According to one of the most widely accepted theories, billions of years ago something the size of Mars crashed into Earth, spewing a huge amount of debris into space that eventually coalesced to form the Moon.
How was the Moon formed?
This is called the giant impact hypothesis, and it provides a clear explanation for some curious properties that the Earth and the Moon have in common, such as the presence of identical minerals.
There’s just one problem. According to a new analysis by planetary scientist Paolo Sossi of the Experimental Planetology group at ETH Zurich in Switzerland, we have absolutely no hard evidence that such a gigantic impact ever occurred.
“Rocks from the Earth’s mantle and those from the Moon are indistinguishable based on every isotopic ratio that traces the material’s provenance in the Solar System,” Sossi explained.
“Since the isotopes of these elements vary widely among planetary materials, if there were evidence of an impact, we would expect to see small differences in their isotopic ratios. However, no such differences have been detected between the Earth and the Moon … so we argue that, in the case of an impact, the two bodies must have been perfectly mixed … or the Moon must have formed in a different way.”
This doesn’t mean, ultimately, that the giant impact didn’t happen. It’s still possible. It’s just that we can’t even assume it happened. How the Moon formed may just be back in the realm of mystery.
The evidence, available on the preprint server arXiv and currently in press for the 2024 Treatise on Geochemistry, is based on a critical review of all relevant geophysical and geochemical evidence to date.
So if there was no collision, where could the Moon have come from? Sossi argues that the Earth and its large satellite simply formed from the same basic material, eliminating the need for any introduction of a hypothetical third body.
Earth and its Moon are unique in the Solar System. It is the only system with two large spherical bodies with differentiated nuclei; the Moon is not much smaller than Mercury and, if it were floating alone, could be considered a planet in its own right.
The Moon is also thought to have played an important role in helping life evolve. It helps stabilize the Earth’s rotation and generates the tides that help the oceans circulate. Without the mass of orbiting rock, Earth would have been a very different place.
Because the Moon is so rare and so important, scientists want to know how it got here. Its origins could shed light on our history and help us assess the likelihood of how often such systems form out there in the wider Universe.
The giant impact hypothesis is not a bad one. In the early days of the Solar System, about 4.5 billion years ago, things were much more chaotic. There were a lot more rocks flying around haphazardly, which we can see from the craters left on the surviving planets and moons. So it’s not unlikely that a newborn Earth was hit by something big enough to create a cloud of impact debris that formed the Moon.
The problem, Sossi and his team found after examining the available evidence, is that the Earth and the Moon are too similar. Elements on Earth and the Moon have identical ratios of isotopes (different forms of the same atom with varying numbers of neutrons in their nuclei). And the more we look, the more we discover.
“Initially, this concordance was found only for oxygen isotopes, but more recently (since the beginning of the 2010s) it has been discovered that this is also the case for chromium and titanium, both elements concentrated exclusively in the rocky part of the body,” explained Sossi.
Furthermore, because the isotopes of these elements vary widely among planetary materials, if there were evidence of an impactor, we would expect to see small differences in their isotopic ratios. However, no such differences have been detected between Earth and the Moon.
“This has since been shown to be true for a number of other elements as well (e.g. iron, calcium, molybdenum, etc.), making the probability that this could have happened by coincidence astronomically small.”
A recent study has shown that the Earth and the Moon are roughly the same age, or formed very close together, about 4.5 billion years ago. Overall, the evidence suggests that the two bodies formed from the same cloud of material.
This is not to say that there could not have been an impact; one model suggests that an impact pulverized the entire equipment of the newborn Earth, whatever it looked like, producing a doughnut of material that merged into two bodies, the Earth and the Moon.
Giant impact models often predict small differences in isotope ratios between Earth and the Moon. Such differences simply do not exist, according to existing data, which means that while we cannot rule out a giant impact, we should rule out models that produce results inconsistent with actual geochemical data.
“The parameter space is essentially infinite,” Sossi said.
We still don’t know how the Earth and the Moon came together from a clump of stardust 4.5 billion years ago. One way to investigate further is to look inside the Moon, and that’s what Sossi and his colleagues are working on.
“The chemical and isotopic evidence is now sufficiently strong to begin questioning the fundamental mechanisms of the Moon’s formation,” Sossi said.
“We are also excited to see how interconnected geophysical, geochemical and dynamical constraints can be in providing a new holistic view of the formation of the Moon. We are able to communicate on common problems across disciplines, which will ultimately help us find a solution to this puzzle.”
The team’s research can be read at arXiv.
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