3.8 to 4 billion years ago, a protoplanet collided with the Moon and formed the Imbrium Basin. The new research also suggests, based on the sizes of other impact basins in the Moon, Mars and Mercury, that the early solar system was likely well stocked with protoplanet-sized asteroids.

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3.8 to 4 billion years ago, a protoplanet collided with the Moon and formed the Imbrium Basin. The new research also suggests, based on the sizes of other impact basins in the Moon, Mars and Mercury, that the early solar system was likely well stocked with protoplanet-sized asteroids.