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動画の字幕をクリックしてすぐ単語の意味を調べられます！
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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.
コツ：単語をクリックしてすぐ意味を調べられます！

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【TED-Ed】【ミニ知識】地球の水はどこから来た？（Where did Earth’s water come from? - Zachary Metz）

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稲葉白兎 2015 年 3 月 29 日 に公開    Tomomi Shima 翻訳    Chiaky チェック

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