The Three Ages of Terrestrial Water: Multiple Origins

A planetary mystery with multiple origins

Water (H₂O) is one of the most abundant molecules after hydrogen (H₂) and carbon monoxide (CO) in the Universe, and its existence on Earth has its origins in the protosolar nebula, the vast reservoir of gas and dust from which the Solar System formed.

Water covers more than 70% of the Earth's surface, but its origins still raise many questions. Has the planet always been hydrated? Is it a legacy of the protosolar nebula, or the result of fortuitous collisions? Today, geochemists and planetary scientists identify three major phases or "ages" in the history of terrestrial water.

First Age: Primordial Water Incorporated into the Solar Nebula

During the formation of the Earth 4.56 billion years ago, a significant portion of the accreted matter already contained hydrated minerals, particularly silicates from carbonaceous chondrites. These water-rich meteorites in the form of phyllosilicates contain up to 10% of their mass in H\(_2\)O.
The inner Earth was thus able to trap water in its mantle from the very beginning of accretion, with sufficient pressure to stabilize this water in a bound form. It is estimated that the mantle could still contain up to 2 to 10 times the mass of the current oceans today.

N.B.: Phyllosilicates are minerals that have a sheet structure where water and hydroxide ions \((OH^-)\) are integrated between the atomic layers. Their presence in carbonaceous meteorites testifies to an ancient interaction with liquid water, making them key tracers for understanding the early phases of hydration of bodies in the Solar System.

Second Age: The Late Heavy Bombardment

Between 4.1 and 3.8 billion years ago, the Earth experienced a massive influx of celestial bodies resulting from planetary reorganization (Nice model). Comets and asteroids rich in ice then delivered water to the surface.
Isotopic analyses of deuterium (\(D/H\)) in the oceans show a signature close to that of certain classes of chondrites (notably CM), but different from that of Oort-type comets.
The bombardment also allowed some of the water trapped in the depths to be degassed by thermal effect, initiating the primitive hydrosphere.

N.B.: Chondrites (CM for "Mighei-type carbonaceous chondrites") are primitive meteorites particularly rich in hydrated minerals, containing up to 10% of bound water. Their hydrogen isotopic composition (\(D/H\)) is very close to that of terrestrial oceans, making them serious candidates as a major source of water for the primitive Earth.

Third Age: Recycling and Tectonic Stabilization

The establishment of plate tectonics

From 3.5 billion years ago, the Earth entered a phase of relative internal and external stability, marked by the gradual establishment of plate tectonics. This unique dynamic system in the Solar System allows for continuous recycling of water between the surface (oceans, crust) and the interior of the planet (upper mantle). This water also influences plate tectonics by reducing fault friction.

The fundamental role of subduction

During subduction, oceanic plates laden with hydrated sediments and basaltic crust penetrate the mantle. Under the effect of increasing pressure and temperature, these materials release their water above the subducting plate, triggering the partial melting of the mantle and feeding the volcanoes of island arcs. This process releases water vapor back into the atmosphere through volcanic degassing.

Hydrated minerals and thermal stability

This deep water cycle is controlled by the thermodynamic equilibria of water-bearing minerals (such as lawsonite, chlorite, or serpentine), which dehydrate at precise pressures (3 to 10 GPa) and temperatures of around 500 to 800 °C. The current geochemical model suggests that about 2 to 3 km\(^3\) of water are subducted each year, while a comparable amount is released by volcanic activity.

A regulatory mechanism on a planetary scale

This mechanism ensures long-term regulation of the volume of the oceans and the global climate via the tectonic thermostat. Water also plays a lubricating role in the sliding of lithospheric plates, lowering their effective viscosity and facilitating mantle convective movements.

A vast reservoir of water in the mantle

This recycling, coupled with the deep storage of water in the mantle transition zones (410–660 km deep), has maintained a stable hydrosphere for more than 3 billion years. Some models estimate that the mantle could contain up to three times the current mass of the oceans, trapped in minerals such as wadsleyite and ringwoodite.

N.B.: Wadsleyite and ringwoodite are magnesium silicate polymorphs capable of incorporating several percent of water in their crystalline structure under high pressures. Their presence in the Earth's mantle transition zone suggests a vast underground water reservoir.

Water on Earth: A Precarious Balance

The total mass of terrestrial water is estimated at about \(1.4 \times 10^{21}\) kg. But only a fraction (0.023%) is available on the surface in liquid form. The rest is trapped in ice, underground aquifers, and hydrated minerals. The Earth remains a unique case with its hydrosphere stable for more than 3 billion years.

Sources: Nature – Marty et al. (2015), Science – Alexander et al. (2012), NASA – Solar System Exploration.