More than four billion years ago, something enormous slammed into the moon hard enough to carve out a scar wider than India, the seventh-largest country in the world by area. This scar, known as the South Pole–Aitken Basin, stretches across the moon’s far side for about 2,000–2,500 kilometers and remains the largest confirmed impact basin on the lunar surface.

Scientists have studied it for decades because it may expose material from deep inside the moon — including pieces of the lunar mantle normally hidden beneath the crust. However, one mystery refused to go away. 

Which direction did the impactor come from, and where did all that excavated material end up?

“The South Pole–Aitken (SPA) basin-forming impact was a critical event in the Moon’s history. Despite being the oldest and largest acknowledged basin, critical details, including the impactor’s size, nature, direction, and fate of the ejecta, remain uncertain,” the new study notes.

Now, a new study argues that the ancient impact likely came from north to south, overturning some earlier interpretations and potentially changing the scientific value of NASA’s future Artemis missions. 

According to the researchers, if their reconstruction is correct, astronauts near the lunar south pole could land within deposits that may contain material excavated from deep inside the moon during the colossal collision. 

This would give scientists a rare chance to study samples originating from the lunar mantle without drilling kilometers into the surface — something planetary scientists have wanted for decades. However, there’s a problem.

A crater that refused to make sense

The problem is that the basin has always looked contradictory. Its elongated, tapered shape points in one direction, while certain crustal features appeared to suggest another.

Previous studies investigated several of these features individually, but the new work attempts to reproduce the basin’s shape, crustal asymmetry, and impact direction within a single impact scenario.

Scientists also struggled to explain strange chemical deposits rich in thorium and iron found southwest of the basin. To solve the puzzle, the researchers created high-resolution 3D simulations of giant asteroid impacts on a moon-like world. 

They tested different impact angles, speeds, sizes, and internal structures of the incoming object to see which combination best reproduced the real basin seen today.

The asteroid that reshaped the moon

One key detail involved whether the impactor was differentiated. In planetary science, that means the object had already separated into layers, with a dense metallic core surrounded by lighter outer material — similar to how Earth has a core and mantle. The team found that this internal structure mattered enormously.

Their best-fitting scenario involved a differentiated object roughly 260 kilometers wide striking the moon from north to south at a shallow angle of around 30 degrees. The asteroid did not completely punch through the lunar crust. Instead, its dense core deformed the surface in a way that produced the basin’s unusual tapered shape.

The simulations showed that the collision unfolded in several stages. First, the incoming object blasted material outward at tremendous speed, excavating deep layers of the moon. Then gravity took over. 

As the unstable crater collapsed inward, part of the basin rebounded unevenly, raising sections of the interior higher than others. Much of the mantle material that had been thrown out eventually fell back into the basin itself rather than escaping far away.

The researchers also tested different impact speeds. When the impactor struck at 10 kilometers per second, the resulting basin became too elongated compared to the real South Pole–Aitken basin. At 16 kilometers per second, the crater turned too circular. The sweet spot appeared near 13 kilometers per second.

That velocity carries another important clue. According to the team, the impactor probably originated from the Mars region of the early solar system rather than from closer to the Venus-Earth region.

In other words, the object that reshaped the moon may have been a leftover planetary building block wandering inward from the outer rocky solar system during the chaotic era of planet formation.

Artemis may land on deep lunar debris

The study also tackled one of the biggest practical questions for future lunar exploration: where the excavated mantle material landed. The simulations revealed a striking butterfly-like ejecta pattern.

Mantle material spread roughly 550 kilometers beyond the basin rim in the downrange direction and about 650 kilometers across the sides, while almost none was deposited uprange.

That finding matters because NASA’s Artemis missions are targeting the moon’s south polar region near the basin’s rim. 

Under older south-to-north impact models, the planned landing region would likely contain little or no mantle ejecta. 

However, “if a north-to-south impact produced SPA, the Artemis III mission may land within the ejecta deposit that contained excavated mantle material by the SPA-forming impact,” the study authors note.

The moon’s hidden interior may soon be within reach

If astronauts eventually recover mantle-bearing material from the SPA ejecta field, the scientific payoff could be enormous. Researchers could directly study the chemistry of the moon’s deep interior, determine when the giant impact occurred, and better understand how rocky worlds evolved in the early solar system.

Samples returned from these regions should therefore reveal the age of SPA and the composition of the lunar mantle.

The work also highlights how planetary scars preserve hidden records of ancient events. Similar giant elliptical basins exist on Mars and even Pluto, meaning the new modeling approach could help scientists reinterpret collisions across the solar system. 

However, the authors acknowledge that even their advanced simulations still cannot capture every fine-scale detail of crustal deformation or ejecta movement. Computer models of impacts this large remain computationally difficult, especially when reconstructing events that happened billions of years ago.

Therefore, the next phase may not rely on simulations alone. If future Artemis missions return samples from the south polar region, scientists could directly test whether the predicted mantle-rich ejecta is really there.

The study is published in the journal Science Advances.