How wet is the Earth?

It seems like it should be easy to figure out the amount of water on the Earth, right? Just sum up the volume of the oceans and youโ€™re 99% done. In fact, you might have seen graphs like this showing how moons like Europa have lots more water than the Earth.

That would seem to work except thereโ€™s a problem; minerals. Many minerals can hold water in their structures. Some of these minerals occur at the Earthโ€™s surface; clay minerals, for example, hold large amounts of water that can be released if the minerals are heated. The same is true for minerals that sit deeper down.

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