1. The Moon

2. Nearest Neighbor The Moon is the closest large object to Earth.
364,000 km away
1738 km radius
The Moon rotates very slowly compared to Earth.
29.5 day period
27.3 day sidereal period
Matches Earth orbit
Earth-Moon from 10 million km (NASA)

3. Craters The surface of the Moon is different than Earth’s.
No atmosphere or oceans
Flat dark lava flows – maria
Cratered highlands
The two sides are different from each other.
Earth side
far side

4. Ejecting a Moon
George Darwin (1878) suggested that the early Earth ejected a blob as it spins.
If this were true the earth and moon would spin four times faster - the problem is angular momentum.
before
after

5. Accreting a Moon
Accretion explains the planets, but can it explain the moon?
The earth and moon should have the same composition.
Matching surface rocks
Unknown interior
If this were true the Moon would have 65% more mass – the problem is density.
Mineral survey in false color

6. Capturing a Moon The Moon could form elsewhere with lower density.
The orbit could bring it near the Earth where it would be captured.
If this were true, it would be more likely to hit the Earth – the problem is the large size of the Moon.

7. Catastropic Impact
The best theory is that an object larger than the moon collided with the earth (1984).
The collision left a cloud of dust that became the moon.

8. Impact Simulation
Computer simulations study collisions at different angles.
Glancing blow required
Amount of ejected material and temperature
F Zavatti

9. Accretion Simulation Simulations study formation from cloud.
Initial disk
Spiral structure
Lunar seed
Accreted Moon
Time is about 100 years.

10. Hypothesis The giant impact hypothesis explains:
Large Earth iron core
Small Moon core
Similar rocks on surface
Lack of water and air on Moon
Lack of thick atmosphere on Earth
Cratering on the Moon
There are still unanswered questions:
Why are the deep Earth and old Earth minerals the same as the Moon?
Why doesn’t the surface of Earth show the signs of complete melting?