General Astronomy/Earth's Moon

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The Moon has no atmosphere due to its weak gravity. It is composed of a 800 km thick lithosphere, covering a core of thin magma.

The Moon has two types of landscapes: bright, cratered highlands in the south, and dark mare basins in the north. One side of the moon always faces the earth; this side has many more mare basins.

Primary lunar craters were caused by meteors. Secondary craters were caused by debris. Lunar craters have rays around the edges. These are caused by the debris from meteors: since the moon has weak gravity and no atmosphere, it sprays out for a long distance.

Erosion is much slower on the moon than on the Earth. It is estimated that Buzz Aldrin's footprint will last 10 million years!

The maria (plural of mare) were caused by big asteroids strikes which ruptured the Lunar crust and released internal lava to flow up into the impact craters and flood-fill the bottom of the crater out to the crater walls. The lava solidified into basalt, creating the large, dark basins. This iron-rich basalt creates tension in the maria basins. This sometimes causes the crust to crack, forming graben.

Early in the moon's history, meteoric impacts caused the surface to melt. The liquid lunar surface underwent differentiation: the heavier iron sunk, while lighter aluminum rose to the top. The core was kept liquid by radioactive heating from uranium, potassium, and thorium.