An in-depth examination of oxygen isotopes within lunar regolith, retrieved from Apollo mission sites, leads planetary scientists to the groundbreaking conclusion that the ceaseless bombardment by meteorites over the past 4 billion years could have contributed only a negligible portion of Earth’s water, necessitating a significant reassessment of prevailing scientific theories.
A close-up view of a portion of a ‘relatively fresh’ crater, looking southeast, as photographed during the third Apollo 15 lunar surface moonwalk. Image credit: NASA.
Prior research had posited that meteorites, during the nascent stages of the Solar System, might have been a pivotal conduit for delivering substantial quantities of water to our planet through their pervasive impacts.
In a recent scholarly endeavor, Dr. Tony Gargano, affiliated with NASA’s Johnson Space Center and the Lunar and Planetary Institute, alongside his research cohort, employed an innovative analytical technique on the fine particulate matter blanketing the Moon’s surface, known as regolith.
Their findings indicated that, even when accounting for the most exceptionally lenient estimations, the water delivered by meteorites since approximately 4 billion years ago could only account for a minute percentage of Earth’s total water reserves.
The Moon functions as a remarkably preserved chronicle of the impact events experienced by the Earth-Moon system throughout eons.
While Earth’s terrestrial crust, subject to dynamic geological processes and meteorological phenomena, tends to obliterate such historical imprints, lunar samples retain them with fidelity.
However, the interpretation of these records is not without its complexities.
Conventional methodologies for dissecting regolith have historically centered on the analysis of elements with a strong affinity for metals.
The recurrent cosmic impacts on the lunar surface can contaminate these elemental signatures, thereby complicating the precise reconstruction of the original composition of the impacting meteoroids.
Enter the domain of triple oxygen isotopes, meticulously precise markers that leverage the fundamental characteristic of oxygen—the most abundant element by mass in rocky materials—to remain unaltered by impact events or other external environmental influences.
These isotopic compositions provide an unclouded perspective on the constituents of meteorites that impacted the Earth-Moon system.
The isotopic oxygen measurements revealed that a minimum of approximately 1% by mass of the regolith comprised material originating from carbonaceous meteorites that underwent partial vaporization upon their lunar impact.
By utilizing the established properties of these types of meteorites, the researchers were able to quantify the volume of water they likely carried.
“The lunar regolith represents one of the rare locales where we can still decipher a cumulative record reflecting the objects that impacted Earth’s vicinity over billions of years,” stated Dr. Gargano.
“The characteristic oxygen-isotope signature enables us to isolate the signal of an impactor from a complex mixture that has been subjected to melting, vaporization, and reworking innumerable times.”
These groundbreaking findings carry significant implications for our understanding of the origins of water on both Earth and the Moon.
When extrapolated by a factor of roughly 20, to account for the considerably higher frequency of impacts experienced by Earth, the integrated water contribution calculated by the model constituted only a minimal fraction of the water present in Earth’s oceans.
This observation presents a considerable challenge to the hypothesis that the late-stage delivery of water-rich meteorites was the primary source of Earth’s hydrological endowment.
“Our conclusions do not assert that meteorites delivered no water,” commented Dr. Justin Simon, a planetary scientist within the Astromaterials Research and Exploration Science Division at NASA Johnson.
“Rather, they indicate that the long-term geological record preserved on the Moon strongly suggests that late meteorite delivery could not have been the principal source of Earth’s oceans.”
In the context of the Moon, the estimated delivery since approximately 4 billion years ago, while insignificant by Earth-ocean standards, is not inconsequential for the lunar body itself.
The Moon’s readily accessible water resources are predominantly concentrated within small, perpetually shadowed regions situated at its north and south poles.
These areas represent some of the most frigid environments within the Solar System, presenting unique opportunities for scientific investigation and potentially serving as vital resources for future lunar endeavors, particularly for NASA missions such as Artemis III and subsequent expeditions.
The specimens subjected to analysis in this investigation were procured from equatorial regions of the Moon on the hemisphere facing Earth, which is where all six Apollo missions achieved their landings.
The geological materials, including rocks and dust, collected over a half-century ago continue to yield novel insights, though their origin is confined to a limited geographical area of the Moon.
Samples anticipated through the Artemis program are poised to unlock a new era of scientific discoveries in the decades ahead.
“I consider myself part of the succeeding wave of Apollo-era scientists – individuals who, while not participants in the original missions, have been rigorously trained utilizing the samples and the profound questions that the Apollo program brought to light,” remarked Dr. Gargano.
“The intrinsic value of the Moon lies in its capacity to furnish us with empirical validation: tangible, physical substances that can be meticulously analyzed in laboratories, thereby grounding our inferences derived from orbital observations and telescopic data.”
“I eagerly anticipate discovering the revelations that the Artemis samples, and consequently the next generation of scientists, will impart concerning our cosmic abode within the Solar System.”
This investigation has been officially published in the Proceedings of the National Academy of Sciences.
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Anthony M. Gargano et al. 2026. Constraints on the impactor flux to the Earth-Moon system from oxygen isotopes of the lunar regolith. PNAS 123 (4): e2531796123; doi: 10.1073/pnas.2531796123
