THE ORIGIN OF THE MOON AND THE SINGLE IMPACT HYPOTHESIS .5.

Previous papers in this series have described the smooth particle hydr odynamics (SPH) method, which has been employed to explore the possibi lity that a major planetary collision may have been responsible for th e formation of the Moon. In those simulations the SPH code used partic les of equal mass and fixed smoothing lengths; I have found that the r esults obtained were reliable regarding what happens to the interiors of the colliding planets. Because the particles placed into the surrou nding 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 v ariable 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 i nfinity. 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 th e Moon formed from a rock disk in orbit around the Earth, then some ot her mechanism would be needed to transport angular momentum and mass o utward in the equatorial plane, so that rack condensates from the hot atmosphere would be precipitated beyond the Roche limit, thus providin g material for collection into the Moon. Most of this atmospheric mate rial was originally in the Impactor and was mixed with terrestrial roc k before evaporation. Recent calculations by R. M. Canup and L. W. Esp osito (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 R oche lobe. On the other hand, higher-angular-momentum collisions can l eave a quite massive body in orbit about the Earth, and this could for m much or essentially all of the Moon. These questions remain challeng ing and require further investigations, (C) 1997 Academic Press.