What are the true origins of water on Earth?

Seas and oceans cover 70% of the earth's surface -

• It may not have been celestial bodies that brought water to Earth, according to a study published by our partner The Conversation.

• Covering more than 70% of the surface of the blue planet, “surface” water would fit in a sphere 1,300 kilometers in diameter.

• The analysis of this theory was carried out by Laurette Piani (cosmochemist) and Guillaume Paris (geochemist), both CNRS research officers at the Nancy Petrographic and Geochemical Research Center, University of Lorraine.

Earth is the only known planet to have large bodies of liquid water on its surface.

This water is essential to us and its presence can seem absolutely natural to us.

But do we really know where water comes from on our planet?

This is a non-trivial question because it has long been thought that the Earth was formed "dry" (without water) because of its proximity to the Sun and therefore the heat which prevailed during its formation.

A combination of cosmic circumstances would have allowed celestial bodies, comets or asteroids, coming from the confines of our solar system to strike young Earth, bringing water there and making it a planet probably unique in the Universe.

A meteorite study recently published by our team reveals that water - or at least its constituents, hydrogen and oxygen - could have simply been present in rocks available in our planet's formation environment.

This could reshuffle the cards on the probability of other blue planets.

Water on Earth, Water on Earth

First, we need to define what water is and where it is.

The “blue planet” is covered with water on more than 70% of its surface.

Seas and oceans represent about 96.5% of the water present in the Earth's outer envelopes (surface and atmosphere), while the remaining 4% corresponds mainly to glaciers and ice caps, groundwater and the whole. other reservoirs such as lakes, rivers, soil moisture, atmospheric vapor, etc.

If this ubiquitous water on the surface of the Earth seems very abundant to us, it is in fact only a small percentage of the total mass of the Earth, on the order of 0.02% - it would be entirely contained in a sphere. 1300 km in diameter, roughly the size of Germany from north to south.

Water is omnipresent on the surface of the earth © MaxwellFury / Pixabay

In fact, the Earth's interior could be the main reservoir of Earth's water, with estimates ranging from 1 to more than 10 times the total amount of water in the oceans.

Although the water in the outer layers is mainly in the well-known form of two hydrogen atoms and one oxygen atom, H²O, what is called “water” inside the Earth rather refers to to hydrogen incorporated in various forms in minerals, lavas and fluids.

This hydrogen can combine with oxygen in minerals to form water, if pressure and temperature conditions allow.

Water represents less than 0.5% of the total mass of our planet.

It remains nonetheless fundamental both for the formation and evolution of our planet and for the emergence, evolution and maintenance of life on its surface.

How did “water” get into the emerging solar system?

In the emerging solar system, hydrogen represented more than 91% of the atoms present, but mainly in the form of hydrogen gas, H².

More rarely, hydrogen could combine with other atoms to form water, methane (CH₄), ammonia (NH₃), etc. To find out if hydrogen can be incorporated into planetary bodies in formation, it is necessary to know the temperature: ice (solid form of water, but also of methane or hydrogen for example) can be incorporated in rocks in formation, unlike gases.

Oxygen, on the other hand, associates easily with silicon or other cations - iron, magnesium, etc.

- to form minerals, whatever the temperature.

However, the temperature decreases with distance from the Sun and during the first millions of years.

Water is the molecule that solidifies first on cooling.

At the very low pressures which then prevailed, the water passes directly from the gaseous to solid state at a temperature of the order of - 120 ° C.

The solar system was originally a cloud of gas and dust from which planets and various bodies formed by agglomeration of dust.

The incorporation of water into planetary bodies depends on the surrounding temperature, at the low pressures of the interplanetary medium: above -120 ° C, water is in vapor form and does not agglomerate with other solids / © Laurette Piani

The Earth and the other rocky planets (Mercury, Venus and Mars) formed in the inner zone of the solar system, close to the Sun, where it is too hot for the water to be stable in the form of ice.

Thus, these planets did not contain water ice in their rock constituents, and are therefore considered to be initially "dry".

But then, why does the Earth now have so much water, within and on its surface?

The historical hypothesis: hydrogen was brought to Earth by meteorites

Some meteorites, called chondrites, come from small asteroids which, unlike planets, have not evolved geologically since the formation of the solar system, and have thus been able to preserve their primary materials.

Among these chondrites, some, such as carbonaceous chondrites, form far enough from the Sun for water ice to be present and to agglomerate with other solids.

Other chondrites, ordinary and enstatite chondrites, come from small asteroids that form closer to the Sun, where water is in the vapor phase and cannot be retained in rocks.

Like the rocky planets, these ordinary and enstatite chondrites are then called "dry".

Therefore, they have been ignored as a source of water for the developing Earth by scientists.

On the left, the Murchison meteorite, a carbonaceous chondrite rich in hydrated minerals and organic matter formed in the outer solar system (0.46 g piece).

On the right, the Sahara meteorite 97096, an enstatite chondrite containing no hydrated minerals, formed in the inner solar system near Earth's orbit.

(70 g piece) / © J. Taylor / Flickr, L. Piani & C. Fieni / MNHN - CC BY-SA 4.0

The historically accepted hypotheses are that the Earth would have formed with dry materials.

The building blocks of water would have been brought by celestial bodies formed far from the sun: comets (a hypothesis thwarted by the measurements of the

space probe

, ESA) or

meteorites

(that is, ie initially containing water ice which forms hydrated minerals with hydrogen in their structure), such as carbonaceous chondrites.

Another hypothesis: the Earth was not that “dry” at first

Our recent study reveals a different story.

We have carried out the analysis of the hydrogen of enstatite chondrites, analogues of the rocks of the Earth in formation.

The hydrogen concentration in these “dry” rocks informs us about the possible presence of water in the Earth in formation.

The isotope ratios, which quantify the different forms of the same atom according to their number of neutrons, are real fingerprints of the chemical elements and make it possible to refine the comparison between the Earth and the enstatite chondrites.

Our measurements show that enstatite chondrites, although they do not contain hydrated minerals, possess significant amounts of hydrogen with an isotope ratio coinciding with that of Earth.

They could therefore explain all of the water contained in the Earth's mantle and part of the surface water (oceans).

This hydrogen would be present in minute quantities in the minerals and carbonaceous compounds which agglomerated to form the enstatite chondrites.

The majority of bricks (H and O) at the origin of the water of the Earth could therefore have been present from the start.

What consequences for a local source of water?

Thus, the Earth would have contained hydrogen and oxygen in large quantities and with the right isotopic compositions from the start of its formation.

If this does not tell us when the oceans appeared on the surface, we now know that the bricks necessary for the appearance of water did not need to be brought by hydrated bodies formed very far from the Sun.

However, it remains to be understood in what form (s) and by what process hydrogen could have been incorporated and preserved in the rocks of the internal solar system.

The presence of hydrogen in the rocks of the internal solar system is all the more important as this hydrogen could have been a source for the water of the other rocky planets of our solar system (Mercury, Venus and Mars) and could, by analogy , represent a water reservoir for planets orbiting other suns.

Science

Coronavirus: Could the oceans be home to tomorrow's epidemics?

Science

Black holes (and their many mysteries) explained to your kids

This analysis was written by Laurette Piani (cosmochemist) and Guillaume Paris (geochemist), both CNRS research officers at the Nancy Petrographic and Geochemical Research Center, University of Lorraine.

The original article was published on The Conversation website.

• Environment

• Water

• Geology

• Planet

• Earth

• Science