Did you know that 70% of our planet is covered in water? This includes the vast oceans, the clouds in the sky, the ice caps at the poles, and even the water inside our bodies. However, only a small portion of this is fresh water. If we gathered all of Earth’s water into a single sphere, it would be about 860 miles (or 1,385 kilometers) in diameter. Water is essential for life on Earth, making our planet truly special.
Was Earth always filled with water? If not, how did it get here? Let’s explore the origins of Earth’s water.
Our solar system began as a cloud of gas and dust that collapsed into a swirling disk. Some of this cloud contained water. In the area where Earth formed, the young sun’s heat was too intense for water vapor to condense on its own. However, tiny dust grains acted as ‘condensation nuclei,’ allowing water droplets to form. These particles, along with the water they carried, eventually came together to form our planet.
As Earth cooled, chemical reactions between rocks in the crust and the early atmosphere created more water molecules. Inside Earth’s mantle, water existed not as H2O molecules but as hydrogen and hydroxyl ions within the rock. It’s still uncertain how much water remains trapped inside Earth, but there could be several oceans’ worth hidden within minerals or even as ice inside diamonds deep underground.
While some of Earth’s water was formed here, much of it likely came from elsewhere in the solar system. Asteroids known as ‘carbonaceous chondrites’ contain water molecules and their basic components, oxygen and hydrogen. In the early solar system, Jupiter might have been closer to the Sun, and its gravity could have sent these watery asteroids toward Earth. Comets, which are rich in water ice, might have also collided with early Earth, bringing more water.
How do scientists determine whether our water came from asteroids or comets? Each water molecule has two hydrogen atoms and one oxygen atom. Some hydrogen atoms have an extra neutron, making them ‘heavy.’ By comparing the ratios of heavy to normal hydrogen in water, scientists found that Earth’s water matches more closely with asteroid water than comet water. Although we can’t be 100% certain, it seems that most of our water came from asteroids.
But where did all this water originate before the solar system? Astronomers have found water throughout the universe, in gas clouds between stars and in other planet-forming disks. Around a supermassive black hole, they’ve even detected a water reservoir 140 trillion times heavier than Earth’s oceans. This cosmic water must have existed at least 12 billion years ago.
The universe’s hydrogen was created during the Big Bang, but oxygen is formed inside stars. Water could have formed as soon as the first stars ignited, about 12.7 billion years ago. Some of the oxygen atoms in the water we drink today might be from these early stars.
Why is understanding the history of water important? The earlier water existed in the universe, the sooner watery planets could form, providing more time for life to evolve. This knowledge helps astronomers estimate the likelihood of life elsewhere in the universe.
Curious about how astronomers search for other worlds? Brilliant.org offers courses on astronomy, including lessons on how scientists explore planets beyond our solar system. These lessons are designed to make learning engaging and accessible, encouraging curiosity and a deeper understanding of the cosmos.
Stay curious and keep exploring the wonders of science!
Using materials like a clear plastic container, soil, small plants, and water, create a mini water cycle model. Observe how water evaporates, condenses, and precipitates within your model. Document your observations and explain how this cycle is essential for maintaining life on Earth.
Research the differences between asteroids and comets, focusing on their composition and role in delivering water to Earth. Create a presentation to share your findings with the class, highlighting why scientists believe asteroids contributed more to Earth’s water supply.
Investigate how water is detected in space. Use online resources to learn about the methods astronomers use to find water in distant galaxies. Write a short report on one significant discovery of water in space and its implications for understanding the universe’s history.
Participate in a class debate on the importance of understanding the history of water in the universe. Prepare arguments for why this knowledge is crucial for estimating the likelihood of life elsewhere and how it impacts our understanding of planetary formation.
Create a comic strip that illustrates the journey of a water molecule from its cosmic origins to becoming part of Earth’s water supply. Use creativity to show the molecule’s travels through space, its arrival on Earth, and its role in the water cycle.
Here’s a sanitized version of the provided YouTube transcript:
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Seventy percent of Earth is covered in water. That includes every drop in the oceans, clouds, ice caps, and even in our bodies. However, even less of that is fresh water. All of Earth’s water combined would fill a sphere only 860 miles (about 1,385 kilometers) in diameter. Water is one of the things that makes Earth special, and without it, life as we know it couldn’t exist.
Was our home always a water-rich planet? If not, how did it all get here? What do we really know about Earth’s origins of water?
[INTRO]
The solar system formed when a cloud of gas and dust collapsed into a swirling disk, and at least some of that cloud contained water. In the orbit where Earth would eventually be, the young sun’s intense heat prevented water vapor from condensing on its own. However, tiny grains of dust acted as ‘condensation nuclei’ to allow droplets to form. These tiny debris particles coalesced into our young planet along with the water they carried.
As Earth cooled, rocks in the crust reacted with chemicals in the early atmosphere to create more water molecules. Water existed inside Earth’s mantle, but not as independent H2O molecules; rather, it separated into hydrogen and hydroxyl ions in the rock. We don’t know exactly how much water is still locked inside Earth today, but there may be several oceans’ worth of primordial water embedded within minerals or even as ice inside diamonds deep underground.
This explains some of Earth’s water, but much—if not most—of the water on the surface today was imported from farther out in the solar system. One type of asteroid called a ‘carbonaceous chondrite’ contains water molecules and their raw ingredients, oxygen and hydrogen. During the early years of the solar system, we think Jupiter may have orbited closer to the Sun than it does today. Its gravity would have flung thousands of these watery asteroids toward the young Earth. Comets also hold a lot of water ice, and there are billions of them zipping around the solar system. If they crashed into early Earth, they could have brought water too.
So how can we tell if most of our water came from asteroids or comets? Every water molecule contains two hydrogen atoms and one oxygen atom. However, a tiny fraction of the hydrogen atoms in water have an extra neutron in their nucleus. Comet water and asteroid water have different ratios of these heavy hydrogens to normal hydrogens. The atomic fingerprint of the water we find on Earth doesn’t quite match what we see in comets, but the water in asteroids is a closer match.
Like many things in science, we can’t be 100% sure yet, but so far it seems that most of our water came from asteroids. So Earth probably got its water in multiple ways: some was built into the planet, Earth made a bit on its own, and the rest was delivered by space rocks.
But where did all this water come from before the solar system? Astronomers have detected water all over the cosmos. They’ve seen it in clouds of gas between stars and in other planet-forming disks. Around one supermassive black hole, they’ve even detected a reservoir of water 140 trillion times heavier than Earth’s oceans. This space fog is far enough away that it, and its water, must have existed at least 12 billion years ago, and maybe earlier.
So how old is the universe’s oldest water? All of the universe’s hydrogen was made during the Big Bang, but oxygen is only produced inside the nuclear furnace of stars. So water could have formed as soon as the very first stars in the universe ignited, about 12.7 billion years ago at the earliest. A tiny fraction of the air we breathe—and the water we drink today—contains some of these very first oxygen atoms.
Why do we care so much about the history of something as seemingly mundane as a glass of water? The earlier there was water in the universe, the earlier there could have been watery planets. And the earlier there were watery planets, the more time there could be for life to evolve. That’s a refreshing thought.
Stay curious. Thank you to Brilliant.org for supporting PBS Digital Studios. Water has been essential to the development of life on Earth, but knowing the age of water and where water might exist in our galaxy is one of the best clues astronomers have for figuring out the odds of extraterrestrial life.
How do astronomers actually look for other worlds, though? If you’re eager for answers to that and other astronomy questions, Brilliant has a course all about astronomy. It includes a breadth of knowledge about the cosmos, and lessons like “Worlds Beyond Earth” teach you the tools that astronomers use to search for and study planets outside our own solar system.
Brilliant’s lessons are broken up into bite-sized pieces, with instruction designed to guide you through problems. Learning about astronomy is about more than just facts and memorization. Staying curious is about learning how to learn anything.
Brilliant – Math and Science Done Right, proud to support It’s Okay to Be Smart. To learn more about Brilliant, go to Brilliant.org/BESMART.
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This version maintains the original content while removing any informal language or unnecessary repetition.
Water – A transparent, tasteless, odorless, and nearly colorless chemical substance that is essential for most plant and animal life on Earth. – Water covers about 71% of Earth’s surface, mostly in seas and oceans.
Planet – A celestial body moving in an elliptical orbit around a star, such as the Earth orbiting the Sun. – Earth is the third planet from the Sun in our solar system.
Solar – Relating to or determined by the sun. – Solar energy is harnessed from the Sun’s rays and can be used to generate electricity.
Asteroids – Small rocky bodies orbiting the sun, mostly found between the orbits of Mars and Jupiter in the asteroid belt. – Scientists study asteroids to learn more about the early solar system.
Comets – Celestial objects consisting of a nucleus of ice and dust, which, when near the sun, develops a “tail” of gas and dust particles pointing away from the sun. – Halley’s Comet is one of the most famous comets and is visible from Earth every 76 years.
Hydrogen – The lightest and most abundant chemical element in the universe, often found in stars and gas giants. – Hydrogen is a primary component of the Sun, where it undergoes fusion to produce energy.
Oxygen – A chemical element that is essential for the respiration of most living organisms and is a major component of Earth’s atmosphere. – Oxygen makes up about 21% of Earth’s atmosphere and is crucial for life.
Universe – All existing matter and space considered as a whole; the cosmos. – The universe is vast and contains billions of galaxies, each with millions of stars.
Stars – Luminous celestial bodies made of plasma, held together by gravity, and generating light and heat from nuclear reactions at their core. – Our Sun is a star that provides the necessary heat and light to sustain life on Earth.
Life – The condition that distinguishes animals and plants from inorganic matter, including the capacity for growth, reproduction, and continual change preceding death. – Scientists search for signs of life on other planets by looking for water and organic molecules.
