South Pole–Aitken Basin

The South Pole–Aitken Basin (SPA) is the largest, deepest, and oldest recognized impact basin on the Moon and one of the largest preserved impact structures in the Solar System, comparable in scale to Hellas Planitia and Utopia Planitia on Mars. Centered near 53° S, 169° W on the southern lunar farside, the elliptical structure extends roughly 2,500 km north-to-south—from the 135-km crater Aitken at 16.8° S down across the lunar south pole to the nearside limb craters Schomberger and Hale—and approximately 2,000 km east-to-west, encompassing nearly one-sixth of the lunar farside hemisphere. With a floor plunging more than 8 km below the lunar reference sphere and a surrounding highland rim rising up to +5 km, the South Pole–Aitken Basin exhibits 13 km of total vertical relief and defines the base of the Pre-Nectarian stratigraphic system (~4.25 to 4.33 billion years ago).

Because synchronous tidal locking keeps the lunar farside permanently oriented away from Earth, the South Pole–Aitken Basin remained unknown until the dawn of spacecraft exploration. The first indication of a giant southern farside basin emerged in the early 1960s from Soviet Luna 3 and Zond 3 photographs and NASA Lunar Orbiter limb imagery, which revealed an extensive arc of high mountains along the southern limb paired with a dark albedo province across the southern farside. In 1968, laser altimetry and Apollo 15 and 16 gamma-ray and X-ray orbital subsatellite tracks across the northern margin near Aitken crater confirmed both a deep topographic depression and an elevated iron concentration.

The dimensions, geochemistry, and internal crustal structure of the basin were established through three generations of lunar polar orbiters:

• Clementine (1994) and Lunar Prospector (1998–1999): Lidar altimetry from Clementine mapped the continuous 2,500-km depression, while the Lunar Prospector gamma-ray and neutron spectrometers documented a distinct geochemical anomaly across the basin floor characterized by elevated iron (FeO ≈ 8–14 wt%, compared to 3–5 wt% in the surrounding feldspathic highlands) and moderate thorium enrichment (Th ≈ 2–4 ppm).
• Kaguya (SELENE), Chandrayaan-1, and Lunar Reconnaissance Orbiter (2007–present): High-resolution laser altimetry by the Lunar Orbiter Laser Altimeter (LOLA) aboard LRO resolved over 50 superimposed secondary basins and craters across the floor (including the Apollo, Poincaré, Planck, Schrödinger, Ingenii, and Von Kármán basins), while the Moon Mineralogy Mapper (M³) aboard Chandrayaan-1 mapped the pyroxene-dominated mineralogy of the floor.
• GRAIL Gravity Mission (2011–2012): Dual spacecraft gravity gradiometry from NASA's Gravity Recovery and Interior Laboratory (GRAIL) revealed that the crust beneath the central South Pole–Aitken Basin has been thinned to 20 to 30 km, compared to 50 to 60 km beneath the northern farside highlands, sitting atop a dense, solidified mafic impact-melt sheet up to 30 to 50 km thick.

A central question of the South Pole–Aitken Basin in planetary science is why an impact structure 2,500 km long did not excavate completely through the lunar crust to flood the floor with green, olivine-rich primitive upper mantle (peridotite or dunite), nor trigger hemisphere-scale lunar mare flood-basalt resurfacing like the smaller nearside Imbrium and Serenitatis basins.

Three-dimensional hydrocode simulations by Peter H. Schultz (1997), Natalia Artemieva, and Brandon C. Johnson et al. (2012, 2016) resolved this paradox by demonstrating that the basin formed via a low-velocity, oblique impact:

1. Elliptical Geometry and North-Northwest Trajectory: The basin's 4:3 elliptical elongation (2,500 × 2,000 km), the asymmetric distribution of thick crustal ejecta piled onto the northern farside highlands, and the southward offset of the thorium/iron anomaly relative to the topographic rim all show that a projectile 170 to 250 km in diameter struck the southern farside from the south-southeast at an oblique angle of 30° to 45° and a modest impact velocity of 10 to 15 km/s.
2. Shallow Excavation Depth: Whereas a vertical, high-velocity (20+ km/s) impact of that size would excavate to depths exceeding 100 to 150 km and distribute primitive mantle olivine across the Moon, a 30°–45° oblique impact excavated primarily laterally, stripping away the upper 15 to 25 km of pure anorthositic crust while melting the underlying lower crust and uppermost mantle in situ.
3. Differentiated Mafic Impact-Melt Sheet: The collision generated a 50-km-deep central sea of superheated silicate impact melt spanning >1,000 km across the basin floor. As this melt sea cooled and crystallized over millions of years, dense olivine cumulates settled to the base while orthopyroxene- and plagioclase-rich noritic and gabbronoritic rocks crystallized in the upper layers exposed on the floor today.

Spectroscopic surveys by Chandrayaan-1 M³ and Kaguya Spectral Profiler demonstrate that the floor of the South Pole–Aitken Basin is dominated by low-calcium pyroxene (orthopyroxene) with diagnostic 1-μm and 2-μm crystal-field absorption bands, corresponding to noritic lower crust and differentiated impact melt rather than upper-crustal plagioclase anorthosite. Only sparse, thin patches of basalt flooding occur inside deep interior craters (such as Apollo, Von Kármán, Ingenii, and Poincaré), covering less than 10% of the basin floor because the surrounding farside crust remained too thick and depleted in radiogenic heat-producing elements compared to the nearside Procellarum KREEP Terrane.

Direct surface exploration of the South Pole–Aitken Basin advanced through China's Lunar Exploration Program:

• Chang'e-4 and the Yutu-2 Rover (2019): On January 3, 2019, Chang'e-4 achieved the first soft landing on the lunar farside inside the 186-km Von Kármán crater (45.44° S, 177.59° E), resting on an Imbrian mare basalt plain covered by ejecta from the adjacent 72-km Finsen crater. Visible and near-infrared spectroscopy by the Yutu-2 rover revealed regolith and boulders dominated by orthopyroxene and olivine derived from the differentiated SPA impact-melt sheet excavated by Finsen.
• Chang'e-6 Farside Sample Return (2024): On June 2, 2024, the Chang'e-6 lander touched down on the southern mare plain inside the 492-km Apollo basin (41.64° S, 153.99° W) in the northeastern sector of the South Pole–Aitken Basin and returned 1,937.3 grams of farside soil and drill core to Earth on June 25, 2024. High-precision lead–lead (Pb–Pb) and Rb–Sr radiometric dating of the returned Chang'e-6 samples revealed two major findings: (i) local farside mare volcanism erupted at 2.807 ± 0.003 Ga (plus high-aluminum basalt clasts at 4.20 Ga) from a strongly depleted mantle source formed after the Giant-Impact Hypothesis lunar magma ocean crystallization, and (ii) noritic and gabbronoritic impact-melt clasts record a major basin-forming impact age of 4.25 Ga, directly anchoring the Pre-Nectarian age of the South Pole–Aitken Basin ~350 million years prior to the 3.9-Ga Late Heavy Bombardment peak.

Because the southern rim of the South Pole–Aitken Basin crosses directly over the lunar south pole, its uplifted massifs—including Mons Mouton, Malapert Massif, and the rim of the 21-km Shackleton crater—stand at elevations of +3 to +5 km alongside deep crater floors that lie in permanent shadow due to the Moon's low axial tilt (1.54° relative to the ecliptic). While high mountain peaks along the SPA rim receive sunlight for >80% to 90% of the lunar year, the adjacent permanently shadowed regions (PSRs) act as cryogenic cold traps at temperatures of 35 to 100 K, sequestering water ice and volatiles delivered by comets, chondrites, and solar-wind space weathering. Consequently, the south-polar rim of the South Pole–Aitken Basin is the primary landing target for NASA's crewed Artemis III and Artemis IV expeditions.

Why isn't the South Pole–Aitken Basin filled with dark lava like Mare Imbrium?

Although the South Pole–Aitken impact thinned the southern farside crust to 20–30 km, the farside mantle lacked the high concentration of heat-producing radioactive elements (potassium, thorium, and uranium) present in the nearside Procellarum KREEP Terrane. Furthermore, the farside crust outside the central floor remained thicker and lower in density than the dense iron-rich basaltic magmas generated in the lunar mantle, preventing widespread hydrostatic magma ascent.

How old is the South Pole–Aitken Basin?

Stratigraphically, the South Pole–Aitken Basin is the oldest recognized multi-ring basin on the Moon, predating the Nectaris basin and superimposed by dozens of Pre-Nectarian craters. Uranium–lead and lead–lead chronometry of noritic impact-melt clasts returned by Chang'e-6 in 2024 places the formation of the South Pole–Aitken Basin at approximately 4.25 to 4.33 billion years ago.

Did the South Pole–Aitken impact create the Moon's nearside–farside asymmetry?

Several geodynamic models propose that the oblique South Pole–Aitken impact contributed directly to the lunar hemispherical dichotomy: its northward-directed ejecta curtain deposited a thick insulating blanket of feldspathic crust across the northern farside highlands, while antipodal seismic and mantle-convection plumes pushed late-stage KREEP residual melts from the lunar magma ocean toward the opposite hemisphere underneath Oceanus Procellarum and Mare Imbrium.

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