High-precision oxygen isotopes in Apollo lunar soils reveal a persistent impactor fingerprint, showing that impacts contributed only a tiny fraction of Earth’s water

WASHINGTON and HOUSTON, Jan. 20, 2026 /PRNewswire/ — A long-standing idea in planetary science is that water-rich meteorites arriving late in Earth’s history could have delivered a major share of Earth’s water. A new study by Universities Space Research Association (USRA) and University of New Mexico argues that the Moon’s surface record sets a hard limit on that possibility: even under generous assumptions, late meteorite delivery since about four billion years ago could only have supplied a small fraction of Earth’s water.

In a paper published in the Proceedings of the National Academy of Sciences, researchers led by Dr. Tony Gargano at USRA’s Lunar and Planetary Institute and the University of New Mexico analyzed a large suite of Apollo lunar regolith samples using high-precision triple oxygen isotopes. Earth has erased most of its early bombardment record through tectonics, and constant crustal recycling. The Moon, by contrast, preserves a continuously accessible archive: lunar regolith, the loose layer of debris produced and reworked by impacts over billions of years.

Ever since the Apollo missions, scientists have tried to read that archive using elements that concentrate in impactors—especially “metal-loving” siderophile elements, which are abundant in meteorites but scarce in the Moon’s silicate crust. But regolith is an especially challenging mixture: impacts can melt, vaporize, and rework material repeatedly, and post-impact geological processes can separate metal from silicate, complicating attempts to reconstruct the type and amount of impactor material.

The new study takes a different approach. Instead of relying on metal-loving tracers, it uses oxygen—the dominant element by mass in rocks—and its triple-isotope “fingerprint” to separate two competing signals that normally get tangled in lunar regolith: (1) the addition of meteorite material and (2) isotopic effects from impact-driven vaporization. From measuring offsets in the oxygen isotope composition of regolith, the team finds that at least ~1% by mass of the regolith reservoir consists of impactor-derived material that are best explained from carbon-rich meteorites that were partially vaporized upon impact.

The team translated these impactor fractions into water-delivery bounds for the Moon and Earth, expressed in Earth-ocean equivalents for scale. For the Moon, the implied delivery since ~ 4 billion years ago is tiny on an Earth-ocean scale. But tiny compared to Earth’s oceans does not mean unimportant for the Moon. Instead, the Moon’s accessible water inventory is concentrated in small, cold-trapped reservoirs, and water is the kind of resource that matters immediately for sustained human presence for important things like life support, radiation shielding, and fuel. In other words, the long-term trickle of impactor-derived water can be negligible for Earth yet still be a meaningful contributor to the Moon’s available water budget.

The researchers then extended the same accounting to Earth, using a commonly applied scaling in which Earth receives substantially more impactor material than the Moon. Even if Earth experienced roughly 20× the impactor flux and even adopting the extreme megaregolith end-member, the cumulative water delivers only a few percent of an Earth ocean at most. That makes it difficult to reconcile the late-delivery of water-rich meteorites as the dominant source of Earth’s water, given that independent estimates yield several ocean-mass equivalents of water in the Earth in total.

“The lunar regolith is one of the rare places we can still interpret a time-integrated record of what was hitting Earth’s neighborhood for billions of years,” said Gargano. “The oxygen-isotope fingerprint lets us pull an impactor signal out of a mixture that’s been melted, vaporized, and reworked countless times.”

“Our results don’t say meteorites delivered no water,” added co-author Dr. Justin Simon from NASA’s Astromaterials Research and Exploration Science (ARES) Division. “They say the Moon’s long-term record makes it very hard for late meteorite delivery to be the dominant source of Earth’s oceans.”

Gargano framed the work as part of a scientific lineage that began with Apollo. “I’m part of the next generation of Apollo scientists—people who didn’t fly the missions, but who were trained on the samples and the questions Apollo made possible,” Gargano said. “The value of the Moon is that it gives us ground truth: real material we can measure in the lab and use to anchor what we infer from meteorites and telescopes.”

“Apollo samples are the reference point for comparing the Moon to the broader solar system,” Gargano added. “When we put lunar soils and meteorites on the same oxygen-isotope scale, we’re testing ideas about what kinds of bodies were supplying water to the inner solar system. That’s ultimately a question about why Earth became habitable, and how the ingredients for life were assembled here in the first place.”

About LPI
The Lunar and Planetary Institute (LPI), operated by Universities Space Research Association, was established during the Apollo program in 1968 to foster international collaboration and to serve as a repository for information gathered during the early years of the space program. Today, the LPI is an intellectual leader in lunar and planetary science. The Institute serves as a scientific forum attracting world-class visiting scientists, postdoctoral fellows, students, and resident experts; supports and serves the research community through newsletters, meetings, and other activities; collects and disseminates planetary data while facilitating the community’s access to NASA astromaterials samples and facilities; engages and excites the public about space science and invests in the development of future generations of scientists. The research carried out at the LPI supports NASA’s efforts to explore the solar system. More information about LPI is available at www.lpi.usra.edu.

About USRA
Founded in 1969, the Universities Space Research Association (USRA) is an independent, nonprofit organization that advances space- and Earth-related science, engineering, and technology through innovative research, education, and workforce development programs. USRA partners with government agencies, academic institutions, and industry to address some of the nation’s most complex scientific and technical challenges.

For more information, visit www.usra.edu.

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