
 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.
 
 The 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.
 
 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). 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.  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.
 
 