Earth's Water: An Extraterrestrial Origin?

Scientific Summary

The Astronoo article examines the origin of Earth's water and the oceans. It confronts internal contributions, from mantle degassing, with extraterrestrial contributions, comets and carbonaceous asteroids. Deuterium/hydrogen (D/H) isotopic ratios serve as tracers to compare Earth's water with cosmic waters. Carbonaceous meteorites and certain comets show values close to those of the oceans, suggesting a mixture of sources. The comparative table synthesizes the potential contributions. The article emphasizes that ocean water probably results from a combination of planetary and cosmic processes, which sheds light on the links between the formation of the solar system and the habitability of Earth.

Where does ocean water come from and what is the extraterrestrial share?

This article is devoted to the origin of Earth's water and the possible role of cosmic contributions. Water covers more than 70% of the globe's surface, but its origin remains debated. Two main families of sources are considered. The first is terrestrial: water would have been trapped in the mantle during the planet's formation, then released by volcanic degassing. The second is extraterrestrial: planetesimals, comets and carbonaceous asteroids would have bombarded the primitive Earth, notably during the Late Heavy Bombardment, bringing volatile compounds. To distinguish these contributions, scientists use the deuterium/hydrogen (D/H) isotopic ratio. Carbonaceous meteorites show values close to those of the oceans, while certain Oort cloud comets also come close. The current answer is therefore nuanced: ocean water appears to result from a mixture of internal and external contributions. This article details the mechanisms, isotopic clues and uncertainties that remain.

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Water, a key substance for life

Water, engine of life and climate

Water covers more than 70% of the Earth's surface and plays a central role in the biochemistry of living organisms, the climate and planetary geodynamics. Its presence is indispensable for life as we know it, and it is involved in the planet's thermal regulation, the transport of nutrients and the formation of oceans and rivers.

A fundamental question for geophysics

The question of the origin of this water is therefore fundamental in geophysics and exobiology. Understanding whether ocean water comes only from internal processes on Earth or whether it includes a share of extraterrestrial origin allows us to retrace the planet's history and to evaluate the probability of the presence of water on other bodies of the solar system.

Terrestrial and extraterrestrial mechanisms

Scientists identify mainly two sources of water for the primitive Earth:

Terrestrial origin: mantle degassing

Water may have formed during the degassing of the Earth's mantle during the planet's initial cooling. It was trapped in the form of mantle water and was progressively released by volcanism, contributing to the formation of the primordial oceans.

Extraterrestrial contributions: planetesimals and Late Heavy Bombardment

Cosmic bodies, mainly planetesimals, bombarded the Earth during the first 500 million years, a period known as the Late Heavy Bombardment. These impacts potentially delivered a significant amount of water and other volatile compounds, enriching the Earth's surface.

These two mechanisms are not mutually exclusive. The exact proportion of each source remains debated, but isotopic models and observations of carbonaceous meteorites and comets allow better estimation of their respective contributions.

Isotopic clues

The deuterium/hydrogen (D/H) ratio

The study of deuterium/hydrogen (D/H) isotopic ratios in terrestrial and extraterrestrial water is a key tool for tracing the origin of water. Carbonaceous meteorites generally show a D/H ratio similar to that of the terrestrial oceans, while certain comets, notably low-activity comets in the Oort cloud, also show close values.

A mixture of internal and external contributions

These data suggest that Earth's water could result from a mixture of internal and external contributions. The local variations in D/H observed in the oceans and polar ice also provide clues about ancient water flows and isotopic fractionation processes linked to the hydrological cycle and past climatic conditions.

Chronology and implications for habitability

Isotopic analyses therefore allow us not only to retrace the origin of water, but also to understand the chronology of its accumulation and its role in the geological and biological evolution of the Earth.

Comparative table of water sources

The table below compares the main potential sources of Earth's water according to their origin and their D/H isotopic ratio.

Comparison of the different sources of Earth's water
Source	Origin	D/H isotopic ratio	Comment
Terrestrial volcanism	Earth's mantle	1.5 × 10^-4	Continuous contribution since Earth's formation
Oort cloud comets	External to the solar system	1.6 × 10^-4	Ratio close to that of Earth's oceans
Carbonaceous asteroids	Main belt	1.4 to 1.5 × 10^-4	Major contribution to the total amount of Earth's water

Synthesis of Earth's water sources

Ocean water probably results from a combination of terrestrial and extraterrestrial sources. Isotopic work tends to show that cometary and asteroidal contributions played a significant role, while mantle degassing provided a continuous flux. This hypothesis emphasizes the interconnection between the formation of the solar system and the emergence of water on Earth.

References

FAQ: Everything to know about the origin of ocean water

Where does ocean water mainly come from?

Ocean water appears to come from a mixture of internal contributions, from Earth's mantle degassing, and extraterrestrial contributions, brought by carbonaceous asteroids and certain comets. No single source suffices to explain all the observed water.

What is the D/H ratio and why is it important?

The deuterium/hydrogen (D/H) ratio measures the proportion of heavy water relative to light water. It serves as an isotopic tracer to compare Earth's water with that of meteorites, comets and planetesimals. A close ratio indicates a potential source.

Did comets bring ocean water?

Certain comets, notably those of the Oort cloud, have a D/H ratio close to that of the oceans. They could therefore have contributed, but their contribution is probably minor compared to carbonaceous asteroids.

Did carbonaceous asteroids play a role?

Yes. Carbonaceous meteorites, from asteroids of the main belt, contain water and volatile compounds. Their D/H ratio is often close to that of Earth's water, which makes them a possible major source.

Is Earth's mantle degassing sufficient to explain the oceans?

Earth's mantle degassing has provided a continuous flux of water throughout Earth's history. However, models suggest that this process alone does not always suffice to explain the current volume of the oceans, hence the hypothesis of complementary extraterrestrial contributions.

Is water present elsewhere in the solar system?

Yes. Traces of liquid water or ice exist on Mars, Europa, Enceladus, Ganymede and in comets. The study of these reservoirs helps to understand the mechanisms of formation and transport of water in the solar system.