The Moon - A Fork After the Giant Impact
Origin & Age
Earth is 4.6 billion years old; the Moon is 4.5 billion. The Moon formed when a Mars-sized body, Theia, struck the early Earth (about 100 million years after formation), and ejecta coalesced into the Moon. This Giant Impact created a body that is similar to Earth in some ways, yet fundamentally different.
Moon Specs
| Item | Value | Versus Earth |
|---|---|---|
| Age | ~4.5 billion years | Earth is 4.6 billion |
| Diameter | 3,474 km | ~1/4 of Earth |
| Mass | 1/81 of Earth | — |
| Gravity | 0.165 g | ~1/6 of Earth |
| Surface temperature | Day +120°C / Night −170°C | Extreme diurnal range |
| Atmosphere | Near-vacuum (trace Ar, He) | Not breathable |
| Can humans move bare-skinned? | No (seconds to loss of consciousness) | Impossible |
| With spacesuit? | Yes (proven by Apollo EVAs) | Feasible |
Magma Ocean → White Highlands & Dark Seas
Right after formation, the Moon was largely molten—a global magma ocean. As it cooled, light plagioclase floated and crystallized into anorthosite crust (the bright highlands), while denser olivine and pyroxene sank to form the mantle. Later, fractures allowed mantle-derived magma to erupt and solidify as basalt, creating the dark maria (“seas”).
Surface Composition & Ratios
By area, the lunar surface is dominated by anorthosite (~80–85%) with basaltic plains (~15–20%). This contrast draws the familiar patterns we see from Earth.
Rock Specs (Materials View)
| Rock | Key traits | Exists on Earth? | Uses (material) | Major sources (on Earth) |
|---|---|---|---|---|
| Anorthosite (Plagioclase-rich; lunar highlands) | Light-colored, low density; product of magma-ocean flotation | Rare (localized) | Decorative stone, monuments, research specimens | Norway, Canada, South Africa (very scarce in Japan) |
| Basalt (Lava plains; lunar maria) | Fe/Mg-rich, dark volcanic rock; solidified lava flows | Abundant | Aggregates, road base, rock wool (insulation), cladding | Japan (Fuji, Aso), Hawaii, Iceland |
Why the Moon Has No Granite
Granite on Earth requires water-assisted re-melting and differentiation, often driven by plate tectonics. The Moon lacks liquid water and plate tectonics, so basalt cannot “evolve” into silica-rich granite on a continental scale.
Environment & Human Activity
With a near-vacuum and extreme temperatures, bare-skin activity is impossible. Spacesuits enable EVA, as demonstrated by the Apollo missions, allowing sampling and surface operations.
Contrast with Earth
The Moon is a simple anorthosite-plus-basalt world with virtually no water or atmosphere. Earth, by contrast, combines water and plate tectonics to build granite continents—the very stage on which culture and civilization arose.
Summary
- Origin: Giant Impact → magma ocean → anorthosite highlands / basaltic seas
- Ratios: anorthosite ~80-85% / basalt ~15-20%
- Human viability: bare-skin no / spacesuit yes
- Key divergence: no water or plates → no granite generation