The team, led by Jay Melosh, Distinguished Professor of Earth, Atmospheric, and Planetary Sciences and a professor of physics, found the crater through an analysis of data from NASA’s Gravity Recovery and Interior Laboratory, or GRAIL, mission, which mapped the distribution of masses beneath the moon’s surface in unprecedented detail. Tiny variations in the moon’s acceleration of gravity measured between a pair of orbiting satellites are applied to detect, characterize and validate the presence of buried craters and other subsurface structures.

“The point of the GRAIL mission was to put the two spacecraft into orbit to map lunar gravity to an unprecedented precision,” says Melosh, who is also a member of the GRAIL science team. “By looking at gravity, we can probe through subsurface layers and better understand what lies beneath.”

Although some of the 124-mile-wide crater is visible at the surface of the moon, most of it is buried and can be seen only through the gravity signatures captured during the GRAIL mission.

The team was testing a new technique that sharpens the GRAIL data to see smaller-scale features, like ridges and valleys, when team members noticed an unusual circular feature.

The Earhart crater, a previously unknown lunar crater, is outlined in the magenta dash circle. A team of researchers at Purdue found the crater through an analysis of data from NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission. The team provisionally named the crater Earhart, after the famous aviator Amelia Earhart.

“We were looking for subsurface anomalies and noticed something that was so big we could not recognize it at first,” says Rohan Sood, a graduate student who first noted the anomaly and later presented the findings at the Lunar and Planetary Science Conference. “We slowly kept zooming out, and it kept getting bigger and bigger. We had to keep going to match what the data was telling us.”

“This very large crater had never been recognized optically,” Melosh says. “We were able to see gravity variations that were similar to what we find around other big craters. Comparing the gravity signatures of known craters to our new anomaly gave us confidence that we were looking at a very big crater below the moon’s surface.”

True teamwork

“It’s important to understand that these pictures or images we’re studying from the GRAIL mission are not easily developed,” says Kathleen Howell, the Hsu Lo Distinguished Professor of Aeronautics and Astronautics, who co-led the research. “Our work is a great example of the engineering and science communities coming together and each side bringing their expertise to the table.”

The Purdue team developed two independent methods of analyzing the gravity data to investigate small-scale structures near the limit of the data’s resolution. The team says the collaboration began long before the data from the GRAIL mission arrived.

“Before we started, there were years of computer science work that involved modeling, analysis and physics to set up systems to eventually analyze the data from the spacecraft,” says David Blair, who served as a PhD graduate assistant on the team.

“This is a prime example of the synergy between engineering and science at Purdue,” says Loïc Chappaz, who also served as a PhD graduate assistant on the team. “We have the science aspect of gathering the data from the spacecraft and then the engineering side of calibrating the data, which requires an understanding of the makeup of those vehicles.”

“The GRAIL mission launched with such promise,” Howell says. “It has now been justified as we have exceeded the initial expectations for this mission and the spacecraft. Our work is just one part of the overall GRAIL mission.”

Looking ahead

The team provisionally named the crater Earhart, after the famous aviator and Purdue advisor Amelia Earhart. Names of planetary features must be submitted and approved by the International Astronomical Union, a process that could take years.

The finding also validates the team's technique, and the group plans to extend the search to the entire moon to reveal other buried craters and small-scale features beneath the surface, Sood says. The search could uncover underground tunnels formed by lava flows, called lava tubes, which have been discussed as a possible shelter for human habitats on the moon. 

“One idea for a next mission is to fly with ground-penetrating radar to try and directly detect the lava tubes, and we may even be able to run such a mission from right here at Purdue,” Melosh says. “Overall, we want to better understand our celestial neighbor and where the moon came from. The moon is a witness to the earliest history of Earth. The more we understand the moon, the more we understand our own planet.”

Colleen Milbury, who served as a postdoctoral research associate on the team, says the findings already have helped other missions navigate in lunar orbit, thanks to fine-tuning of the gravitational data.

“This is new and exciting, especially that we get to work with data coming from current missions to understand current conditions on the moon and in space,” Milbury says.