moon: Formation and Evolution

Formation and Evolution

It is now most commonly believed that moon formed when an object (sometimes called Theia after the mother of Selene, goddess of the moon) collided with the young earth. One theory holds that when a Mars-sized body impacted the earth the cores of the earth and object merged in the earth while material from the crust and mantle was blasted into orbit around the earth and later accreted to form the moon. Another theory holds that the body was larger and faster, delivering a glancing blow and contributing relatively little material to the earth-moon system that it created. After the moon's crust formed, subsequent impact of very large meteorites depressed the mare basins, at the same time thrusting up the surrounding crust to form the highlands. The mare basins later filled with lava flow, which in turn was covered by a thin layer of lunar “soil”—fine rock dust pulverized by the very slow mechanisms of lunar erosion (thermal cycling, solar wind, and micrometeorites). The craters were probably also formed by meteorite bombardment rather than by internal volcanic action as once believed. The rays surrounding the craters are material ejected during the impacts that formed the craters. The moon's rock types are correlated with its major geological periods.

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