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The Development of Earth's Hydrosphere

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

What is Earth's hydrosphere? And where did all that water come from? Learn how scientists answer these questions, and then take a quiz to test your knowledge.

What is the Hydrosphere?

If aliens were approaching Earth right now, what would be the first thing they'd see? From millions of miles away, Earth appears as a small, blue dot. The earth has been described as the blue planet, because it's covered in water -- a full 71% of Earth's surface is covered with water. It is by far Earth's most distinctive feature, though we sometimes don't think about water a lot since we're land mammals. But how did it get that water?

The blue planet
The blue planet

Earth's hydrosphere is made up of all the water on Earth's surface, from oceans and seas to lakes and rivers. By some definitions, you should even include the water in clouds. But the earth hasn't always been here - like the entire universe, it formed billions of years ago. In this lesson, were going to talk about how Earth's hydrosphere developed.

The water cycle shows the ways that water moves between different parts of the hydrosphere: rain, evaporation, gravity, etc.
The water cycle shows the ways that water moves between different parts of the hydrosphere: rain, evaporation, gravity etc.

Development of the Earth's Hydrosphere

Scientists are pretty confident that around 3.8 billion years ago, Earth had a similar amount of water compared to today. And we know that Earth formed about 4.6 billion years ago. It is the period in between these dates where Earth's hydrosphere formed.

Earth as a whole formed when a 'disk' of rock, dust, and ionized particles condensed and came together. Some of these particles and rocks may have contained up to 20% water from elements that formed in previous, now-dead stars. In these early days, as the heavy elements started to sink towards the center to produce Earth's iron core, Earth was larger and the surface was far enough away from the center to be significantly cooler. This allowed lightweight water to outgas, finding its way out of rocks and rising towards the surface. The low temperatures at the surface made it harder for water to escape into space.

Once the earth as a whole cooled and contracted enough for the surface to reach a stable state below 100° C (the boiling point of water), the water would have condensed to form an ocean.

An example of how a solar system forms from a disk.
An example of how a solar system forms

The chemical mix found in the ocean changed gradually over time as Earth settled into a state more like what we know today. The liquid water dissolved minerals from rocks in the same way it does today, releasing sodium, magnesium, iron, and calcium to make a salty ocean. The hydrosphere was probably similar to modern water from around 1.5 to 2 billion years ago; it seems it took a while to settle.

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