How An Impact Created A Bulls-Eye On The Moon
published during a new moon.
10/31/2016

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The Orientale basin on the moon was formed when a large asteroid impacted the lunar surface. Colors in this image show gravity data. Areas in red are more dense than areas in blue. Credit: Ernest Wright, NASA/GSFC Scientific Visualization Studio

A long time ago, in the space right above our planet, an object nearly 40 miles across traveling at speeds of over 33,000 miles per hour slammed into the moon. The event, which took place a mind-boggling 3.8 billion years ago, left a distinct bulls-eye mark 580 miles across that scientists call the Orientale Basin. Until now, not much was known about how it formed.

In two papers published recently in Science, researchers announced that data from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission had uncovered how the unique three-ringed structure of the Orientale basin came to be. The data helped them model how large and how fast the object would have been traveling when it hit the moon, in addition to what happened to the lunar surface right after the collision.

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Orientale Basin as seen from NASA’s Lunar Reconnaissance Orbiter. Credit: NASA/GSFC/Arizona State University

The two GRAIL spacecraft measured small variations in the moon’s gravity. Just like on Earth, the moon’s gravity isn’t constant everywhere–it might be stronger or weaker depending on where you’re standing, which can give researchers hints of what lies under the surface.

In this case, the researchers found that the initial impact of the object caused a huge crater to form just after impact, but the force of the collision was so strong that it sent the moon’s surface flowing like the surface of a pool after a cannonball. In the aftermath, the initial crater was obliterated, and the surface quickly settled into the bulls-eye structure that we see today.

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Gravity Map of Orientale Basin. Credit: NASA/JPL-Caltech

“This was a really intense process,” Brandon Johnson, an author on both of the papers said. “These several-kilometer cliffs and the central ring all formed within minutes of the initial impact.”

The collision was so strong that it sent 816,000 cubic miles of material from the moon flying into space. That’s enough to fill up the Great Lakes 153 times over.

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Expedition 47 Flight Engineer Tim Peake of the European Space Agency took this striking photograph of the moon from his vantage point aboard the International Space Station on March 28, 2016.  Credit: ESA/NASA

Similar ring-like structures are seen elsewhere in the Solar System and researchers hope that the insights into how the Orientale basin formed will eventually help them decode the history of other impacts on other rocky surfaces, like Mars, where the craters are less well-preserved.

The twin GRAIL probes, Ebb and Flow were purposefully crashed into a lunar mountain after their mission was completed in 2012. But as these new studies show, both are still making a mark on research.