The 2,000-kilometer South Pole-Aitken Basin bears witness to a massive impact in the early days of the moon. Planetary researchers have now reconstructed how it came to be using a simulation. Accordingly, the impactor was around 260 kilometers in size, came from the region of Mars and hit the moon at a flat angle from the south. The new data also shows whether the planned landing zone for the Artemis lunar mission could reveal more about the impact and its consequences.
The South Pole Aitken Basin, which is more than 2,000 kilometers in size, is by far the largest impact crater on the Moon and one of the largest in the solar system. Measurement data from lunar probes show that lunar mantle material reached the surface in this region. There is also a mass anomaly deep beneath the impact depression – possibly a relic of the former impactor. According to some planetary researchers, the primeval impact could even be the reason why the halves of the moon differ so greatly geologically and geochemically.
How did the South Pole-Aitken Basin form?
But how the impact happened four billion years ago is largely unclear. “Crucial details are missing, including the size of the impactor, its composition, its trajectory and the fate of the ejected ejecta,” report Shigeru Wakita of Purdue University in Indiana and his colleagues. For a long time it wasn’t even clear from which direction the impact came – whether from the north or south. Due to the elliptical shape of the South Pole-Aitken Basin, both would be conceivable.
However, given plans by the USA and China to soon send astronauts to the moon and build lunar stations there, such information is important – for example, in order to select a geologically interesting landing site. That’s why Wakita and his team have now reconstructed the moon impact using a high-resolution 3D model simulation. To do this, they examined different impactor sizes, compositions, impact angles and speeds.
Huge chunks with a dense core
It turned out that the best fit for the current shape and size of the South Pole-Aitken Basin was an object around 260 kilometers in size that came from the south at an angle of around 30 degrees. The celestial body hit the lunar surface at a speed of around 13 kilometers per second – this is rather slow compared to previous assumptions. “Because the impactor hit at a shallow angle, it did not fully penetrate the moon,” Wakita and his team explain. This created the large but rather shallow pool.
There is also new knowledge about the structure of the celestial body: the huge chunk must have had a more massive core and a more porous, less dense shell. “An undifferentiated impactor would have left a more circular impact basin,” report the researchers. The asymmetrical crust thickness in front of and behind the South Pole-Aitken Basin also speaks against an undifferentiated chunk. “We assume that the core of the impactor occupied around half of its radius – that corresponds to almost 30 percent of its mass,” said the team.
The originator came from the Mars environment
There are also initial indications of where the chunk once came from: the speed and angle of impact suggest that the impactor followed a low orbit similar to Earth’s orbit before hitting the moon. “The most likely origin of this celestial body is in the area of Mars rather than in the area between Earth and Venus,” write Wakita and his colleagues. The originator of the South Pole-Aitken Basin could therefore be similar in composition to Mars and enstatite chondrites – rather rare meteorites that contain a particularly high amount of magnesium silicate.
“Samples from the South Pole-Aitken Basin and its surroundings could reveal more about how the impactor was composed and where it came from,” the researchers write. Such samples could provide future lunar missions – including as part of NASA’s Artemis program or China’s Chang’e program.
Artemis: Landing site south of the pool would be ideal
Wakita and his team have very specific tips for future lunar missions. They have determined where material from the impactor, but also ejected lunar mantle material, could most likely be found. “We found that the ejecta show a butterfly-like pattern: the mantle material is distributed around 550 kilometers beyond the southern edge of the basin and around 650 kilometers in the transverse direction,” the researchers report. North of the South Pole-Aitken Basin, however, there are hardly any ejecta.
“If NASA’s Artemis mission lands in the south polar region of the moon as planned, the astronauts could sample ejecta from the south pole Aitken impact,” write Wakita and his colleagues. Accordingly, there could be a layer of ejected mantle material more than 350 meters thick in the planned landing region. If the astronauts bring samples of this back to Earth, this could help to date the impact more precisely and perhaps also narrow down the origin of the cause.
Source: Shigeru Wakita (Purdue University, Lafayette, USA) et al., Science Advances, 2026; doi: 10.1126/sciadv.aea1984