The Origin of the Moon and the Single-Impact Hypothesis I

Recent ly the s ingle-impact hypothes is for forming the Moon has gained some favorable attention. We present in this paper a series of three-dimensional numerical s imulat ions of an impact be tween the protoear th and an object about 0.1 o f its mass . For computat ional convenience both objects were a s sumed to be composed of granite. We studied the effects on the ou tcome of the collision of varying the impact parameter , the initial internal energy, and the relative velocity. The resul ts show that if the impact parameter is large enough so that the center of the impactor approximate ly grazes the limb of the protoearth, the impactor is not completely dest royed; part of it forms a c lump in a large elliptical orbit about the Earth. This c lump does not collide with the Ear th , s ince the effects, first, of vapor pressure gradients during the impact , and later, of angular m o m e n t u m transfer due to the rotation of the deformed Earth, have modified the ballistic trajectory. However , s ince the orbit of the c lump comes close to the Earth (within the Roche limit) the c lump will be des t royed and spread out to form a disk around the Earth. The amoun t of angular m o m e n t u m in the E a r t h M o o n sys t em thus obtained tends to fall short of the observed amount ; this deficiency would be eliminated if the mass of the impactor were somewhat greater than the one a s s u m e d here. The scenario for making the Moon from a single-impact event is supported by these s imulat ions. © 1986 Academic Press, Inc.