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Evolution of solar system
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 in
earth
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 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.

ôThe 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.

ôThe 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 æsuccessfulÆ 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 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.  ôTerrestrial, Lunar, and Interplanetary Rock
Fragmentation.ö

Hartmann, W. K. 1977.  ôLarge Planetesimals in the Early Solar System.ö
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.