Formation of the Moon

Suggests that the Moon formed at the same time as the Earth from the same material.
Critique: The Earth and Moon have different compositions:
- Earth has a metallic core, rocky mantle, and crust.
- Moon consists predominantly of rock.
Challenges arise: Where does the metal go if they formed together?

Proposes that the Moon formed independently in space and was later captured by Earth's gravity.
Critique: Similarities in the composition of Earth and Moon rocks:
- The crust and mantle rocks of the Earth are very similar to those of the Moon.
- Suggests a problematic origin since such similarities are unlikely if the Moon was captured.

Most widely accepted explanation:
The Earth, having already differentiated, was struck by a Mars-sized body.
This impact:
- Ejected significant amounts of the Earth’s crust and mantle into space.
- Caused the mixing of the two bodies (the Earth and the impacting body’s core).
The remaining debris:
- Gathered in orbit around Earth due to its gravitational pull.
- Eventually coalesced to form the Moon.
Explains similarities:
- The rocks making up the Moon originated from the Earth’s crust and mantle which were blasted into space during the impact.
- Iron is too heavy to have escaped into space and thus remains part of Earth’s core.

Moon formation occurred over 4 billion years ago.
After its formation, the Moon was bombarded by meteorites:
- Resulted in numerous craters across the surface.
Mares (dark basalt plains on the Moon) formed about 3 billion years ago.
- Initially, the Moon had no mares, which later developed from basaltic flows.
Continued impacts:
- The Moon is still being hit, but most current impacts are minor micrometeorites which smooth the surface over time.
- This smoothing causes the mares and craters to become less distinct.
Overall, the Moon's surface has remained relatively stable, with minimal changes except for the effects of micrometeorite impacts.