The Characterization of Lunar Surface Impact Erosion and Solar Wind Sputter Processes on the Lunar Surface

A wide variety of processes have been instrumental in the evolutionary progression of the lunar surface towards its present state. During the early history of the Moon, chemical differentiation, volcanism and, particularly for the mare areas, infilling by magma flow have been major mass transport factors but, since further cooling some 3 x 10 to the power of 9 years ago, the development of surface features has been primarily determined by impact erosion from remnants of the primeval stellar condensation and matter associated with asteroids and comets. As a result of the evolutionary decrement of interplanetary matter impact erosion is time dependent but even so at a particular epoch the relative importance of impact erosion to solar wind sputter erosion in the determination of the topology of a feature will be dependent on the scale of the feature examined. An impact crater formed on the lunar surface is normally exposed to the 400 km s-1 solar wind spectrum which will result in sputter erosion of surface atoms. Under varying angles of incidence during the lunar cycle, features exposed to the solar wind will be gradually smoothed and will eventually disappear. A similar erosion effect is the abrasion of large craters due to the formation within them of much smaller craters, i.e. a process similar to sand blasting. We also have the possibility during these quasi-continuous processes of single and complete obliteration by an impacting particle at least as large as that which caused the original crater. Thus we may characterize any particular crater with three erosion lifetimes: (1) the sputter lifetime, (2) the (small particle) abrasion lifetime, and (3) the (large particle) erasure lifetime.