Astronomers Now Know How the Moon Got Its Bull's-Eye Crater

A major lunar mystery may have just been solved. Orientale basin — a 580-mile-wide crater — is one of the moon’s most striking features and resembles a large bull’s-eye. After pondering its origins for decades, scientists now think they’ve discovered how the crater’s concentric rings formed, and it’s more complicated than just a giant meteor pummeling into the surface.

A pair of articles published in the new issue of Science, explains how researchers used data from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) spacecraft to examine the lunar subsurface in order to piece together the giant crater’s history.

“Big impacts like the one that formed Orientale were the most important drivers of change on planetary crusts in the early solar system,” says Brandon Johnson, lead author of one of the papers and a co-author of the other. “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.”

Maria Zuber, a a geophysicist at the Massachusetts Institute of Technology and lead on the gravity field study, explained that they used GRAIL much like an X-ray machine. The spacecraft mapped the interior of the planet by using its own gravity. The lunar interior is made up of a myriad of materials, each with their own temperature, density, and composition. The surface also has a variety of features, and GRAIL measures the gravitational differences around all of these objects to map out what’s underneath.

“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,” Jim Head, a geologist at Brown University in Rhode Island, a GRAIL scientist and a co-author of the new research, said 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.”

A color-coded map that shows the strength of surface gravity around the Orientale basin.
A color-coded map that shows the strength of surface gravity around the Orientale basin. 

By analyzing the GRAIL data, the researchers concluded that an object about 40 miles (64 km) across, struck the Moon while traveling at roughly 9 miles per second. The data also indicated that the three concentric rings surrounding the crater were not caused by the initial impact. Instead, they were formed by processes happening below the surface in response to the impact.

As the crust rebounded following the impact, molten rocks below the surface flowed toward the point of impact, causing the crust above to rise up, forming the jagged cliffs that make up the outer two rings.

The innermost ring, however, was formed by a different process. When the crust rebounds, it can form a central peak. But in Orientale’s case, that central peak was so massive that it was unstable, eventually collapsing into a ring-like structure.

Multi-ring craters exist all over the solar system, and future missions modeled after GRAIL could be sent to other planetary bodies to better understand their inner-workings. When similar craters formed on the Earth, the first single-celled organisms were emerging. By studying craters similar to Orientale, on places like Mars, it could give us a better understanding of whether or not life existed.