Giant Impact Basins on the Moon Fascinate Lunar Geologists

Orientale Basin on the Moon, a gravity map.

The early history of the Earth-moon system was a very violent one. It came just over a billion or so years after the Sun and planets began to form. First, the Moon itself was created by the collision of a Mars-sized object with the infant Earth. Then, about 3.8 billion years ago, both worlds were bombarded by debris left over from the creation of the planets. Mars and Mercury still bear the scars from their impacts, too. On the Moon, the giant Orientale Basin remains as a silent witness to this period, called the "Late Heavy Bombardment". During that time, the Moon was pummeled with objects from space, and volcanoes flowed freely as well.

The History of Orientale Basin

The Orientale basin was formed by a giant impact some 3.8 billion years ago. It is what planetary scientists call a "multi-ring" impact basin. The rings formed as shock waves rippled across the surface as a result of the collision. The surface was heated and softened, and as it cooled, the ripple rings were "frozen" into place in the rock. The 3-ringed basin itself is about 930 kilometers (580 miles) across.

The impact that created Orientale played an important role in the early geologic history of the Moon. It was extremely disruptive and changed it in several ways: fractured rock layers, the rocks melted under the heat, and the crust was shaken hard. The event blasted out material that fell back to the surface. As it did, older surface features were destroyed or covered up. The layers of "ejecta" help scientists determine the age of surface features. Because so many objects slammed into the young Moon, it's a very complex story to figure out.

GRAIL Studies Orientale

The Gravity Recovery and Interior Laboratory (GRAIL) twin probes mapped variations in the Moon's gravitational field. The data they gathered tell scientists about the interior arrangement of the Moon and provided details for maps of the concentrations of mass.

GRAIL performed close-up gravity scans of the Orientale basin to help scientists figure out the concentrations of mass in the region. What the planetary science team wanted to figure out was the size of the original impact basin. So, they searched for indications of the initial crater. It turned out that the original splashdown region was somewhere between the size of the two innermost rings surrounding the basin. There is no trace of the rim of that original crater, however. Instead, the surface rebounded (bounced up and down) after the impact, and the material that fell back to the Moon obliterated any trace of the original crater.

The main impact excavated about 816,000 cubic miles of material. That's about 153 times the volume of the Great Lakes in the U.S. It all fell back to the Moon, and along with the surface melting, pretty well wiped out the original impact crater ring.

GRAIL Solves a Mystery

One thing that intrigued scientists before GRAIL did its work was the lack of any interior material from the Moon that would have flowed up from beneath the surface. This would have happened as the impactor "punched into" the Moon and dug deep beneath the surface. It turns out that the initial crater likely collapsed very quickly, which sent material around the edges flowing and tumbling into the crater. That would have covered up any mantle rock that might have flowed up as a result of the impact. This explains why the rocks in Orientale basin have a very similar chemical make up as the other surface rocks on the Moon.

The GRAIL team used the spacecraft's data to model how the rings formed around the original impact site and will continue to analyze the data to understand the details of the impact and its aftermath. The GRAIL probes were essentially gravitometers that measured minute variations of the gravitational field of the Moon as they passed over during their orbits. The more massive a region is, the greater its gravitational pull.

These were the first in-depth studies of the gravitational field of the Moon. The GRAIL probes were launched in 2011 and ended their mission in 2012. The observations they made help planetary scientists understand the formation of impact basins and their multiple rings elsewhere on the Moon, and on other worlds in the solar system. Impacts have played a role throughout solar system history, affecting all planets, ​including Earth.

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Petersen, Carolyn Collins. "Giant Impact Basins on the Moon Fascinate Lunar Geologists." ThoughtCo, Aug. 27, 2020, Petersen, Carolyn Collins. (2020, August 27). Giant Impact Basins on the Moon Fascinate Lunar Geologists. Retrieved from Petersen, Carolyn Collins. "Giant Impact Basins on the Moon Fascinate Lunar Geologists." ThoughtCo. (accessed January 26, 2021).