Origins of the Moon

There are several theories that might explain how the Moon was created. Three have gained popularity over the years but all have faults. Any theory in order to gain acceptance and be plausible must be able to answer all open questions without raising any new ones. No existing theory currently is capable of doing that.

Any theory of lunar creation must be capable of explaining several issues that no existing theory can do with complete certainty. The first issue that must be explained is the close relationship in size between Earth and the Moon. This is unique among major bodies in the solar system and so must be explained by an event that is rather rare. Secondly is the fact that the samples returned from the Moon exhibit the same oxygen isotope composition as those examined on Earth. This means that the Moon formed in the same environment as Earth did. Rock samples examined on Mars have very different oxygen isotopes compositions for example. By inference, this means that the Moon and Earth formed in close proximity. Third is the fact that the Moon orbits Earth in a stable, nearly circular, prograde path. Fourth, such a theory must explain the dramatic difference in density between the two bodies. The density of Earth is 5.5 grams per cubic centimeter, the Moon is only 3.3 grams per cc. This is because the Moon is almost completely lacking in iron, while Earth is the most iron-rich body in the solar system. Finally if a theory proposes that the Moon is spun off from the primordial Earth, it must account for the lack of an impact scar somewhere on Earth's surface.

One of the earliest theories is that the Moon condensed out of the solar system's primordial debris cloud in the vicinity of Earth. But if this were true, then the Moon would be made of most of the same material that Earth is and should exhibit similar composition. But it does not, the Moon is almost a completely iron-free zone. The Moon's low density relative to Earth, the lack of any iron in the rocks and the fact that the Moon had no measurable magnetic field all illustrate this fact. The strength of this theory is that it readily explains the stable nature of the Earth-Moon system in a stable circular orbit and the Moon's tidal lock to Earth. It also explains the similarity in oxygen isotopes.

A second theory proposes that the Moon formed separately in the inner solar system and wandered too close to Earth and was captured by its gravity. But this theory fails to pass even a prima fascia test. The Sun is by far the dominant gravitational force in the inner solar system. For an object to be captured in a prograde orbit (counterclockwise as viewed from above Earth's north pole), it would have to approach from behind. As it neared Earth, it would be accelerated and flung back out of Earth's gravitational field into a solar orbit15. This "slingshot" effect is frequently used to boost deep space probes into the outer solar system. To be captured by a planet, an object must approach from the front, so the gravity of the capturing planet would act to slow the would-be Moon down. But this would in turn create an orbit that is both highly elliptical and retrograde in nature. This theory also cannot explain, except by chance, the Moon's lack of iron. It also does not explain why lunar samples contain similar oxygen isotopes. If the Moon formed elsewhere, it should exhibit differences in this measure in the same way that Mars does.

The leading theory today was formulated some 25 years ago. It suggests that late in Earth's formation, a large planetary sized body impacted Earth. The impact dislodged a massive amount of mantle material from both planets with sufficient velocity to enter Earth orbit. These materials then coalesced into the Moon. The theory satisfies the oxygen isotope matter because the Moon would have been formed from material, most of which came from Earth. Since the impact occurred late in Earth's formation, most of Earth's iron would have sunken into the core; this also explains why the Moon lacks iron. It also explains the Moon's neat and orderly orbit. It also accommodates the unique nature of the Earth-Moon system because such an impact event would be so rare. It is plausible that no other planet has

15 The dominance of the Sun in this manner was clearly illustrated recently by an object that Earth's gravity captured briefly but after the object made several orbits of Earth, it drifted back out into a solar orbit again. What some thought might be a new "moon" of Earth was later identified as the third stage of the Saturn V that was used to launch Apollo 12.

undergone such an impact and that may be why no other planet has such a large moon. But this theory too leaves open items to be answered. First is what became of the impacting planet? To dislodge enough material from Earth to create the Moon, the impacting planet had to have been nearly the size of Mars! If so, then where did all that material go? And if the impactor had an iron core of its own, then where is the iron? Proponents of the impact theory postulate that any iron could have been merged into Earth's core and computer models do support this idea. But what became of the rest of the impactor? If it settled onto Earth's surface we should then be able to detect large quantities of rock that are not native to Earth (oxygen isotopes again). If it remained in space, it should have formed a ring around Earth. There is none to be found. Had it been coalesced into the Moon, we would see the differences in oxygen isotopes in the Moon's rocks. The final issue with the impact theory is the lack of a plausible explanation for an impact site. Though billions of years of erosion would have reduced the size of the wound, it would still be clearly evident. It is not. Some have suggested the Pacific basin as an impact site, but the depths of the ocean and composition of the seabed are no different than they are in the Atlantic or Indian Oceans. Despite its shortcomings however the impactor theory continues to hold up as the best current model on how the Moon formed.

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