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5.31 Origin of the Moon

Where did the Moon come from? This is one of those questions that has plagued children and adults for all of history. Ancient myths explained the Moon's formation with stories as varied as a woman banished to reside in the sky, a bird's egg, a frog, and a "hare in the Moon." Today, these stories are still of cultural interest, but bland and boring next to the potential realities of the Moon's origins that science has revealed. While early myths only needed to explain the simple fact that there is a moon, modern theories must explain additional details as well. Samples from the Apollo and Soviet expeditions, combined with Earth-based observations, have shown:

• As a whole, the Moon is deficient in iron compared to the Earth.

• The Moon is made out of material similar to the Earth's mantle.

• The lunar surface is deficient in water and other volatile compounds, compared to the Earth.

• Very large bodies hit the Moon early in its history.

• Although the evidence is masked by erosion and geological upheaval, very large bodies also hit the Earth.

• The proportions of different oxygen isotopes in lunar rocks (e.g. O16, O18, etc.) are exactly the same as in terrestrial minerals, but different from those in other parts of the Solar System.

Based on these facts and our understanding that the early Solar System experienced a violent period of heavy bombardments, a short list of possible theories has emerged:

• Rotational Fission: George Darwin, son of Sir Charles Darwin, proposed in 1880 that the Moon was "spun-off" from an early, rapidly spinning Earth, and that the Moon emerged from what is now the Pacific Ocean. This theory is no longer really considered possible because we now understand the ocean basin is significantly younger than the moon. This model also doesn't explain the current angular momentum in the Earth-Moon system.

• Binary Accretion: that the Moon was formed from the "left-overs" of Earth's proto-planetary material, and the two co-formed side-by-side. This theory fails to explain how the volatile elements of the Earth and Moon are so different.

• Capture: that the Moon formed in an orbit similar to the Earth and was captured by its gravity into a stable orbit. This theory also fails to completely match the data: the Oxygen isotopic ratios would be expected to be different on both worlds if the Earth and the Moon formed separately. This theory also is very difficult to model, with a third body potentially being needed to explain the capture.

• Giant Impact: that the Moon was formed in a catastrophic impact event between a large (Mars-sized) body and an early-Earth approximately 50 million years after the start of the solar system.


Earth's Moon. Click here for original source URL


The giant impactor model is the most widely accepted today. Looked at more in detail, we start with the idea that planets in the solar system formed from collisions and aggregations of many smaller bodies into larger bodies. The young Sun was very hot, and the rock fragments flying through space were semi-molten. When these pieces collided, their semi-molten nature combined in combination their mutual gravitational attraction caused these pieces to bind together like gravitationally and chemically bound bundles of bread dough. Over time, what started as dust and gravel sized flying particulate matter grew into proto-planets.

Scientists think that an interplanetary body the size of Mars crashed into the Earth at a late stage of the Earth's formation. Computer models of the event have clarified details of the process. This giant impact occurred after the iron in each body had differentiated into a central core. The smaller impactor and the Earth's mantle were completely disrupted, but the Earth's core remained relatively unaffected. The iron in the core of the impacting body sank through the molten proto planet and joined the iron at the Earth's core. Lighter debris from Earth's mantle and the impactor's mantle mixed together and were ejected into a swarm of material around the Earth, where it formed a ring of material that eventually aggregated to form the Moon.

This hypothesis neatly fits the available facts, but it is only as good as the data. If new facts emerge during future observations, this hypothesis may go into the same "Historical Explanation" category as the "Bird's Egg" model of the past. For now though, this is the leading theory and it eloquently explains the Moon's deficiency of iron (since all the iron would be in the Earth's core) and the similarity between the Moon's composition and the composition of the Earth's mantle (since the Moon would actually have formed from the Earth's mantle debris). The impact scenario also explains the Moon's relative lack of water: the impact would have heated the debris to such a high temperature that volatiles like water vapor would have escaped into space. The Earth retained those gases with its larger gravity. Terrestrial and lunar oxygen isotopes match because much of the debris was from the Earth itself, and since the impacting interplanetary body would have originally formed at about the same distance from the Sun.

The hypothesis of a catastrophic origin of the Moon leads to an obvious question: where is the scar of this enormous impact on the Earth? The reality is, the impact occurred so early in the Earth's history that the planet had not completely solidified. The semi-molten rock of the Earth's surface would have flowed under the influence of gravity to fill in any large crater. The impact was then followed by over four billion years of geological activity that would have worn away any remaining traces.

The debate over the origin of the Moon reveals the process of science. There were always a number of ways to explain a given set of data, and scientists keep working on the problem until they find a solution that matches all the facts. In this case, some scientists proposed that the Moon formed separately, and then was gently captured by the gravity of the larger Earth. Not only is this very unlikely from a dynamical perspective, but this theory can't explain the compositions of the Earth and the Moon as neatly as the hypothesis of a catastrophic origin, so it has been largely set aside as not feasible.

Science is unable to provide an answer for every question from the distant past. The historical record is incomplete, and the evidence is fragmentary. Although we may have very good and plausible ideas about ancient events — the death of the dinosaurs, the formation of the Moon — we may never be absolutely sure. Most of the progress we make in science is based on evidence such as the list of facts given above. Discovering the correct interpretation of these facts is part of what makes science exciting. But science also moves forward using new investigative techniques.


Far side of the Moon, as seen by the Clementine spacecraft. Click here for original source URL.