Where does Earth’s water come from?

Within the Solar System, the Earth is located very close to the so-called «ice line«, sometimes called the frost line or snow line. This line is located in the main asteroid belt between the orbits of Jupiter and Mars, and from there the temperature of the planets is cool enough for the lightest compounds of hydrogen, such as water (H2O), methane (CH4) or ammonia (NH3 ), are permanently in solid state.

One of the most widespread theories suggests that these light compounds were pushed by the solar wind during the formation of the Solar System, pushing them away from the Sun, while other heavier compounds, such as metallic compounds and silicates, stayed closer.

This theory explains why water is relatively scarce within the ice line: on Mercury there is no water, on Venus traces appear in the atmosphere, and on Mars there is only a thin layer of ice in the polar regions. Only planet Earth breaks this rule within the ice line. 71% of the Earth’s surface is covered by seas and oceans and half of that mass of water exceeds 3 km in depth.

Theories about the origin of terrestrial water

There are several theories that try to explain why there is a mass of water on Earth that is much larger than the rest of the planets that are inside the ice line. Until now, the most widespread theory held that the planet’s volatile elements came in carbonaceous comets and chondrites (a very old type of meteorite) that carried primitive ice since the formation of the Solar System. Today this theory is very weak and there is a tendency to think of a closer origin, including the geological and biological activity of planet Earth itself.

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1.- Terrestrial origin

Oceans are known to have existed on Earth between 100 and 500 million years after Earth’s formation, about 4.54 billion years ago. At that time, it is estimated that the Earth’s radius was half that of today, but it would have had enough gravity to maintain an atmosphere with water vapour.

This first water vapor must have come from the interior of the earth, where the lighter substances tended to rise to the upper layers and the heavier ones (iron, nickel) tended to sink towards the deeper layers. However, the amount of water from the Earth’s interior expelled by volcanic activity does not seem to be enough to explain the enormous amount of water on the planet or to explain the formation of stable masses of water on its surface.

Another theory suggests that, in addition to the water inside the Earth, water could have been formed on the surface by the reaction between hydrogen and oxygen from the outgassing of magma. But there is not enough evidence that there was enough free hydrogen and oxygen at that time.

2.- Comets and asteroids

Comets and asteroids closest to the Sun are very dry, with little water. As we move further away, these objects begin to have higher water content. So, in the main asteroid beltBetween Jupiter and Mars, asteroids have a significant amount of water. The outermost asteroids in this belt can contain up to 17% ice.

If we continue moving away from the Sun and reaching the Kuiper belt, beyond Neptune, the water content of asteroids could reach 50%. The Oort Cloud asteroids, also from the group of trans-Neptunian objects, may be made entirely of ice, methane and ammonia and formed near the Sun in the early stages of the formation of the Solar System.

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The contribution of water by collision of these objects It is the most accepted theory for a long time. Earth’s water was believed to come from distant comets and a type of primitive meteorite called carbonaceous chondrites.

However, a study by researchers at the Carnegie Institution of Washington compared the content of deuterium (an isotope of hydrogen) in the water of some of the best-known comets believed to come from the Oort cloud, such as Comet Halley, and the content of Earth water deuterium. The deuterium content in comets far from the Sun has been found to be much higher than that of Earth’s water, which is more similar to the deuterium content found in the ice of much closer and younger main-belt asteroids.

Therefore, according to this study, it is most likely that Earth’s main body of water comes from the asteroid belt. and not from distant comets and meteorites as previously thought.

3.- Biological activity

Throughout the history of the planet, the volume of water has been increasing, probably due to the biological processes of living beings. One theory holds that most of Earth’s water comes from the biological activity of sulfur-oxidizing bacteria. These bacteria, present in the first oceans, would use Hydrogen Sulfide (HdoisS, or hydrogen sulfide) as a substrate in a photosynthetic process that produces water among the end products.

The origin of Earth’s water still seems somewhat diffuse, but it is more likely that it has a multiple origin through a complex process involving the aquifers of the solar system, the reactions of the original atmosphere and the biological activity of the living beings that inhabit the Earth. planet. All this to create unique features in the Solar System that make our very existence possible.

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71% of the Earth’s surface is covered by large bodies of water whose origin is not entirely certain Diagram of the position of the main asteroid belt in the Solar System Diagram illustrating the position of the Kuiper belt and the Oort Cloud Halley’s Comet, believed to be coming from the Oort Cloud, is one of the sources for studying the origin of Water on Earth

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