The origin of the moon and the single impact hypothesis .5.

Previous papers in this series have described the smooth particle hydrodynamics (SPH) method, which has been employed to explore the possibility that a major planetary collision may have been responsible for the formation of the Moon. In those simulations the SPH code used particles of equal mass and fixed smoothing lengths; I have found that the results obtained were reliable regarding what happens to the interiors of the colliding planets. Because the particles placed into the surrounding space were isolated rather than overlapping, however, that part of the calculation was unreliable. Ten additional cases have been run with 5000 particles in the Protoearth and 5000 in the Impactor, with variable smoothing lengths. Three of the cases had Protoearth/Impactor mass ratios of 5:5, 6:4, and 7:3. The other cases had a mass ratio of 8:2 and a variety of angular momenta. All cases had zero velocity at infinity. In every case the product of the collision became surrounded by evaporated particles of rock vapor, forming an extended atmosphere; however, relatively little mass extended beyond the Roche lobe. If the Moon formed from a rock disk in orbit around the Earth, then some other mechanism would be needed to transport angular momentum and mass outward in the equatorial plane, so that rack condensates from the hot atmosphere would be precipitated beyond the Roche limit, thus providing material for collection into the Moon. Most of this atmospheric material was originally in the Impactor and was mixed with terrestrial rock before evaporation. Recent calculations by R. M. Canup and L. W. Esposito (1996, Icarus 119, 427-446) have shown that it is very difficult to form the Moon from a gaseous disk largely confined to within the Roche lobe. On the other hand, higher-angular-momentum collisions can leave a quite massive body in orbit about the Earth, and this could form much or essentially all of the Moon. These questions remain challenging and require further investigations, (C) 1997 Academic Press.