Space & Innovation

Secrets of Moon's Mysterious Ringed Basin Revealed

Two lunar probes recently mapped the gravity of the Orientale Basin, which sits on the very edge of the near side of the moon.

Just how a giant bullseye-shaped basin formed on the moon has intrigued scientists for years.

Now, thanks to data collected by NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission, they may have the answer.

The Orientale Basin sits on the very edge of the near side of the moon, so it has been difficult to observe.

But close fly-bys by two lunar probes in 2012 mapped the gravity of the area, revealing detail about the interior structure of the basin not previously accessible to scientists.

The data, published in two papers in the journal Science, indicated the multi-ringed basin was formed by geological faulting caused by the impact of a large asteroid.

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The findings could help scientists better understand how multi-ringed impact basins formed on the Moon as well as other planets during a critical time in the evolution of the solar system, said Katarina Miljkovic of Curtin University, who was a co-author on both studies.

"We've shown how the rings form, not just the rings of Orientale, but the exact mechanism for ring formation on the moon, and in general that should work on any other body as well," Miljkovic said.

The Orientale basin, shot by Luna Orbiter 4 in 1967. Credit: NASA

The Orientale Basin was formed nearly 3.8 billion years ago, during a time when the moon was being bombarded by asteroids.

It has three concentric rings, which up until now have been not been fully observed.

"GRAIL's given us not only a high resolution gravity map of Orientale, but also a pretty great gravity map of the entire moon," Miljkovic said.

The data, published in the first study, revealed that while the outermost of the crater's three rings has a diameter of more than 930 kilometers (578 miles), there was also a smaller transient crater that is not visible today.

Computer modelling in the second study indicated a crater that matched the GRAIL data would have been formed when a 40-mile-wide asteroid slammed into the moon.

"That's a really big rock. And it hits the moon at 15 kilometers per second, it's pretty supersonic," Miljkovic said.

"You have this extremely violent process. The entire basin forms within two hours, real time."

According to their model, the impact would have initially formed a bowl-shaped crater about 400-500 kilometers (250-310 miles) wide and 100 kilometers (62 miles) deep.

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After this violent geological event, the walls of the transient crater begin to collapse as it moves back towards gravitational equilibrium.

"The process is so violent a larger portion of the surface feels this pressure going out of excavation, and then the other rings which are beyond this inner depression start faulting," Miljkovic said.

She said under the study's modelling, the energy produced by the collision spread outward through the surface, causing "normal faults."

These are a type of geologic fault occurring on the moon when warm mantle material that flows during the collapse of a transient crater pulls and pushes the cool crust.

This pushing and pulling elevates the rings of crust around the impact crater, creating the 'bullseye' effect we can see on the surface of the moon today.

"Where the rings form, those are regions of localized high strain, and because of that high strain, or difference in strains in the region, you form these rings and plains," Miljkovic said.

While the study focused on the Moon, Miljkovic said the research could provide some insight into what happened on Earth during the Late Heavy Bombardment, as well as how the distinctive rings around other craters - like the Caloris Planitia on Mercury and the Valhalla crater on Jupiter's moon Callisto - were formed.

"Because Earth and the moon are basically one system, we can prove that a lot of those impacts on the Moon also happened on the Earth as well, it tells us a lot about the evolution of the Earth as well," she said.

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This article originally appeared on ABC Science Online.