The moon is the only natural satellite of earth and its nearest neighbor in space. Its distance from the earth is about 384,400km. It is the brightest object in the night sky, bit it gives off no light of its own. The light that is visible is what the moon is reflecting from the sun. It has a diameter of 3476km and a mass of 7.35*1022kg. Throughout history, it has had many names; Luna, as named by the Romans, and Selene or Artemis by the Greeks. As a result of its size and composition, the Moon is sometimes classified as a terrestrial "planet" along with Mercury, Venus, Earth and Mars. Origin of the Moon Before the modern age of space exploration, scientists had three major theories for the origin of the moon: (1) fission from the earth, (2) formation in earth orbit, and, (3) formation far from earth. Then, in 1975, having studied moon rocks and close-up pictures of the moon, scientists proposed what has come to be regarded as the most probable of the theories of formation, planetesimal impact or giant impact theory. (1) Formation by Fission from the Earth The modern version of this theory proposes that the moon was spun off from the earth when the earth was young and rotating rapidly on its axis. This idea gained support partly because the density of the moon is the same as that of the rocks just below the crust, or upper mantle, of the earth. A major difficulty with this theory is that the angular momentum of the earth, in order to achieve rotational instability, would have to have been much greater than the angular momentum of the present earth-moon system. (2) Formation in Orbit Near the Earth This theory proposes that the earth and moon, and all other bodies of the solar system, condensed independently out of the huge cloud of cold gases and solid particles that constituted the primordial solar nebula. Much of this material finally collected at the center to form the sun. (3) Formation Far from Earth According to this theory, independent formation of the earth and moon, as in the above theory, is assumed; but the moon is supposed to have formed at a different place in the solar system, far from earth. The orbits of the earth and moon then, it is surmised, carried them near each other so that the moon was pulled into permanent orbit about the earth. The Planetesimal Impact Theory, the modern version of the fission theory, was published in 1975. This theory proposes that early in the earth's history, well over 4 billion years ago, the earth was struck by a large body called a planetesimal, about the size of Mars. The catastrophic impact blasted portions of the earth and the planetesimal into earth orbit, where debris from the impact eventually coalesced to form the moon. This theory, after years of research on moon rocks in the 1970s and 1980s, has become the most widely accepted one for the moon's origin. The major problem with the theory is that it would seem to require that the earth melted throughout, following the impact, whereas the earth's geochemistry does not indicate such a radical melting. As the Apollo project progressed, it became noteworthy that few scientists working on the project were changing their minds about which of these three theories they believed was most likely correct, and each of the theories had its vocal advocates. In the years immediately following the Apollo project, this division of opinion continued to exist. One observer of the scene, a psychologist, concluded that the scientists studying the Moon were extremely dogmatic and largely immune to persuasion by scientific evidence. But the facts were that the scientific evidence did not single out any one of these theories. Each one of them had several grave difficulties as well as one or more points in its favor. In the mid-1970s, other ideas began to emerge. William K. Hartmann and D.R. Davis (Planetary Sciences Institute in Tucson AZ) pointed out that the Earth, in the course of its accumulation, would undergo some major collisions 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 indicated 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 "
" theory, but everyone agreed that it was relatively untested and that it would be appropriate to reserve judgment on it until a lot of testing has been conducted. The next step clearly called for numerical simulations on supercomputers. 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, they 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. Simulations for a variety of different initial conditions were conducted and it was concluded 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). Additional 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 Earth-Moon system has lost angular momentum due to solar tides, but the amount is uncertain. These calculations are still in progress and will probably take 1 or 2 years more to complete Bibliography GIANT IMPACT THEORY OF THE ORIGIN OF THE MOON, A.G.W. Cameron, Harvard-Smithsonian Center for Astrophysics, Cambridge MA 02138, PLANETARY GEOSCIENCES-1988, NASA SP-498 EARTH'S ROTATION RATE MAY BE DUE TO EARLY COLLISIONS, Paula Cleggett-Haleim, Michael Mewhinney, Ames Research Center, Mountain View, Calif. RELEASE: 93-012 Hartmann, W. K. 1969. "Terrestrial, Lunar, and Interplanetary Rock Fragmentation." Hartmann, W. K. 1977. "Large Planetesimals in the Early Solar System." 1 "Landmarks of the Moon," Microsoft® Encarta® 96 Encyclopedia. © 1993-1995 Microsoft Corporation. All rights reserved. 2 "Characteristics of the Moon," Microsoft® Encarta® 96 Encyclopedia. © 1993-1995 Microsoft Corporation. All rights reserved.