The Moon

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)

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

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

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

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.

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.

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

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

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?