What is the strongest evidence for the giant impact origin of the Moon?

• A. Similar mantle composition with volatile depletion
• B. Isotopic signature identical to Earth's core
• C. Moon rocks show high organic content
• D. Rock density much higher than Earth rocks

Answer: (B)

The strongest evidence for a giant-impact origin comes from the isotopic fingerprints: the Moon and Earth share nearly identical isotopic compositions, especially for oxygen, and this close match extends to other elements as well. If the Moon formed largely from a body with its own distinct chemistry, you’d expect its isotopic ratios to differ from Earth's. In the giant-impact picture, material from Earth's mantle is mixed into the disk that becomes the Moon, producing a satellite with Earth-like isotopes. Volatile depletion and high-temperature processing are expected consequences of such a collision and are supportive but not as diagnostic as the isotopic match. The Moon does not show unusually high organic content, and its rock densities don’t point to a clear, separate origin. A claim about isotopic similarity to Earth’s core would be mismatched with the strongest, most diagnostic signal, which is similarity to Earth's mantle/crust composition.