The Giant-Impact Hypothesis for the Moon’s Formation Is in Doubt

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For decades, astronomers and scientists have relied on a theory of how the Earth-moon system formed known as the giant-impact hypothesis: A large chunk of rock nicknamed Theia (mother of Selene, the moon goddess in Greek mythology) slammed into the Earth not long after it formed. This impact would have liquefied both Theia and the Earth, while simultaneously ejecting a huge amount of material.

Over time, conventional thinking goes, the Earth cooled again and became rocky, while the huge lump of ejected rock formed the moon. There’s even an explanation for why the moon is thicker on one side than the other. A second, smaller moon may have briefly formed, before its orbit destabilized and it impacted on the far side of our own moon.

The problem with the giant-impact hypothesis is it’s increasingly difficult to square with data. A new paper posits the conventional great impact hypothesis isn’t quite right, and argues for an entirely new theory of Earth-moon formation.

Let’s back up a moment and start with an uncontestable fact: The moon–our moon–is damned odd. First, we’re the only rocky planet with a moon of significant size at all; Mercury and Venus have no moons, and the moons of Mars are either asteroids captured by that planet’s gravity or the remnants of a giant impact event that struck Mars in its own distant past. For example, Deimos, which orbits Mars, is little more than a dot, roughly as bright as Venus in our own sky.

The moon is about 1 percent the mass of Earth, while the combined mass of all the moons of the outer planets is no more than one tenth of one percent of their parent planets. The moon is also responsible for 80 percent of the angular momentum in the Earth-moon system, while in the other planets, this value is less than 1 percent. And unlike every other planet or satellite in the solar system, it appears to be made of exactly the same isotopes in the same ratios we observe on Earth.


That’s highly unusual in and of itself. As the early solar system formed, the lighter isotopes were dispersed by stellar wind, explaining why the inner planets are rocky while the outer planets are gas giants and so-called ice giants (Uranus, Neptune, and the still-hypothetical Planet IX). Each of the planets and satellites contains its own unique mixture of isotopic ratios, which is why we can sometimes identify the origins of various meteorites; a meteorite from Mars has a distinctly different isotopic ratio than a meteor from the asteroid belt.

The problem with the giant-impact hypothesis is that it has difficulty accounting for why the isotopic ratios on the moon look exactly like the ones we see on Earth. Over at The Atlantic, Rebecca Boyle steps through the various potential options, including new work by scientist Sarah Stewart and her student, Simon Lock. Stewart and Lock have offered an intriguing option that posits a new idea for how the Earth and moon may have coalesced after a massive impact. They propose Theia struck the Earth and thoroughly vaporized it, forming a torus of molten rock and vaporized material. As the lava-bagel spun, the outer edge moved much more quickly than the inner region, and never completely differentiated from it. They’ve named this hypothetical structure a synestia, syn from the Greek “together” and Hestia, the Greek goddess of the home, hearth, and architecture.


The inner, rocky planet eventually became the Earth, while the faster-moving cloud of vaporized rock coalesced into the moon. This explains both the high conservation of angular momentum and the isotope mixing, though this is a still a theoretical model and not one we’ve observed in the real-world to date. Then again, our data on planetary formation is still extremely limited. We’ve directly observed geologic activity on Mercury and captured planets in the process of forming around a distant star, but this kind of impact and mixing is far more difficult to see, particularly around a planet as small as ours to start with.

A synestia isn’t the only way to theoretically create an Earth-moon system. Boyle steps through some other hypotheses, including the idea that Theia was a body with near-identical isotope ratios to Earth to start with, or that the Earth may have been subject to multiple large impacts that collectively broke off and mixed enough material to create the moon. Given our lack of time travel, it may not be a theory we can ever test. Then again, spacecraft like Kepler have pushed back the boundaries of observed astronomy by leaps and bounds, confirming some of our ideas about how planets and stars form and disrupting others.

It’s entirely possible that if we keep an eye out (and keep building bigger telescopes) we will catch the above process in action around a different star. To be sure, most of these new ideas are variants on the standard giant-impact hypothesis, not wholesale repudiations of it. But they still represent several challenges to the conventional explanation.

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