ABSTRACT

Hypervelocity cosmic bodies impacting on the lunar surface create volumes of vaporized rock materials which react in thermal agitation with the lunar soils. Water drops falling onto a surface of hot silt provide a model for the thermal agitation process and produce several distinctive types of craters which appear to have exact analogues on the lunar surface. Surprisingly, the interaction between the silt and vaporizing water is not always the same and at least three types of reactions occur, each producing a distinctive crater morphology. In one type of reaction, the blast triggers the formation of an outwardly expanding, rolling ring of vapor and rock debris. The expanding ring digs a crater which has slump features around the rim and the inwardly rolling motion piles materials up in the center to form a cone. If the depth of silt is shallow, the same reaction creates a flat-bottomed crater with slump rings. A second type of reaction occurs when the vaporized materials rotate horizontally in a swirling motion. This type of reaction creates concentric multiple craters and chains, or lines of coalesced craters. Swirl-like slumps of debris are left in the bottom of some of the craters, while in others, the wall may be breached and the material flows away leaving a rille-like trail. A third reaction occurs when the impact explodes violently and the materials are blown from the crater. These craters have blast rays and generally have smoothly sloping bottoms which contain a minimum of debris. Photographs of all these features observed in the model studies are shown and compared with photographs of analogous features on the surface of the moon. Implications as to the origin of the lunar surface are discussed.