Billions of years ago, according to the theory, an object about the size of Mars collided with Earth, ejecting a large amount of debris into space that eventually came together to form the Moon.
This is known as the giant impact hypothesis, which offers a tidy explanation for some peculiar properties that Earth and the Moon share, such as identical minerals.
However, there’s one issue. A new analysis led by planetary scientist Paolo Sossi of the Experimental Planetology group at ETH Zürich in Switzerland shows we have no clear evidence that such a giant impact ever occurred.
“Rocks from the Earth’s mantle and the Moon are indistinguishable on the basis of every isotopic ratio that tracks the provenance of material in the Solar System,” Sossi told ScienceAlert.
“Because the isotopes of these elements vary widely among planetary materials, were there any trace of an impactor, we would expect to see small differences in their isotopic ratios. Yet, no such differences between the Earth and the Moon have been detected … hence we argue that, in the event of an impactor, the two bodies must have been perfectly mixed … or the Moon must have formed in a different way.”
This doesn’t entirely rule out the giant impact hypothesis. It’s still a possibility. But at the same time, we can’t really assume it happened either. The origin of the Moon might remain an unsolved mystery for now.
The evidence, available on the preprint server arXiv and due for publication in the 2024 Treatise on Geochemistry, is based on a comprehensive review of all relevant geophysical and geochemical data gathered so far.
So, if no collision took place, where did the Moon come from? Sossi suggests that Earth and its large satellite may have formed from the same basic material, eliminating the need for a hypothetical third body.
Earth and the Moon are a unique pair in the Solar System. It’s the only system with two large spherical bodies with differentiated cores. The Moon is not much smaller than Mercury and, if it were on its own, might even be considered a planet.
The Moon is also thought to have played a key role in the development of life. It helps stabilize Earth’s rotation and generates tides that help circulate the oceans. Without the Moon, Earth might have been a very different world.
Because the Moon is both rare and crucial, scientists are eager to understand how it came to be. Understanding its origins could provide insights into our own history and help us estimate how often similar systems might form elsewhere in the Universe.
The giant impact hypothesis is not a bad idea. In the early Solar System, about 4.5 billion years ago, things were much more chaotic, with numerous rocks flying around, as seen from the craters left on surviving planets and moons. So, it’s not far-fetched to think a young Earth could have been hit by something large enough to create the debris that formed the Moon.
The issue, as Sossi and his team have uncovered through their analysis, is that Earth and the Moon are too alike. Elements on both bodies have identical ratios of isotopes — different versions of the same atom with varying neutron numbers. And the more we investigate, the more this holds true.
“Initially, this concordance was found only for the isotopes of oxygen, but more recently (since the early 2010s) this has been found to be the case for chromium and titanium – both of which are elements that are concentrated solely in the rocky part of the body,” Sossi explained.
“Furthermore, because the isotopes of these elements vary widely among planetary materials, were there any trace of an impactor, we would expect to see small differences in their isotopic ratios. Yet, no such differences between the Earth and the Moon have been detected.
“This has subsequently also been shown to be true for a host of other elements (e.g., iron, calcium, molybdenum, etc.) making the likelihood that this could have arisen through coincidence astronomically small.”
Recent research has shown that Earth and the Moon are also nearly the same age, having formed around 4.5 billion years ago. This combined evidence suggests that both bodies originated from the same cloud of material.
This doesn’t mean an impact couldn’t have occurred. One model proposes that an impact pulverized the entire early Earth, forming a donut of material that later became Earth and the Moon.
Yet, giant impact models often predict slight variations in isotope ratios between Earth and the Moon, which existing data does not support. While we can’t completely dismiss a giant impact, models producing results inconsistent with current geochemical data should be reconsidered.
Still, “The parameter space is essentially infinite,” Sossi said.
We simply don’t know how Earth and the Moon formed from a clump of stardust 4.5 billion years ago. One way to investigate further is by examining what lies within the Moon, which Sossi and his colleagues are currently exploring.
“The chemical and isotopic evidence is now sufficiently strong to start questioning the fundamental mechanisms of Moon formation,” Sossi said.
“We’re also pleased to see how interrelated the 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 mutual problems across disciplines, which will ultimately help us in finding a solution to this conundrum.”
The team’s research can be found on arXiv.
