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  • It has no taste, color or smell, and we often look right through it.

  • It covers over 70% of the Earth,

  • cycling from the oceans and rivers to the clouds and back again.

  • It even makes up about 60% of our bodies.

  • With all this water around and inside us,

  • it's easy to take its presence for granted.

  • But in the rest of the solar system, liquid water is almost impossible to find.

  • So how did our planet end up with so much of this substance

  • and where did it come from?

  • As you probably know,

  • a water molecule consists of two basic parts.

  • Hydrogen, the simplest of all elements,

  • has been around since close to the beginning of our universe.

  • Oxygen entered the scene several hundred million years later

  • after stars began to form.

  • The massive pressure at the center of these fiery infernos was so great

  • that hydrogen atoms fused together to form helium.

  • Helium, in turn, fused to form heavier elements,

  • like beryllium, carbon and oxygen in a process known as nucleosynthesis.

  • When stars eventually collapsed and exploded into supernovas,

  • these new elements were spread across the universe

  • and combined into new compounds, like the now familiar H2O.

  • These water molecules were present in the dusty cloud

  • that formed our solar system

  • and more collided with our planet after its formation.

  • But there's a big question that we don't have the answer to:

  • how much water arrived on Earth, and when?

  • If, as one theory goes,

  • relatively small amounts of water were present on Earth when the rock formed,

  • the high temperatures and lack of any surrounding atmosphere

  • would have caused it to evaporate back into space.

  • Water would have been unable to remain on the planet

  • until hundreds of millions of years later

  • when our first atmosphere formed through a process called outgassing.

  • This occurred when molten rock in the Earth's core

  • released volcanic gasses to the surface,

  • creating a layer that could then trap escaping water.

  • So how then did water get back to the planet?

  • Scientists have long suspected

  • that much of it was brought by ice-bearing comets,

  • or more likely asteroids that bombarded the Earth over millions of years.

  • Recent research has challenged this theory.

  • In examining carbonaceous chondrite meterorites

  • that formed shorty after the birth of our solar system,

  • scientists have found that not only did they contain water,

  • but their mineral chemical composition matched rocks on Earth

  • and samples from an asteroid that formed at the same time as our planet.

  • This suggests that the Earth may have accumulated

  • a substantial amount of water early on that was able to stay put,

  • despite the lack of an atmosphere,

  • though asteroids may have brought more over the eons.

  • If this turns out to be true,

  • life may have formed much earlier than previously thought.

  • So we do not yet definitively know whether the water on Earth

  • came from its initial formation,

  • later impacts,

  • or some combination of the two.

  • Regardless, the water that runs from our showers, drinking fountains and faucets

  • is something that didn't just come from a nearby lake or river,

  • but first underwent a cosmic and chaotic journey to get here.

It has no taste, color or smell, and we often look right through it.

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B1 US TED-Ed water earth formed planet solar system

【TED-Ed】Where did Earth’s water come from? - Zachary Metz

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    稲葉白兎 posted on 2015/03/29
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