Theories of The Origin of the Moon
The Moon is the only natural satellite of Earth. The distance from Earth is 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 and Artemis 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. 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.
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.
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.
First 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.
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 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