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  • Unlike every other planet in our solar system, Earth's surface is 70% liquid water,

  • which while useful for life, is also kind of weird,

  • because everything we know about how and when our planet formed says Earth's surface should be born dry.

  • The story goes like this: our solar system formed from the collapse of a large cloud of dust and gas.

  • The dense blob of gas at the center ignited to form the sun,

  • which as a young, unstable star unleashed a fierce solar wind. Over time this stream of charged particles

  • pushed the remaining gas cloud farther and farther out, leaving only solid particles behind

  • to clump together into rocks, planetesimals,

  • and finally, the rocky planets of the inner solar system that we know today.

  • And here's the problem: water, in the form of ice, couldn't have been one of the solid particles that stuck around,

  • because the early inner solar system was far too hot for frozen water,

  • and any water vapor would have been blasted away by the solar wind.

  • So if Earth didn't start off with water, how did we end up with such splendid oceans?

  • We know H2O wasn't manufactured here over the eons, because natural processes like combustion,

  • breathing and photosynthesis create and destroy roughly equal amounts of water - and either way,

  • the amounts in question are so miniscule that they can't account for the abundance of water on the planet.

  • Since Earth's water was neither part of the original package nor manufactured here,

  • it must have flown in from far away, on meteoroids or comets or other bodies originating in the

  • outer solar system where they were far enough from the Sun for frozen water to survive.

  • Comets, being dirty iceballs, are a logical candidate for the source of our water,

  • but were ruled out when we discovered that they are far richer in heavy hydrogen (that's hydrogen

  • with a neutron as well as a proton in its nucleus) than Earth water.

  • For every million hydrogen atoms in Earth water, about 150 are heavy ones, while typical comet water has

  • twice that many. These mismatched chemical signatures suggest that Earth's water

  • could not have arrived on comets.

  • It turns out that the most likely source for Earth's water is a type of meteorite called

  • a carbonaceous chondrite. "Chondrite" is just the name given to the class of stony meteoroids

  • that most commonly strike the Earth. But only the carbonaceous chondrites contain water

  • - as well as lots of carbon, if you couldn't tell from their name. They have water in them

  • because they formed out beyond the sun's "frost line", and what's more, their water has levels

  • of heavy hydrogen similar to that of earth water, strongly suggesting that these earth-crashers

  • are the source of our ice caps, clouds, rivers, and oceans.

  • And thus the water that turned our planet into a blue marble came, quite literally,

  • out of the blue.

Unlike every other planet in our solar system, Earth's surface is 70% liquid water,

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