Water is essential for life on Earth and is one of our most precious natural resources. But considering how our planet formed, it is quite surprising how much water we still have. The Earth aggregated from a cloud of gas and dust – a protoplanetary disk – and was incandescently hot for the first few million years. Its surface was kept molten by impacts from comets and asteroids. Earth’s interior was also (and still is) kept liquid by a combination of gravitational heating and the decay of radioactive isotopes. That means that if there were any initial water (and organic…

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