Planetary scientists examining oxygen isotopes in lunar soil from the Apollo missions have determined that 4 billion years of meteorite impacts may have contributed only a minimal amount of Earth’s water. This insight prompts a reevaluation of established theories regarding water’s origins on our planet.
Close-up of a relatively new crater to the southeast, captured during Apollo 15’s third lunar walk. Image credit: NASA.
Previous research suggested that meteorites significantly contributed to Earth’s water supply due to their impact during the solar system’s infancy.
In a groundbreaking study, Dr. Tony Gargano from NASA’s Johnson Space Center and the Lunar and Planetary Institute, along with colleagues, employed a novel technique to analyze the lunar surface debris known as regolith.
Findings indicated that even under optimistic conditions, meteorite collisions from approximately 4 billion years ago may have delivered only a small percentage of Earth’s water.
The Moon acts as a historical archive, documenting the tumultuous events that the Earth-Moon system has endured over eons.
While Earth’s dynamic geology and atmosphere erase these records, lunar samples have retained valuable information.
However, this preservation is not without its challenges.
Traditional regolith studies have focused on metal-preferring elements, which can be obscured by continuous impacts on the Moon, complicating efforts to reconstruct original meteorite compositions.
Oxygen triple isotopes offer highly precise “fingerprints” since oxygen, being the most abundant element in rocks, remains untouched by external forces.
These isotopes facilitate a deeper understanding of the meteorite compositions that impacted the Earth-Moon system.
Oxygen isotope analyses revealed that approximately 1% of the regolith’s mass consists of carbon-rich material from meteorites that partially vaporized upon impact.
With this knowledge, researchers calculated the potential water content carried by these meteorites.
“The lunar regolith uniquely allows us to interpret a time-integrated record of impacts in Earth’s vicinity over billions of years,” explained Dr. Gargano.
“By applying oxygen isotope fingerprints, we can extract impactor signals from materials that have undergone melting, evaporation, and reprocessing.”
This significant finding alters our understanding of water sources on both Earth and the Moon.
When adjusted to account for global impacts, the cumulative water indicated in the model equates to only a minor fraction of the Earth’s oceanic water volume.
This discrepancy challenges the theory that water-rich meteorites delivered the bulk of Earth’s water.
“Our results don’t rule out meteorites as a water source,” noted Dr. Justin Simon, a planetary scientist at NASA Johnson’s Celestial Materials Research and Exploration Sciences Division.
“However, the Moon’s long-term record indicates that the slow influx of meteorites cannot significantly account for Earth’s oceans.”
While the implied water contribution from around 4 billion years ago is minimal in the context of Earth’s oceans, it remains notable for the Moon.
The Moon’s available water is concentrated in small, permanently shadowed areas at the poles.
These regions, among the coldest in the solar system, present unique opportunities for scientific research and exploration resources as NASA prepares for crewed missions to the Moon with Artemis III and subsequent missions.
The samples analyzed in this study were collected from near the lunar equator, where all six Apollo missions landed.
Rocks and dust gathered over half a century ago continue to yield valuable insights, albeit from a limited lunar area.
Future samples collected through Artemis are expected to unlock a new wave of discoveries in the years ahead.
“I consider myself part of the next generation of Apollo scientists, trained in the questions and insights enabled by the Apollo missions,” said Dr. Gargano.
“The Moon provides tangible evidence that we can examine in the lab, serving as a benchmark for what we learn from orbital data and telescopes.”
“I eagerly anticipate the information that upcoming Artemis samples will reveal about our place in the solar system.”
The findings of this study will be published in Proceedings of the National Academy of Sciences.
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Anthony M. Gargano et al. 2026. Constraints on impactor flux from lunar regolith oxygen isotopes to the Earth-Moon system. PNAS 123 (4): e2531796123; doi: 10.1073/pnas.2531796123
Source: www.sci.news
