The theory is that billions of years ago, something roughly the size of Mars collided with Earth, throwing a bunch of soil into space that eventually coalesced to form the Moon. This is called the giant impact hypothesis, and it gives us a clear explanation for the various interesting properties that Earth and the Moon share, such as the same minerals.

There’s just one problem: According to a new analysis by planetary scientist Paolo Sosi of the Experimental Planetology Group at ETH Zurich in Switzerland, we have absolutely no clear evidence that such a massive impact ever occurred.

“When looking at isotope ratios that indicate the origin of the material in the Solar System, rocks from the mantle of the Earth and the Moon are indistinguishable from each other,” Sossi told ScienceAlert.

“Since the isotopes of these elements vary greatly in different planetary materials, if there were any impact traces we would expect to see small differences in their isotope ratios. However, no such difference was found between the Earth and the Moon… so we argued that in the event of a collision the two bodies must have been perfectly mixed or the Moon must have formed in some other way.”

This certainly doesn’t mean a giant coup. Negative It’s still possible. It’s just… I mean, we can’t guess what happened. YesPerhaps the formation of the Moon has once again found itself in the realm of mystery.

Evidence available on the arXiv preprint server and currently published “Review on Geochemistry” 2024, based on a critical review of all relevant geophysical and geochemical evidence to date.

So where could the Moon have come from if the collision hadn’t happened? Sossi argues that Earth and its large moon are made of the same basic material, eliminating the need to add a hypothetical third body.

The Earth and its Moon are unique in the Solar System. They are a single system of two large spherical bodies with differentiated cores; the Moon is not much smaller than Mercury and could be considered a planet in its own right if it could navigate on its own.

The Moon is also believed to have played a key role in the development of life. It helps balance the Earth’s rotation and creates tides that help circulate the oceans. Without the orbiting rock, the Earth would be 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 own history and help us estimate how often such systems form in the wider universe.

The giant impact hypothesis isn’t bad. Around 4.5 billion years ago, at the beginning of the solar system, things were much more chaotic. There were a lot more rocks floating around, either freely or unintentionally, as evidenced by the craters left on the surviving planets and moons. So it’s possible that something big enough to hit the tiny Earth to create the debris cloud that formed the Moon.

The problem, Sossi and his team discovered after examining the available evidence, is that the Earth and the Moon more The elements on Earth and the Moon have the same isotope ratio—different forms of the same atom with different numbers of neutrons in their nuclei. And the more we look, the more we find.

“At first, this similarity was found only for oxygen isotopes, but recently (since the early 2010s) it has been found to be true also for chromium and titanium – both elements concentrated only in the rocky part of the body,” explained Sosi

“Furthermore, since the isotopes of these elements vary greatly in different planetary materials, if there were any impact traces we would expect to see small differences in their isotope ratios. However, no such differences were found between the Earth and the Moon.

“This was later shown to be true for a number of other elements (such as iron, calcium, molybdenum, etc.), making the chances of it arising by chance astronomically small.”

A recent study showed that the Earth and Moon are also about the same age, or formed very close together, about 4.5 billion years ago. Taken together, the evidence suggests that the two bodies formed from the same cloud of matter.

That doesn’t mean a hit couldn’t have happened; one model suggests the collision pulverized the entire set and the ridiculous mass that Earth used to be, forming a doughnut of material where the two bodies, Earth and the Moon, merged.

But giant impact models typically predict small differences in isotope ratios between the Earth and the Moon. These differences are not present according to current data, meaning that a giant impact cannot be ruled out, and that we must eliminate models that produce results that are inconsistent with real geochemical data.

Even so, “the parameter space is essentially infinite,” Sossi said.

We don’t know how the Earth and the Moon formed together from a clump of stardust 4.5 billion years ago. One way to investigate this further is to look at what happens at the center of the Moon, and that’s what Saucy and his colleagues are working on.

“The chemical and isotopic evidence is now strong enough to begin to question the fundamental mechanisms of Moon formation,” Sossi said.

“We are also excited to see how geophysical, geochemical, and dynamical constraints can be linked to create a new, holistic view of the Moon’s formation. We can communicate about common problems across disciplines, which will ultimately help us find a solution to this puzzle.” The team’s research is available to read on arXiv.