Spectral properties (0.40 to 0.75 microns) of soils exposed at the Viking 1 landing site

The bidirectional reflectance and photometric function (Hapke, 1981) was determined for seven patches of soil located near the Viking Lander 1 spacecraft. The soil photometric function is strongly backscattering and has a prominent opposition effect such that the ratio of reflectances at 1° and 10° phase angles averages 1.25, 1.24, and 1.19 in blue, green, and red wavelengths, respectively. The reflectance of the soil also exhibits a wavelength dependence as a function of phase angle. For instance, red/blue ratios can vary by up to 33% as the phase angle increases from 10° to 75°. Estimates of soil reflectance at a 5° phase angle averaged over the blue and green passbands of the Viking Lander cameras are 0.11 and 0.17, respectively, while estimates of soil reflectance averaged over the red channel range from 0.30 to 0.39. There is little need to call upon mineralogical variations to explain the subtle color variations exposed at the landing sites. Rather, brightness and color variations within the soil can be correlated with particle size, with finer‐grained soil being brighter and redder than coarser‐grained soil. When compared to earth‐based reflectance data, the soil at the Viking 1 site is most like Martian bright areas. Such a result is consistent with the Lander soil being part of a globally homogenized soil unit.