Why are Earth and Moon isotopically nearly identical?
In plain words
In the standard giant-impact simulations, most of the Moon comes from the impactor, which should have had a different chemical fingerprint. Yet Moon and Earth rocks have the same isotope ratios to a few parts per million.
Precise statement
Canonical giant-impact simulations (impactor mass about 0.1 Earth masses) place mostly impactor material in the protolunar disk, yet the Earth-Moon difference in Delta-17O is about $-1 \pm 5\ \mathrm{ppm}$, Ti and Cr isotopes also match, and $182\mathrm{W}$ matches once the different amounts of late-accreted material on Earth and Moon are corrected for. Identify the formation scenario that jointly reproduces the isotopic match, the angular momentum, the lunar mass and the small lunar core. An answer is a simulated scenario meeting all four constraints with plausible initial conditions.
What would settle it
Impact and disk-evolution simulations with full mixing physics reproducing all four constraints, checked against new isotope systems.
Status in the literature
Unverified note
2025 to 2026 proposals include multiple-impact pathways and impacts on a differentially rotating proto-Earth (arXiv:2512.10757, arXiv:2504.12122); none accepted.
Related problems
- More general than Could the Earth-Moon system have shed a large angular momentum excess?
- More general than Was the Moon-forming impactor isotopically Earth-like?