Apr
19
2011

From Whence the Moon Comes?

By Brady Anderson

Growing up as a boy in a small farming community in rural Nebraska one of my favorite things to do was to stare at the night sky. It was always so beautiful. I would often fantasize of traveling from planet to planet to uphold intergalactic peace as a Jedi knight. Among these imaginations I always wondered where all these planets came from. I especially always wondered where the moon came from. Was it really made of cheese? Did a man truly live there? Could a cow jump over it? And was there actually anything worth seeing on its dark side? Well, now that I am older I don’t look at the sky as often as I used too—darn city lights!—but I do still wonder how exactly the moon come about. As it turns out many in the scientific community have also been concerned with this question and over the years there have been a few different theories as to the origin of the moon.

If found some theories about how the moon was formed on the Planetary Science Institute’s website. One theory is that the moon was a planet that was form around the same time as the earth was formed but was pulled into orbit around the earth due to its smaller density. This theory was largely discredited because the moon does not have any iron. The lack of iron is evidence that the moon was not formed from the same process as the earth was because if it was it would contain iron as the earth does. In response to this lack of iron another theory suggested that the moon formed in some other part of the solar system where there was little iron and was then somehow captured into earth’s orbit. This was disproved when lunar rocks were analyzed and showed the same isotope composition as the earth. The fact that they have the same isotope composition gives evidence that the earth and the moon were formed in the same area of the solar system. Another theory suggested that the moon the early earth was spinning so quickly that the moon flung off the earth. However, it has been proven that the angular momentum and energy need for such a thing to happen are impossible given what we know about the size and composition of the earth and moon.

The leading theory of the moon’s origin is an idea put forward by Dr. William K. Hartman and Dr. Donald R. Davis (http://www.psi.edu/projects/moon/moon.html). They proposed that the moon was formed by a body about the size of mars impacting the earth some around 60 million years after the earth’s initial accretion (Earth System History Third Edition, p. 248). Apparently this celestial body blasted into the earth with such speed and impact that some of the mantle of the body was separated to form what is now the moon. The Planetary Science Institute (http://www.psi.edu/projects/moon/moon.html) shows a computer simulation of what the impact probably looked like.

This impact theory explains several facts about the moon and so lend to its probability. First, the moon does not have any water whatsoever. The theory suggests this is so because water and other compounds were expelled from the moon during its formation. Second, the moon has a small metallic core. This is explained by the theory because when the mars sized body impacted the earth its core sank into the earth and became part of its core, but the body’s mantle exploded from it to form the small core of the moon. Third, the moon has a feldspar-rich outer layer. The theory accounts for this because of the heat involved in the impact formation of the moon. The early moon would have had a magma ocean early on much like the early earth (Earth System History Third Edition, p. 248-249).

So there you have it! The moon was most likely formed from an impact of a smaller celestial body on the earth. It was definitely not made of cheese.

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