Origin of Water: Where did Earth’s water come from?

Most of Earth’s water is in the oceans. But where did Earth’s water come from?

Some of it came from inside the planet. When Earth formed 4.6 billion years ago, it was a hot, molten blob. As it cooled, water vapor escaped from the interior and condensed in the atmosphere.

But the oceans hold much more water than could have come from Earth. So, where did the rest come from?

The prevailing theory is that a giant object, perhaps a comet or a meteor, slammed into Earth about 4.5 billion years ago. The impact vaporized the ocean that was already here and sent water vapor shooting into the atmosphere. This new water eventually condensed and fell back to Earth, creating the oceans we have today.

This theory explains why the oceans are salty. When water vapor condenses, it leaves behind any minerals it was carrying. So the water that fell back to Earth after the impact was saturated with salt.

It also explains why Earth’s water is the same as the water on other planets in our solar system: comets and meteorites were likely responsible for delivering water to all of them.

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