Earth May Once Have Had Two Moons

AaronW writes "According to a story at space.com, Earth may once have had two moons. The smaller moon, estimated to be 750 miles (1200km) wide and only 4% of the mass of the larger moon, crashed into the far side of the larger moon which caused the features we see today on the moon. The surface of the far side of the moon is quite different than the side facing the earth, having a different composition and a much rougher terrain."

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Re:Question for those more knowledgable than I

[gcnaddict]

The explanation given by the paper is that it would've been far more likely for a trojan satellite (one which shares the orbit of our known Moon with the Earth) to have gently crashed into the Moon at a rate of just a few, perhaps one or two, miles per second, which is a collission speed so gentle as to be in absolutely miniscule ranges of probability with an asteroid impact. The net effect of such a slow impact wouldn't be a crater; rather, it would be roughly the same as mashing a clump of dirt on a bigger ball of clay.

As for the far-side bit, the moon wasn't always tidally locked. Tidal locking happens with lots of time.