The Moon is the only natural satellite of Earth.
The distance from Earthis about 384,400km with a diameter of 3476km and a mass of 7. 35*1022kg. Through history it has had many names: Called Luna by the Romans, Selene andArtemis by the Greeks. And of course, has been known through prehistoric times.
It is the second brightest object in the sky after the Sun. Due to 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: fission from the earth; formation inearth orbit; and formation far from earth. Then, in 1975, having studied moonrocks and close-up pictures of the moon, scientists proposed what has come to be regarded as the most probable of the theories of formation, planetesimalimpact or giant impact theory. 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.
Thisidea gained support partly because the density of the moon is the same as thatof the rocks just below the crust, or upper mantle, of the earth. A majordifficulty with this theory is that the angular momentum of the earth, in order toachieve rotational instability, would have to have been much greater than theangular momentum of the present earth-moon system. Formation in Orbit Near the EarthThis theory proposes that the earth and moon, and all other bodies of thesolar system, condensed independently out of the huge cloud of cold gases andsolid particles that constituted the primordial solar nebula. Much of thismaterial finally collected at the center to form the sun.
Formation Far from Earth According to this theory, independent formation of the earth and moon, asin 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 theearth and moon then, it is surmised, carried them near each other so that the moonwas pulled into permanent orbit about the earth. Planetesimal Impact First published in 1975, this theory proposes that early in the earth’shistory, well over 4 billion years ago, the earth was struck by a large body calleda planetesimal, about the size of Mars. The catastrophic impact blastedportions of the earth and the planetesimal into earth orbit, where debris from theimpact eventually coalesced to form the moon. This theory, after years of researchon 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 itwould seem to require that the earth melted throughout, following the impact,whereas the earth’s geochemistry does not indicate such a radical melting. Planetesimal Impact Theory (Giant Impact Theory)As the Apollo project progressed, it became noteworthy that few scientists working on the project were changing their minds about which of these threetheories they believed was most likely correct, and each of the theorieshad its vocal advocates. In the years immediately following the Apollo project,this division of opinion continued to exist. One observer of the scene, apsychologist, concluded that the scientists studying the Moon were extremely dogmatic andlargely immune to persuasion by scientific evidence. But the facts werethat the scientific evidence did not single out any one of these theories.
Each oneof 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 theEarth, in the course of its accumulation, would undergo some major collisions withother bodies that have a substantial fraction of its mass and that thesecollision would produce large vapor clouds that they believe might play a role in theformation 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 themass of Mars would be needed to give the Earth the present angular momentum ofthe Earth-Moon system, and they also pointed out that such a collision would produce a large vapor cloud that would leave a substantial amount ofmaterial in orbit about the Earth, the dissipation of which could be expected to formthe 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 communityduring the next few years. However, in 1984, when a scientific conference on theorigin of the Moon was organized in Kona, Hawaii, a surprising number of paperswere submitted that discussed various aspects of the giant impact theory. At thesamemeeting, the three classical theories of formation of the Moon werediscussed 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 thatit was relatively untested and that it would be appropriate to reservejudgement onit until a lot of testing has been conducted.
The next step clearly calledfor 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 (Universityof Arizona, Tucson, AZ) undertook such simulations. They have used an unconventional technique called smooth particle hydrodynamics to simulatethe planetary collision in three dimensions. With this technique, we havefollowed a simulated collision (with some set of initial conditions) for many hours ofreal 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 therelative amounts of rock and iron that would be in each of these different massfractions.
We have carried out simulations for a variety of different initialconditions and have shown that a “successful” simulation was possible if the impactingbody had a mass not very different from 1. 2 Mars masses, that the collision occurredwith approximately the present angular momentum of the Earth-Moon system, and that the impacting body was initially in an orbit not very different fromthat 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 thisform). This contrasts with the Earth, a typical terrestrial planet in bulkcomposition, which has about one-third of its mass in the form of the iron core. Thus, asimulation could not be regarded as successful unless the material leftin orbit was iron free or nearly so and was substantially in excess of the mass ofthe Moon.
This uniqueness highly constrains the conditions that must be imposedon the planetary collision scenario. If the Moon had a composition typical ofother terrestrial planets, it would be far more difficult to determine theconditions 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 andthat a collision of slightly more than a Mars mass with the Earth, with theEarth-Moon angular momentum in the collision, would put almost 2 Moon masses of rockinto orbit, forming a disk of material that is a necessary precursor to theformation 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 thatare dedicated to them.
Subsequent calculations have been done at Harvard. The first set ofcalculations was intended to determine whether the revised hydrodynamics code reproducedprevious results (and it did). Subsequent calculations have been directedtoward determining whether “successful” outcomes are possible with a wider rangeof initial conditions than were first used. The results indicate that theimpactor must approach the Earth with a velocity (at large distances) of not more thanabout 5 kilometers. This restricts the orbit of the impactor to lie near that ofthe Earth. It has also been found that collisions involving larger impactors with morethan the Earth-Moon angular momentum can give “successful” outcomes.
This initial condition is reasonable because it is known that the Earth-Moon system haslost 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 moreto completeBibliographyGIANT 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-498EARTH’S ROTATION RATE MAY BE DUE TO EARLY COLLISIONS, Paula Cleggett-Haleim, Michael Mewhinney, Ames Research Center, Mountain View, Calif. RELEASE: 93-012Hartmann, 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.
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