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Origin of the Moon

by Yvonne Rathbone
©2000

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I'm writing this piece as a synopsis for an amazing show I saw on the Discovery Channel. The original show aired in December of 1999. I came in during the middle of it so I didn't catch the name, but it was narrated by Patrick Stewart. Anyway, in the interest of deepening our relationship with the Moon through scientific fact, I thought I'd share some of the history of the moon's origin as some planetary scientists see it.

Scientists have been trying to come up with a model for how we got our moon. Turns out our moon is rather amazing in being so big and singular. Models they've run that use scenarios we thought were obviously true, such as particulate matter coalescing naturally into an Earth and a Moon, don't work. That just gives us a Venus situation - no moon. Only big planets like the gas giants get moons of any size. Terrestrial planets only get the occasional asteroid a la Mars. So the fact that our moon is so big and beautiful is a mystery.

The way the scientists went about figuring out the origin of the moon was to keep running simulations until they got one that resulted in our situation. Their first thing to check out was some sort of collision with a rogue planet. But most collisions just sent out dust that eventually fell back to earth with no moons forming. Some collisions made two moons, but these were always very unstable, much smaller than our moon and fell apart within a very short 100,000 years.

Finally they tried a very oblique collision, basically a near miss between a rogue planet, which they named Orpheus, and what they named Earth 1. The collision goes something like this: Orpheus nicks Earth 1. This slows down Orpheus enough that it can't pull away from Earth 1's gravity. It moves slightly past Earth and arcs back, hitting Earth directly, but at a slower velocity. This collision completely disrupts Earth 1's crust and much of it's mantle and even mixes the iron cores of the two planets. Truly, the result can be considered a different planet. The dust that is thrown up due to this collision is thrust far enough away from the Earth that it can't just fall back. Eventually it coalesces into our Moon. So the origin of the Moon and the origin of Earth as we know it are one and the same.

Some more amazing things about this scenario: Before this collision, scientists believe that the Earth's surface was covered with much more water. The amount of land we have today is the direct result of the formation of the moon.

And that's not the only way the moon makes life on Earth possible. Because of it's size, the Moon exerts a stabilizing gravitational pull on the Earth. The 26 degree tilt of the Earth stays that way because of the Moon. Mars, which has only two teeny moons, wobbles all over the place. Sometimes it's poles point to the sun, sometimes it rotates at a 90 degree angle to the sun. This means no seasons. On Earth, however, our planet is held at the same angle and so we have seasons. The Moon, not the Sun, is the source of our seasons!

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