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It's a common phrase we hear: "The amount of water on Earth is constant." It sounds simple, but how do scientists know this to be true? The answer lies in the water cycle—a continuous, global process that moves and recycles water.
The idea that water is constant is based on the principle of a closed system. For the most part, Earth doesn't lose or gain significant amounts of matter, including water, from space. The water we have today is the same water that existed billions of years ago. It just moves around and changes form.
The idea that water is constant is based on the principle of a closed system. For the most part, Earth doesn't lose or gain significant amounts of matter, including water, from space. The water we have today is the same water that existed billions of years ago. It just moves around and changes form.