Class 5: The Earth’s Moon Today’s class: What are the main properties of the Moon? Some fascinating facts about the Moon’s rotation and orbit… How did the Moon form?

Fact Sheet: The Earth’s Moon Earth-Moon distance = 384,000 km (range is 363, ,000 km). Orbital period = 27.3 days. Rotational period = 27.3 days. Radius = 1738 km (0.27R  ). Mass = 7.410 22 kg (0.012M  ). Density = 3340 kg/m 3 (Earth’s density is 5500 kg/m 3 ). No detectable global magnetic field.

I : Properties of the Moon Oldest known rocks are billion years (so the Moon is at least this old!) Moon has much less iron than Earth: Moon is much less dense. No global magnetic field. In Earth, iron is mostly in the core – comprises 16% of the planet’s volume Any iron core in the Moon must be less than 3% of its volume.

… structure of the Earth

Lunar composition Moon rocks very similar to the Earth’s mantle, except… Very few volatile elements or compounds (e.g., water, sodium, zinc, lead…) High boiling-point (refractory) elements in greater abundance: aluminum, titanium, calcium… Recent missions that studied the Moon’s polar region show evidence for H 2 O ice! Located in deep polar craters – the ice is in perpetual shadow, but there’s not much.

II : Tidal effects in the Earth- Moon system Tidal effects are responsible for… Twice-daily rising and falling of tidal waters. Synchronizing the rotation and orbit of the Moon around the Earth. Gradually slowing down the rotation of the Earth.  Our day is increasing by 2 ms/century! Gradually increasing the Earth-Moon distance.  The Moon is moving away at 4 cm/year!

III : Theories of the formation of the Moon Simultaneous formation theory Moon formed from same material as Earth, at same time and under same conditions. Fission theory The Earth was spinning so fast in the past that large fraction tore away to form Moon. Capture theory Moon formed elsewhere and was captured. Giant impact theory A Mars-size body struck Earth in the past and the Moon formed from the debris.