Hypotheses for the Formation of Earth's Moon

When one looks at the conditions which make advanced life on the Earth possible, one of the surprises is the importance of the Moon. The present standard model is that the Moon was formed when Earth experienced a collision with a Mars-sized body early in its history. It is appropriate to examine alternative hypotheses to evaluate the strength of the standard model. The alternative hypotheses advanced have been:

There are major problems with all three alternative scenarios for the formation of the Moon, but they offer looks at some interesting physical concepts.

The Moon's mass was thrown off by a rapidly spinning proto-earth.

Hypothesis for Moon formation

This scenario envisions the early Earth in its molten state spinning rapidly and forming a tidal bulge at the equator which led to the separation of the mass of the Moon and its coalescence as a separate body. Sometimes called the "fission" model, this hypothesis was put forward by George Darwin in 1879. Part of the model was the proposal that a resonance with the Sun and solar tides which gave the necessary extra boost to separate off a large molten mass which then solidified to form the Moon. In support of this hypothesis, Osmond Fischer proposed that the Pacific Ocean represents the unhealed scar where the Moon used to be. Further support came from the fact that the Moon's density is very nearly equal to the density of the Earth's mantle.

The arguments against this hypothesis involve a number of interesting studies.

1. The Pacific Ocean basin is much too young to have been the Moon's exit point. Radiometric dating of the sea floor gives an age of about 200 million years compared to the projected age of the Earth at around 4.5 x 10⁹ years. Dating of the oldest Moon rocks gives an age of the Moon of about 4.4 x 10⁹ years.
2. "The angular momentum in the Earth-Moon system is insufficient by a factor of three for the proposed primordial Earth to have formed the tidal bulge necessary for fission."(Dalrymple) No reasonable mechanism has been found for the loss of enough angular momentum after separation to account for the deficit.
3. Tidal friction in the Earth would dampen a tidal bulge before it could become large enough to throw off part of its mass.
4. The "mass of the daughter planet produced by fission should have been about one fifth the mass of the Earth."(Dalyrmple) No plausible mechanism can bridge the gap between one fifth of the Earth's mass and the observed mass of M_Earth/81.3 for the Moon.
5. The newly formed Moon would be inside the Roche limit relative to the Earth and would be destroyed by tidal forces.
6. No plausible scenario would give the fission fragment (Moon) the orbital velocity necessary to remain in orbit about the Earth. Some matter might escape entirely, but most would fall back to Earth.
7. If the Moon were the result from the fission of an equatorial tidal bulge, then its plane of orbit should be that of the equator. But the Moon's orbit varies from 18.5° to 28.5°.

   Alternative hypotheses for Moon formation

The Moon formed elsewhere and was captured by Earth's gravity.

Hypothesis for Moon formation

While advocates of this model differ in their proposed origin of the interloper, the fact that the age of the Moon is similar to the solar system makes it likely that the source of the material is from within the solar system. Dalrymple points out that the oxygen isotope concentration is identical in the Earth and Moon and the undifferentiated meteorites, but different from that in more primitive meteorites. This would seem to restrict the source of the Moon to the same general neighborhood of the solar system as the Earth, but the bulk composition of the Earth with its large iron core is greatly different from that of the Moon.

Can the Earth just capture a neighbor that wanders by and make a moon of it? Not without losing a lot of energy! A smaller body that encounters the gravity of a larger body will experience the slingshot effect, more formally known as a gravity assist maneuver, and will escape the encounter traveling in another direction.

The Earth and Moon formed together as a planetary pair.

Hypothesis for Moon formation

Often called the "double planet hypothesis", this proposal is that the Earth and Moon formed at about the same time in the same region of the solar system and were close enough together to form a bound system with each other. It certainly seems like a reasonable hypothesis.

The first challenge to this hypothesis is the great difference in composition of the Earth and Moon. If they formed at about the same time in the same region of space by the same mechanisms, then they would be expected to be very similar in structure and composition. But the Earth has an average density of 5.5 gm/cm³ and the Moon has a much lower overall density, 3.34 gm/cm³, indicating that it lacks the iron core. The densities of Mercury, Venus, and Mars are 5.4 gm/cm³, 5.24 gm/cm³, and 3.94 gm/cm³ respectively. Even the majority of meteorites have densities greater than the Moon and many are mostly iron, suggesting that a large iron content is part of the expected composition of objects formed in the Earth's part of the solar system.

Arguments in favor of this hypothesis include the fact that the Moon's composition is very similar to material in the Earth's mantle. Furthermore, the oxygen isotopic content of the Moon rocks is identical to that in Earth rocks and the differentiated meteorites, suggesting that it formed in the same part of the solar system.

Attempts to resolve the problem of the composition differences between Earth and Moon include the proposal that the Moon accreted from a ring of plantesimals that formed around the Earth soon after its formation. A variation on this theme has the Earth capturing a small moon which then grew by accretion. But the Earth would presumably have grown by the same accretion process, and there does not seem to be a clear path to explaining the great difference in composition under the constraints of the double planet hypothesis.