Moon's largest scar may put its deepest secrets within reach of Artemis astronauts

05-26-2026

Purdue-led simulations suggest an iron-cored asteroid helped carve the Moon's South Pole-Aitken basin, scattering deep lunar material near future exploration sites.

Billions of years ago, something enormous slammed into the far side of the Moon and left behind a scar so large it stretches more than 1,200 miles across the lunar surface.

That ancient feature, known as the South Pole-Aitken basin, is the oldest and largest known impact basin on the Moon. It also may hold some of the best clues to what lies deep inside Earth's nearest neighbor. New Purdue University-led research suggests that the impact that formed the basin did more than reshape the Moon's surface. It may have excavated material from the Moon's deep interior and scattered some of it near the lunar south pole, where future Artemis astronauts are expected to explore.

The discovery, led by Shigeru Wakita, research scientist in Purdue University's Department of Earth, Atmospheric, and Planetary Sciences (EAPS), and co-authored by Brandon Johnson, professor in EAPS, has been published in Science Advances.

"We simulated the formation of the Moon's oldest and largest basin," Johnson said. "We found that formation of the massive egg-shaped basin requires an impact by a body with an iron core. We also find that the impact excavates the Moon's deep interior and that some of that material lands near the Moon's south pole where Artemis astronauts are expected to explore."

An animation of the decapitation of an impactor. The top panel represents the differentiated impactor and the bottom panel represents the undifferentiated impactor. (Video provided by/Brandon Johnson)

 

The team used numerical models called shock physics codes to recreate how the South Pole-Aitken basin may have formed. Their simulations show that the basin's unusual tapered, egglike shape is best explained by a 260-kilometer-diameter, or about 160-mile-wide, differentiated impactor striking the Moon from north to south. A differentiated body is one that had separated into layers, including a dense iron core.

That iron core appears to matter. Johnson compares it to throwing two different objects into snow.

An animation of our best-fitting SPA formation. Green color represents the impactor core, cream color represents the crust, and grey color represents the mantle, respectively. (Video provided by/Brandon Johnson)

 

"When you throw a rock in the snow, it penetrates deeper than a snowball," Johnson said. "In this case, the iron core of the impactor is like the rock and causes further excavation of the crater in the downrange direction when compared to an impactor without a dense core. That is what produces the elongated egg-like shape of the basin."

In other words, not all impacts are equal. A rocky object without a dense core would have carved the Moon differently. But an impactor with an iron core could drive deeper into the lunar surface, changing the pattern of excavation and helping explain why the South Pole-Aitken basin has the shape scientists observe today.

The research also helps solve a central mystery of the basin by showing where material excavated by the impact may have ended up. According to the simulations, the impact sent lunar mantle material outward and downrange. Much of that material collapsed back into the basin interior, but some may have been deposited near the Moon's south pole. That makes the region especially important for future lunar exploration because astronauts could potentially collect samples that originated far below the Moon's surface.

Those samples could help scientists uncover what the Moon’s interior is made of while also revealing when the South Pole-Aitken basin formed. "This suggests rocks returned by the Artemis astronauts will provide valuable insight about the composition of the Moons interior and allow us to determine the age of the SPA basin," Johnson said. "This age will help scientist understand the early bombardment history of the Earth and Moon."

That early bombardment history is a record of the violent period when asteroids and other debris struck young planets and moons throughout the inner solar system. Because Earth's surface has been reshaped by plate tectonics, erosion and weather, much of that ancient impact record has been erased here. The Moon, however, preserves a clearer archive of those early collisions.

Differentiated impactor strikes Moon-like target. Color represents crust, mantle, and core,
respectively. (Video provided by/Brandon Johnson)

 

Dating the South Pole-Aitken basin would give scientists a major anchor point in that timeline. It could help reveal when large impacts were occurring and how those collisions shaped both the Moon and Earth during their early history.

The work also adds new urgency to the scientific promise of future lunar sample return. The lunar south pole already is a major target for exploration because of its unusual lighting conditions, permanently shadowed regions and potential resources. Purdue's research suggests the area also may contain pieces of the Moon's deep interior, delivered there by the largest impact in lunar history.

"We hope it will help as we plan for astronauts to return to the Moon," Johnson said. "Our work can help determine the best place to sample materials that can tell us about the bombardment history of the Moon as well as its interior composition two major outstanding questions in lunar science."

At Purdue, this work was made possible through advanced computing resources. Wakita ran the simulations using high-performance computing through Purdue's Rosen Center for Advanced Computing. These tools allow planetary scientists to model violent events that cannot be recreated at full scale in a laboratory, including impacts large enough to reshape an entire world.

The study reflects Purdue's growing strength in planetary science and lunar exploration research. Purdue EAPS researchers are helping scientists better understand how planets and moons formed, how impacts shaped their surfaces and how future missions can make the most of the samples astronauts and robotic missions return.

For Wakita and Johnson, the South Pole-Aitken basin is not only an ancient scar but also a map. If future astronauts collect the right rocks, they may bring home pieces of the Moon's deep past, revealing what lies beneath its surface and helping scientists better understand the violent history shared by the Earth and Moon.

The research was funded by the NASA Solar System Exploration Research Virtual Institute Center for Lunar Origin and Evolution.

 

About the Department of Earth, Atmospheric, and Planetary Sciences at Purdue University

The Department of Earth, Atmospheric, and Planetary Sciences (EAPS) combines four of Purdue’s most interdisciplinary programs: geology and geophysics, environmental sciences, atmospheric sciences, and planetary sciences. EAPS conducts world-class research; educates undergraduate and graduate students; and provides our college, university, state and country with the information necessary to understand the world and universe around us. Our research is globally recognized; our students are highly valued by graduate schools and employers; and our alumni continue to make significant contributions in academia, industry, and federal and state government.

Written by: David Siple, communications specialist, Department of Earth, Atmospheric, and Planetary Sciences at Purdue University