The mysterious origin of water on Earth

Either the Moon-forming impact was insufficient to vapourize all existing water or brought to Earth by some other means.

Nearly 70 percent of the Earth’s surface is covered with water and scientists are confused about the origin of this vitalizing liquid. The extreme conditions present during the formation of the Earth might have made it impossible for water to exist. Several past studies have provided evidence that water was brought to the Earth by impacts of heavenly bodies from the early solar system. New research suggests that asteroids, as well as the gas from which the solar system was formed, might be responsible for the origin of the water. 

Our solar system was formed roughly 4.5 billion years ago from a massive cloud of gas and dust called the solar nebula. Mercury, Venus, Earth, and Mars are rocky planets formed in the inner solar system during a chaotic phase of collapsing gas, dust, and rock collisions. This process of collapsing and collision is referred to as accretion.

The conditions immediately following the creation were volatile inside the inner solar system. The continuous bombardment of meteorites and space debris created from the gas-dust cloud, strong volcanic eruptions, and so on, onto the inner planets were common. The high temperatures must not have allowed a stable environment for water, ice, or other volatile compounds. Furthermore, one theory claims that the Earth was impacted by a Mars-sized rock during the early times, resulting in the formation of the Earth’s satellite, the Moon.

“It was long thought that Earth’s water did not originate from the planet’s region of the protoplanetary disk. It was hypothesized that water and other volatiles must have been delivered to Earth from the outer Solar System later in its history.” – Kevin J. Zahnle, Marko Gacesa, David C. Catling

Theories about the origin of water

For a long time, Earth scientists have been pondering this subject. What is the actual source of water? If the water was formed with the formation of Earth, wouldn’t it have been destroyed by the Moon-forming impact? Or if it was present in the material which accreted into Earth, why is there a difference in isotropic ratios in Earth’s oceans and water in the space rocks?

Two things can be deduced from this. Either the Moon-forming impact was insufficient to vapourize all existing water or brought to Earth by some other means!

Key Concepts

The key here is to know about hydrogen since hydrogen is the primary ingredient of water. Scientists basically need to know the one trustworthy source of hydrogen. In order to know that, the researchers have to consider all reservoirs of hydrogen present on the Earth.

 Secondly, it is vital to know the chemical fingerprint of water on Earth and in our solar system. In our case, chemical fingerprint means the ratio of Deuterium to Protium – isotopes* of hydrogen. We will use the term “isotropic ratio” or “[D/H] ratio” to denote the chemical fingerprint from now on.

*Isotopes of an element contain atoms with the same number of electrons and protons but a different number of neutrons. An ordinary hydrogen or protium atom has one electron and a single proton. Deuterium contains one electron, a proton, and a neutron in its nucleus.


Comets are bodies of frozen gas, rock, dust, and water. A theory proposed comets as the source of water. Chemical analysis of Halley’s Comet back in 1986 revealed otherwise. The [D/H] ratio was measured to be inconsistent with Earth’s oceans’ existing [D/H] ratio. Actually, it was higher as compared to that of the Earth. The theory about comets delivering water to Earth can be ruled out as the deuterium to hydrogen measured in comets is higher than in Earth’s oceans. Furthermore, additional missions to other comets showed that the [D/H] ratio was too high in the comets. According to simulations, comets are expected to contribute less than 10% of the water delivered to Earth.

A theory proposed comets as the source of water. Chemical analysis of Halley's Comet back in 1986 revealed otherwise.
A theory proposed comets as the source of water. Chemical analysis of Halley’s Comet back in 1986 revealed otherwise.

Accumulation after moon-forming impact

Another theory has been proposed to explain the mystery of the origin of the water. It suggests that the water on Earth was delivered later after the moon-forming impact. Research revealed that the matter accreted after the moon forming impact was found to be 1%. This implies that this new material accumulated after the effect was very rich in water content, or this theory does not fit well with the observations.

Asteroidal Impacts

In the early solar system, the conditions were quite chaotic. One explanation for the heavy bombardment of asteroids on earth was that in the early solar system, giant gas planets, i.e., Jupiter, Saturn, etc. migrated from outer areas of the solar system towards their current position. This migration perturbed the asteroid belt and triggered the asteroid bombardment. Models of the early solar system accurately reproduce the conditions present in the early solar system and are also consistent with observations. 

A class of asteroids called the Carbonaceous Chondrites, or C-type, was thought to be the major contributor to bringing water to the newly formed Earth. Analysis of these chondrites showed that the [D/H] ratio in the asteroids is consistent with that of the Earth’s ocean. Furthermore, a group of space rocks originating from one of the largest asteroids, Vesta, known as Eucrite Chondrites, have a similar ratio of heavy to ordinary hydrogen.

However, this hypothesis could not explain the lesser [D/H] ratio in the Earth’s mantle. Moreover, the analysis of the ratio of isotopes of hydrogen and noble gases in the Earth’s atmosphere and the mantle is also different. This points to the fact that the hydrogen on the Earth may not have arrived from one source. 

Now you may ask why matching the isotropic ratios from the Earth’s mantle is essential. Earlier, we mentioned in the key concepts that knowing about all the hydrogen on Earth is necessary. 

Space Weathering

Space weathering is a phenomenon in which charged particles from the sun- mostly hydrogen ions- penetrate, up to a few nanometers, the surface of space rocks and react with the elements present there.

The isotropic ratio matched some of the asteroids, but it failed to explain the less deuterium present in the deep layers of the Earth. This led researchers to believe that there must be another source of water. 

Hayabusa spacecraft, in 2010, brought back samples of rock from the Itokawa asteroid, an S-type (silicate or stony) asteroid that orbits close to the sun compared to the C-type. Scientists from the University of Glasgow utilized Atom Probe Tomography, a novel procedure to study the atomic structure of the asteroid samples one atom at a time. In the case of the Itokawa asteroid, the solar particles reacted with the Oxygen trapped below the fine-grain dust on the asteroid. With that, the team found water below the surface of the asteroid samples. They concluded from the results that one cubic meter of this space rock would contain at least 20L of water.

The water from space weathering is also isotropically light, meaning it has a lower concentration of heavy deuterium. Therefore, S-type asteroids can bring water to the Earth.

Studies showed that rocks deep within the Earth’s mantle have 25% less heavy hydrogen than ordinary hydrogen.
Studies showed that rocks deep within the Earth’s mantle have 25% less heavy hydrogen than ordinary hydrogen.

Solar Nebula

Another team of scientists explained the difference in the isotropic ratios in the Earth’s mantle. The team studied rocks deep inside the mantle brought up via volcanic activity. These rocks had the oldest water samples preserved in them. Their analysis showed that the [D/H] ratio deep inside the mantle was low and had deficient concentrations of heavy deuterium. Studies showed that rocks deep within the Earth’s mantle have 25% less heavy hydrogen than ordinary hydrogen.

They suggest that the water was formed during the formation of the Earth in the solar nebula – the cloud of gas from which the solar system was formed. The researchers also argued that the isotropic ratios could change with time. The reason for this is the lighter hydrogen gets stripped away by solar radiation from the atmosphere leaving behind the heavier deuterium and hence the higher concentration of deuterium in the Earth’s oceans. A similar analysis of lunar rock samples also showed similar results.

Mixed Recipe?

A new study backed the solar nebula theory. A group of scientists led by Peter Buseck at Arizona State University (ASU) has devised a novel suggestion. On October 9, 2018, the new peer-reviewed research was published in the Journal of Geophysical Research: Planets. They theorized that the water might have originated from the solar nebula as well as asteroids.

They modeled the formation of the Earth and suggested that the Earth formed from coalescing water-logged space rocks. During formation, the lighter or ordinary hydrogen from the solar nebula reacted with the molten iron on the early earth and sank towards the center. The heavier isotope didn’t respond and stayed in the upper layers. They also suggested that the core has the largest hydrogen reservoirs on Earth.

Additionally, they said that the rest of the water in the Earth’s oceans was brought to the Earth via asteroid bombardment.

To back this theory up, Laurette Piani and her colleagues looked at a unique space rock called Enstatite Chondrite (EC), whose composition study showed that they were formed instead in the inner side of the solar system. The study also reveals that these chondrites have enough hydrogen to be brought to the Earth. Surprisingly, the isotopic ratio of EC was also consistent with that of the Earth’s mantle. 

With these models, scientists estimated the amount of hydrogen delivered to earth. The findings revealed that the asteroidal impact and some contributions from the solar nebula account for a large portion of the hydrogen contribution.

“For every 100 molecules of Earth’s water, there are one or two coming from the solar nebula,” – Jun Wu, lead author of the study.

Significance (Why?)

Now the question you may ask is why it is essential to know the origin of the water. New planets are being discovered every day. According to the NASA Exoplanet Archive, there are more than 5 thousand potential candidates to the date of writing this article. Out of these, more than 2700 bodies have been confirmed as a planet by the Kepler mission, more than 540 confirmed by the K2 mission, and more than 250 planets confirmed by the TESS mission.  Scientists have confirmed the presence of planets in the habitable zone. 

Piani suggests these findings can extrapolate to other star systems with planets in their habitable zone. This means the potential planets orbiting within the Goldilocks zone may have undergone the same process during their formation. This also suggests that if these planets have experienced such conditions, there must be a chance of finding liquid water.

Moreover, space weathering suggests that solar radiation creates water on space rocks. This also means that there is a chance of finding water that may have formed similarly in an extrasolar system. Furthermore, this knowledge can apply to future space travel as we know that water forms in such types of space rocks.

Finally, the new results have implications for rocky exoplanets orbiting other stars. Many such worlds have now been discovered, and if there is a greater chance for some of them to have liquid water, that also increases the chances of those planets being habitable. The new work, based on computer modeling, may have implications for rocky worlds orbiting distant stars.

According to the researchers:

“Our results suggest that forming water is likely inevitable on sufficiently large rocky planets in extrasolar systems.”

As Piani tells OpenMind, “this material would have been present for the formation of the other rocky planets.”

Concluding remarks 

The origin of water has always been a mystery for scientists. Countless studies have been published to determine water’s genesis correctly. Several theories came up to explain the possibilities. Some scientists claimed that water was formed during the formation of the Earth. On the other hand, theories about water originating far away in the solar system which was brought to Earth by specific means. Most theories failed to explain the difference in the isotropic ratios of hydrogen in the Earth’s oceans and lower layers.

Finally, the scientists combined the two hypotheses of water origin, i.e., asteroids and solar nebula. This theory explains the presence of much of the Earth’s water reservoirs. Understanding the water’s origin is significant for astronomers as these theories can be extrapolated to other solar systems. With that, scientists can figure out the possibility of the existence of water in the extrasolar systems and exoplanets.


Also, read: Human Biology: the water in you!

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