The Giant Impact and the Formation of the Moon

Details The Giant Impact and the Formation of the Moon The Giant Impact and the Formation of the Moon

Jan 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998lpico.957....3c&link_type=abstract

Origin of the Earth and Moon, Proceedings of the Conference held 1-3 December, 1998 in Monterey, California. LPI Contribution N

Computer Science

Angular Momentum, Earth-Moon System, Impactors, Lunar Evolution, Selenology, Hydrodynamics, Solar System Evolution

Scientific paper

The earlier simulations by Cameron of giant impact scenarios for the formation of the Moon covered only a limited range of parameter space. It was then assumed that at impact the Earth was nearly fully formed and the impactor was rather small, and the total mass of the impact was about 1 Earth. The angular momentum in the simulated collisions usually approached twice that in the present Earth-Moon system (= 2 rem), but this was not varied in a systematic way. In summary, a limited part of parameter space was randomly sampled. During the last couple of years, with increased computing power available, it has been possible to carry out a systematic survey of a larger part of parameter space. The calculations of the giant impact have used Smooth Particle Hydrodynamics (SPH). The survey assumed a proto-Earth-to-impactor mass ratio of 7:3. The number of particles involved in the survey was 10,000. After the survey had found the approximate optimum parameters required to leave about two lunar masses of material in orbit following the giant impact, more detailed calculations were carried out in that parameter region using 20,000 particles. For the three most nearly optimum cases, calculations are currently under way using 100,000 particles. The first survey varied the angular momentum of the collision for a total collisional mass of 1 Earth and an angular momentum range of 1.0 to 3.25 fem. In a reasonable range of 1.0 to 1.25 lem the amount of mass left in orbit was much too small to from the Moon; the orbiting mass increased most of the way to the upper limit, and then increasing amounts of mass escaped from the system. Beyond 3.25 lem no collision took place. The total colliding mass needed to be reduced. The second survey chose a total colliding mass of 0.5 Earth and an angular momentum range of 0.5-1.0 lem (beyond which no collision took place). The same general pattern was observed; the orbiting mass increased toward the upper limit of angular momentum. However, the instances of higher angular momentum were unacceptable for forming the Moon because of the high content of metallic Fe among the orbiting debris. The giant impact scenario, in which the collision occurs when the proto-Earth is only half assembled is the only scenario so far identified that can yield a lunar-sized Moon leaving the Earth-Moon system with an angular momentum equal to that of the current Earth-Moon system.

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