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."

Comments

Re:Speak Proper Yoda, Please

[YodasEvilTwin]

You mean "Two Moons, Earth May Once Have Had" or "Had Two Moons Once, Earth May Have." Yoda often places the predicate (minus the helper verb or all verbs) before the subject -- "Lost a planet, Master Obi-Wan has" -- or otherwise rearranges phrases. He doesn't jumble words around randomly.

Re:Question for those more knowledgable than I

[bberens]

When you consider the gravitational field rather than line of sight then the "shadow" that Earth casts is really quite large.

Gravity not visible surface

[perpenso]

I don't think you can use the observable earth as an indication of the protection offered. I would expect that the earth's "gravity well" (sorry, a more proper term escapes me at the moment) would be more relevant. That said I am quite suspicious of your overall approach. It seems to be commonly accepted that Jupiter provides the earth with significant protection, now consider the percentage of the sky hemisphere that jupiter occupies.

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.