Did a giant collision with the early Earth make the Moon?
A giant impact on the early Earth is rated Likely. Whether the classic one-hit version fits the Moon rocks is still disputed, and a hollow Moon is disproven.
- LikelyTheoryThe Moon formed from a giant impact on the early Earth
- DisputedTheoryThe classic version, a Mars-sized impactor and a Moon made mostly from it, fits the Moon rocks
- ✕ DisprovenTheoryThe Moon is hollow, or an artificial object
How the Apollo Moon rocks turned a giant collision with the early Earth into the leading answer, and why the rocks still argue with the classic version.
The question Where did the Moon come from, and what do the Moon rocks show?
What the records say
The documented story holds up: 13 of 13 timeline steps are Confirmed by the records (score 100).
5 theories checked: 1 disproven, 2 unverified, 1 disputed, 1 likely. A theory is rated by its key claim.
Yuan and colleagues suggest pieces of Theia may lie deep inside Earth
confirmedConfirmed as a record of what this document says, not that it is true. See the step
Down the rabbit hole
The ground line is the documented timeline. Each tunnel is a theory that branches off it; every chamber is a step, coloured by how well the records support it. The deepest chamber is the theory’s key claim. Tap one to open it.
Main timeline
Oldest first- Before 1969confirmed
Three ideas lead before Apollo: capture, fission and co-formation
A captured Moon, a Moon flung off a fast-spinning Earth, or a Moon that grew in orbit beside it. A 2004 review says each failed to account for one or more of the main features of the Earth–Moon system.
T2Canup review, 2004p. 442T2Planetary Science Institute page“What were some earlier ideas?”involved party: William K. Hartmann, co-author of the giant-impact hypothesis - Jul 1969–Dec 1972confirmed
Six Apollo landings bring back 382 kg of Moon rock and soil
Twelve astronauts collected 2,196 documented samples, from 21.5 kg on Apollo 11 (July 1969) to 110.5 kg on Apollo 17 (December 1972).
T1NASA, “The Lunar Sample Collection”Introduction; Basic Description; Table 2T1NASA, Apollo 11 mission overviewT1NASA, Apollo 17 mission page - 20 Nov 1969confirmed
Apollo 12 crashes its spent lunar module and the Moon’s seismometer records it for nearly an hour
The ascent stage hit 75.9 km from the seismometer. The signal built up over about 7 minutes; the report says “The total signal duration was approximately 55 min.”
T1Apollo 12 Preliminary Science Report, 1970pp. 44, 46 (fig. 3-8) - 1974–1976confirmed
Two teams propose that a giant impact made the Moon
Hartmann and Davis (Icarus, 1975) suggested a large body hit the early Earth and threw out iron-poor rock that formed the Moon. Cameron and Ward (1976) argued for a roughly Mars-sized impactor, which could also explain Earth’s fast early spin.
T2Canup review, 2004p. 442T1Hartmann and Davis, Icarus, 1975title and dateT2Planetary Science Institute page“How did the theory develop?”involved party: William K. Hartmann, co-author of the giant-impact hypothesis - 1984confirmed
A conference in Kona, Hawaii, makes the giant impact the favourite
The 2004 review says the idea had been “largely ignored” for a decade, and emerged at Kona “as much a result of problems with all the other ideas as to its own still-untested strengths.”
T2Canup review, 2004p. 442T2Planetary Science Institute page“How did the theory develop?”involved party: William K. Hartmann, co-author of the giant-impact hypothesis - Aug 2001confirmed
Simulations make an iron-poor Moon from a smaller, more likely impactor
Canup and Asphaug reported a class of impacts that yields an iron-poor Moon and the present masses and angular momentum of the Earth–Moon system.
T1Canup and Asphaug, Nature, 2001abstract - 12 Oct 2001confirmed
Moon rocks turn out to have Earth’s oxygen isotope fingerprint
Measuring 31 samples from five Apollo missions, Wiechert and colleagues found the lunar oxygen line identical to Earth’s within uncertainties. They said that fits the impact model only if the proto-Earth and the impactor, named Theia, were made of an identical mix.
T1Wiechert et al., Science, 2001abstract - Apr 2012confirmed
Titanium in Moon rocks also matches Earth’s
The Moon’s 50Ti/47Ti ratio matched Earth’s within about four parts per million. The authors wrote that “Geochemical evidence continues to challenge giant impact models”, which have much of the Moon made from Theia.
T1Zhang et al., Nature Geoscience, 2012abstract - Oct 2012confirmed
Science publishes two new impact models to explain the match
According to the journal’s summary, Ćuk and Stewart showed the system’s angular momentum could have halved after the impact, allowing a fast-spinning early Earth; Canup modelled impactors close to the proto-Earth’s own mass.
T2Science editors’ summary, Oct 2012editors’ summary - Jun 2014confirmed
A team reports a small oxygen difference and calls it Theia’s trace
Herwartz and colleagues measured a 12 ± 3 parts per million difference between Earth and the Moon, which they said “supports the giant impact hypothesis of Moon formation.”
T1Herwartz et al., Science, 2014abstract - Jan 2016confirmed
Another team finds no oxygen difference at all
Young and colleagues measured a difference of −1 ± 5 parts per million, “indistinguishable”, and concluded the results “favor vigorous mixing during the giant impact”.
T1Young et al., Science, 2016abstractSources disagreeHerwartz et al. (2014) report a 12 ± 3 ppm Earth–Moon oxygen difference; Young et al. (2016) report −1 ± 5 ppm. Both are shown. - 2017–2022confirmed
New papers propose many impacts, a vapour cloud, or a Moon formed within hours
Rufu and colleagues (2017) modelled a Moon built from moonlets of many smaller impacts. Lock and colleagues (2018) proposed a Moon formed inside a hot, spinning vapour structure they named a synestia. Kegerreis and colleagues (2022) found an impact can place a Moon-like body in orbit straight away.
T1Rufu et al., Nature Geoscience, 2017abstractT1Lock et al., JGR Planets, 2018abstractT1Kegerreis et al., ApJ Letters, 2022abstract - Nov 2023confirmed
Yuan and colleagues suggest pieces of Theia may lie deep inside Earth
Their models suggest two continent-sized zones near Earth’s core, seen in seismic images, “may represent buried relics” of Theia’s mantle.
T1Yuan et al., Nature, 2023abstract
Branches
Go down oneA branch is only as strong as its weakest step so far. The count is how many sources back each step.
The giant impact
Did a planet-sized body hit the early Earth and make the Moon?
First presented by Hartmann and Davis at a 1974 conference on satellites and published in Icarus in 1975; Cameron and Ward set out a Mars-sized version in 1976.
- 61,788 YouTube views, “Where Did Our Moon Come From? We Asked a NASA Scientist” (NASA) as of 17 Oct 2026 T3YouTube: NASA, “Where Did Our Moon Come From?”
- 132,568 English Wikipedia page views, “Giant-impact hypothesis”, Oct 2025–Sep 2026 as of 17 Oct 2026 T3Wikipedia page views: “Giant-impact hypothesis”
It explains why the Moon has so little iron
Earth’s iron had already sunk to its core, so the debris came from rocky mantles.
T2Planetary Science Institute page“Why this is a good hypothesis”involved party: William K. Hartmann, co-author of the giant-impact hypothesisThe premise: confirmedTrue as far as modelling goes: the 2001 simulations produce an iron-poor Moon, and the review lists the iron depletion as a feature the older ideas could not explain.
T1Canup and Asphaug, Nature, 2001abstractT2Canup review, 2004pp. 441–442It explains how the system got its spin
A grazing hit by a Mars-sized body could account for Earth’s fast early rotation.
T2Canup review, 2004p. 442The premise: confirmedThe 2001 simulations matched the system’s angular momentum. Later papers argue the angular momentum may have changed after the impact, which opens other versions.
T1Canup and Asphaug, Nature, 2001abstractT2Science editors’ summary, Oct 2012editors’ summaryMoon rocks share Earth’s oxygen fingerprint, unlike meteorites and Mars rocks
The page says this shows the Moon formed from material from Earth’s neighbourhood.
T2Planetary Science Institute page“Why this is a good hypothesis”involved party: William K. Hartmann, co-author of the giant-impact hypothesisThe premise: confirmedTrue: the oxygen isotopes match. But it is also the classic version’s main problem, since a Moon made mostly of Theia should differ.
T1Wiechert et al., Science, 2001abstractT1Young et al., Science, 2016abstractT1Zhang et al., Nature Geoscience, 2012abstract
- confirmed
Any theory has to explain an iron-poor Moon and a fast-spinning system
The Moon’s low density means it is “severely depleted in iron”; it holds 1% of Earth’s mass; and the system’s angular momentum would give Earth a day of about 4 hours.
T2Canup review, 2004p. 441T1Canup and Asphaug, Nature, 2001abstractStartStrength so far:confirmed· 2 sources on this step - confirmed
Impact simulations can produce those features
Canup and Asphaug’s 2001 simulations produced an iron-poor Moon with the present masses and angular momentum. The 2004 review called the result “a reasonably consistent dynamical account”.
T1Canup and Asphaug, Nature, 2001abstractT2Canup review, 2004abstract= HoldsStrength so far:confirmed· 2 sources on this step - confirmed
But Moon rocks match Earth more closely than the classic version predicts
Classic models put much of the Moon’s material from Theia, yet the oxygen and titanium isotopes match Earth’s. A 2022 paper says such models “struggle to explain” this.
T1Zhang et al., Nature Geoscience, 2012abstractT1Kegerreis et al., ApJ Letters, 2022abstract= HoldsStrength so far:confirmed· 2 sources on this step - likely
Key claim: the Moon formed from a giant impact on the early Earth
Every recent model read here is a version of an impact, and none of the records contradicts one. It rests on simulations and inference, not on a direct record, and which impact it was is still argued, so it is held at Likely.
T2Canup review, 2004abstract; p. 442T1Canup and Asphaug, Nature, 2001abstractT1Kegerreis et al., ApJ Letters, 2022abstractT1Lock et al., JGR Planets, 2018abstract▼ WeakerStrength so far:likely· 4 sources on this step
Bullshit meter
- No step is Unverified or Disproven.
- Key claim contradicted by tier 1? No. No tier-1 record contradicts an impact origin. The open argument is about which kind of impact, so the claim is held at Likely.
Fluff meter: possibly planted
- Can’t be traced to an original record. Absent: traced to the 1975 and 2001 papers and the 2004 review.
- First appeared from new or anonymous accounts. Not measured.
- Coordinated identical wording. Not measured.
- Cited source doesn’t exist. Absent: the papers exist.
- Later contradicted. Not measured for the general claim; the classic version is challenged (see the isotope theory).
● present · ○ absent · – not measured. Lists signals only. It never accuses anyone.
Personal interest meter
- The Planetary Science Institute page, arguing for the impact and against the older ideas was designed by William K. Hartmann, who co-proposed the impact in 1975. T2Planetary Science Institute pageinvolved party: William K. Hartmann, co-author of the giant-impact hypothesis
“Has a documented interest” only, each one sourced. Never an implication of guilt.
Correlation meter
- Theia, the impactor named in the 2001 oxygen study (main timeline), the isotope theory and the 2023 deep-mantle study. T1Wiechert et al., Science, 2001abstract
- Apollo samples, the 382 kg returned 1969–1972 (main timeline) are the rocks measured in the oxygen and titanium studies. T1NASA, “The Lunar Sample Collection”Table 2
- Apollo seismometers, recorded the 1969 crash (main timeline, hollow-Moon theory) and supplied the data re-analysed in 2011. T1Weber et al., Science, 2011abstract
- Angular momentum, is the test classic fission failed (older-ideas theory) and the limit the 2012 papers loosened (newer impact versions). T2Canup review, 2004p. 442
Links to other cases appear once those cases exist.
Who worked on this
This case was assembled by the site from public records. Contributions are not open yet. Nothing here is invented, so the counts are zero.
This is the first published version of the case.
Sources
- T2Robin M. Canup, “Dynamics of Lunar Formation”, Annual Review of Astronomy and Astrophysics 42 (2004), pp. 441–475 (an academic review article). doi.org · copy read: www2.boulder.swri.edu
- T2involved party: William K. Hartmann, co-author of the giant-impact hypothesis “The Origin of the Moon”, Planetary Science Institute, page design by Gregg Herres and William K. Hartmann (undated web page). www.psi.edu
- T1William K. Hartmann and Donald R. Davis, “Satellite-sized planetesimals and lunar origin”, Icarus, April 1975 (title, authors and date read from the bibliographic record; the paper itself was not read). doi.org · copy read: api.openalex.org
- T1Charles Meyer Jr., “The Lunar Sample Collection”, Planetary Materials Branch, NASA Johnson Space Center (undated document), Introduction, Basic Description and Table 2. astromaterials.jsc.nasa.gov
- T1“Apollo 11 Mission Overview”, NASA History. www.nasa.gov
- T1“Apollo 17”, NASA mission page. www.nasa.gov
- T1Gary V. Latham and others, “Passive Seismic Experiment”, chapter 3 of the Apollo 12 Preliminary Science Report, NASA SP-235 (1970), pp. 39–53. ntrs.nasa.gov · copy read: an.rsl.wustl.edu
- T1Robin M. Canup and Erik Asphaug, “Origin of the Moon in a giant impact near the end of the Earth’s formation”, Nature, August 2001 (abstract read). www.nature.com
- T1Uwe Wiechert, Alex N. Halliday and others, “Oxygen Isotopes and the Moon-Forming Giant Impact”, Science, 12 Oct 2001 (abstract read). doi.org · copy read: api.openalex.org
- T1Junjun Zhang, Nicolas Dauphas and others, “The proto-Earth as a significant source of lunar material”, Nature Geoscience, April 2012 (abstract read). www.nature.com
- T2“Forming the Moon from Earth”, the Science editors’ summary of Matija Ćuk and Sarah T. Stewart, “Making the Moon from a Fast-Spinning Earth”, and Robin M. Canup, “Forming a Moon with an Earth-like Composition via a Giant Impact”, Science, Oct 2012 (the two papers themselves were not read). doi.org · copy read: api.openalex.org
- T1Daniel Herwartz, Andreas Pack, Bjarne Friedrichs and Addi Bischoff, “Identification of the giant impactor Theia in lunar rocks”, Science, June 2014 (abstract read). doi.org · copy read: api.openalex.org
- T1Edward D. Young and others, “Oxygen isotopic evidence for vigorous mixing during the Moon-forming giant impact”, Science, January 2016 (abstract read). doi.org · copy read: api.openalex.org
- T1Raluca Rufu, Oded Aharonson and Hagai B. Perets, “A multiple-impact origin for the Moon”, Nature Geoscience, 2017 (abstract read). www.nature.com
- T1Simon J. Lock, Sarah T. Stewart and others, “The Origin of the Moon Within a Terrestrial Synestia”, Journal of Geophysical Research: Planets, February 2018 (abstract read). doi.org · copy read: api.openalex.org
- T1Jacob A. Kegerreis and others, “Immediate Origin of the Moon as a Post-impact Satellite”, The Astrophysical Journal Letters, October 2022 (abstract read). doi.org · copy read: api.openalex.org
- T1Qian Yuan, Mingming Li, Steven J. Desch and others, “Moon-forming impactor as a source of Earth’s basal mantle anomalies”, Nature, November 2023 (abstract read). www.nature.com
- T1Renee C. Weber, Pei-Ying Lin, Edward J. Garnero, Quentin Williams and Philippe Lognonné, “Seismic Detection of the Lunar Core”, Science, January 2011 (abstract read). doi.org · copy read: api.openalex.org
- T3“Hollow Moon”, English Wikipedia (read 17 Oct 2026; a record of what the claim says and where it is said to come from). en.wikipedia.org
- T3“Origin of the Moon”, English Wikipedia, sections “Capture” and “Physical fission” (read 17 Oct 2026). en.wikipedia.org
- T3“Where Did Our Moon Come From? We Asked a NASA Scientist”, NASA, YouTube, 14 Dec 2022. www.youtube.com
- T3Wikimedia pageviews API, English Wikipedia article “Giant-impact hypothesis”, human (user) views, monthly, Oct 2025 to Sep 2026. pageviews.wmcloud.org · copy read: wikimedia.org
- T3Wikimedia pageviews API, English Wikipedia article “Theia (planet)”, human (user) views, monthly, Oct 2025 to Sep 2026. pageviews.wmcloud.org · copy read: wikimedia.org
- T3Wikimedia pageviews API, English Wikipedia article “Synestia”, human (user) views, monthly, Oct 2025 to Sep 2026. pageviews.wmcloud.org · copy read: wikimedia.org
- T3Wikimedia pageviews API, English Wikipedia article “Hollow Moon”, human (user) views, monthly, Oct 2025 to Sep 2026. pageviews.wmcloud.org · copy read: wikimedia.org