum is often observed to decrease in reflectance toward longer wavelengths. This negative continuum slope is associated with both bright and dark regions, but is much more pronounced in the latter. Laboratory experiments were performed to investigate the properties of the continuum slope for thin ferric coatings of fine particles on dark substrates. Several interrelated causes of the negative continuum slope include the presence of very thin ferric coatings on dark substrates and reflectance behavior that is dependent on surface texture, viewing geometry, and wavelength. This reflectance behavior is characterized by the preferential reflectance of longer wavelengths in the forward direction and is observed to various degrees for tightly packed particulate surfaces, for materials with very small grain-sizes, and for surfaces otherwise smooth on the order of or smaller than the wavelength of the measured light. This wavelength-dependent directional reflectance produces a negative continuum slope when viewed at backscattering geometries. Variations in continuum slope are caused by varying ferric coating thickness, varying surface texture, and varying geometry of the measurement. The ISM imaging spectrometer data of the Olympus Mons region reveal surface features in the form of annuli and arcs on and near the flanks of the volcano that are defined by spatially coherent continuum slope values. These ISM data provide a unique opportunity to evaluate the spatial variation of Martian surface properties in the context of the thin ferric coatings experiments. The spatial distribution of the spectral annuli and arcs is consistent with eolian transported particulates and with the formation of duricrust or duricrust-like surface texture. © 1993 by Academic Press, Inc.