the moon

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 within
earth’s orbit; and 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.

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.

Planetesimal Impact

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
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