Moon rocks quietly carry a verdict: the Moon looks chemically like Earth’s outer layers, not like a visiting body from elsewhere in the Solar System. Their oxygen isotopes fall on precisely the same line as terrestrial rocks, a match so tight that a random captured object would almost certainly miss it.
Under the microscope, Apollo samples show silicate minerals forged in an intensely hot magma ocean, but they are depleted in volatile elements that small, wandering worlds tend to keep. The bulk composition mirrors Earth’s silicate mantle rather than its metal core, implying crust and mantle material were stripped and reassembled. Trace-element partitioning, a staple of geochemistry, reinforces that the Moon and Earth once shared a single differentiated reservoir.
Titanium and tungsten isotope signatures add a second layer of evidence. These act like a planetary bar code, and the code in lunar basalts is effectively the same as in terrestrial basalts, while meteorites from distant asteroids show clear divergence. That pattern aligns with a giant impact scenario in which Earth’s outer shell was blasted into orbit and later cooled into the Moon, leaving behind a subtle but consistent chemical echo of its terrestrial birth.
