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Roger Weller, geology instructor

Earth and Moon
by Richard G. Martinez
Physical Geology
Fall 2007


Creation of the Earth and the Moon


Last Quarter Moon


The Moon is the only natural satellite of the Earth. Other than the Sun, the Moon is the second brightest object in the sky. The Moon is one forth the size of Earth. The Moon orbits around the Earth at about every 29.5 Earth days. That is about once per month. The moon always shows the same face towards the Earth. The moons orbit around the Earth is exactly as long as one spin around its axis (1 lunar day). There are several theories to how the creation of the Earth-moon system formed and the “Capture Theory” will be the first to look at.


The Capture Theory:

          This theory state that the Moon was formed somewhere else in the Solar System. It had slowed down by debris close to the Earth, and was captured by the force of gravity from Earth. The problem to this theory is that the encounter would have been at a high velocity to where being captured would not happen. Mostly, the samples of rocks brought back shows that the Earth and Moon have about the same amount of oxygen isotopes which make them similar to one another.

The Coaccreation Theory:

          This theory states that the Moon was formed from the accreation disc of solid objects. This cannot explain the angular momentum and the difference between the chemical composition of the earth and Moon. If they formed at the same place, their compositions would have been similar. The Earth is about 1.6 times denser than the Moon.

The Fission Theory:

          This theory came from George H. Darwin, son of Charles Darwin. It states that the Earth spun and flattened so quickly, that a large piece of material was ejected and eventually became the Moon. The Pacific Ocean and its basin are sometimes thought as to where this piece came from. This would explain why both compositions are similar, but the angular momentum is to small to fit into this model.


The idea in a nutshell:

At the time Earth formed 4.5 billion years ago, other smaller planetary bodies were also growing. One of these hit earth late in Earth's growth process, blowing out rocky debris. A fraction of that debris went into orbit around the Earth and aggregated into the moon.


The Giant Impact, copyright William K. Hartman


The Collision Theory:

          This is the most popular theory, and another known as the “Giant Impact Theory.” It is stated that when the Earth cooled down, a mars sized protoplanetary hit it melted the crust again, and the matter that was thrown into space gathered together and slowly formed the Moon. This would help explain why the Earth’s crust and the Moon have the same density and materials.









Five Hours After Impact, based on computer modeling by A. Cameron, W. Benz, J. Melosh, and others.
 Copyright William K. Hartmann




         Giant impacts were not uncommon during the late stages of the formation of the Terrestrial planets. After the system developed, there were billions of years of bombardment from meteors asteroids and comets. The surface of the Moon is evidence to this process. This is very common to objects in our Solar System.





Works  Sited