How can I teach the Moon's formation in class?
Present the giant impact hypothesis, show the evidence from Moon rocks and meteorites, and let pupils model the impact and evaluate competing ideas.
The leading explanation for the Moon's origin is the giant impact hypothesis. It is a good topic for evaluating evidence.
The idea. About 4.5 billion years ago, a planet-sized body, often called Theia, perhaps similar in size to Mars, collided with the young Earth. The impact threw molten and vaporised rock into orbit around Earth, which gathered to form the Moon.
Evidence. - Moon rocks: Apollo samples show the Moon has a similar composition to Earth's mantle but is poor in iron and volatile elements, consistent with forming from hot, vaporised material. - Small core: the Moon's iron core is small, suggesting it formed mainly from rocky outer material. - Magma ocean: the Moon's highland crust of anorthosite suggests it was once covered in molten rock, consistent with a hot formation. - Isotopes: the Moon's oxygen isotopes are almost identical to Earth's. Meteorites from Mars, Vesta and other bodies differ, which shows how distinctive this similarity is. - Computer models show that such an impact can produce a Moon-like body.
Puzzles. Why are Earth and Moon so similar if part of the Moon came from Theia? Scientists propose thorough mixing, or that Theia formed near Earth.
Older ideas. Pupils can evaluate earlier hypotheses: the Moon splitting from a fast-spinning Earth, forming alongside Earth, or being captured. Each has problems.
Activities. Pupils sort evidence cards, model the impact with clay balls, or create an animation storyboard.
This topic links astronomy, geology and the nature of scientific models.
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