Where Did Earth's Water Come From?

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0:00Unlike every other planet in our solar system, Earth's surface is 70% liquid water, which

0:05while useful for life, is also kind of weird, because everything we know about how and when

0:10our planet formed says Earth's surface should be bone dry.

0:13The story goes like this: our solar system formed from the collapse of a large cloud

0:17of dust and gas. The dense blob of gas at the center ignited to form the sun, which

0:22as a young, unstable star unleashed a fierce solar wind. Over time this stream of charged

0:28particles pushed the remaining gas cloud farther and farther out, leaving only solid particles

0:33behind to clump together into rocks, planetesimals, and finally, the rocky planets of the inner

0:38solar system that we know today.

0:40And here's the problem: water, in the form of ice, couldn't have been one of the solid

0:44particles that stuck around, because the early inner solar system was far too hot for frozen

0:49water, and any water vapor would have been blasted away by the solar wind.

0:53So if Earth didn't start off with water, how did we end up with such splendid oceans? We

0:57know H2O wasn't manufactured here over the eons, because natural processes like combustion,

1:03breathing and photosynthesis create and destroy roughly equal amounts of water - and either

1:08way, the amounts in question are so miniscule that they can't account for the abundance

1:12of water on the planet.

1:14Since Earth's water was neither part of the original package nor manufactured here, it

1:18must have flown in from far away, on meteoroids or comets or other bodies originating in the

1:22outer solar system where they were far enough from the Sun for frozen water to survive.

1:26Comets, being dirty iceballs, are a logical candidate for the source of our water, but

1:31were ruled out when we discovered that they are far richer in heavy hydrogen (that's hydrogen

1:36with a neutron as well as a proton in its nucleus) than Earth water. For every million

1:40hydrogen atoms in Earth water, about 150 are heavy ones, while typical comet water has

1:45twice that many. These mismatched chemical signatures suggest that Earth's water could

1:49not have arrived on comets.

1:51It turns out that the most likely source for Earth's water is a type of meteorite called

1:54a carbonaceous chondrite. "Chondrite" is just the name given to the class of stony meteoroids

1:59that most commonly strike the Earth. But only the carbonaceous chondrites contain water

2:03- as well as lots of carbon, if you couldn't tell from their name. They have water in them

2:07because they formed out beyond the sun's "frost line", and what's more, their water has levels

2:11of heavy hydrogen similar to that of earth water, strongly suggesting that these earth-crashers

2:16are the source of our ice caps, clouds, rivers, and oceans.

2:20And thus the water that turned our planet into a blue marble came, quite literally,

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