Water is essential to life as we know it and it seems completely normal to have water all
around us.
Yet Earth is the only known planet to be covered by oceans.
Do we know exactly where its water came from?
This is not a simple question: it was long thought that Earth formed dry – without
water, because of its proximity to the Sun and the high temperatures when the solar system
formed.
In this model, water could have been brought to Earth by comets or asteroids colliding
with the Earth.
Such a complex origin for water would likely mean that our planet is unique in the universe.
However, in a 2020 study, we showed that water – or at least its components, hydrogen and
oxygen – may have been present in the rocks that initially formed the Earth.
If that is so indeed, other “blue planets” with liquid water are more likely to exist
elsewhere.
Liquid water covers more than 70% of Earth’s surface, with about 95.6% of it in oceans
and seas, and the remaining 4% in glaciers, ice caps, groundwater, lakes, rivers, soil
humidity, and the atmosphere.
But most of Earth’s water is deep underground: between one and ten times the volume of the
oceans are contained in the mantle.
At the surface of the Earth, “water” means two hydrogens for each oxygen (H2O), whereas
what we call “water” in the mantle corresponds to hydrogen incorporated in minerals, magmas
and fluids.
This hydrogen can bond with surrounding oxygen to form water at the appropriate temperature
and pressure conditions.
While water represents less than 0.5% of the mass of the Earth, it is key to the evolution
of the planet itself and to life at its surface.
In the early solar system, there was a lot of hydrogen, mainly in the form of dihydrogen
gas (H2), or bonded with oxygen atoms to form water (H2O).
However, Earth and the other rocky planets (Mercury, Venus, and Mars) formed near the
Sun, where it was too hot for water to incorporate into rock as ice: it just would have evaporated.
So why does the Earth now have so much water, both in its mantle and on its surface?
Some meteorites, called chondrites, come from small asteroids that, unlike the planets,
have not geologically evolved since their formation.
They are good witnesses of the first millions of years of the solar system.
The carbonaceous chondrites for instance formed far enough away from the Sun to initially
contain water ice (all of which has since been incorporated into hydrated minerals through
hydrothermal alteration).
Contrastingly, ordinary and enstatite chondrites formed closer to the Sun where water was gaseous
and was incorporated in large amounts into rocks: like the rocky planets, ordinary and
enstatite chondrites are considered to be “dry”.
Until now, the accepted hypothesis was that Earth formed from dry materials, and that
its water was delivered by celestial bodies that formed further from the sun: hydrated
meteorites, such as carbonaceous chondrites, or comets – although this last hypothesis
was recently thwarted by the ESA space probe Rosetta.
Our study tells a different story.
We analysed the hydrogen in enstatite chondrites.
Remember that these are among our best analogues for the rocks that formed Earth, so the hydrogen
concentrations in these “dry” rocks hint at the possible presence of water during Earth’s
formation.
We compared the Earth's composition and that of enstatite chondrites by looking at the
amounts of various isotopes (atoms of the same element but containing different numbers
of neutrons).
We find that, although enstatite chondrites do not contain hydrated minerals, they do
contain small amounts of hydrogen with an isotopic ratio consistent with the Earth’s.
Hydrogen is thought to have been present in trace amounts (<0.1%) in the minerals and
organic compounds that agglomerated to form enstatite chondrites, explaining where most
of the water contained in Earth’s mantle and in part of the oceans comes from.
The majority of Earth’s water (more precisely its elements, hydrogen and oxygen) may thus
have been present from the beginning.
This does not tell us when the oceans appeared on Earth’s surface, but we now know that
Earth’s water was not necessarily delivered by hydrated bodies that formed very far from
the Sun.
However, we do not yet understand in what form and by what process hydrogen was incorporated
and stored in rocks of the inner solar system.
The presence of hydrogen in inner solar system rocks is particularly important because it
could have been a water source for the other rocky planets (Mercury, Venus, and Mars).
Similar rocks could then represent a source of water for planets orbiting other suns,
a condition to develop life, at least life as we
know it.