Earth's Water: An Extraterrestrial Origin?

Water, a Key Substance for Life

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 essential for life as we know it, and it is involved in the thermal regulation of the planet, the transport of nutrients, and the formation of oceans and rivers.

The question of the origin of this water is therefore fundamental in geophysics and exobiology. Understanding whether ocean water comes solely from internal processes on Earth or whether it includes a part of extraterrestrial origin helps trace the history of the planet and assess the probability of water presence on other bodies in the solar system.

Terrestrial and Extraterrestrial Mechanisms

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

• Terrestrial origin: Water may have formed during the degassing of the Earth's mantle during the planet's initial cooling. It was trapped as mantle water and gradually released by volcanism, contributing to the formation of primordial oceans.
• Extraterrestrial contributions: Cosmic bodies, mainly planetesimals, bombarded 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 help better estimate their respective contributions.

Isotopic Clues

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 have a D/H ratio similar to that of Earth's oceans, while some comets, particularly those with low activity in the Oort Cloud, also show close values.

This data suggests that Earth's water may result from a mix of internal and external contributions. Local variations in D/H observed in oceans and polar ice also provide clues about ancient water flows and isotopic fractionation processes related to the hydrological cycle and past climatic conditions.

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

Comparative Table of Water Sources

The water in the oceans likely results from a combination of terrestrial and extraterrestrial sources. Isotopic studies tend to show that cometary and asteroid contributions played a significant role, while mantle degassing provided a continuous flow. This hypothesis highlights the interconnection between the formation of the solar system and the emergence of water on Earth.