For decades, a widely accepted scientific explanation for Earth’s water has been that it was delivered by water-rich meteorites, especially carbonaceous chondrites, during the planet’s early formation. This hypothesis was strongly supported by similarities in isotopic ratios, especially the ratio of deuterium to hydrogen (D/H) between Earth's water and that found in certain meteorites. 1 However, a new study led by researchers at the University of Oxford, published in "Icarus," is challenging our understanding of the origin of water on Earth. 2
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The Meteorite Hypothesis
According to the meteorite delivery view, the early Earth, formed from dry, rocky materials close to the Sun, was initially waterless. Water was delivered later by impacts from distant, water-rich asteroids and comets. This idea was supported by the close match between the D/H ratios measured in Earth's oceans and those found in certain classes of meteorites, suggesting a shared origin. 1, 3
A New Approach Using Advanced Isotope Analysis
A recent Oxford-led study challenges the theory that Earth's water arrived via water-rich meteorites by employing state-of-the-art isotope analysis and modeling techniques. The research team focused on enstatite chondrites, a rare type of meteorite with a chemical composition analogous to the early Earth. The researchers analyzed the elemental and isotopic composition of these meteorites, specifically a specimen known as LAR 12252, collected from Antarctica, to trace the origin of hydrogen, which is the essential ingredient for water. 2
The team used X-ray Absorption Near Edge Structure (XANES) spectroscopy at the Diamond Light Source synchrotron to distinguish between hydrogen intrinsic to the meteorite and hydrogen introduced through terrestrial contamination. Their findings suggested that the hydrogen present in the meteorite's fine-grained matrix was both abundant and native, not a result of contamination. The hydrogen content in this matrix was five times higher than in other parts of the meteorite, and it was primarily present as hydrogen sulfide, which is a form that is unlikely to be introduced after the meteorite's arrival on Earth. 2
Key Findings Suggest Earth's Water is Largely Native
The study shows a clear discrepancy between the D/H ratios in Earth's deep mantle and those found in most meteorites. Although ocean water and some meteorites share similar D/H ratios, hydrogen extracted from samples deep within the boundary between the core and mantle exhibits lower levels of deuterium. This suggests that a significant portion of Earth's hydrogen did not come from asteroidal sources but was present from the planet's very beginnings.
This finding is reinforced by other recent research, which has traced the isotopic signature of water back to the earliest phases of the solar system. According to that research, studies of ancient meteorites and solar nebula gas reveal that two distinct reservoirs of hydrogen existed in the young solar system: one comprising solar gas (the primordial hydrogen-rich gas left over from the Sun's formation) and another enriched in water vapor, created by a massive influx of interstellar water into the inner solar system. The early existence of this gas with Earth-like isotopic composition implies that Earth's water was present before the accretion of the planet's first building blocks. 4, 5
Implications on Planetary Formation and Habitability Studies
The evidence undermines the long-standing view that Earth's water was delivered primarily by asteroids and comets after the planet's formation. Instead, it suggests that water is a natural outcome of the materials from which Earth formed.
Earth appears to have retained significant amounts of hydrogen from the solar nebula cloud of gas and dust that surrounded the young Sun during its accretion. This hydrogen, incorporated into the planet's interior, later combined with oxygen to form water.
If Earth could acquire and retain water in this way, the implication is that it is plausible that other rocky planets in our solar system and around other stars could also possess native water. This shifts the focus of planetary habitability studies, suggesting that water may be more common on rocky planets than previously thought.
It also prompts a reevaluation of Mars and Venus. Mars, with its ancient river valleys and apparent underground ice reservoirs, and Venus, which may once have had oceans, could have also trapped water during their formation phases. The loss of that water, therefore, might relate more to later atmospheric escape processes than to a failure to initially acquire it.
Next Steps in Research
This study opens new possibilities for future research. Scientists are now keen to investigate the specific mechanisms by which hydrogen from the solar nebula could have been incorporated and retained within growing planetary bodies. Comparative studies will also be crucial. For instance, researchers can test whether early water acquisition was a common feature or unique to Earth by analyzing the isotopic signatures of other solar system bodies, including Mars, asteroids, and Mercury.
References and Further Reading
- Piani, L., Yurimoto, H., & Remusat, L. (2018). A dual origin for water in carbonaceous asteroids revealed by CM chondrites. Nature astronomy. https://doi.org/10.1038/s41550-018-0413-4
- Barrett, T. J., Bryson, J. F., & Geraki, K. (2025). The source of hydrogen in Earth's building blocks. Icarus. https://doi.org/10.1016/j.icarus.2025.116588
- Piani, L., Marrocchi, Y., Rigaudier, T., Vacher, L. G., Thomassin, D., & Marty, B. (2020). Earth's water may have been inherited from material similar to enstatite chondrite meteorites. Science. https://doi.org/10.1126/science.aba1948
- Aléon, J., Lévy, D., Aléon-Toppani, A., Bureau, H., Khodja, H., & Brisset, F. (2022). Determination of the initial hydrogen isotopic composition of the solar system. Nature Astronomy. https://www.nature.com/articles/s41550-021-01595-7
- Earth's water was around before Earth. (2022) CNRS. Available at: https://www.cnrs.fr/en/press/earths-water-was-around-earth
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