The fission model is a historically significant, though now superseded, theory on the origin of Earth’s Moon. It proposed that a young, molten Earth rotated so rapidly that a piece of its mass was ejected into space, which eventually coalesced to form the Moon.
Conceptual Basis of the Fission Model
The model was formally proposed by George Darwin in 1879. He theorized that the early, molten Earth was spinning so fast it became distorted and elongated at its equator. This instability, possibly amplified by solar tides, led to a portion of Earth’s mass being thrown into orbit. Gravitational forces would have then pulled this material together into a sphere.
As the Earth’s rotation slowed over eons due to tidal friction, the Moon would have receded to its current distance. A later modification suggested the event was triggered by rotational instability as Earth’s core and mantle differentiated. This process required a rotational period of about 2.6 hours. Material flung beyond Earth’s Roche limit—the distance where tidal forces would tear a body apart—could then safely accrete to form the Moon.
Early Supporting Arguments
One argument for the fission model was the Moon’s lower density compared to Earth. This discrepancy was explained by the theory, which proposed the Moon formed from Earth’s lighter crust and mantle, leaving the dense iron core behind.
Another now-disproven idea was that the Pacific Ocean basin was the scar left by the Moon’s departure. The apparent fit of the basin’s shape fueled this speculation for decades. These arguments were convincing at the time, accounting for the Moon’s composition and offering a potential origin point on Earth’s surface.
Insurmountable Scientific Hurdles
The fission model failed to overcome several scientific problems. The primary issue is the angular momentum of the Earth-Moon system. For Earth to have spun fast enough to eject the Moon—requiring a day of only two to three hours—the system’s total angular momentum would have been much higher than it is today. No known mechanism could account for dissipating that much energy to slow the system to its current state.
Analysis of Apollo lunar samples revealed compositional problems. While oxygen isotopic ratios are similar to Earth’s, lunar rocks are severely depleted in volatile elements like water, sodium, and potassium. A simple ejection of mantle material would not produce this chemical signature. These physical and chemical inconsistencies led scientists to seek other explanations.
The Prevailing Giant-Impact Explanation
The leading theory for the Moon’s formation is now the Giant-Impact Hypothesis. This model proposes that a Mars-sized protoplanet named Theia collided with the early Earth about 4.5 billion years ago. The impact ejected a large amount of debris, mostly from the mantles of both bodies, into orbit. This cloud of material then coalesced to form the Moon.
This hypothesis resolves the issues of the fission model. A glancing blow from a large impactor would impart the correct amount of angular momentum to the Earth-Moon system. It also explains the Moon’s small iron core and lower density, as it formed from the lighter mantle materials of the two bodies.
The energy and heat from the impact provide a mechanism for the depletion of volatile elements, which would have vaporized and been driven into space. The model also accounts for isotopic similarities and differences, as the Moon is a mixture of materials from both early Earth and Theia.