Theories abound to address how our moon was formed. After we brought moon rocks back from the lunar excursions of the 60s, certain facts became clear, like the striking similarity between Earth’s chemical composition and that of the moon. Like the blind men with the elephant, we’re still at a loss for how to explain them in a way that satisfies the skepticism of the entire scientific community. But every good theory advances us closer to the much-sought-after point where our predictions and our observations agree. The latest, which is still open for debate: the Moon may have coalesced out of many smaller moonlets produced by many smaller impacts.
The argument is straightforward: In this history, the moon resulted not from a single Great Impact, but from a series of more like 20 impacts with smaller objects, across a period of a few million years. Each time something hit the Earth, it chipped off debris, which formed into a disk in a matter of hours as it rotated away from Earth. The disks, known as moonlets, aggregated over several hundred years to assemble the moon.
Whether this theory holds up in light of the chemical composition of both orbiting bodies is an open question. Both a single Great Impact and a series of smaller unpleasantries have the potential power to explain what happened to form our moon, because both hypotheses make predictions that can be tested. Lead author Ralaca Rufu points out that the team reached these hypotheses using the model the Great Impact authors originally used to test their own theory, back in 1984.
Neither is the moon made of green cheese.
“We see that the moon and Earth are very similar in a lot of ways, in a lot of signatures that we measure— oxygen, tungsten, and so on,” Rufu told the Atlantic. “So you cannot explain the composition of the moon with a foreign material.”
The introduction of a little bit of foreign material would produce a telltale difference in proportional “contamination:” The Earth would have less of whatever it was than the Moon, which would tell us conclusively something hit us. We kept some of it and the Moon got some of it, and proportionally the foreign content of the Earth and Moon would be different. That’s one of those facts: we don’t have that telltale compositional difference. But there are other theories in play that show how you can get to the lunar chemistry we see by way of an impact that causes zero mixing, or with a mushier impact that causes total mixing.
The Great Impact hypothesis is the current heavyweight in the field of lunar-origin explanation. Its current best says that shortly after the Earth formed, an itinerant planet (Theia) between the sizes of Mars and Mercury hit the nascent Earth in one great big impact that sloshed out a messy bolus of molten rock that eventually fell together and became the Moon.
In a zero-mixing scenario, the planet that hit us would’ve had to leave little trace of itself, because the Moon and Earth are so similar in composition; this might mean that the thing that hit us was already quite solid and cold itself, and our crust was also solid enough that a glancing impact could have ruptured the Earth and squeezed out a plume of magma. This is supported by the fact that our orbit is not wildly out of wing. Instead of T-boning us, the inbound planet would have had to bounce off the Earth at a glancing angle, sailing off into interstellar space instead of slowing us down too much or hanging around to further perturb the inner solar system.
But if a cold, solid body hit us and then left without leaving much behind, wouldn’t this mean that there wasn’t much surface mixing, so the plume of magma would be most similar in composition to the chemical makeup of the Earth’s mantle and/or core? We don’t even really have core or mantle samples of our own planet, to know how to answer that. Furthermore, what happens to our planet’s magma when it hits the vacuum of space? Does it cool in ways that explain the Moon’s composition?
The far side of the Moon — and Earth.
In a total-mixing scenario, a very young and still only barely solid Earth gets slugged by a slow, mostly molten Theia at a glancing angle, causing the two to mingle, spin and eventually slosh out the moon out of the now thoroughly-mixed Earth-Theia system. It all depends on the chemistry of the collision. Scientists backing this theory contend that dumping a ton of silica (Theia’s core) into the Earth’s early metal-sulfide and carbide core chemistry could force the now-larger Earth’s bulk chemical composition to undergo a shift as profound as the Oxygen Catastrophe: forming lighter metal silicates and carbonates, and then rejecting them to the crust.
For the total-mixing scenario to hold, the Moon must have formed before the Earth was quite cool from the impact, and it must have formed out of the wholly commingled bulk of Earth/Theia. Theia didn’t fly away, in this scenario; it was smashed into Earth like two balls of Play-Doh, mixed completely, and then the Moon pinched off from that. We can test this by observing planets colliding elsewhere — no big — or by otherwise conclusively proving that the carbon-silicon chemistry described explains the partitioning of lunar and geochemistry.
The Moon Moon moonlet model still leaves unanswered several important questions, like how the moonlets finally aggregated into our moon. But Rufu isn’t ready to scrap the theory just yet. “I would like readers and scientists alike to not say that this is wrong because you were taught in school that it’s a giant impact,” she said to the Atlantic. “Try to be open-minded.” Whatever the answers here, the way to find out conclusively will involve further sampling of both the Moon’s subsurface composition and the Earth’s mantle and core. Isotopic analysis has settled many a debate, and this one may be no exception.