A Planet-Sized Asteroid Helped Create the Man in the Moon

Monday, 25 July 2016 - 1:07PM
NASA
Moon
Monday, 25 July 2016 - 1:07PM
A Planet-Sized Asteroid Helped Create the Man in the Moon
3.8 billion years ago, a huge asteroid crashed into the Moon and created the Imbrium crater, affectionately known as the right eye of the Man in the Moon. Now, new research estimating the asteroid's size claims that it was the size of a protoplanet, which is two times larger and 10 times more massive than previous estimates.

The accepted estimates prior to this research used computer models rather than any physical evidence, and found that the asteroid was approximately 50 miles in diameter. This new study, partially funded by NASA and published in Nature, looked at physical features of the Imbrium Basin as well, and found that the asteroid was likely 150 miles across, which is large enough to be classified as a protoplanet.

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"We show that Imbrium was likely formed by an absolutely enormous object, large enough to be classified as a protoplanet," Pete Schultz, professor of earth, environmental and planetary sciences at Brown University, said in a statement. "This is the first estimate for the Imbrium impactor's size that is based largely on the geological features we see on the Moon."
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The basin, which measures about 750 miles in diameter, has a set of grooves that radiate from the crater's center, which told researchers that the asteroid hit the Moon at an oblique angle rather than head-on. But researchers were mystified by a second set of grooves in the surface that don't radiate from the basin's center, but rather have their own point of origin, which they concluded must be the original point of impact.

In order to estimate the size of the impactor, Schultz performed impact experiments at the Vertical Gun Range at the NASA Ames Research Center. He modeled the impact by simulating it with many small projectiles moving at 16,000 miles per hour, and found that the asteroid would break apart upon initial impact. As a result, he could determine the approximate size of the asteroid by observing the groove trajectories, which would reflect the pieces of the impactor breaking off from either side. Using this method, he came up with the estimate of 150 miles, or 250 kilometers, in diameter, which is approximately the length of New Jersey.

Opening quote
"That's actually a low-end estimate," Schultz said. "It's possible that it could have been as large as 300 kilometers."
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In addition to telling us about this particular impact, this study may yield more information about the impact of the Late Heavy Bombardment, which occurred around this time. Schultz and his colleagues concluded that protoplanet-sized asteroids may have been fairly common in our early Solar System.

Opening quote
"The large basins we see on the Moon and elsewhere are the record of lost giants," Schultz said.
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As a result, we can use the surface of the Moon in order to determine the behavior of asteroids and other impactors during that relatively opaque time period:

Opening quote
"The Moon still holds clues that can affect our interpretation of the entire solar system," he said. "Its scarred face can tell us quite a lot about what was happening in our neighborhood 3.8 billion years ago."
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