Theories Of The Origin Of The Moon — страница 2

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with other bodies that have a substantial fraction of its mass and that these collision would produce large vapor clouds that they believe might play a role in the formation of the Moon. A.G.W. Cameron and William R. Ward (Harvard University, Cambridge MA) pointed out that a collision with a body having at least the mass of Mars would be needed to give the Earth the present angular momentum of the Earth-Moon system, and they also pointed out that such a collision would produce a large vapor cloud that would leave a substantial amount of material in orbit about the Earth, the dissipation of which could be expected to form the Moon. The Giant Impact Theory of the origin of the Moon has emerged from these suggestions. These ideas attracted relatively little comment in the scientific

community during the next few years. However, in 1984, when a scientific conference on the origin of the Moon was organized in Kona, Hawaii, a surprising number of papers were submitted that discussed various aspects of the giant impact theory. At the same meeting, the three classical theories of formation of the Moon were discussed in depth, and it was clear that all continued to present grave difficulties. The giant impact theory emerged as the "fashionable" theory, but everyone agreed that it was relatively untested and that it would be appropriate to reserve judgement on it until a lot of testing has been conducted. The next step clearly called for numerical simulations on supercomputers. ?The author in collaboration with Willy Benz (Harvard), Wayne L.Slattery at

(Los Alamos National Laboratory, Los Alamos NM), and H. Jay Melosh (University of Arizona, Tucson, AZ) undertook such simulations. They have used an unconventional technique called smooth particle hydrodynamics to simulate the planetary collision in three dimensions. With this technique, we have followed a simulated collision (with some set of initial conditions) for many hours of real time, determining the amount of mass that would escape from the Earth-Moon system, the amount of mass that would be left in orbit, as well as the relative amounts of rock and iron that would be in each of these different mass fractions. We have carried out simulations for a variety of different initial conditions and have shown that a "successful" simulation was possible if the impacting

body had a mass not very different from 1.2 Mars masses, that the collision occurred with approximately the present angular momentum of the Earth-Moon system, and that the impacting body was initially in an orbit not very different from that of the Earth. ?The Moon is a compositionally unique body, having not more than 4% of its mass in the form of an iron core (more likely only 2% of its mass in this form). This contrasts with the Earth, a typical terrestrial planet in bulk composition, which has about one-third of its mass in the form of the iron core. Thus, a simulation could not be regarded as ?successful? unless the material left in orbit was iron free or nearly so and was substantially in excess of the mass of the Moon. This uniqueness highly constrains the conditions that

must be imposed on the planetary collision scenario. If the Moon had a composition typical of other terrestrial planets, it would be far more difficult to determine the conditions that led to its formation. The early part of this work was done using Los Alamos Cray X-MP computers. This work established that the giant impact theory was indeed promising and that a collision of slightly more than a Mars mass with the Earth, with the Earth-Moon angular momentum in the collision, would put almost 2 Moon masses of rock into orbit, forming a disk of material that is a necessary precursor to the formation of the Moon from much of this rock. Further development of the hydrodynamics code made it possible to do the calculations on fast small computers that are dedicated to them. Subsequent

calculations have been done at Harvard. The first set of calculations was intended to determine whether the revised hydrodynamics code reproduced previous results (and it did). Subsequent calculations have been directed toward determining whether "successful" outcomes are possible with a wider range of initial conditions than were first used. The results indicate that the impactor must approach the Earth with a velocity (at large distances) of not more than about 5 kilometers. This restricts the orbit of the impactor to lie near that of the Earth. It has also been found that collisions involving larger impactors with more than the Earth-Moon angular momentum can give "successful" outcomes. This initial condition is reasonable because it is known that the