The Collision Model of the Moon's Formation

The collection and analysis of rocks brought back from the Moon during the Apollo spaceflight program in 1969 renewed interest in the process by which the Moon formed. One theory, the collision model, was that Earth, while it was still in the process of forming, was hit by a projectile that was Mars-sized (half the diameter of Earth). While some debris from this collision was ejected into space, other debris remained in Earth's orbit, where it formed a thin ring of rocks. The Moon then formed from this ring by accretion, the processes of collision and sticking responsible for building most bodies in the solar system. Consistent with this model was the finding that lunar samples were depleted of elements that vaporize readily, such as zinc, as would have occurred as a result of a violent impact. Stuck in the gas phase, these elements would have been swept into space. Apollo's samples were also exceedingly dry, water being among the lost substances.

Another remarkable property of lunar samples is that they are highly depleted in the siderophile, or iron-loving elements that tend to concentrate in the metallic iron cores of planets. When planetary cores form by the molten iron sinking to the planet's center, the siderophile elements (such as platinum, gold, and iridium) are incorporated into the falling iron and are highly depleted from the crustal and mantle materials left above (the mantle is the layer above the core, while the crust is the thin outermost layer). That the lunar rocks were depleted in siderophile elements was peculiar, however, because the Moon cannot have a substantial iron core. The mean density of the Moon is 3.4 times the density of water, very similar to that of rocks on the lunar surface and much lower than that of Earth, which is 5.5 times the mean density of water. If it had a substantial core of dense metallic iron, the mean density of the Moon would be higher than is observed.

The collision model solves the siderophile mystery by suggesting that metallic cores had formed in both Earth and the impact projectile before the collision. In the collision, the iron cores of both bodies ended up in the center of Earth, and the debris ejected into orbit was mainly from the mantles of both bodies. The preceding burying of siderophiles in planetary cores explains why gold and platinum are so rare on the Moon and in the crustal rocks of Earth. The impact ejection of mantle materials from both the giant impactor and the target Earth is consistent with some of the remarkable similarities between the Moon's trace elements (elements that exist in only low concentrations) and those of rocks from Earth's mantle. It is also consistent with Earth and the Moon having identical ratios of the isotopes (variations of a single element) of the elements that are present on both of them.

A collisional origin is very appealing, but did it actually happen? For an impact to eject enough material to form the Moon, the colliding body has to be huge, a Mars-sized body. Theoretical modeling of planet formation by planetary scientist George Wetherill showed that a natural consequence of the accretion process is that several large bodies do form in each planet's accretion zone (the area in which the planet-forming material accumulates). The growth process includes the impact of several bodies, each of which carries more than 10 percent of the mass of the final planet.

In the case of Earth, these big bodies were the size of Mars and larger. Their impact not only ejected material into space but also injected considerable amounts of heat into Earth's mantle. This heat input and great violence led to the forging of Earth's core during the accretion phase, before the planet was fully formed. Core formation requires high internal temperatures so that blobs of molten iron descend through the mantle to reach the core. Celestial objects that form through the accretion of small bodies form a core only after long-term buildup of radioactive heat from the decay of uranium, potassium, and thorium. In Earth's case, the early heat from the accretion of large bodies led to core formation as accretion occurred. Both Earth and the large projectile that struck it had differentiated (separated into layers) and already had metal cores at the time of collision.