The moon’s biggest and most ancient crater is more circular than previously thought

The South Pole-Aitken basin is the moon’s oldest and largest visible crater—a massive geological wound 4 billion years old that preserves secrets about the moon’s early history, much like a lunar time capsule.

Based on some features of the basin, researchers thought that the crater was shaped like an oval or ellipse. For years, scientists believed this enormous crater was formed by an object striking the moon from a shallow angle, possibly as extreme as a rock skipping across water. Under this theory, very little debris from the impact would have sprayed across the lunar South Pole, which is the landing region for the upcoming Artemis missions to return humans to the lunar surface.

But a new University of Maryland-led study published in the journal Earth and Planetary Science Letters suggests that the impact may have been much more direct, leading to a much rounder crater—a finding that challenges our current understanding of the moon’s history, with significant implications for NASA’s future missions to the moon.

“It’s challenging to study the South Pole-Aitken basin holistically due to its sheer enormousness, which is why scientists are still trying to learn its shape and size. In addition, 4 billion years have passed since the basin was originally formed and many other impacts have obscured its original appearance,” explained the study’s lead author, Hannes Bernhardt, an assistant research scientist in UMD’s Department of Geology.

“Our work challenges many existing ideas about how this massive impact occurred and distributed materials, but we are now a step closer to better understanding the moon’s early history and evolution over time.”

Using high-resolution data from NASA’s Lunar Reconnaissance Orbiter, Bernhardt and his team developed an innovative approach to understanding the South Pole-Aitken basin’s complex structure. They identified and analyzed more than 200 mountain formations scattered around the basin, geologic features that the team suspected were ancient remnants of the original impact.

From the distribution and shapes of those mountain-like features, the team realized that the impact should have created a more circular crater from which significant chunks of planet-forming material were dispersed across the moon’s surface, including the South Pole region.

“A rounder, more circular shape indicates that an object struck the moon’s surface at a more vertical angle, possibly similar to dropping a rock straight down onto the ground,” Bernhardt said. “This circular impact implies that debris from the impact is more equally distributed around it than was originally thought, which means that Artemis astronauts or robots in the South Pole region may be able to closely study rocks from deep within the moon’s mantle or crust—materials that are typically impossible for us to access.”

These lunar rocks could provide crucial insights into the moon’s chemical composition and help validate theories about how the moon may have been created from a massive collision between Earth and another planet-sized object.

Recently, India’s Chandrayaan 3 rover detected minerals indicative of impact debris coming from the mantle close to the South Pole, supporting the UMD team’s theory about a more vertical impact forming a circular basin that would be required to spray such material in that area.

Bernhardt believes that his team’s research provides critical information for future moon missions, helping mission planners and astronauts identify areas to explore and what materials they may encounter. A thick layer rich in materials from the lower crust and upper mantle could offer unprecedented access to the moon’s complex geological history, potentially shedding light not just on the moon’s formation but also the transformative events that shaped our solar system.

“One of the most exciting implications of our research is how it is applicable to missions to the moon and beyond,” Bernhardt said. “Astronauts exploring the lunar South Pole might have easier access to ancient lunar materials that could help us understand how the moon and our solar system came to be.”