Silicate rocks in powdered form can be classified as belonging to the broad categories of rock glasses, crystalline acidic rocks, or crystalline basic‐ultrabasic rocks, on the basis of spectral reflectance properties in the 0.4‐ to 2.0‐μ wavelength region. Distinguishing characteristics are the ratio of reflected light at 0.7 μ to that at 0.4 μ, here termed R/B, the change in R/B with albedo, the maximum albedo of a rock powder, and the occurrence of broad absorption bands in the reflectance spectrums between 0.8 and 2.0 μ. Spectral reflectance properties depend strongly on particle opacity which, in turn, is controlled by chemical composition, mineralogy, and particle size; but reflectance also is affected in a regular way by particle packing, particle shape, and illumination geometry. The experimental results have potential application in determining gross compositional trends on planetary surfaces, but they also emphasize that many variables must be considered in any geologic interpretation of spectral reflectance data.

Lunar Orbiter II photographed a 13-meter boulder which has rolled down the inner slope of a 3-kilometer crater leaving a track 6 meters wide. A static-bearing . strength of 4 x 10(6) dynes per square centimeter at 75-centimeter depth is estimated from these data if certain assumptions are made.

Differences in mean particle size of disturbed versus undisturbed materials in the vicinity of the Surveyor 1 footpads explain the albedo differences seen in Surveyor 1 footpad photographs. Decrease of albedo with increase in particle size is shown by laboratory measurements reported elsewhere and agrees with the Surveyor 1 observation that the darker ejecta is coarser than the surrounding finer grained brighter material. This interpretation of the data also implies that, at the location of Surveyor 1, there exists a lunar surface layer which is very fine grained and less than 5 cm thick. The material at the landing site is basic or ultrabasic in composition.