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Theories of the origin of the Moon

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Joseph KramerBy Joseph Kramer

Astronomy Columnist

I really hope readers have enjoyed exploring the relationship between our temporary home and its closest celestial neighbor that seems to have such a significant impact on our lives in the ‘here and now’. It really is something to appreciate. Not just the physical bodies themselves and their interactions with each other but also the laws that govern their behavior. The laws that transcend our tiny corner of the incomprehensibly enormous ‘everything’. For this last article in our lunar exploration I would like discuss modern theories and ideas concerning the origins of our Moon. Natural satellites are formed in many various ways. Mars for example has two moons, Phobos and Deimos. Both of these we once asteroids that were captured by Mars’ gravity. This very well could be a common method of natural satellite formation. When we consider other satellites however, such as the gorgeous rings of Saturn, we still have a lot to learn to understand their origins.
Let’s get elbow deep in some ideas, I don’t have a lot of words available. The Earths density is 5.52 g/cm^3 which is much greater than the Moons 3.35 g/cm^3. This fact alone eliminates the idea that the Earth and Moon formed around the same time, out of the same material (double planet theory). However, the crust of the Earth has a density similar to that of the Moon.


In 1878 and idea was presented that suggest the material that forms the Moon had ‘ejected’ from the Earth by Sir George Howard Darwin (the son of Charles Darwin). The idea is that the fast rotation and solar tides forced a piece of the large basin of the Pacific Ocean to break away. This is referred to as the ‘fission hypothesis’.
In the early 20th century, the capture theory was presented, in which just like Mars’ moons, celestial a celestial object was captured by Earth’s gravity. A significant problem with this theory is that the way a celestial object can be completely captured by another, and not just have their orbit redefined, is if they come in contact with each other or if a third object with a relevant gravitation force is present, slowing down one of the objects to orbital speed.
In the 1970s, a new theory was proposed by A.W. Cameron and William Ward of Harvard. To my understanding, this is the most widely accepted theory of Lunar Origins today. But there are some significant problems with it. The Large Impact Theory simply states that the Moon formed as the result of an impact between a large object and the Earth. In the past, I have heard our early planet referred to as ‘Earth Mach One’ and ‘Earth Mach Two’ which refers to our planet prior to this collision and then after it. (Not that this rhetoric has any significance into understanding this). This is also referred to as the collisional ejection theory. Simulations show many reasons why this theory could be accurate which would result in today’s environment such as rotational speed of the Earth, the ratio of iron in the Moon and Earth, the plane the Moon orbits the Earth around, the rotational rate of the Moon and its orbital period. Just a few years ago, this would have confirmed the origin of the Moon. But like every human, scientists do not know everything and acknowledge they could be wrong. Disclaimer: I am not a chemist and know very little about it. I only know enough to apply it to astronomy and cosmology to a small degree. What I am about to explain is very recent development in the field.
The oxygen isotope ratio of a celestial body can very well be considered its ‘fingerprint’. This is a thing that can distinguish one object from another with great accuracy. It should be considered a type of identifying signature. If the large impact theory were accurate, that would mean one body with a specific isotope ratio, collided with the Earth that has our known isotope ratio. The result would be two bodies with different isotope ratios In 2001, an examination of the Lunar samples from the Apollo mission determine the Lunar isotope ratio to be unexpectedly similar to terrestrial isotope ratios, which is completely different from any other objects composition in our solar system. It has been shown that the likelihood of the object that would have collided with the Earth, in this large impact theory, would have had less than one percent chance of having the chemical composition to create this similarity between the Earth and the Moon. Since this discovery, there have been studies that both strengthen and weaken the large impact theory.
In conclusion, one day after our Sun becomes a red giant and scorches all life on Earth shortly before its own destruction, when we are a space faring species, the knowledge of the origins of our Moon may be irrelevant. We will discuss our home planet in this detail to satisfy the same curiosity that fuels the reason we discuss the origins of our ability for language or the domestication of dogs today. It will not have any practical implication. From our perspective today, this is still a very interesting idea in our quest to simply know all of the unknown. Who would have known that when we look at the Moon, something we are all so used to, that we can explore these great ideas in such detail, and in doing so, gain a greater understanding about our own origins. I wish you all clear skies. All you have to do is look up, I promise, you won’t regret it.

Joseph Kramer is an Army Aviation Officer. He is an amateur astronomer and contributing member of Central Arkansas Astronomical Society.