V. Optical, thermal, radio and radar properties Of the Moon’s surface Infrared evidence of differential surface processes on the Moon

The following summarizes certain previously unpublished inferences regarding the lunar surface that were included in a more extensive oral presentation. Infrared (Shorthill & Saari 1961; Murray & Wildey 1964) and radar (Pettengill & Henry 1962) observations of the Moon acquired in 1960–62 demonstrated that, in some cases at least, conspicuously bright craters like Tycho also are characterized by the presence of more consolidated material at or very near the surface and by considerably rougher terrain on the metre scale. Interpreting the bright craters generally as younger—and less aged—than the less conspicuous craters leads to the conclusion that the process of modification operative on the lunar surface not only gradually reduce the visible reflectivity to the average back­ground level but also smooth and insulate the surface materials. Recent observations of the infrared emission during a lunar eclipse (Saari & Shorthill 1965) and during the lunar night time (Murray, Westphal & Wildey 1967) reveal further an unexpected degree of variability in thermal properties geographically. The infrared anomalies observed during lunar light time and eclipses generally correspond and are distributed quite nonuniformly. For instance, Mare Tranquillitatis exhibits a much higher surface density of anomalies than does Mare Serenitatis. Also, Mare Crisium is characterized by a small, but real, enhancement of night time infrared emission throughout; similar enhancements are also apparent on some portions of other maria surfaces during an eclipse. Both the nonuniform distribution of infrared anomalies and the nonuniform low level enhancements imply processes on the lunar surface which in some areas preferentially produce or expose material of lower than average thermal inertia (more consolidated material) and/or in other areas preferentially remove or cover such material. Specifically, either random impact must be more effective in exposing consolidated rock in Mare Tranquillitatis than in Mare Serenitatis because of intrinsic physical differences in the host rocks of the two maria, or there has been a more rapid covering process operative in Mare Serenitatis. The broad, low level enhancements require similar selective formation or removal processes. These inferences would seem to be most compatible with a terrain characterized by a range of lithologies and, possibly, by periodic extrusion of thin blankets of new materials. It may be of importance to search for any correlation between the distribution of non-thermal visible emission and the distribution of infrared and other anomalies because the same differential surface processes may control the magnitude and distribution of both sets of phenomena.