Some 4.5 billion years ago, a Mars-sized rock is theorized to have collided with the proto-Earth. The collision is one of the only ways to create an Earth-Moon system with the properties we observe today. It also may have partially re-liquefied Earth’s surface, destroyed Chaotian property values, and created an atmosphere of plasma metal vapor around both our planet and the enormous cloud of angry debris that now surrounded it.
But there have always been some unresolved issues with the Giant Impact Hypothesis (hereafter abbreviated GIH). The chief problem is in answering the question of what happened to Theia — the Mars-sized impactor that supposedly struck our planet (named for the mother of Selene, the moon goddess, in Greek mythology). In the conventional explanation for the GIH, the material from Theia becomes the basis for much of the Moon, mixed with some material from Earth.
The problem is, some of the evidence points in both directions. The oxygen isotopic ratio found on the Moon is essentially identical to Earth. Oxygen isotope ratios, which we can measure with great precision, are different for each body in the solar system. The only reason for the Earth and Moon to align in the way that they do is if they are made from the same “stuff.” But if the Moon is made from an impactor as theorized, the ratios of siderophile elements (metal-loving elements) should be different than they are. Specifically, we should find more of them than we practically do.
A new paper from planetary scientist Kevin Righter of the Astromaterials Research and Exploration Science Division (ARES) at NASA challenges the idea that the Moon formed principally from Theia. Righter built a model to compare the concentrations of 14 specific siderophile elements in the final Earth-Moon system by controlling various aspects of the initial collision based on what we know about the Moon today. Ultimately, the model in which the bulk of the Moon’s material came from Earth is a much better fit for the disposition of siderophile elements that we see today than any theorized model in which the Moon is mostly composed of the likely “ingredients” of Theia. The video below shows the evolution of the Moon’s surface (in our best current understanding) from after its initial formation until the present day.
“Researchers have analyzed small subsets of these elements in the past, but this is the first time that all 14 elements were modeled together to analyze the Earth-Moon system,” Righter said. “By simulating the main processes contributing to the Moon’s formation and early differentiation, we were able to predict the level of each element that should be present in the Moon’s mantle.”
Righter than compared his model to the actual moon rocks found by the Apollo astronauts and found a strong match for 9 of the 14 volatile siderophiles found in the rock samples. In the case of the other five, Righter believes they may have migrated out of the gas cloud created by the initial impact and dissipated away from the system or Theia, explaining why the Moon is more deficient in these materials than it ought to be.
One of the problems with reconciling these differences is that it’s rather difficult to provide enough energy to completely liquify and remix rock in a homogeneous way. This is where the molten rock atmosphere idea I referred to earlier comes in — one theory to explain the similarities in composition between the Earth and the Moon, dating back to 2007, is that a common plasma-metal vapor metal atmosphere may have formed between the two bodies post-impact, mixing them together and resulting in the configuration we see today.
Ultimately, this seems like the kind of problem that could be substantially illuminated by a deeper exploration of lunar geology. There are fundamental questions about whether or not the moon rocks retrieved by the Apollo astronauts represent an accurate picture of the moon’s geological history. While NASA deliberately chose landing sites at different craters and areas in an attempt to retrieve rock samples that would reflect different points in time, later research has suggested the impact that created the Imbrium Basin might have been large enough to deposit debris at all of the sites the original Apollo astronauts visited. Instead of sampling rocks from throughout the Moon’s history, we may have only sampled the same event.
Even if that’s true, it wouldn’t mean the GIH is false — no other theory for how the Moon could have formed addresses as many peculiarities and problems in the Earth-Moon system as the GIH does, and no option is as broadly supported. But additional data points gathered from beneath the lunar surface or from areas where the Imbrium impact could not have thrown debris could shed considerable light on this subject.