Earth once had two moons

Earth may have once had two moons, but one was destroyed in a slow-motion collision that left our current lunar orb lumpier on one side than the other.

Astronomers have long been puzzled by the differences between the side of the moon that always faces Earth—the near side—and the side that always faces away, the far side. The topography of the near side is relatively low and flat, while that of the far side is high and mountainous with a much thicker crust.

According to a new computer model, this discrepancy can be explained if a smaller “companion moon” collided with our moon’s far side early in its history. Such a collision would have left the far side splattered with especially hard rocky material that now forms the current lunar highlands.

For the theory to work, the smaller moon must have crashed into the larger one at about 4,400 miles (7,081 kilometers) an hour. Learn more here, here, here or here.

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