The Moon

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
  Earths.
• 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 doesnt the surface of Earth show the signs
  of complete melting?