S&T's Dr. Greg Story says there's evidence to support a theory that there was a collision that melted two planets together and threw off matter to become that big luminous object in the sky that gives wolves and werewolves a fit.

The previous column of this series gave the currently accepted theory of how our moon came to be.  That theory says that the moon was born in a collision between the Earth and a smaller planet about 4.4 billion years ago.  The collision melted the two planets, which merged together to form our present day Earth.  The energy from the collision also ejected a large amount of material which went into orbit around the new Earth.  Over a period of about a thousand years that material clumped together to form our moon.  Today we'll lay out the evidence for this theory.

The Age of the Moon

Thanks to NASA’s Apollo program, which landed humans on the moon in 1969, we have moon rocks. Certain types of radioactive atoms in rocks can be used to determine how long ago the rocks solidified. Using this type of radioactive dating, the moon rocks have been found to be 4.4 billion years old.  The oldest rocks on Earth have also been found to be 4.4 billion years old.  On the other hand, we have found that asteroids, which are space rocks left over from the formation of our solar system, are 4.55 billion years old.  So it is believed that the Earth was formed 4.55 years ago, making it 150 million years older than the oldest known rocks on Earth.  Remember that radioactive dating tells you when a rock solidified, which suggests that some event caused the Earth to become completely molten and then re-solidify 4.4 billion years ago.  This evidence points to a collision between worlds 4.4 billion years ago that both melted the planets and ejected the material which formed the moon.

The Composition of the Moon

The chemical composition of moon rocks is very similar to Earth rocks.  You might think rocks are rocks, but scientists can tell the difference between rocks from Earth and rocks from Mars, due to the differences in chemical composition.  We know the composition of Martian rocks because we have landed robotic probes on Mars to run tests.  So the fact that moon rocks are very similar to Earth rocks strongly suggests that the moon was once part of the Earth.

Secondly, when planets form, the heavier elements tend to sink to the middle of the planets.  We know that the rocky planets in our solar system, Mercury, Venus, Earth, and Mars, all have large metallic cores made mostly of iron and nickel.  The moon has virtually no metallic core.  This fact also supports the collision theory of its origin.  In a collision between planets, the material which would be ejected would come from the outer crusts of the planets, as opposed to the cores.  So the ejected material that ended up becoming the moon would be particularly low in iron and nickel, leading to an abnormally small metallic core, which is just what the moon has.   

The Conclusion

When we want to understand events that happened before recorded history we use the scientific evidence we can gather to find the most likely explanation. Just like any detective work, we may not be able to say with absolute certainty what really happened, but based on the evidence outlined above, the collision theory appears to best fit the facts we know about the moon.