Lunar regolith suggests meteorites supplied only a small share of Earth's water.

A NASA study using triple oxygen isotope measurements of Apollo lunar regolith reports that meteorite delivery since about four billion years ago could have supplied only a small fraction of Earth's ocean water.

NASA researchers analyzed Apollo lunar regolith with high-precision triple oxygen isotope measurements to clarify the Moon's record of meteorite impacts and the timing of water delivery. The team, led by Tony Gargano, found that roughly 1% by mass of the regolith contains material from carbon-rich meteorites that was partially vaporized on impact. Using known properties of those meteorites, the researchers estimated how much water such impacts could have delivered since about four billion years ago. The study concludes that, even under generous assumptions, meteorite delivery in that period would make up only a small fraction of Earth's ocean water. Key findings: - The study used triple oxygen isotope “fingerprints” on Apollo regolith to isolate impactor material that is otherwise altered by repeated impacts. - Measurements indicate about 1% by mass of the sampled regolith is derived from carbon-rich meteorite material that experienced partial vaporization on impact. - When scaled to higher Earth impact rates, the cumulative water estimated from these meteorites amounts to only a small percent of Earth’s ocean volume. - Authors state the results do not rule out any meteorite-delivered water but make it difficult for late meteorite delivery to be the dominant source of Earth’s oceans. - The analyzed samples come from equatorial near-side Apollo landing sites; researchers note that Artemis-returned samples will expand the geographic and geologic coverage in future work. Summary: The findings narrow the possible contribution of late meteorite delivery to Earth's oceans by placing an upper bound on water supplied since about four billion years ago. For the Moon, the implied delivery is small on an Earth-ocean scale but may be meaningful for localized polar cold traps. Researchers identify upcoming Artemis sample returns as the next opportunity to test and extend these conclusions.