It's our closest neighbor in the solar system and the only one we've set human feet on so far. But there's still plenty of mystery surrounding our orbital partner, the moon.
—Image courtesy NASA
Perhaps one of the biggest questions is why we have a lone natural satellite, and a pretty big one at that.
Today's prevailing theory is that the moon is made from bits of Earth that broke off during a collision between our planet and a Mars-size body about 60 million years after Earth was born. This material formed a ring around Earth that condensed into the moon.
The theory explains the moon's size as well as why Earth's mantle and the moon both lack iron—because the heavy element would have sunk down to form a core.
The trouble is, if the moon has a core, why does it have no magnetic field?
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