Where Did Earth’s Water Come From
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Earth’s Unique Water Composition
Unlike every other planet in our solar system, Earth’s surface is 70% liquid water. This is not only useful for life but also somewhat peculiar. Everything we know about how and when our planet formed indicates that Earth’s surface should be bone dry.
The Formation of Our Solar System
The story of our solar system’s formation begins with the collapse of a large cloud of dust and gas. The dense blob of gas at the center ignited to form the sun, which, as a young, unstable star, unleashed a fierce solar wind. Over time, this stream of charged particles pushed the remaining gas cloud farther and farther out, leaving only solid particles behind. These particles clumped together into rocks, planetesimals, and finally, the rocky planets of the inner solar system that we know today.
The Problem with Water Formation
Here’s the problem: water, in the form of ice, couldn’t have been one of the solid particles that stuck around. The early inner solar system was far too hot for frozen water, and any water vapor would have been blasted away by the solar wind. So if Earth didn’t start off with water, how did we end up with such splendid oceans?
The Source of Earth’s Water
We know H2O wasn’t manufactured here over the eons, because natural processes like combustion, breathing, and photosynthesis create and destroy roughly equal amounts of water. Furthermore, the amounts in question are so minuscule that they can’t account for the abundance of water on the planet. Since Earth’s water was neither part of the original package nor manufactured here, it must have flown in from far away, on meteoroids or comets or other bodies originating in the outer solar system where they were far enough from the Sun for frozen water to survive.
Comets and Carbonaceous Chondrites
Comets, being dirty iceballs, are a logical candidate for the source of our water, but were ruled out when we discovered that they are far richer in heavy hydrogen than Earth water. For every million hydrogen atoms in Earth water, about 150 are heavy ones, while typical comet water has twice that many. These mismatched chemical signatures suggest that Earth’s water could not have arrived on comets.
It turns out that the most likely source for Earth’s water is a type of meteorite called a carbonaceous chondrite. “Chondrite” is just the name given to the class of stony meteoroids that most commonly strike the Earth. But only the carbonaceous chondrites contain water – as well as lots of carbon, if you couldn’t tell from their name. They have water in them because they formed out beyond the sun’s “frost line”, and what’s more, their water has levels of heavy hydrogen similar to that of earth water, strongly suggesting that these earth-crashers are the source of our ice caps, clouds, rivers, and oceans.
The Origin of Earth’s Water
And thus, the water that turned our planet into a blue marble came, quite literally, out of the blue.
Discussion Questions
- Why is Earth’s composition of liquid water considered peculiar compared to other planets in our solar system?
- How did the solar system’s formation contribute to Earth’s potential lack of water?
- Why couldn’t water, in the form of ice, have been present during the early stages of the inner solar system?
- How do natural processes like combustion, breathing, and photosynthesis impact the abundance of water on Earth?
- What possible sources from the outer solar system could have brought water to Earth?
- Why were comets initially thought to be the source of Earth’s water, and why were they ruled out as a potential source?
- What are carbonaceous chondrites, and why are they the most likely source of Earth’s water?
- How does the presence of heavy hydrogen in the water found in carbonaceous chondrites support the theory that they are the source of Earth’s water?
Lesson Vocabulary
earth’s – belonging to or associated with the planet Earth – The earth’s atmosphere is made up of various gases.
unique – being the only one of its kind; unlike anything else – The snowflake had a unique pattern that had never been seen before.
water – a transparent, odorless, tasteless liquid that is essential for all forms of life – The plants need water to grow and thrive.
composition – the nature of something’s ingredients or constituents; the way in which a whole or mixture is made up – The composition of the painting was a perfect blend of colors and shapes.
formation – the process of forming or being formed – The formation of a rainbow occurs when sunlight is refracted through raindrops.
solar system – the collection of eight planets and their moons in orbit around the sun, together with smaller bodies in the form of asteroids, meteoroids, and comets – Our solar system includes planets like Earth, Mars, and Jupiter.
problem – a matter or situation regarded as unwelcome or harmful and needing to be dealt with and overcome – The math problem seemed difficult at first, but with practice, it became easier to solve.
source – a place, person, or thing from which something comes or can be obtained – The library is a great source of information for research papers.
comets – a celestial object consisting of a nucleus of ice and dust and, when near the sun, a “tail” of gas and dust particles pointing away from the sun – Halley’s Comet is one of the most famous comets that can be observed from Earth.
carbonaceous chondrites – a type of stony meteorite that contains organic compounds and water – Scientists study carbonaceous chondrites to learn more about the early solar system.
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- Categories: Earth & Space Science, TED Ed, Video Lessons
- Keywords: carbonaceous chondrites, comets, composition, earth, formation, problem, solar system, source, unique, water
