Around 3.8 billion years ago, an asteroid more than 150 miles across, roughly equal to the length of New Jersey, slammed into the Moon and created the Imbrium Basin -- the right eye of the fabled Man in the Moon. This new size estimate, published in the journal Nature, suggests an Imbrium impactor that was two times larger in diameter and 10 times more massive than previous estimates.
"We show that Imbrium was likely formed by an absolutely enormous object, large enough to be classified as a protoplanet," said Pete Schultz, professor of earth, environmental and planetary sciences at Brown University. "This is the first estimate for the Imbrium impactor's size that is based largely on the geological features we see on the Moon."
Previous estimates, Schultz said, were based solely on computer models and yielded a size estimate of only about 50 miles in diameter.
These new findings help to explain some of the puzzling geological features that surround the Imbrium Basin. The work 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.