If the moon formed from a chunk of earth why is moon rock so unique?

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So as I understand it the moon was formed when a chunk of earth was blown into orbit when a mars like planet collided with earth, if this is the case why is the composition of moon rock not similar to the composition of rock found on earth, when the moon if just a piece of the earth its self?

In: Planetary Science

7 Answers

Anonymous 0 Comments

Based on your question, it sounds like you think the Moon came from something like a wedge or chunk *broken off* from the Earth. Scientists currently think what actually happened is, a planetary body had a *glancing* collision with the Earth, liquefying the proto-Earth’s surface and most of the impactor’s material and tearing the impactor apart. This created a huge spray of material around the Earth, coming from *both* the impactor and the proto-Earth. The Earth reformed and cooled; *most* of the sprayed-out material fell back to it, but a portion of it was able to form a stable orbit as a satellite.

Because of this glancing blow, and the liquefaction of (part of) both bodies, the material of the two planets intermingled. The Moon’s development changed some of its composition (as others noted, the heavy stuff sank to the core, for example), but it’s got a mix of both the impactor’s material and Earth’s material, just like the Earth today does.

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