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The Question

(Submitted April 21, 1998)

How have the moon rocks from Apollo helped scientists learn about the composition and history of the moon?

The Answer

Moon rocks have told scientists many things.

For example, Moon rocks lack both iron and easily vaporized 'volatile' elements. This suggested the theory that the Moon was produced when a huge planetesimal, perhaps as big as Mars, slammed into the still-forming Earth, ripping material out of its crust. The crustal material formed a ring around Earth which then coalesced into the Moon. The iron of Earth had already sunk down to the core before the collision, so there is not much iron on the Moon. The extreme heat of the impact vaporized the volatile elements. So the composition of the Moon rocks tells us about how it was formed. Such a violent event also had a substantial effect on Earth, so by studying the Moon, we now understand more about our own planet.

As another example; you can tell when a rock was formed by various methods (including looking at the decay of radioactive elements in the material). The ages of the Moon rocks, many of which were formed by later meteor collisions, tells us the history of how often big meteors fall. This reveals an epoch of heavy bombardment which ended about 4 billion years ago. Almost immediately after the end of this epoch (as close as we can tell from the fossil record) life appeared on Earth. The fact that life appeared so quickly might mean that life is easy to make. This, in turn, would mean that most hospitable planets in the universe have life, and we might not be alone.

David Palmer
for Ask an Astrophysicist

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