Spectrophotometric studies of the photometric function, composition, and distribution of the surface materials of Mars

Appendix 2, along with the entire paper, is available in microfiche. Order from the American Geophysical Union, 1707 L Street; N.W., Washington, D.C. 20036. Document J72-002; $1.00. Payment must accompany order. During the 1969 opposition an IR (λλ 0.60–2.27 μm) mapping program of the surface of Mars was carried out at a spatial resolution of 300–500 km. Spectral albedo curves were obtained for 150 mare and desert areas. These curves fall into two well-defined and fairly uniform color-albedo groups corresponding to mare and desert units. The lack of areas with color and albedo that are distinctively different from those of these groups is interpreted as an indication that the surface of Mars consists of basically only two types of materials. Laboratory comparison data support a model of the Martian regolith that suggests that the general color of these surface units is due to a limonite stain coating the particles of lithic soils. Color differences between the two units in the 1-μm region are interpreted as an indication that the proposed lithic soils of the maria are richer in pyroxene, olivine, or both than the lithic soils of the deserts. Features in the spectral albedo curves at 1.6 and 2.1 μm are interpreted as being due to the H2O ice on the surface or in the Martian atmosphere. The data are also used to show that the photometric properties of the Martian deserts can be described empirically by using the Minnaert function, at least for wavelengths between 0.60 and 2.27 μm and for phase angles up to ∼20°. Values of k, the limb-darkening coefficient of the Minnaert function, are given for those ranges of wavelengths and phase angles.