Free-air gravitational anomalies and shaded topographic relief of the moon's 930-km-diameter Orientale impact basin © Ernest Wright, NASA/GSFC Scientific Visualization Studio

Secrets of ringed Moon crater revealed

Two new studies have revealed the structural secrets of Orientale basin, a huge bull’s eye-shaped crater on the surface of the Moon.

The Orientale basin is one of the youngest, largest and best-preserved craters on the Moon. However, only recently has a mission by NASA’s Gravity Recovery and Interior Laboratory (GRAIL) been able to pass spacecraft close enough to reveal the 580-mile diameter crater in unprecedented detail. This has allowed two new studies to shed light on the processes in its formation, giving rise to its three concentric rings of rock.

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A transient appearance

One of the great mysteries of the Orientale basin, and focus of the first study, is the size and location of the transient crater, which is the first depression created after the impact throws material away from the surface.

In order to do this, scientists from Brown University combined the new GRAIL data with high-resolution maps of the Moon’s gravitational field. They quickly disproved the theory that one of the rings was the transient crater, as the rebound of the surface following large collisions normally destroys any trace of the initial impact point. However, although not visible on the surface, the GRAIL data was still able to identify the transient crater between its inner two rings.

“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,” says Jim Head, a geologist at Brown University. “It’s like trying to understand how the human body works by just looking at the surface.

“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.”

Identification of the transient crater also enabled the researchers to estimate how much material was displaced as a result of the impact. This further clarified which parts of the Moon’s crust were removed, which parts of the mantle and interior were elevated, and how much material was resultantly distributed around the Moon.

A model with great impact

In the second study, a team of researchers utilised the GRAIL data to develop a computer model reconstructing the impact and its aftermath. Estimating that the impact object was 40 miles across and travelling at nine miles per second, they were able to accurately recreate Orientale’s ringed appearance.

“This was a really intense process,” says lead researcher Brandon Johnson. “These several-kilometre cliffs and the central ring all formed within minutes of the initial impact.”

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This is the first time a model has been able to reproduce the rings, with GRAIL providing the essential the foundation for doing so. Equipped with this new understanding, other multi-ring basins in the Solar System can now be investigated in a similar way, providing further insights into how large impacts influenced the evolution of the Moon, Mars and Earth.