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Two New Studies Explain The Formation Of A Bullseye Crater On The Moon

On the southwestern border of the near side of the Moon, there’s a crater like no other. The Mare Orientale is a multi-ringed impact basin with three concentric series of regions that often remind onlookers of a bullseye. It was formed 3.8 billion years ago, and scientists finally know how.

A 64-kilometer (40-mile) asteroid hit the Moon at a neck-breaking speed of 15 kilometers (9.3 miles) per second. On impact, it created a crater between 320 and 460 kilometers (200 and 285 miles) in diameter. The rocks were not stable enough and, over time, the subsequent geological movements generated the outer rings, with the largest reaching 930 kilometers (580 miles) across.

Two studies were published this week in Science on the Mare Orientale basin. In the first one, led by Maria Zuber from MIT, the team uses data from the Gravitational Recovery and Interior Laboratory(GRAIL) to investigate what lays beneath the crater. This has allowed the scientists to find out the size of the original crater, the volume of material redistributed by the impact (a staggering 3.4 million cubic kilometers/815,000 cubic miles), and also to make important conclusions on how these ringed craters form. The study indicates that the presence of preexisting faults plays a crucial role in the terrain changes.

"In the past, our view of Orientale basin was largely related to its surface features, but we didn't know what the subsurface structure looked like in detail. It's like trying to understand how the human body works by just looking at the surface," said co-author Jim Head, a geologist at Brown and a GRAIL science team member, in a statement. "The beauty of the GRAIL data is that it is like putting Orientale in an X-ray machine and learning in great detail what the surface features correspond to in the subsurface."

The second study, led by Brandon Johnson of Brown University, used the GRAIL data to simulate the impact that formed the crater. According to the paper, the impact has created a 390-kilometer (242-mile) crater in diameter that's 180 kilometers (112 miles) deep. The flow of weak material from the lunar interior was crucial to the collapse of the edge of the crater and the formation of the external rings.

Although the Mare Orientale is not the only multi-ringed crater on our satellite, this discovery has implications far beyond the Moon.

"Big impacts like the one that formed Orientale were the most important drivers of change on planetary crusts in the early solar system," added Johnson. "Thanks to the tremendous data supplied by GRAIL, we have a much better idea of how these basins form, and we can apply that knowledge to big basins on other planets and moons."

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