Posted on 11/25/2025 12:05:25 PM PST by Red Badger
Artist’s impression of the collision between the early Earth and Theia. Since Theia originated in the inner Solar System, in this perspective the Sun can be seen in the background. (Credit: MPS / Mark A. Garlick)
In A Nutshell
* Inner Solar System origins: By measuring iron isotopes in Moon rocks and meteorites, researchers determined Theia probably formed closer to the Sun than Earth did, not in the distant outer Solar System.
* Identical twins: Earth and the Moon have virtually identical chemical signatures, but both fall outside the range of any meteorites we’ve found—suggesting they incorporated exotic material from the inner Solar System’s innermost regions.
* The chemistry tells the story: Different elements record different chapters of Earth’s formation. Iron and molybdenum track the final stages when Theia hit, while zirconium captures the whole accretion history—together, they pinpoint where Theia came from.
* Solving a decades-old puzzle: Scientists have long wondered why Earth and Moon rocks look so similar despite models predicting the Moon formed mostly from Theia. The answer: both bodies drew from overlapping neighborhoods of the inner Solar System.
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When it comes to the origin of our planet’s moon, the giant-impact theory is the most widely accepted premise.
According to the theory, a massive planet-sized body slammed into the young Earth around 4.5 billion years ago, creating an explosion so violent it birthed our Moon. Scientists have long known about this catastrophic collision, but a fundamental question has lingered unanswered for decades. Where did this cosmic wrecking ball come from?
Research that traced the likely origins of Theia, the Mars-sized impactor that forever changed Earth’s destiny, finally offers some cosmic clarity. By measuring iron isotopes in lunar samples brought back by Apollo astronauts and comparing them with meteorites from across the solar system, a team led by scientists from the University of Chicago and Germany’s Max Planck Institute discovered something unexpected. Models point to Theia forming closer to the Sun than Earth did, originating from the inner Solar System rather than the outer reaches where icy bodies roam.
Published in the journal Science, the work goes a long way toward resolving a cosmic puzzle that has divided planetary scientists for years, with major consequences for understanding how planets form, migrate, and occasionally collide during the chaotic early days of a solar system’s evolution.
Tracing Chemical Fingerprints Across the Solar System
The research team took a different approach to tracking Theia’s origins. Instead of looking at just one element, they measured iron isotopes in 15 terrestrial rocks, 6 lunar samples from the Apollo missions, and 20 meteorites. These measurements showed that Earth and the Moon have virtually identical iron isotopic compositions, but both lie at one extreme end of the range found in meteorites.
Meteorites are typically divided into two major groups that represent different regions of the early Solar System. Noncarbonaceous meteorites come from the inner Solar System, while carbonaceous chondrites represent the outer Solar System beyond Jupiter’s orbit. Earth and the Moon‘s composition falls outside the range of noncarbonaceous meteorites but on the same trend line, suggesting they incorporated material not found in any meteorites scientists have collected.
Lead researcher Timo Hopp and his colleagues didn’t stop with iron. They combined their new measurements with existing data for other elements including zirconium, molybdenum, chromium, and ruthenium. Each element tells part of Earth’s accretion story because they behave differently during planet formation.
The research indicates both proto-Earth and Theia formed chiefly via inner Solar System material. (Credit: Jurik Peter on Shutterstock)
How Different Elements Record Different Chapters of Earth’s History
When planets form, certain elements prefer to sink into the metallic core rather than remain in the rocky mantle. Iron and molybdenum are moderately siderophile, meaning they have an affinity for metal and predominantly settled into Earth’s core. The iron and molybdenum in Earth’s mantle today reflect only the final stages of the planet’s growth, roughly the last 40% and 10% of material that accreted, respectively.
Theia’s impact happened near the end of Earth’s formation, so it contributed a substantial amount of these core-loving elements to Earth’s mantle. If Theia had come from a very different region of the Solar System than proto-Earth, scientists would expect to see differences between Earth and Moon rocks for elements like iron and molybdenum. But they found essentially identical compositions.
Meanwhile, zirconium behaves as a lithophile element, preferring to stay in the rocky mantle rather than sinking to the core. Zirconium’s isotopic signature in Earth’s mantle reflects the planet’s entire accretion history, not just the final stages. The fact that Earth’s mantle shows unusual compositions for both types of elements provided critical constraints.
Calculating Theia’s Origins
The researchers performed detailed calculations to determine what combinations of Theia and proto-Earth compositions could produce the Earth and Moon we see today. They tested different scenarios, including models where Theia ranged from 5% to 50% of Earth’s current mass.
Most scenarios led to an impossible conclusion: either Theia or proto-Earth would need to have had isotopic compositions far more extreme than anything measured in meteorites. Yet, one scenario fit the data elegantly. If proto-Earth had a composition similar to enstatite chondrites, a type of inner Solar System meteorite, and Theia was even more enriched in certain isotopic signatures than typical inner Solar System materials, all the pieces fell into place.
Theia appears to have been enriched in material produced by the slow neutron capture process, a particular type of nuclear reaction that occurs in certain stars. Variations in this material among inner Solar System bodies may reflect a gradient in composition with distance from the Sun, with objects closer to the Sun being more enriched.
The Earth as Seen from the Surface of the Moon. It’s theorized Theia hit proto-Earth at an angle, producing a catastrophic impact that vaporized rock and created an orbiting ring of debris around the young planet. Eventually, that material formed our moon. (Credit: muratart/Shutterstock)
Why Theia’s Origins Change Our Understanding of Moon Formation
The work challenges some popular ideas about how the Moon formed. The most widely cited models predict that the Moon consists mostly of material from Theia, with little contribution from Earth’s mantle. If that were true and Theia came from the outer Solar System, Earth and Moon rocks should show detectable differences in composition. They don’t.
The research suggests both proto-Earth and Theia formed predominantly from inner Solar System material, and that Theia’s journey probably began closer to the Sun than Earth’s orbit before gravitational interactions eventually sent it on a collision course with our planet. Some ancient Earth rocks show similar isotopic enrichments in ruthenium, an element delivered primarily during the final 0.5% of Earth’s accretion, providing independent evidence that Theia had an unusual composition enriched in material from the inner Solar System’s innermost regions.
The work also helps explain why detecting Theia’s signature has proven so difficult. Previous measurements of oxygen, calcium, titanium, chromium, and tungsten isotopes in lunar samples showed no measurable differences from Earth rocks. This isotopic similarity has been one of the biggest puzzles in planetary science. The new research suggests that both Earth and Theia sampled overlapping but not identical regions of the inner Solar System, with Theia forming just a bit closer to the Sun.
The Moon-forming impact was one of the most catastrophic events in our planet’s history. Theia, roughly the size of Mars, delivered a glancing blow to proto-Earth. The collision was energetic enough to vaporize rock and create a disk of debris orbiting Earth. Over time, this material coalesced to form the Moon.
Understanding where Theia came from helps scientists reconstruct the orbital dynamics of the early Solar System. Planets didn’t form in tidy, circular orbits and then stay put. Instead, gravitational interactions caused massive bodies to migrate over millions of years, occasionally leading to spectacular collisions. Theia’s origins in the inner Solar System suggest that even dramatic impacts like the one that created our Moon typically involve bodies from the same neighborhood of the Solar System rather than objects that traveled from distant orbits.
The study involved years of intensive laboratory work. Measuring iron isotopes at the precision required involves separating iron from rock samples through ion exchange chromatography, then analyzing it using multicollector inductively coupled plasma mass spectrometry. The measurements must account for interference from cosmic rays that bombarded some lunar samples after they formed, creating artificial isotopic variations that could confuse the results.
Three lunar samples showed no effects from cosmic ray exposure and had iron isotopic compositions indistinguishable from Earth’s mantle. The team used the average of these samples as representative of the Moon’s original composition before it was exposed to space radiation at the lunar surface. The approach opens new avenues for understanding other planets and moons throughout our Solar System and beyond.
Paper Summary
Study Limitations
The study relies on mass balance calculations that involve several assumptions about the giant impact process. The calculations assume that Theia was the last major impact on Earth and that core formation followed specific redox conditions. Different assumptions about how much of Theia’s core mixed with Earth’s mantle would change some quantitative details, though the authors’ modeling still points toward Theia’s inner Solar System origins across various scenarios. The research also depends on the assumption that variations in isotopic compositions among meteorites reflect a spatial distribution in the early Solar System, which, while well-supported, is an inference rather than direct observation. Additionally, the correction for late accretion’s contribution to Earth’s molybdenum inventory introduces some uncertainty, though the authors show that using different assumptions about late-accreted material does not substantially change the results.
Funding and Disclosures
The research was supported by multiple NASA grants (80NSSC20K1409, 80NSSC23K1022, 80NSSC21K0380, 80NSSC20K0821, and 80NSSC23K1163), National Science Foundation grant EAR-2001098, Department of Energy grant DE-SC0022451, Deutsche Forschungsgemeinschaft project ID 263649064, and European Research Council advanced grant number 101019380. The authors declared no competing interests. Lunar samples were provided by CAPTEM and NASA’s curation programs. Meteorite samples came from multiple institutions including the Field Museum of Natural History in Chicago, the Smithsonian National Museum of Natural History, and NASA. Antarctic meteorite samples were recovered through the Antarctic Search for Meteorites program funded by the NSF and NASA.
Publication Details
The paper “The Moon-forming impactor Theia originated from the inner Solar System” was authored by Timo Hopp, Nicolas Dauphas, Maud Boyet, Seth A. Jacobson, and Thorsten Kleine. It was published in Science on November 20, 2025. Hopp is affiliated with the Department of the Geophysical Sciences at the University of Chicago and the Max-Planck-Institut für Sonnensystemforschung in Göttingen, Germany. Dauphas holds positions at the University of Chicago and the University of Hong Kong. Boyet is with Laboratoire Magmas et Volcans at Université Clermont Auvergne in France. Jacobson is at Michigan State University’s Department of Earth and Environmental Sciences. Kleine is at the Max-Planck-Institut für Sonnensystemforschung. The paper’s DOI is 10.1126/science.ado0623.
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Moon Ping!..............
Not Theia. Tiamat.
Tia Leone...............
It’s hollow and used to spy on us.
Despite all the scientific findings, its still a theory. A theory that assumes the solar system resembles the current configuration, omits data that shows the Moon is older than the solar system, ignores plasma discharges. And much more.
But it does keep everything nice and neat, so no paradigms are stepped on, helps keep closed minds closed, while forcing shut young open minds.
Ain’t science grand!
So, if it was spalled off from Earth, they will find signs of early life in the fossil records on the moon?!
It would have happened waaaay before that occurred...............
I guess this means the Earth is also made of green cheese.
So where did it come from?
Can we ask the earth to show us where on the globe the big bad space rock hit us?
The idea that we can derive how our particular Solar System got itself together, 4-plus billion years after that all chaos happened, is IMO sort of.... ummm.... "silly"?
But it does get the grant money.
Where did they get the moon rocks from? Stanley Kubrick?
Hmmm...
Going to have to turn in my tin foil hat for one made of zirconium?
Where did -who- get the moon rocks from?
The Moon rocks came from the Moon, where else? You remember, we sent a couple of guys up there in a spacecraft and they brought some back.
Some materials found on Earth have been determined to probably have come from the Moon via collisions that threw material into space, although that's not 100% guaranteed; it's inferred from characteristics.
Where have all the planets gone....long time passing
An artist’s rendering that leads a science article is always a waving red flag.
That means they are speculating.
No.
Interesting.
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