Origin of Water on Earth: How Did it Get Here?

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Water is an incredibly precious resource, and it might seem pretty convenient that it’s found all over the world (water covers around 70% of the Earth’s surface), given our reliance on it for survival.

However, the Earth’s oceans, streams, rivers, and lakes are no accident. These water bodies exist because of a long list of processes that occurred way, way back – and there’s a lot we still don’t know about a time so long ago.

Here, we’ve discussed what scientists know so far about how water came to be on Earth, dating right back to the planet’s formation.

📌 Key Takeaways:

  • Earth’s water is thought to have formed on Earth shortly after its formation, around 4.5 billion years ago.
  • There are several theories about where Earth’s water came from. The main theory is that water on Earth is the result of collisions with comets and asteroids during the solar system’s early period.
  • Earth is one of the few planets formed with water because it’s at an optimal distance from the sun, its geological processes help to sustain water, and its protective atmosphere prevents water from flying off into space.

🤔 How Did Water Originate on Earth? What We Know So Far

Sadly, time travel hasn’t yet been invented, so it’s impossible to say exactly when and how water originated on our planet. However, based on scientific research, here’s what we know about the Earth’s formation, and how surface and groundwater sources came to be.

Planet Formation

Several billion years ago, when the Earth and other planets were formed, scientists believe that it all started with a protoplanetary disk (a donut-shaped disk of tiny dust grains, ice, and gas), which swirled around the sun – newly formed at that time.

The grains of gas, dust, and ice began having interactions with one another, forming clumps that grew in size and became planetesimals (small rock-type objects that play a role in planet formation).

Was there water to be found this early on? Possibly – there likely would have been water molecules in the protoplanetary disk, but during this period, temperatures were too hot, and water remained in its vaporized form. Plus, early Earth had no atmosphere, so any water vapor that did manage to condense into liquid form would have simply been carried back into space.

This aspect of Earth’s formation has scientists scratching their heads, wondering how our oceans arrived if it wasn’t possible for the Earth to form with its water bodies already in place.

Planet formation

The Influence of Comets and Asteroids

Planetary scientists believe that, if Earth’s oceans didn’t form when the Earth’s surface was formed, they would have had to arrive later via asteroids and comets.

Asteroids and comets are two distinct celestial objects in our solar system. Asteroids are rocky, metallic bodies that orbit the sun. They’re found throughout the solar system, but they’re most commonly found in the asteroid belt between Jupiter and Mars. Comets are icy bodies composed of water, gases, dust, and rocky material. They have highly elliptical orbits, often taking them far from the sun and then close to it, in the inner solar system.

Both comets and asteroids can harbor ice, and scientists theorize that in the early solar system days, these may have at least partially contributed to the depositing of water on the Earth’s surface.

But there’s a lot that scientists had to ask themselves when it came to this process. What was the timeline? Was all the water deposited on Earth at once? Or did many events happen over a longer period?

Let’s get technical.

We can get a better understanding of whether or not asteroids and comets were responsible for the Earth’s oceans by comparing the chemical makeup of the Earth to that of these celestial objects.

We know that water has 8 protons from its oxygen molecule and 2 from its 2 hydrogen atoms, totaling 10 protons. In comparison, water usually only has 8 neutrons from its oxygen molecule (nothing from its hydrogen atoms). But we also know that some isotopes of water have additional neutrons.

A 2014 study in the journal Science discussed water molecules with different numbers of neutrons, specifically focusing on those that were on meteorites that we believe fell to Earth from Vesta (an ancient asteroid). Vesta still exists today in the asteroid belt and is heavily cratered, which suggests a collision-filled past. Scientists discovered the same distribution of isotopes on the Vesta rock samples as those found on Earth. This doesn’t mean we can jump to conclusions and say for certain that Vesta was the Earth’s water source, but it does suggest that asteroids with similar compositions, that existed back in the early solar system period, were responsible.

Hyperactive Comets

If scientists became pretty certain that water on Earth came from asteroids, you might be wondering how comets fit into the equation.

This possibility comes from several research cases and studies, although originally, due to the discovery that comets have a different ratio of regular water to “heavy” water (made from deuterium, which was thought to have been formed as a result of the Big Bang) compared to Earth, it was theorized that comets could only be responsible for around 10% (at most) of the Earth’s water.

However, this theory was questioned in 2018, when planetary scientists made a close passage of the comet 46P/Wirtanen and were able to get a closer look at the comet’s surface, giving them a better understanding of its isotopic makeup. Here, the scientists found evidence that contested the original findings from previous observations: the 46P/Wirtanen had similar ratios of hydrogen and deuterium as those found in the Earth’s mantle.

How have scientists made sense of these findings? Comet 46P/Wirtanen has been classed as a “hyperactive” comet, which essentially means that as it approaches the sun, it releases more water than a regular comet, giving it isotope ratios that are closer in similarity to those found on Earth. It’s a rarer comet type, but it’s still a contender for being at least a part of the reason why early Earth eventually came to hold water.

Hyperactive comet theory

🌏 Why is Earth One of the Few Water-Bearing Planets?

So, why is the Earth one of the only planets that has large quantities of water?

Before we answer this question, we need to clarify that the Earth isn’t the only planet in the solar system that we know to have water. Planets including Neptune and Uranus, as well as other celestial bodies (including Enceladus and Europa, the famous ice moons), are also known to be water-bearing.

With that said, there are a few reasons that contribute to the abundance of water on Earth’s surface:

It’s at an Ideal Distance from the Sun

One of the reasons why Earth has abundant water sources, and why it’s able to hold onto its water, is that it’s situated in the “habitable zone” or so-called “Goldilocks zone” of our solar system. At the Earth’s distance from the sun, temperatures are optimal for liquid water to exist. It’s not too hot, like Venus, where water would evaporate, nor too cold, like Mars, where water would freeze.

Its Geological Processes Keep Water as a Liquid

We also know that Earth’s geological processes, including plate tectonics and volcanic activity, have helped maintain a stable climate and keep water in its liquid form. These processes recycle carbon dioxide and other greenhouse gases, preventing extreme fluctuations in temperature that could cause water to evaporate on a massive scale.

Plate tectonics and volcanic activity

It Has Abundant Water Sources

Another factor that probably helps Earth to sustain its water resources is simply the sheer number of sources available. Our planet has a significant amount of water stored in its oceans, lakes, rivers, and underground aquifers, which we know to be a result of billions of years of accumulation and retention.

It has a Protective Atmosphere

Finally, Earth’s atmosphere plays an important role in maintaining surface temperatures that are conducive to liquid water. Plus, our planet traps heat through the greenhouse effect, preventing excessive heat loss into space. The earth’s atmosphere allows the water cycle – the continuous movement of water around the planet – to occur.

📑 Final Word

Hopefully, this article has helped you to learn more about Earth’s water, and the strongest theories about how liquid water came to be on Earth. We still don’t know much about the time frame in which the Earth’s water came to be, and it’s likely that we’ll never know the full story of how our planet now holds such an abundance of water.

If you have any other questions about the planet’s water, check out our FAQ section below.


Where did water on Earth come from?

Although nobody knows for certain where Earth’s water came from, scientists believe that the vast majority of water was formed on the planet’s surface as a result of interactions with icy asteroids and comets several billion years ago. Another theory is that interactions between magma oceans and hydrogen present in the planet’s early atmosphere helped to form water on Earth.

How is water created in nature?

In nature, water is primarily created through a natural process called the water cycle (or hydrologic cycle). This continuous cycle begins with the evaporation of water from the planet’s surface, driven by heat from the sun. The water vapor rises into the atmosphere, where it cools and condenses to form clouds. The clouds eventually release precipitation, which falls back to the surface of the Earth and can either replenish bodies of water like rivers and lakes or seep into the ground to recharge underground aquifers. Water is also formed through chemical reactions between hydrogen and oxygen atoms in the atmosphere, where two hydrogen atoms combine with one oxygen atom to form a water molecule.

What did we drink before water?

Humans didn’t drink anything before water for one simple reason: water existed on the planet’s surface long before humans did. Water is essential to survival, so human existence would have been impossible if the Earth never accumulated water after the planet’s formation some 4.5 billion years ago.

Who first discovered water?

If you’re asking who discovered water back in the very beginning days of early life on Earth, that’s unfortunately impossible to answer. It’s likely that living beings were drinking water instinctively to sustain life rather than discovering it as some sort of miracle nectar. If you’re wondering who discovered the composition of water, we have a few scientists to thank for this, including Henry Cavendish (1731 – 1810), who discovered hydrogen, and then went on to discover water’s composition a few years later.

When did it become safe to drink water?

Water has always, in a physical sense, been safe to drink for humans. Water did not become adapted to be suitable for humans to drink; instead, humans were always adapted to drink water as a survival resource. But if you’re wondering when the possibility of drinking water containing microorganisms was no longer a threat, water chlorination was introduced after 1908, and this method of water disinfection is still used widely today.

How did early humans not get sick from water?

We can only guess how early humans avoided getting sick from water. One theory is that they used natural filtration methods like sand or charcoal. Or, they may have consumed water from animal bladders or stomachs, which acted as natural containers and may have had some antibacterial properties. With that said, we shouldn’t assume that waterborne diseases simply weren’t a risk back then – it’s likely that they still contributed to high mortality rates in early human populations.

What year was the first water?

We can’t say exactly when water first came to be on Earth during the early solar system days because this period was so long ago (around 4.5 billion years to be exact). However, according to specialists in planetary science, liquid water existed on Earth shortly after it was formed, around 4.5 billion years ago.

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