How long has there been water on Earth?
How and when in the history of Earth did the first water appear?
Water has always been present on Earth, in one form or another. It was present, as ice and in water-bearing minerals called clays, in the planetesimals that the Earth was built from, and more water arrived in the form of comets. Some water, in the form of vapor and ice crystals, has always been present in the atmosphere of the Earth. Water vapor is also released from the Earth's interior by volcanoes.
But, I am being a little coy . . . I am guessing that you are wondering when Earth first had liquid water. The answer is probably earlier than you think! The Earth is about four and a half billion years old. It's difficult to study what the Earth was like back then, because the Earth's crust is constantly being recycled, and very few rocks are that old. However, we do have a little bit of rock that is 4,400,000,000 years old, found on the continent of Australia. This rock shows evidence that the surface of the Earth was solid and cool when this rock solidified, just 600,000,000 years after the Earth was formed!
There was even liquid water present where this rock solidified!
So as far back as we have geological evidence of what the Earth was like, there was liquid water present.
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