Transcript The Moon
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 Earth side • The two sides are different from each other. far side Ejecting a Moon • George Darwin (1878) suggested that the early Earth ejected a blob as it spins. before after • If this were true the earth and moon would spin four times faster - the problem is angular momentum. 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?