South Pole-Aitken Basin
The largest confirmed impact crater on the moon — 2,500 km wide, up to 8.2 km deep
NASA/LROC south polar mosaic · South pole at center
Diameter
2,500 km
Max depth
8.2 km
range 6.2–8.2 km
Formed
~4.25–4.3 Ga
confirmed by Chang’e-6
Crust below
~30 km
thinned from 60–80 km
Overview
The solar system’s largest known impact structure
SPA Basin formed ~4.25–4.3 billion years ago — confirmed by samples returned by China’s Chang’e-6 mission in 2024. Because lava never flooded it like the near-side maria, it preserves an extraordinarily ancient surface: a direct window into the early solar system.
Largest Crater on the Moon
Spanning nearly a quarter of the Moon’s circumference, the South Pole-Aitken Basin is the largest, deepest, and oldest confirmed impact structure on the moon. Invisible from Earth, it has rewritten our understanding of how planets form — and may hold the key to a permanent human presence in space.
The 4 Pillars of SPA Basin Science
Continental scale
2,500 km in diameter — wide enough to swallow the entire contiguous United States, with room to spare on either coast.
Everest-class drop
6.2–8.2 km deep. Standing on the basin floor at its deepest point, Mount Everest would barely crest the distant rim.
Pre-Imbrian relic
Formed ~4.25 billion years ago during the Late Heavy Bombardment — precisely dated in 2024 by Chang’e-6 norite sample analysis.
Stripped to the bone
The impact thinned the lunar crust from a normal 60–80 km down to just ~30 km, exposing ancient deep-crustal and mantle-influenced material.
A Window Into the Lunar Interior
The SPA Basin is effectively a cosmic drill site. Current best estimates place the impactor at 350–400 km in diameter — larger than many dwarf planets — striking at 12–16 km/s. The energy released punched through the lunar crust and deep into the lunar mantle, leaving the basin floor chemically distinct from the rest of the Moon: enriched in iron, clinopyroxene, and thorium — signatures normally buried far beneath the surface.
Unlike the dark “seas” (Maria) on the near side, the SPA Basin was never fully resurfaced by lava flows. Chang’e-6 samples returned in 2024 confirmed the presence of hematite and maghemite — iron oxide minerals produced by the intense heat and transient oxygen-rich vapour generated by the original impact — a finding that challenges the long-held assumption that the lunar surface is entirely chemically reduced.
Data from NASA’s GRAIL mission (2011–2012) revealed a massive subsurface mass concentration beneath the SPA Basin floor. Scientists hypothesise this may be the dense metallic remnant of the original impactor — its iron-rich core, now buried deep in the lunar mantle, still exerting a measurable gravitational pull on spacecraft in lunar orbit. It is, in essence, the bullet still lodged in the wound — though this remains an active area of research.
SPA Basin depth vs. Earth and solar system landmarks — scaled to Mariana Trench (11 km max)
Why Does the SPA Basin Matter?
The SPA Basin is not merely a geological curiosity — it is arguably the most strategically important location in the inner solar system for scientific exploration.
Water ice
Permanently shadowed regions near the south pole rank among the coldest places in the solar system. Water ice trapped there for billions of years could supply fuel and life support for future human missions.
Deep-crust samples
The SPA floor exposes deep crustal and mantle-influenced minerals unreachable anywhere else on the Moon. Analysing them could rewrite models of how rocky planets formed and differentiated in the early solar system.
Artemis gateway
NASA’s Artemis programme targets the lunar south polar region. The SPA Basin’s rim hosts candidate landing sites balancing scientific value, solar power access, and proximity to ice deposits.
Mission Logistics: Robotic Surface Recon
The first soft landing anywhere on the lunar far side. The Yutu-2 rover performed the first in-situ chemical analysis of the basin floor, detecting clinopyroxene and orthopyroxene — minerals consistent with deep crustal and mantle-influenced material excavated by the original impact.
The first ever sample return from the lunar far side, retrieving 1,935 grams of material from within the SPA Basin. Analysis of impact-melt norite clasts precisely dated the basin to 4.25 billion years ago, and revealed hematite and maghemite — iron oxides formed by the impact event — challenging assumptions about lunar surface chemistry.
The Lunar Reconnaissance Orbiter produced definitive topographic maps of the basin. GRAIL’s twin spacecraft measured gravity anomalies revealing the thinned crust (~30 km vs 60–80 km in surrounding highlands) and a subsurface mass concentration — possibly the metallic remnant of the original impactor.
Frequently Asked Questions
What is the largest crater on the Moon?
Why is the SPA Basin on the far side of the Moon?
What minerals did Chang’e-6 find in the SPA Basin?
What is the GRAIL gravity anomaly beneath the SPA Basin?
Why haven’t humans visited the SPA Basin yet?
The Ultimate Scientific Objective
The South Pole-Aitken Basin is not a destination at the edge of our ambitions — it is the destination. No other location in the inner solar system offers the same combination of ancient deep-crustal material, preserved impact history, accessible water ice, and strategic value for a permanent human presence.
The 1,935 grams Chang’e-6 returned in 2024 are only the beginning. Every gram of material from the SPA floor is a message in a bottle from the solar system’s violent infancy — 4.25 billion years old, finally being opened. The hematite discovered in those samples alone has rewritten what we thought we knew about the Moon’s surface chemistry.
The side of the Moon we cannot see from Earth holds the greatest scientific prize in our cosmic neighbourhood. We are only just beginning to read it.
Further Reconnaissance
Technical Intelligence & Global Mapping Tools
🌗 The Far Side Guide
The SPA Basin is the defining geological feature of the far side. Analyze the tidal locking mechanics that keep this giant crater hidden from Earth.
📍 South Pole Recon
The southern rim of the SPA Basin is the target for NASA's Artemis III. Analyze the topography and water-ice "Cold Traps" found at 89.9°S.
🗺️ The Lunar 100 Guide
Identify the other 99 most significant features on the lunar surface. A technical field guide for high-fidelity telescope observation.