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Title: Why are Earth and Moon isotopically nearly identical?
Plain: 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: 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.
Settled by: Impact and disk-evolution simulations with full mixing physics reproducing all four constraints, checked against new isotope systems.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: open
Status 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.
Posed since: (empty)