H_2O-chemists might call it dihydrogen monoxide, or even oxidane. But most of us just know it by its street name: water. In a contest for the world's most versatile molecule, H_2O would surely rank near the top. Almost every weather textbook displays a diagram of the hydrological cycle: water evaporates from the sea, forms clouds that then bring rain and snow to the plains and mountains, and then returns to the oceans via rivers in an ever-repeating process. In some ways it is just that basic, but closer examination reveals how water's many unusual properties make for a vastly more complex and fascinating journey. Water is abundant in our universe: one nebula in Orion produces 60 times the volume of Earth's oceans every day. In our own solar system, deep layers of ice or subsurface oceans likely envelop three moons of both Jupiter and Saturn, and just maybe Neptune's moon Triton. Our Blue Planet is 71% covered by water. It is a matter of debate as to how Earth's 330 million cubic miles of water came to be here. Perhaps it was delivered by icy comets lured from the deep space Oort cloud, or from the water-rich minerals of asteroids bombarding our molten protoplanet, later released by innumerable volcanic eruptions. Regardless, the water molecule is essential to our world.
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