NASA: GRAIL Mission Explains Moon’s Bull’s-Eye Crater

Scientists have finally discovered how the moon's mysterious bull's-eye crater came to be.

The crater, named Orientale impact basin, is located along the moon's southwestern limb -- the left-hand edge as seen from the Earth -- and was formed 3.8 billion years ago. The basin is circled by three concentric rings, and the crater itself is known as the largest and most preserved example of a multi-ring basin, about 580 miles (930 kilometers) wide.

Using data from NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission, scientists were able to identify the powerful impacts that created the basins like Orientale and how these influenced the moon's geology. The two research papers were published in the journal Science.

"We use gravity to map the interior of a planet in ways somewhat analogous to an X-ray," Maria Zuber, a geophysicist at the Massachusetts Institute of Technology (MIT) and author of the gravity field study, said in a report by Space.com.

According to Zuber, a planet or moon's interior is composed of various materials with different temperatures and density, as well as mountains or craters on the surface, which means that the gravitational field around these objects is not the same all around. Studying the variations in gravity around the moon could provide insights about the moon's interior. Data from GRAIL helped scientists determine what the surface features correspond to in the subsurface.

A second study led by Brandon Johnson, a geologist at Brown University, used data from GRAIL to determine how the rings around Orientale formed. The findings suggested that an object about 40 miles (64 km) across traveling at a speed of about 9 miles per second, had struck the moon's surface.

However, the scientists found that none of the rings of Orientale represents the initial transient crater or the dent caused by the initial impact. Instead, scientists discovered that after an impact, the surface violently rebounds and obliterates signs of the impact.

"Big impacts like the one that formed Orientale were the most important drivers of change on planetary crusts in the early solar system," Johnson said.

The GRAIL mission consists of twin probes, Ebb and Flow, which had circled the moon since Jan. 1, 2012 with the scientific goal of studying the moon's structure and surface composition. The mission ended in 2012 when the twin spacecraft was crashed into the moon's surface.