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R. M. Canup (Southwest Research Institue), E. Asphaug (University of California Santa Cruz)
The leading hypothesis for lunar origin is the giant impact theory, which proposes that the Moon formed from debris ejected into bound earth orbit when early Earth collided with a roughly Mars-sized protoplanet (Hartmann & Davis 1975; Cameron & Ward 1976). Simulations of potential lunar-forming impacts using a 3-D Lagrangian method known as Smooth Particle Hydrodynamics (or SPH; Lucy 1977) have been performed in numerous works (Benz et al 1986, 1987, 1989; Cameron & Benz 1991; Cameron 1997, 2000, 2001).
Numerical resolutions now achievable with SPH allow for the material placed into orbit as a result of the impact-typically representing a few percent of the total mass-to be resolved with 102 to 103 SPH particles. Recent works (Cameron 1997, 2000, 2001) have identified only a limited class of impacts capable of placing a sufficient amount of material into orbit to yield the Moon: those that involve a collisional angular momentum twice that of the current Earth-Moon system, or that require that the lunar-forming impact occurred when Earth was only about half formed. Both of these scenarios are restrictive and somewhat problematic.
Here we are performing a new series of SPH simulations of potential satellite-forming impacts. In particular, we are exploring the dependence of collisional outcome on the equation of state, the mass ratio of the impactor-to-target, and the pre-impact partitioning of angular momentum into spin vs. impact parameter. One objective is to test the sensitivity of the empirical scaling relationships observed in Cameron's results (Canup et al. 2001) to a broader variation in impact conditions. Interestingly, our initial results suggest a wider range of impacts than those described to date by Cameron may also be successful lunar forming candidates.