Earth’s Moon is our closest and most regular celestial companion. It is also a relatively large one – especially in the inner solar system, where only Mars has other moons, and both are very small. This makes it all the more frustrating that the origins of the Moon, very early on in the history of the Earth (about 4.5 billion years ago, when the Earth itself was only a few million years old), defy easy explanation. The most common explanation today, the so-called giant impact theory or giant impact hypothesis, states that the Moon consists of debris thrown off during a giant collision between the early Earth and another planet-sized object. However, this hypothesis has not (yet) been able to fully explain all of the data we have about the early inner solar system.
– Giant Impact Theory –
Around the turn of the 20th century, scientists estimated that based on Newtonian physics the Moon was actually getting farther away from the Earth, albeit very, very slowly. This was confirmed by laser ranging experiments once the first Cold War-era spacecraft landed on the Moon. The finding was an unexpected one, implying that the Moon was at one point much closer to the Earth than it is now. This is the opposite path from the one that would be expected if the Moon were an early solar system body of its own, captured by Earth’s gravity. (For instance, in several tens of millions of years, models predict that Mars’s moon Phobos will be drawn in and impact its surface.)
Followed to its logical conclusion, albeit one which the mathematical models initially could not completely validate, this discovery implied that at some point the Moon must have emerged from the surface of the Earth – presumably through some sort of intensely traumatic event early in the solar system’s history. The most likely explanation is that a protoplanet roughly the size of Mars crashed into Earth. The core of this planet, hypothetically named Theia by astronomers, would have sunk into Earth’s own iron core. However, large amounts of debris from the surface of both Earth and Theia would have been thrown off into orbit. This material would then have coalesced into a body of its own, our Moon.
The first manned landings on the Moon indicated that the oxygen isotope ratios in the surface rocks of the Moon were similar to those here on Earth, indicating that the two bodies have a common origin (consistent with the giant impact theory). This, along with other assumed facts about the Moon (like the presence of a very small iron core, if any) can be outcomes from a modelled interplanetary collision, if the appropriate velocity and angle of impact are used in the calculations. At least in theory, therefore, the Moon could have formed from an ancient collision, as the giant impact hypothesis states.
– Criticism and Alternatives –
However, the above conclusion results from only a selective treatment of evidence. There are other facts we know about the Moon and the Earth which seem to rule out the giant impact theory. For example, we currently have no evidence for a lava ocean on the surface of the Earth at that point in its history – which should have resulted from the collision with Theia. Moreover, despite the oxygen isotope ratios, the proportion of other minerals present in the Moon’s composition do not seem likely to emerge from the mantle of Earth, implying that at least some other factor contributed to the Moon’s formation.
In fact, even the oxygen isotope ratios themselves are not nearly as conclusive as they may appear at first glance. If the Moon formed after a collision with Theia, than the ratios should not simply match those on Earth: they should be altered to reflect the different material taken from Theia, which they are not. (This last point operates on the assumption that Theia’s composition was different than Earth’s, but is borne out by the fact that today all planets’ compositions differ somewhat.)
Nevertheless, no competing theory is able to account for more of the evidence at once than the giant impact theory – which is why it remains the currently accepted theory. The other two standard models of moon formation predict that moons can form either through the accretion of dust and debris in orbit of a planet (much in the same way planets form in orbit around the Sun), or are free-floating pre-planetary objects in their own right which are captured by another planet’s gravity and pulled into orbit. Currently no strong body of evidence supports either of these explanations either.
For this reason, today the most likely answer to the question of where the moon came from is that it is the result of a nearly catastrophic impact early in the solar system’s history, between Earth and a Mars-sized planet named Theia. However, the Earth-Theia collision does not explain all of the evidence (or at least, does not yet), and therefore a more powerful explanation may yet be devised.