The Moon — The Far Side Anomaly: A Different World
The Moon — The Far Side Anomaly: A Different World
[edit | edit source]Overview
[edit | edit source]The far side of the Moon — the hemisphere permanently turned away from Earth by tidal locking — is one of the most anomalous features in planetary science. The two hemispheres of the Moon are so different in geological character that they might almost be described as different worlds that happen to share the same body.
The Asymmetry in Numbers
[edit | edit source]| Feature | Near side (Earth-facing) | Far side (hidden) |
|---|---|---|
| Maria (dark volcanic plains) | ~31% of surface | ~2% of surface |
| Average crustal thickness | ~60 km | ~100 km |
| Major impact basins flooded with lava | Many (Imbrium, Serenitatis, Crisium) | Essentially none, despite having South Pole-Aitken Basin |
| Ancient volcanic activity | Extensive and prolonged | Very limited |
| Mascons | Abundant beneath near-side basins | Fewer and different character |
| Topography | More subdued; large flat areas | Rougher; more uniformly cratered |
| Thorium concentration | Elevated (KREEP terrain) | Much lower |
The Most Anomalous Asymmetry
[edit | edit source]The most puzzling aspect of the near-side/far-side asymmetry is this: the far side has the South Pole-Aitken Basin — the largest confirmed impact structure in the solar system, approximately 2,500 km across and 8 km deep. This enormous impact basin should have been flooded with volcanic lava, just as the near-side basins were. It was not. The South Pole-Aitken Basin is the biggest impact crater in the solar system and it contains essentially no volcanic mare — while smaller basins on the near side are full of it.
The implication: whatever drove the near-side volcanic flooding was not simply the availability of large basins or the energy of large impacts. It was something specific to the near side — something about the near-side geology that enabled volcanic eruptions that the far-side could not generate despite having an even larger excavated basin.
Proposed Explanations
[edit | edit source]The KREEP hypothesis: The near side of the Moon contains elevated concentrations of a suite of elements called KREEP (Potassium — K; Rare Earth Elements; Phosphorus). These elements are incompatible with common lunar minerals and become concentrated in the residual liquid of a crystallising magma ocean — the last material to solidify. If the KREEP-rich material (which contains heat-producing radioactive elements including thorium and uranium) is concentrated on the near side, it could have provided the heat source that drove near-side volcanism for much longer than far-side volcanism.
Why KREEP is concentrated on the near side is itself unexplained — it represents an asymmetry in the Moon's original formation.
The giant farside impact hypothesis: A 2022 study proposed that an ancient enormous impact on the far side could have caused molten material to migrate to the near side, creating the KREEP concentration. This is speculative and the ancient impact has not been identified.
Tidal heating: Earth's gravitational tidal forces preferentially heat the near side; this additional heat source may have extended near-side volcanic activity.
Why the Far Side Is Hidden
[edit | edit source]The tidal locking that keeps the far side permanently hidden from Earth is, in the hollow/spaceship moon framework, among the most discussed features. The argument: the dramatically different face — with its thicker crust, its absence of volcanic plains, its enormous and anomalous impact basin containing a buried metallic mass — is exactly the face that cannot be seen from Earth. The most anomalous, least understood, and most geologically different hemisphere of our closest neighbour has been hidden from human observation for the entirety of recorded history.
China's Chang'e 4 mission achieved the first soft landing on the far side in 2019, providing the first ground-truth observations from this hidden world. The data confirms it is geologically different from the near side but has not yet resolved the question of why.
