Clues buried in Moon dust are helping scientists revisit one of the oldest questions: where did Earth’s water come from? For years, a dominant theory proposed that water-rich meteorites delivered vast amounts of water during the early stages of the Solar System’s evolution. These meteorites, crashing into the young Earth, were thought to have been key contributors to the formation of Earth’s oceans.
But new research led by Tony Gargano, a postdoctoral fellow at NASA’s Johnson Space Center and the Lunar and Planetary Institute, challenges that view. The team analyzed regolith from Apollo lunar missions using a novel approach, providing the clearest evidence yet that the volume of water delivered by meteorites over the past four billion years is minimal.
The Moon As An Impact Archive
The Moon’s surface serves as a unique record of the Earth-Moon system’s impact history. Unlike Earth, whose surface is constantly reshaped by tectonic activity and weather, the Moon retains a nearly untouched record of meteorite strikes.
“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 Tony Gargano,
As reported in NASA’s statement, to examine this record the researchers used triple oxygen isotope analysis, a method that focuses on oxygen, the dominant element in rocks, and one that is largely unaffected by impacts or external conditions. This approach allowed the team to isolate and identify meteorite material embedded in the Moon’s surface, even after repeated bombardment and reworking of the soil. The analysis showed that only about 1% (by mass) of the regolith came from carbon-rich meteorites.
Limited Water Contribution From Meteorites
According to the study, published in Proceedings of the National Academy of Sciences, even under generous assumptions, meteorite delivery over the last four billion years could have supplied only a small fraction of the water found in Earth’s oceans.
The findings contradict the idea that late-stage meteorite impacts were a major source of Earth’s water. NASA planetary scientist Justin Simon explained:
“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.”
The research used the known water content of carbonaceous meteorites to calculate potential delivery rates, scaling the findings from lunar data to Earth’s size and impact rate. Even after multiplying the lunar values by roughly twenty to match Earth’s exposure, the resulting water volume remained insufficient to account for ocean levels.
How Apollo Samples Still Shape Lunar Science Today
The team’s conclusions are based on regolith samples collected during the six Apollo missions, all from near the lunar equator on the side facing Earth. These samples, gathered over fifty years ago, continue to yield new insights.
“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,” said Gargano.
According to NASA, upcoming missions like Artemis III will provide access to new lunar terrains, including permanently shadowed regions near the Moon’s poles. These regions, among the coldest in the Solar System, could hold further clues about water delivery in the Earth-Moon system.