Where Did The Earth’s Water Come From?


43. Where Did the Earth’s Water Come From? I’m Fraser Cain, the publisher of Universe
Today. This question comes from Andrew Bumford and
Steven Stormont. In a previous episode I’ve talked about how
the entire Solar System collapsed down from a cloud of hydrogen and helium left over from
the Big Bang. And yet, we stand here on planet Earth, with
all its water. So, how did that H20 get to our planet? The hydrogen came from the solar nebula, but
where did the oxygen come from? Here’s the amazing part. The oxygen came from stars that lived and
died before our Sun was even born. When those stars puffed out their final breaths of oxygen,
carbon and other “metals”, they seeded new nebulae with the raw material for new worlds. We owe our very existence to the dead stars
that came before. When our Sun dies, it’ll give up some of its
heavier elements to the next generation of stars. So, mix hydrogen together with this donated
oxygen, and you’ll get H20. It doesn’t take any special process or encouragement,
when those two elements come together, water is the result. But how did it get from being spread across
the early Solar System to concentrating here on Earth, and filling up our oceans, lakes
and rivers? The exact mechanism is a mystery. Astronomers
don’t know for sure, but there are a few theories: Idea #1: impacts. Take a look at the craters on the Moon and
you’ll see that the Solar System was a busy place, long ago. Approximately 3.8 to 4.1 billion years ago
was the Late Heavy Bombardment period, when the entire inner Solar System was pummeled
by asteroids. The surfaces of the planets and their moons
were heated to molten slag because of the non-stop impacts. These impactors could have been comets or
asteroids. Comets are 80% water, and would deliver vast
amounts of water to Earth, but they’re also volatile, and would have a difficult time
surviving the harsh radiation of the young Sun. Asteroids have a lower ratio of water, but
they could protect that water a little better, delivering less with each catastrophic impact. Astronomers have also found many hybrid objects
which contain large amounts of both rock and water. It’s hard to classify them either way. Idea #2 is that large amounts of water just
came directly from the solar nebula. As we orbited around the young Sun, it passed through
the water-rich material in the nebula and scooped it up. Gravitational interactions between the planets
would have transferred material around the Solar System, and it would have added to the
Earth’s volume of water over hundreds of millions of years. Of course, it’s entirely possible that the
answer is “all of the above”. Asteroids and comets and the early solar nebula all delivered
water to the Earth. Where did the Earth’s water come from? Astronomers
don’t know for sure. But I’m sure glad the water is here; life here wouldn’t exist without
it. Thanks for watching.

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