UNM researchers analyze moon record, challenging Earth’s water origins

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 argues that the Moon's surface record sets a hard limit on that possibility: even under generous assumptions, late meteorite delivery since about 4 billion years ago could only have supplied a small fraction of Earth's water.

In a paper published in the Proceedings to the National Academy of Sciences, researchers led by Tony Gargano, Ph.D., at the 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 lunar regolith, which is a collection of loose 'soil' and broken rock at the surface, acts like a long-term mixing layer," said Gargano. "It captures impact debris, stirs it in, and preserves those additions for immense spans of time. That is why it is such a powerful archive. It lets us study a time-averaged record of what was hitting the Earth-Moon system."

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.

"Triple oxygen isotopes give us a more direct and quantitative way to approach the problem. Oxygen is the dominant element in most rocks, and the triple-isotope framework helps us distinguish true mixing between different reservoirs from the isotopic effects of impact-driven vaporization," said Gargano. "In practice, that lets us isolate an impactor fingerprint from a regolith that has a complicated history, with fewer assumptions and a clearer chain from measurement to interpretation."

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.

"Our results don't say meteorites delivered no water. 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."

- Tony Gargano, Ph.D.

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. The main takeaway from our study is that Earth's water budget is hard, if not impossible, to explain if we only consider a single, late delivery pathway from water-rich impactors from the outer solar system. Even though some meteorite types carry a lot of water, their broader chemical and isotopic fingerprints are quite exotic relative to Earth. Habitability models have to satisfy such empirical constraints, and our study adds a constraint that future theories will need to reproduce."

"Our results don't say meteorites delivered no water," added Simon. "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."

Apollo samples also matter because the Moon preserves that impact story across deep time in a way Earth does not. The Moon does not just tell us about the Moon. It preserves an accessible record of the impact environment of the inner solar system, which helped set the boundary conditions under which Earth became habitable. There is still real wonder in that. Scientists have rocks collected decades ago, from another world, and they are still capable of changing how we think about the origin of Earth's water and the conditions that made life possible.

"What modern techniques add to this amazing legacy of scientific exploration is precision and interpretive power. We can now resolve subtle isotopic signals that allow quantitative tests of formation and habitability models," said Gargano. "That is why Apollo science keeps evolving. The samples are the same, but our ability to interrogate them, and the questions we can ask of them, are fundamentally better."

In addition to his research findings, Gargano is equally proud of what scientists are doing in terms of training and outreach because it captures that same arc: taking something that feels distant and making it tangible and impactful to our lives.

"At UNM, I have been training Albuquerque high schoolers in planetary science and geochemistry, including senior Brooklyn Bird and junior Violet Delu from the Bosque School," said Gargano. "These students are getting hands-on training in geochemistry using UNM's unique collection of Astromaterials, and they are learning the physical craft of laboratory science: how to prepare and handle samples, how to make high-quality measurements, and how to think clearly about uncertainty and reproducibility.

"But the deeper lesson is the transformation that happens when a student realizes they can hold a piece of another world, make a measurement, and pull meaning out of it. They learn how a chemical signal becomes a geologic story, and how that story scales up into an explanation for how a planetary body evolved to become the way it is. Experiences like that change what students think is possible for themselves. They build confidence, technical ability, and a sense of belonging in a field that can otherwise feel out of reach."

Bird and Delu will both be presenting their independent research projects at the 57th Lunar and Planetary Science Conference this spring and will also be educators to their peers and younger students through Bosque School outreach events. This is a model Gargano is excited to carry forward to other places in the country, so that more underserved students can gain access to world-class research experiences and obtain skill sets in geochemistry that open doors for them internationally.