Approximately 4.3 billion years ago, during the early formation of our solar system, a massive asteroid collided with the far side of the moon, resulting in the creation of the South Pole-Aitken Basin—an enormous crater. This feature, the largest on the moon, spans over 1,200 miles in length and 1,000 miles in width. Its rectangular shape is attributed to a glancing impact rather than a direct hit. Challenging previous beliefs that the basin was formed by an asteroid coming from the south, recent research indicates that the narrowing shape of the basin towards the south suggests an impact from the north.
The South Pole-Aitken Impact Basin on the far side of the Moon was formed by a southward impact. Image credit: Jeff Andrews-Hanna / University of Arizona / NASA / National Astronomical Observatory of Japan.
“The downstream edge of the basin should have a thick layer of material that was excavated from the moon’s interior by the impact, while the upper edge should not,” explained Dr. Jeffrey Andrews-Hanna, a planetary scientist at the University of Arizona.
“This suggests that the Artemis mission will target the downrange rim of the basin, an ideal site to examine the moon’s largest and oldest impact basins, where most of the ejecta, consisting of material from deep within the moon, are likely to be gathered.”
Historically, it has been believed that early moons were molten due to the energy released during their formation, resulting in a magma ocean that enveloped the entire moon.
As this magma ocean solidified, heavy minerals settled to create the Moon’s mantle, while lighter minerals floated upwards to form the Earth’s crust.
Nevertheless, certain elements were not incorporated into the solid mantle and crust, but instead became concentrated in the last liquid remnants of the magma ocean.
These “residual” elements, including potassium, rare earth elements, and phosphorus, are collectively known as KREEP.
Dr. Andrews-Hanna and his team noted that these elements appear to be especially abundant on the moon’s near side.
“If you’ve ever frozen a can of soda, you might have noticed that high fructose corn syrup doesn’t freeze all the way through and instead accumulates at the bottom of the liquid,” remarked Dr. Andrews-Hanna.
“We believe a similar phenomenon occurred on the moon with KREEP.”
“Over millions of years, as it cooled, the magma ocean crystallized into the crust and mantle.”
“Eventually, only a small amount of liquid remained trapped between the mantle and the crust, which is this KREEP-rich material.”
“The abundance of KREEP’s heat-producing elements somehow concentrated on the moon’s near side, causing it to heat up and initiate intense volcanic activity, thus creating the dark volcanic plains visible from Earth.”
“However, the process by which this KREEP-rich material became concentrated on the near side and how it evolved remains an enigma.”
“The moon’s crust is considerably thicker on the far side compared to the near side that faces Earth, a discrepancy that continues to puzzle scientists.”
“This asymmetry influences various aspects of the moon’s development, including the final stages of the magma ocean.”
“Our hypothesis posits that as the far side’s crust thickened, the underlying magma ocean was forced outward, akin to squeezing toothpaste from a tube, causing most of it to accumulate on the near side.”
A recent investigation of the Antarctic Aitken Basin has uncovered unexpected asymmetries supporting this scenario. The western ejecta blanket is rich in radioactive thorium, while the eastern side is not.
This indicates that the rift left by the impact formed a conduit through the moon’s crust, near the boundary separating the “normal” crust from the underlying layers that contain the final remnants of the KREEP-rich magma ocean.
“Our research shows that the distribution and composition of these materials align with predictions derived from modeling the later stages of magma ocean evolution,” stated Dr. Andrews-Hanna.
“The last remnants of the Moon’s magma ocean have reached the near side, where the concentration of radioactive elements is at its peak.”
“However, prior to this, there may have been a thin, patchy layer of magma ocean beneath parts of the far side, explaining the presence of radioactive ejecta on one flank of the Antarctic Aitken Basin.”
For further information, refer to the study published in the journal Nature.
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JC Andrews-Hanna et al. 2025. The southern impact excavated a magma ocean in the Moon’s South Pole Aitken Basin. Nature 646, 297-302; doi: 10.1038/s41586-025-09582-y
Source: www.sci.news
