Simulations of a late lunar-forming impact

Results of about 100 hydrodynamic simulations of potential Moon-forming impacts are presented, focusing on the "late impact" scenario in which the lunar forming impact occurs near the very end of Earth's accretion (Canup and Asphaug, 2001, Nature 412, 708-712). A new equation of state is utilized that includes a treatment of molecular vapor ("M-ANEOS"; Melosh, 2000, in: Proc. Lunar Planet. Sci. Conf. 31st, p. 1903). The sensitivity of impact outcome to collision conditions is assessed, in particular how the mass, angular momentum, composition and origin (target vs. impactor) of the material placed into circumterrestrial orbit vary with impact angle, speed, impactor-to-target mass ratio, and initial thermal state of the colliding objects. The most favorable conditions for producing a sufficiently massive and iron-depleted protolunar disk involve collisions with an impact angle near 45 degrees and an impactor velocity at infinity <4 km/sec. For a total mass and angular momentum near to that of the current Earth-Moon system, such impacts typically place about a lunar mass of material into orbits exterior to the Roche limit, with the orbiting material composed of 10 to 30% vapor by mass. In all cases, the vast majority of the orbiting material originates from the impactor, consistent with previous findings. By mapping the end fate (escaping, orbiting, or in the planet) of each particle and the peak temperature it experiences during the impact onto the figure of the initial objects, it is shown that in the successful collisions, the impactor material that ends up in orbit is primarily that portion of the object that was heated the least, having avoided direct collision with the Earth. Using these and previous results as a guide, a continuous suite of impact conditions intermediate to the "late impact" (Canup and Asphaug, 2001, Nature 412, 708-712) and "early Earth" (Cameron, 2000, in: Canup, R.M., Righter, K. (Eds.), Origin of the Earth and Moon, pp. 133-144; 2001, Meteorit. Planet. Sci. 36, 9-22) scenarios is identified that should also produce iron-poor, ∼ lunar-sized satellites and a system angular momentum similar to that of the Earth-Moon system. Among these, those that leave the Earth >95% accreted after the Moon-forming impact are favored here, implying a giant impactor mass between 0.11 and 0.14 Earth masses.