Lunar surface roughness: Shadowing and thermal emission

A statistical model of the moon's surface roughness is used in an attempt to explain the deviations of the observed gross infrared thermal emissive properties of the moon from those characterizing a smooth Lambertian surface. Comparison of theory and experiment suggests that the thermal brightness variation across the full moon disk is affected by large-scale relief of rms slope 10°–20°. The method requires development of a self-shadowing theory of random rough surfaces, which can be used, in a different context, to determine local rms surface slope from the amount of shadow visible in a moon photograph. It is suggested that this technique may be particularly useful in the rapid analysis of Lunar Orbiter photography. Analysis of an earth-based photograph of a typical highland region yields 9° for the rms slope of large-scale roughness. The rms slopes deduced agree with those found in radar studies of the moon.