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Did Earth’s First Water Come from Comets, Volcanoes or Asteroids?

First Water - Origin of Water Volcanoes or Comets

Earth’s first water source

You drink, bathe, and swim in water. But what if I told you that it all came from melted comets? Or from an asteroid? Would you believe it?

The 3 main theories for the origin of Earth’s water are:

  • MELTED COMETS: Water on our surface is from melted comets.
  • INSIDE EARTH: Water came from inside the rocks that made up the Earth itself.
  • ASTEROIDS: Asteroids mixed with ice transported water to Earth.

We have 326 million trillion gallons of water on Earth. We figure Earth’s water originated over 4 billion years ago in the Hadean Eon. But where did it all come from?

Hadean Eon Timeline

Is Earth’s source of water from melted comets?

Comet Anatomy

The first theory is that water comes from inside our solar system. If you look around it, there’s plenty of water in the solar system.

For example, 2/3 of Uranus and Neptune are ice. And the Oort Cloud is comets. They’re far enough from the sun that they weren’t blasted by the sun. So it stayed as ice.

If you look at the surface area of Earth, water covers about 71% of the planet. Some are salt water and some are freshwater.

But if you look at Earth’s mass including its interior, water accounts for only 0.02% of the planet’s mass. It’s just a wet coating on the surface. Because water is very abundant in space, comets needed to transport only 0.02% of this mass to Earth. This makes water from comets a plausible theory.

So how did the space ice get here? Comets could have transported it like a cargo ship. But water on Earth is different from water found in comets. That’s right! There are different types of water depending on how many protons and neutrons. Deuterium or “heavy water” is an isotope of water. Deuterium has an extra neutron in it. And comets have a higher ratio of deuterium. For every 10,000 water molecules on Earth, there are 3 deuterium atoms.

From studying comets like the Rosetta Comet, they have a higher ratio of deuterium. Because there’s a higher concentration of deuterium in comets, it makes this theory less likely. But we haven’t measured many comets. And other comets are more consistent with Earth.

Did volcanoes ‘degas’ water from Earth’s interior?

Volcano Degassing

The simplest explanation is often the best. It’s the theory that water existed inside the rocks that made up the Earth itself.

Because Earth’s interior contains minerals with hydrogen and oxygen, volcanoes continually ‘degassed’ or ‘outgassing’ releasing H2O as water vapor.

During Earth’s early history, magma flowed from the mantle to the crust from tectonic activity. Heat flow was nearly 3 times as high as it is today.

Because of the intense heat, water would have evaporated into space. But Earth’s surrounding atmosphere gradually cooled down. Earth’s surface cooled below the boiling point of water. Instead of water evaporating into space, rain began draining into ocean basins.

Did water originate from our solar system’s asteroid belt?


The last source is the asteroid belt between the planet Mars and Jupiter. There’s ice scattered all around the asteroid belt.

Ice is mixed in with asteroids. Asteroids are 20% water with liquid water jumbled inside the rock. Asteroids are an even better candidate because the ratio of deuterium to normal hydrogen water is much closer to the water we have on Earth.

But some asteroids match the atomic properties of water we have on Earth. And others don’t.

Earth’s first oceans

Earth is the only planet we know that has liquid water. But Earth wasn’t always the big, blue marble that we know today. When Earth formed, it was hot and dry. It was blasted by solar radiation. Because of the intense heat, any water would have evaporated into space. So this means water must’ve come later.

Later, Earth started developing an atmosphere. Because we’re wrapped in a blanket of air (oxygen, nitrogen, etc.) it helps keep the water inside. There wasn’t an atmosphere when Earth first formed so it must’ve been dry.

Earth was born with water, dust, and rubble from an early supernova. But most of it evaporated into space. So this means the source of water came from somewhere else. If you leave your laundry to dry. Then, you come back, and it’s wet. There must have been a source of water.

Whether water came from comets or volcanoes, Earth’s cooling was necessary. And it was over millions of years this process continued. Water condensed, formed oceans, and remained. And because Earth had oceans, this is where life could begin. Eventually, evolution began with the last universal common ancestor called LUCA.

Did Earth’s First Water Come from Comets, Volcanoes, or Asteroids?

Earth has water, but where did it come from? There are many different theories about where Earth’s earliest water came from.

Scientists have debated whether Earth’s first water came from comets, volcanoes, or asteroids.

If you have any questions or comments, please use the comment form below to let us know what your concerns are. We’re always here to help.

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  1. I need some help. Why aren’t the exact dates listed for when the first water appeared?

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