Measuring some 2,500km (1,600 miles) in diameter, the South Pole-Aitken Basin is an immense impact crater on the far side of the Moon.
Up to 8.2km (5.1mi) deep in some places, it’s the oldest and deepest basin on the Moon, and indeed one of the largest known impact craters in the entire Solar System.
The South Pole-Aitken Basin is about 4 billion years old and is a site of particular scientific interest.
Not only because China’s Chang’e 6 mission visited the South Pole-Aitken Basin and brought back samples to Earth.
Not only because NASA’s Artemis III mission intends to follow suit in 2026.
But also because the South Pole-Aitken Basin appears to have a different mineral composition than other areas of the Moon, and because of the 2019 discovery of a large mass lurking just below its surface.
Discovery of the South Pole-Aitken Basin
The basin gets its name from two features that lie on its outer rim: the lunar South Pole and, to the north, the crater Aitken.
It is believed to have been formed 4.2-4.3 billion years ago, but it remained entirely unknown to humankind until the early 1960s, when its presence was first detected in images sent back by the Russian lunar probes Luna 3 and Zond 3.
By the end of that decade, higher-resolution, widefield images from the US Lunar Orbiter programme confirmed the existence of this huge impact feature, and began to give scientists an idea of its great depth, but it wasn’t until 1994 that the basin was fully mapped by NASA’s Clementine mission.
Formation and geology
In terms of its geology, the South Pole-Aitken Basin is interesting in several ways.
Firstly, the Moon’s crust is at its thinnest in this region: just 30km (18.6mi) thick, compared to an average 50km (31mi) elsewhere on the Moon.
That’s because the huge impact that formed the basin dislodged material from the crust, creating the mountains that form its rim, which are among the tallest on the Moon, with elevations of up to 8,500m (27,900ft).
For comparison, Earth’s tallest mountain, Mount Everest, stands around 8,850m (29,000ft) tall.
Secondly, the composition of the basin appears different from what has been encountered on the near side of the Moon, in lunar highland regions or in confirmed lunar meteorites, with elevated levels of iron, titanium and thorium.
What’s beneath the South Pole-Aitken basin?
A 2019 study published in the journal Geophysical Research Letters found an unusually large mass of material hidden beneath the South Pole-Aitken Basin.
While some scientists suggested a volcanic origin for this mass, most believe the likely explanation is that this mass is a remnant of the asteroid or other body that caused the basin to form in the first place.
Further study is required, which is why, with the Chang’e 4 mission having achieved the first successful touchdown in the basin in 2019, China’s Chang’e 6 mission was dispatched to the region in May 2024.
Chang’e 6 touched down in the Apollo crater, which lies towards the basins’s northeastern rim, on 1 June, and by 25 June had returned our first-ever samples from the lunar far side to Earth.
Lunar scientists in China and elsewhere are currently examining and analysing these samples.
Needless to say, the results of that research are, at time of writing, eagerly anticipated.
NASA’s Artemis III mission, meanwhile, is currently scheduled to make the first crewed Moon landing since 1972 in 2026.
And while a landing site has not yet been chosen, most of the 13 candidate sites proposed lie within the South Pole-Aitken Basin.
So expect to be hearing much more about this region over the next few years.