Comments on the Origin of the Moon

For over a century, it has been realized that there are some aspects about the Moon. The oddness was increased in the time of the Apollo landings, when rock samples were returned to Earth. These unusual features of the Moon translate into difficulties in understanding the origin of the Moon.

Some of the attributes of the Moon that enter into consideration are as follows.

It is large relative to the size of the planet it orbits. Mercury and Venus have no moons; Mars has two very small moons.
Its density is significantly less than that of the Earth; about 3.3 rather than 5.5 grams per cubic centimeter.
The material that makes up the Moon rocks is in many ways quite similar to the Earth, and in other ways quite different. The relative abundance of isotopes of oxygen, O16, O17, and O18, is essentially identical to that of the Earth, whereas other objects in the solar system have measureably different oxygen isotopes.
Moon rocks differ from Earth rocks in that they are deficient in volatile materials. Volatiles are materials, such as water, that are driven off when matter is heated. This indicates that that material that formed the Moon was subjected to an intense heating episode.

Prior to the Apollo program, there were three primary theories for the origin of the Moon. These were fission, coformation, and capture from elsewhere in the solar system. I won’t go into a lot of discussion of them because the whole set of information available after the Apollo landings showed that none of them was viable.

The suggestion that was put forth which is, to date, the best possibility is the Giant Impact Theory. According to this viewpoint, the early Earth formed and had enough time for differentiation to occur. The process of differentiation is the process by which dense materials like iron and nickel settle to the center of the planet, leaving the light material in the outer parts of the planet.

At this point (the theory goes) the early Earth was hit by a large planetesimal about the size of the planet Mars, and a large amount of mantle material was thrown out into space, where it later collected to form the Moon. An artists’s conception is shown on p 180, and shows how it was that the Moon would have formed from lighter rocks, and thus have a lower density than the Earth. It also can explain the low abundance of volatile materials as due to the heating that occurred in the impact event. At the moment, this seems the best suggestion for the origin of the Moon.