The role of ballistic erosion and sedimentation in lunar stratigraphy

Abstract

Many of the lunar surface formations have been emplaced by impact craters. Two mechanisms have been proposed for transport of crater ejecta; both the base surge and ballistic transport mechanisms are reviewed in this paper. Formation of base surges associated with underwater and underground explosion craters and with volcanic events all require the presence of an atmosphere in the area where ejecta impacts. Ejecta impacts and mixes with air and forms an aerosol cloud that carries the dust outward and deposits it on preexisting terrain. Because the moon contains no appreciable atmosphere, it is concluded that the base surge mechanism of separation and transport of fine‐grained crater ejecta is not a viable lunar process. Studies of laboratory impact craters, high‐explosion and nuclear craters, and lunar craters, and theoretical studies of formation of impact craters indicate that material is ejected from impact craters at low angles to the surface. Calculated ejecta positions at constant times after impact of the body that produced Copernicus crater are similar to shapes that are observed when laboratory craters are formed. Results are used to construct a model of emplacement of deposits of craters of all sizes. Particles are emplaced in low‐angle trajectories around the crater from the base of the expanding truncated cone. All of the ejecta of a small crater is emplaced at such a low velocity that the deposit consists entirely of crater ejecta. Therefore the deposit that surrounds the small lunar crater is entirely crater ejecta. When large craters are formed a significant fraction of the crater's ejecta has velocity high enough to crater preexisting terrain when it impacts. It craters preexisting terrain and mixes it with primary crater ejecta. The mixture of debris moves laterally away from the crater a short distance and forms the crater's deposit. It is concluded that deposits of large craters contain local preexisting material as well as crater ejecta. The cratering model is used to synthesize many lunar observations. For example, dunes around small lunar craters can be explained as a result of interaction of the flow with material ejected from secondary craters produced later at greater radial distances. Deceleration lobes and other features also are related to the emplacement model. Mantled Imbrium sculpture is explained as a result of production of the sculpture by secondary cratering due to passage of the conical sheet of ejecta and subsequent mantling of ejecta of secondary craters produced earlier nearer the crater. Model results, coupled with lunar observations, suggest that lunar smooth plains are in some places the erosional products of secondary craters of many highland craters and in some places they were emplaced by basins and consist of basin ejecta mixed with regional and local material. In some places they are predominantly deposits of local primary craters. The small low‐albedo smooth pondlike deposits that surround many lunar craters may be impact melts. If they are and if they were emplaced from ballistic trajectories, then the angles of ejection must have been much higher than the angles of ejection of the other ejecta. Such a bimodal pattern of crater ejecta may be explained if impact occurs in a layered target. This suggests that impact melts in crater deposits may result from stratigraphic layering within the area of the crater.