The Moon ’s largest and oldest impact crater, the South Pole–Aitken (SPA) basin, has long been a source of fascination for scientists trying to understand the lunar surface and its history. New research led by planetary scientist Jeffrey Andrews-Hanna from the University of Arizona, recently published in Nature, overturns some of the long-standing beliefs about how this colossal crater was formed. This study reveals that the asteroid which carved the basin did not come from the south, as was previously thought, but from the north, fundamentally reshaping our view of the Moon’s early history and providing new clues for future exploration. The basin itself, spanning nearly a quarter of the Moon’s surface on the far side, preserves a key chapter in the Moon’s evolution, offering insights into its fiery beginnings and volcanic past.
How a northern asteroid changed the story: North vs South
For decades, the accepted view, supported by NASA and other space research bodies, was that the asteroid that created the SPA basin struck the Moon from a southern direction. The large basin stretching from the Moon’s south pole to the Aitken crater was thought to bear the marks of an impactor arriving from below the lunar south pole. This was inferred mainly from the distribution of ejecta (rock and debris blasted from the impact) and the shape of the basin. The shape was thought to be largely symmetrical, with a thicker accumulation of debris towards the northern edge of the basin.
The new study challenges this narrative. By comparing detailed topographical data, gravitational measurements, and crust thickness models, the team found that the basin’s shape narrows towards the south, similar to a teardrop or avocado. This narrowing means the projectile came from the north, hitting the surface at a glancing angle and excavating material predominantly towards the south. "We have outlined the South Pole-Aitken basin using various methods," Andrews-Hanna states, "regardless of the method employed, the shape consistently narrowed towards the south." This reorientation of the impact direction provides a fresh understanding of the basin’s formation and the geological processes triggered by the collision.
The Moon’s magma ocean: Clues from the South Pole–Aitken basin
Earlier models of the Moon’s formation held that it was covered by a vast global magma ocean after its creation, where heavier minerals sank to form the mantle and lighter ones floated to form the crust. Elements like potassium, rare earth elements, and phosphorus, collectively called KREEP , were believed to concentrate in the final crystallising zones of this magma ocean.
The new findings suggest that the SPA impact excavated deeply into this ancient magma ocean, creating a "window" into the Moon’s interior layers. The impact redistributed these heat-producing radioactive elements unevenly across the lunar surface. This partly explains the mysterious asymmetry observed between the near side and far side of the Moon. The near side is marked by vast volcanic plains, or mare, rich in KREEP, while the far side is rough and heavily cratered.
As Andrews-Hanna explains, “The ancient South Pole–Aitken impact basin provides a key data point for our understanding of the evolution of the Moon, as it formed during the earliest pre-Nectarian epoch of lunar history, excavated more deeply than any other known impact basin and is found on the lunar far side, about which less is known than the well-explored near side.” This suggests the SPA basin might hold some of the oldest and most pristine samples of the Moon’s interior yet to be studied.
NASA’s Artemis program and future Moon missions
NASA's Artemis programme plans to send astronauts to explore the Moon's south pole, where the SPA basin rim is a proposed landing site. Previously, scientific interest centred on the basin as a challenging but tantalising frontier for sample collection. Now, with the new understanding of the impact’s southward excavation direction, the specific landing sites are recognised as lying on the downrange rim, where the ejecta rich in thorium and other radioactive elements are thickly deposited. "These results have important implications for the upcoming human exploration of the lunar south pole by Artemis, as proposed landing sites are now recognised to sit on the downrange rim and thorium-rich impact ejecta of the basin," the report confirms.
This means the Artemis missions could collect lunar materials from far deeper inside the Moon than ever before, offering unprecedented insights into its formation and geological history. Such samples could help resolve long-standing questions about the Moon's volcanic activity and the distribution of elements key to its heat and evolution.
Challenging NASA’s long-held beliefs about the Moon’s oldest basin
According to NASA’s older understanding, the SPA basin, more than 4 billion years old, was the biggest and deepest known impact crater in the solar system, about 2,500 kilometres wide and 10 kilometres deep. It was thought to have formed from an impactor arriving from the south, whose immense collision possibly even caused the Moon’s axis to shift. Earlier NASA studies emphasised the basin’s role in creating the dichotomy between the Moon's near side, rich in volcanic plains, and its heavily cratered far side.
Additionally, NASA data had identified a mysterious mass beneath the SPA basin floor, five times the size of Hawaii’s Big Island, which could be the metallic remains of the asteroid or leftover oxides concentrating as the magma ocean cooled. However, the new research highlights how the distribution of elements around the basin is asymmetrical, with more radioactive elements such as thorium found on the western flank than the eastern, a nuance not fully accounted for before.
This updated model also reframes interpretations of the basin’s ejecta pattern and crust thickness variations. Instead of a simple southerly impact, the north-to-south glancing blow explains why ejecta and crustal anomalies vary across the basin. This adds precision to understanding the Moon’s early environment, magma ocean dynamics, and the subsequent volcanic activity differences between lunar hemispheres.
Also Read | NASA warns! 44-foot Massive asteroid 2025 TC speeding past Earth today at 58,100 km/h; how safe are we really
How a northern asteroid changed the story: North vs South
For decades, the accepted view, supported by NASA and other space research bodies, was that the asteroid that created the SPA basin struck the Moon from a southern direction. The large basin stretching from the Moon’s south pole to the Aitken crater was thought to bear the marks of an impactor arriving from below the lunar south pole. This was inferred mainly from the distribution of ejecta (rock and debris blasted from the impact) and the shape of the basin. The shape was thought to be largely symmetrical, with a thicker accumulation of debris towards the northern edge of the basin.
The new study challenges this narrative. By comparing detailed topographical data, gravitational measurements, and crust thickness models, the team found that the basin’s shape narrows towards the south, similar to a teardrop or avocado. This narrowing means the projectile came from the north, hitting the surface at a glancing angle and excavating material predominantly towards the south. "We have outlined the South Pole-Aitken basin using various methods," Andrews-Hanna states, "regardless of the method employed, the shape consistently narrowed towards the south." This reorientation of the impact direction provides a fresh understanding of the basin’s formation and the geological processes triggered by the collision.
The Moon’s magma ocean: Clues from the South Pole–Aitken basin
Earlier models of the Moon’s formation held that it was covered by a vast global magma ocean after its creation, where heavier minerals sank to form the mantle and lighter ones floated to form the crust. Elements like potassium, rare earth elements, and phosphorus, collectively called KREEP , were believed to concentrate in the final crystallising zones of this magma ocean.
The new findings suggest that the SPA impact excavated deeply into this ancient magma ocean, creating a "window" into the Moon’s interior layers. The impact redistributed these heat-producing radioactive elements unevenly across the lunar surface. This partly explains the mysterious asymmetry observed between the near side and far side of the Moon. The near side is marked by vast volcanic plains, or mare, rich in KREEP, while the far side is rough and heavily cratered.
As Andrews-Hanna explains, “The ancient South Pole–Aitken impact basin provides a key data point for our understanding of the evolution of the Moon, as it formed during the earliest pre-Nectarian epoch of lunar history, excavated more deeply than any other known impact basin and is found on the lunar far side, about which less is known than the well-explored near side.” This suggests the SPA basin might hold some of the oldest and most pristine samples of the Moon’s interior yet to be studied.
NASA’s Artemis program and future Moon missions
NASA's Artemis programme plans to send astronauts to explore the Moon's south pole, where the SPA basin rim is a proposed landing site. Previously, scientific interest centred on the basin as a challenging but tantalising frontier for sample collection. Now, with the new understanding of the impact’s southward excavation direction, the specific landing sites are recognised as lying on the downrange rim, where the ejecta rich in thorium and other radioactive elements are thickly deposited. "These results have important implications for the upcoming human exploration of the lunar south pole by Artemis, as proposed landing sites are now recognised to sit on the downrange rim and thorium-rich impact ejecta of the basin," the report confirms.
This means the Artemis missions could collect lunar materials from far deeper inside the Moon than ever before, offering unprecedented insights into its formation and geological history. Such samples could help resolve long-standing questions about the Moon's volcanic activity and the distribution of elements key to its heat and evolution.
Challenging NASA’s long-held beliefs about the Moon’s oldest basin
According to NASA’s older understanding, the SPA basin, more than 4 billion years old, was the biggest and deepest known impact crater in the solar system, about 2,500 kilometres wide and 10 kilometres deep. It was thought to have formed from an impactor arriving from the south, whose immense collision possibly even caused the Moon’s axis to shift. Earlier NASA studies emphasised the basin’s role in creating the dichotomy between the Moon's near side, rich in volcanic plains, and its heavily cratered far side.
Additionally, NASA data had identified a mysterious mass beneath the SPA basin floor, five times the size of Hawaii’s Big Island, which could be the metallic remains of the asteroid or leftover oxides concentrating as the magma ocean cooled. However, the new research highlights how the distribution of elements around the basin is asymmetrical, with more radioactive elements such as thorium found on the western flank than the eastern, a nuance not fully accounted for before.
This updated model also reframes interpretations of the basin’s ejecta pattern and crust thickness variations. Instead of a simple southerly impact, the north-to-south glancing blow explains why ejecta and crustal anomalies vary across the basin. This adds precision to understanding the Moon’s early environment, magma ocean dynamics, and the subsequent volcanic activity differences between lunar hemispheres.
Also Read | NASA warns! 44-foot Massive asteroid 2025 TC speeding past Earth today at 58,100 km/h; how safe are we really
You may also like
Women's World Cup: Kapp's All-round Brilliance, Wolvaardt's Class Help South Africa Eliminate Pakistan
Defence Minister Rajnath Singh to address Navy's Commanders' Conference
Always counting calories? Top cardiologist says food is more than energy, urges rethinking nutrition for true health
Workers hit by stealth tax as Rachel Reeves launches new £12bn raid
Kevin Danso breaks silence after leaving Tottenham opponent with gruesome injury
