For centuries, astronomers thought Mercury always had the same hemisphere facing the Sun, similar to the situation with the Moon and Earth. With the advent of the Space Age, however, they found the actual situation is very odd-- three days on Mercury equals two Mercurian years.
Now, they might know why. Caloris Basin is a huge impact site on Mercury-- exactly the right size, age, and location to mark the spot a giant asteroid crashed into the planet, knocking its rotation from being tidally locked on the Sun to its current strange rotation.
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