Forming a Moon with an Earth-like composition via a Giant Impact

In the giant impact theory, the Moon forms from debris ejected into an Earth-orbiting disk by the collision of a large planet with the early Earth. Prior impact simulations generally predict that most of the disk material originates from the impacting planet (e.g., 1-3). However, the Earth and Moon have identical oxygen isotope compositions (4). This has been a challenge for the impact theory, because the impactor's composition would have likely differed from that of the Earth (e.g., 5). Here we simulate impacts involving much larger impactors than previously considered. In this case, the impactor's composition strongly contaminates the composition of both the target planet and the disk. We find these impacts can produce a massive disk with an essentially identical composition as the final planet's mantle, consistent with Earth-Moon compositional similarities (6). However such impacts leave the Earth-Moon system with a substantial excess of angular momentum, typically by a factor of two. Ćuk & Stewart (7) have recently proposed that the Earth-Moon system angular momentum could have been reduced by this amount due to capture of the Moon into the evection resonance with the Sun. Alternatively, we also consider whether large impactors into retrograde rotating targets could produce close disk-planet compositional agreement with a reduced final system angular momentum, which would relax the degree of angular momentum removal required after the impact. (1) R. M. Canup, E. Asphaug, Nature 412, 708 (2001); (2) R. M. Canup, Icarus 168, 433 (2004) ; (3) R. M. Canup, Icarus 196, 518 (2008); (4) U. Wiechert, A. N. Halliday, D. -C. Lee, G. A. Synder, L. A. Taylor, D. Rumble, Science 294, 345-348 (2001); (5) K. Pahlevan, D. J. Stevenson, Earth Plan. Sci. Let. 262, 438 (2007); (6) R. M. Canup, submitted (2012); (7) M. Ćuk, M., S. T. Stewart, Early Solar System Impact Bombardment II, Feb. 1-3, Houston, 4006 (2012).