Have you ever wondered about the origin of water on Earth? Water may have formed from hydrogen and oxygen inside the atmosphere of Earth or it could have been there all the way from the formation of Earth or it could have extraterrestrial origin and came to Earth from comet’s impacts. What do you think?

Origin of Water
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How is Water Formed?

Water covers 2/3 of the Earth’s surface. Without water, life would not have been possible on Earth. This is the reason why scientists search for the presence of water on other planets. Presence of water can confirm the probability of Earth-like life forms on an alien planet.

Chemically, water is a compound formed by the combination of two molecules of hydrogen and one molecule of oxygen. This reaction is highly exothermic although it requires heat for initiation.

This means, reaction between hydrogen and oxygen begins when there is sufficient heat for activation and after the reaction happens a large amount of heat is produced.

For example – hydrogen and oxygen can mix during lightening to give water. Since this reaction yields a large amount of heat energy and produces water as a byproduct, it is used by space stations for producing electricity. The energy plants which produce energy in such fashion are called fuel cells and produce clean energy.

Hydrogen is abundant in the Universe while oxygen is not. Where did the oxygen come from? The oxygen might have come as a part of stardust. Oxygen can be formed inside the core of stars and like any other rare elements, it could have come to Earth later.

But, the formation of water is a highly exothermic process. So, the formation of such a tremendous amount of water could have released a lot amount of heat. This heat would not let liquid water form on the surface. So, for the origin of water on Earth, oxygen and hydrogen could not have combined by the reaction explained above. However, some water may have such origin.

Origin of Water on Earth

We were not here when the water originated on Earth. So, we have to rely on different pieces of evidence from study of geology to know the most convincing explanation. On the basis of shreds of evidence, there can be two hypotheses about the origin of water on Earth. Both are supported by facts.

One states that water have been accumulated from the interplanetary dust which formed Earth and had existed as long as Earth has the common origin hypothesis. Meanwhile, the other one states that the Earth was dry when it formed and water came with different water-rich asteroids which hit Earth, the Interstellar origin hypothesis.

Interstellar Origin Hypothesis

This hypothesis assumes that Earth was very hot when it formed. This temperature was too much for Earth to harbour any forms of water. So, water could not have formed on Earth during the formation of Earth itself.

Even if it did, it would be lost in space as it does by photolysis and also due to extreme heat. Further, the moon forming impact would have destroyed most of Earth’s water.

This leads us to interstellar origin hypothesis and it assumes water is obtained by Earth when water-rich interstellar objects, mainly comets which hit Earth’s surface. The study of water ice contained on the surface of icy comets and asteroids shows a good match.

But, Earth’s water surface does not completely contain water of such origin. Only, some percentage of water was accumulated in such a way. Hence, the origin of water on Earth is partly due to it.

Common Origin Hypothesis

This hypothesis supports that the Earth’s water did not come from any other extraterrestrial source but it was here from the very beginning of Earth’s formation. There are various large masses of water vapour in space.

There exists a reservoir which contains as much as 140 trillion times of water contained by whole Earth in the Universe. So, it is not uncanny to consider that water was there from the beginning.

This assumption is based on the recent study of oldest rocks found on Earth, one of carbonaceous chondrite called Apatite. These rocks formed along with the Solar system and contains some fraction of water in it. The water has composition of both deuterium (isotopic hydrogen with atomic mass 2) and hydrogen.

The isotopic hydrogen ratio was calculated for Earth’s rock and Vesta’s (an asteroid formed along with Earth) rock and it matched. Further results from other isotopes like carbon and nitrogen isotopes confirmed that these two formed at the same time as the solar system. Eventually, this confirmed Earth formed with most of the Earthly water along with it.

The Actual Origin of Water on Earth

From the above facts, it can be insinuated that the formation of water on Earth was not completed due to a single cause. Both hypotheses are supported well by experimental evidences and both must be true.

So, lets put it this way. The origin of water on Earth is diverse. Most of Earth’s water was originated from the interplanetary dust that made up Earth. Later there was various water-rich comet and asteroid impact on Earth, which also accounted for origin of water. Also, the formation of water by combination of hydrogen and oxygen has some share of water present on the Earth.

Though there was a significant loss of water due to photolysis and moon formation impact, Earth’s gravity, magnetic field and its distance from the Sun played a major role in protecting water in liquid form.

Also, the cooling by tectonic plate movement secured the presence of liquid water. Finally, here we are, the advanced species that evolved step by step from the very water that asteroids and comets brought or the one that came as a result of the death of some star or a quasar in interplanetary dust.

Ashwin Khadka is a PhD Scholar in Nano Energy and Thermofluid Lab in Korea University, Republic of Korea under Korean Government Scholarship Program. He has a Masters Degree in Physics from Tribhuvan University, Kathmandu, Nepal. He is a science enthusiast, researcher and writer.