The National Science Foundation.
NARRATOR: A hellish, fiery wasteland.
A molten planet hostile to life.
Yet somehow, amazingly, this is where we got our start.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] How?
How did the universe, our planet,
how did we ourselves come to be?
How did the first sparks of life take hold here?
Are we alone in the cosmos?
Where did all the stars and galaxies come from?
These questions are as ancient as human curiosity itself.
And on Origins, a four-part NOVA miniseries,
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] we'll hunt for the answers.
This search takes unexpected twists and turns.
Imagine meteors delivering Earth's oceans from outer space.
( loud explosion )
Descend into a toxic underworld,
where bizarre creatures hold clues to how life got its start.
And picture the view when the newborn Moon,
200,000 miles closer to Earth than today,
loomed large in the night sky.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] This cosmic quest takes us back in time
to within moments of the Big Bang itself
and retraces the events that created us,
this place we call home,
and perhaps life elsewhere in the cosmos.
Coming up tonight, the beginnings of planet Earth.
MAN: If you look under your bed, you find that little bits of dust
collect together into dust balls.
And something like that
must be what happened in the solar system, too.
NARRATOR: What started as a giant ball of debris floating in space
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] turned into Earth.
But 4 1/2 billion years ago,
it wasn't exactly "home sweet home."
MAN: The earth, at some point, was totally molten--
a big droplet of melt just floating in space.
How did it change from a raging inferno like this
to a place we all know and love?
It seems a series of massive disasters
were the best thing to hit the infant planet.
MAN: We all hear about the impact
65 million years ago that wiped out the dinosaurs.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] You're getting that kind of impact
something like once a month on the early Earth.
NARRATOR: And more clues are embedded within these rocks--
fragments left over from the first hours of Earth's life.
MAN: Very little is left behind
from the earth's earliest time period.
But what is left behind has revealed to us
a planet much more complicated than we ever thought.
NARRATOR: New discoveries rewrite the story
of how our planet was born, on this episode of Origins
on NOVA right now.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] NARRATOR: In its infancy, Earth was a primeval hell...
a lifeless planet
bombarded by massive asteroids and comets.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] The Moon, much closer to Earth, loomed large in the sky.
Instead of water,
red hot lava streamed across the surface of our planet.
Volcanoes spewed noxious gases into the primitive atmosphere.
Scorched and battered, Earth was a planet under siege.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF]
Yet somehow, the world we call home
emerged from these violent origins.
So how did Earth make such an astonishing transformation?
How did it change from a raging inferno like this...
to a place we all know and love--
with firm ground beneath our feet,
air we can breathe,
and water covering nearly three-quarters of its surface?
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] A place where life could take hold
and evolve into complex organisms like you and me?
Well, it turns out Earth became a habitable planet
only after a series of devastating disasters
in its early years.
And to see how this happened, let's imagine
<http://www.pbs.org/nova/origins/life.html>[type:PROGRAM][name:a brief history of life][0B27] all of Earth's 4 1/2 billion-year history
condensed into a single day--
just 24 hours on an ordinary clock or watch
like this.
If we start right now,
then the first humans walked the earth only 30 seconds ago.
Dinosaurs began roaming the planet just before 11:00 p.m.
The first multicelled animals evolved at 9:05.
Before that, mostly single-celled organisms existed,
and we think the first of those appeared
around 4:00 in the morning.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] Earth was born at midnight on this 24-hour clock,
4 1/2 billion years ago.
But its violent history began well before that
when huge, ancient stars
that had reached the ends of their lives exploded.
These supernovas cooked up
all the chemical elements we know today
including iron, carbon, gold,
and even radioactive elements like uranium.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF]
Over time, gravity took hold, and this cloud of stardust
collapsed into an enormous rotating disc,
the solar nebula.
In the center of this disk, temperature and pressure rose,
and a star-- our sun-- was born.
Eventually, gases like hydrogen and helium
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] would be swept to the far reaches of the disc.
But closer to the Sun were dust grains
made of the heavier elements.
MAN: They're circling around
the early Sun in little racetracks,
and occasionally grains traveling nearby will collide.
Something like this happens in your house.
If you look under your bed,
you find that little bits of dust
are collecting together into big, large dust balls.
And something like that must be what happened
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] in the solar system, too.
If they collide slowly, it can add up to a larger object
and gradually grow.
TYSON: With enough collisions, dust grew into pebbles
and pebbles grew into rocks.
And as the rocks grew larger, so did the collisions.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] ZOLENSKY: If they collide head on
or at higher velocities, they'll actually break apart.
Like shooting a gun at a wall.
TYSON: But other times, the rocks stuck together.
And the larger they got, the stronger their gravity became.
Because of the gravitational attraction
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] between these bodies, you coalesce.
Instead of just making a mess-- and you do make a mess as well--
you build bigger things,
because gravity holds things together.
TYSON: In time, gravity shaped them into small, round planets,
or planetesimals-- just a few miles across.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] ZOLENSKY: Gradually they grow
from golf-ball size to rugby-ball size
and then house size, then township size.
And then one or two of these objects will get large
faster than anything else
and become the big boys on the block.
TYSON: Eventually, some of these planetesimals
grew as big as our moon.
And then... they combined
to form the four small, rocky planets
closest to the Sun:
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] Mercury, Venus, Mars and Earth.
But the early earth bore little resemblance
to the planet we're all familiar with.
And today, working out exactly what Earth was like
as a newborn planet is no easy task.
It's like looking at me as an adult
and trying to figure out what I was like as a baby.
( baby crying )
When was I born?
How much did I weigh?
Now, a snapshot will give you a pretty good idea
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] of what I looked like when I was young.
But the earth was born
4 1/2 billion years ago,
and hardly anything survives from that time
to tell us about our planet's infancy.
That's because at midnight on the clock,
the newborn planet was nothing but a fiery ball of rock
covered with lava.
STEVENSON: As you go back to these very earliest times,
the first few hundred million years,
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] the earth was so energetic
and was recycling materials so vigorously
and melting material
that rocks from that period have not survived.
TYSON: So to reconstruct the story of Earth's infancy,
we look for clues, not from the ground but from outer space.
More than a hundred million miles from Earth,
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] between Mars and Jupiter,
lies a region called the asteroid belt.
Here, trillions of asteroids--
enormous rocks left over from planet building--
are held in orbit.
Every now and then, a fragment of one of these asteroids
is knocked out of orbit
and set on a collision course with Earth.
Called meteors, they can have a big impact.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] ( news theme music )
This exclusive report is about an object from space
buried in ice, described as a scientific mother lode.
We take you first to the northwest corner of British Columbia
near the Alaska border.
TYSON: Here, a massive meteor plunged through the atmosphere,
leaving a streak across the sky.
A local bush pilot discovered the debris
scattered across this lake,
which was frozen over at the time.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF]
Realizing the importance of the find,
he mailed a few fragments
to NASA meteorite expert Michael Zolensky.
ZOLENSKY: He sent samples down frozen in a case.
I had a real problem getting through U.S. customs
because they wanted to thaw these out.
They were concerned that they contained
deadly pathogens from Canada or something.
TYSON: Zolensky immediately recognized it as a carbonaceous chondrite,
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] a carbon-rich meteorite
formed from the very same stardust that built the earth.
ZOLENSKY: The last time we had a major fall of a carbonaceous chondrite
was 30 years ago, so that means it's about one time in a career
you have this happening to you,
and to have it happen to me in my career
while I was still young enough to take advantage of it
was a very exciting thing for me.
TYSON: A team of scientists scrambled
to collect as much of the meteorite as possible.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] This was the opportunity of a lifetime.
More than 400 fragments strewn across the frozen lake
could each contain clues to the very beginning of Earth.
The scientists hoped
that inside, the fragments would be uncontaminated,
in the same pristine condition as when they formed
4 1/2 billion years ago.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] If it lives up to expectations,
this meteorite could reveal the exact chemistry
of the dust grains that built the newborn Earth.
STEVENSON: Meteorites are a window on the past.
And they tell us something about the conditions
in which the solid planets formed.
ZOLENSKY: This particular meteorite is really special.
In the first place, it has
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] the highest carbon content of any meteorite
and the highest amount
of these preserved interstellar stardust grains
of any meteorite
and it has a very high water content as well.
TYSON: In addition, about 90 other elements have been identified.
And already they are providing
a chemical fingerprint of early Earth.
Oxidation payload...
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] TYSON: And within this meteorite are radioactive elements
that decay at a precisely known rate,
allowing scientists to calculate the meteorite's age.
And since most meteorites formed at the same time as the planets
and from the same material,
the age of the meteorite gives you the age of Earth
and its neighbors.
ZOLENSKY: If you date meteorites,
what you find is that almost all meteorites have the same age,
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] about 4.55 billion years old.
They're all the same, it's pretty monotonous.
Within a couple of tens of millions of years
to hundreds of millions of years,
they all have exactly the same age.
And so what we do is take the oldest of these ages
and use that as the initial age of the solar system.
TYSON: That narrow range of ages indicates
that all meteorites and planets coalesced extremely quickly
in the early days of the solar system.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF]
But Earth had barely taken shape
before the first of several major disasters
struck the young planet.
Earth's gravity was pulling in
huge quantities of debris from space.
A continual bombardment
that generated enormous amounts of heat on the surface.
At the same time, radioactive elements
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] trapped deep within the earth were decaying,
producing even more heat
and roasting the planet from the inside.
The combined effect was catastrophic.
By eight minutes after midnight on our 24-hour clock,
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] the planet had become a raging furnace.
And when the temperature reached thousands of degrees,
dense metals such as iron and nickel
in Earth's rocky surface melted.
The outer part of the earth
would have been completely molten.
We call that a magma ocean.
It's a liquid rock ocean hundreds of kilometers thick.
ZOLENSKY: We think the earth at some point was a big droplet of melt
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] just floating in space.
When you have a totally molten object like this,
the heaviest elements, and that includes things like iron,
would sink to the center of this droplet,
and the lightest elements,
things rich in carbon and water for instance--
your light elements-- would float to the top
and float there like algae on a lake.
TYSON: The global migration of the elements,
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] known as the "iron catastrophe,"
would have a profound effect on the future of our planet.
The sinking iron accumulated at Earth's center,
where it created a molten core twice the size of the Moon.
The liquid iron is constantly swirling and flowing.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] And even today, this motion generates electric currents
which turned our planet into a giant magnet
with north and south poles.
The core is still in constant motion.
And we can see evidence of Earth's liquid iron core
on the cold, snowy wastes of arctic Canada.
( yelping )
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF]
MAN: The magnetic field is constantly fluctuating
on a minute-to-minute or even second-to-second basis,
and one result of this is the fact
that it causes the magnetic pole
to actually move randomly over the course of a day.
TYSON: Every few years, geologist Larry Newitt sets out
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] in search of the precise location
of the magnetic north pole, or north on a compass.
Newitt spends days at a time on the ice
in temperatures as low as minus 50 degrees Fahrenheit.
The geographic North Pole is in a fixed position.
But the magnetic pole is always on the move.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] Over the last century,
its position has changed dramatically.
NEWITT: Get going.
TYSON: To identify the pole's current position,
Newitt measures the strength and direction
of the magnetic field at about eight different sites,
then closes in on it.
NEWITT: 89... 13.0.
Since we don't know where the pole is,
we can't just go there and take a reading,
so we surround it, and then I determine its location
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] by a process of, well, what amounts to triangulation.
2-6-8... 35.0.
TYSON: At the time of the most recent survey,
the pole had moved 125 miles off the Canadian coast.
And Newitt and his colleagues
have discovered something curious.
Its movement is picking up speed.
NEWITT: Over much of the past hundred years
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] it's been around ten kilometers per year.
But since about 1970, it's started to accelerate
and now it's moving along at about 40 kilometers per year.
If this keeps up, it'll reach Siberia
in about another 40 or 50 years.
But, of course, that's a rather dangerous extrapolation;
we don't really know where it's going to go.
TYSON: Without Earth's liquid iron core, life would be in trouble.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] This swirling ball of molten iron
is what generates the magnetic field around our planet.
And we need that magnetic field
because every day, a deadly stream
of electrically charged particles bombards the earth.
Ejected by the Sun in monstrous solar flares...
these particles hurtle through space
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] at about a million miles an hour,
forming what is known as the solar wind.
Some think that if the solar wind ever reached our planet,
it would strip away the atmosphere.
But Earth's magnetic field creates a protective shield
that deflects these deadly particles.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] And you don't have to travel far
to see the fate of a planet that lost its shield.
Four billion years ago, Mars had a liquid iron core
and a magnetic field just like Earth's.
Mars built up a thick atmosphere
and supported liquid water on its surface.
The planet may have even been home
to primitive forms of life.
But Mars is just a fraction the size of the earth,
so it cooled more rapidly.
And as it cooled, its molten iron core hardened.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] As a result, Mars stopped generating its magnetic shield
and, according to one theory,
this left its atmosphere to be scoured away by the solar wind.
Today, the surface of Mars is a barren desert.
Mars is a stark reminder of what our world could have become
if its iron core had cooled,
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] because without a magnetic shield,
a planet is left prey to the solar wind
and life as we know it could never flourish.
The time had reached 16 minutes after midnight.
The iron catastrophe was over.
But even with the formation
of Earth's core and magnetic shield,
our planet remained a hostile and alien world.
Volcanoes spewed clouds of noxious gases,
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] and the planet was enveloped in a suffocating atmosphere
of carbon dioxide, nitrogen and steam.
With no oxygen to breathe
and no ozone layer to block the lethal ultraviolet radiation,
this was not a hospitable place for life,
at least life as we know it.
And in the midst of this hellish brew, the Moon was born.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF]
Beginning when I was about 11 years old,
I used to climb the stairs
to the roof of this apartment building
where my family lived, here in New York City,
a building prophetically named the Skyview Apartments.
And with simple binoculars, just like these,
I'd gaze up above the streetlights
beyond the buildings and into the night sky.
And nothing will ever capture the excitement I felt
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] when I first turned my binoculars on the Moon.
When I saw that the Moon was packed
with mountains and valleys and craters,
I thought I'd discovered an entire new world.
And then I began to wonder:
Where did the Moon come from and how did it get there?
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF]
Well, little did I know that about the same time,
the mystery of the Moon's origin was also attracting
the attention of a scientist named Bill Hartmann.
HARTMANN: I'm always looking at the Moon and thinking about its phases.
And when I was a little kid, I had a telescope--
I used to be out there drawing craters on the Moon
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] and was very excited I could even see these craters
and mountains and so on,
so it's always had a special interest for me.
TYSON: Hartmann has been studying the Moon for the last 40 years.
And when he began his career in the late 1960s,
he and many other planetary scientists hoped
that NASA's Apollo missions
would solve the mystery of how the Moon formed.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] HARTMANN: One of the pitches
to sell that program scientifically
was that we were going to be able to go to the Moon
and find these old rocks from 4.5 billion years ago
and they were going to tell us everything
about the origin of the Moon.
MAN ( over radio ): Old Orion is finally here, Houston.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] Fantastic!
TYSON: The Apollo astronauts collected hundreds of rocks
from the Moon's surface.
ASTRONAUT: Look at the size of that biggie.
ASTRONAUT 2: It is a biggie, isn't it?
This... this one right here?
That's it-- you got it right there.
TYSON: Scientists calculated their age using radioactive dating.
To their astonishment, they discovered that the Moon
was millions of years younger than Earth.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] And those same rocks held another secret.
HARTMANN: I think the biggest single surprise
was that the materials on the Moon
had exactly the same chemistry as the earth
and different from any samples that we have
anywhere else in the solar system.
So that pretty well forced the idea
that the Moon has to have formed from the same basic material
as the earth.
TYSON: But even more mysterious
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] was that the Moon rocks contained very little iron,
just like the rocks on Earth's surface.
In a flash of inspiration,
Hartmann and a colleague came up with a controversial new theory
for the formation of the Moon.
HARTMANN: We came up with this very simple idea
that maybe as the earth was forming
at our distance from the Sun,
somewhere nearby, made out of the same material,
was a second largest body, which got pretty big
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] before it finally plowed into the earth.
TYSON: They proposed that about 50 million years
after Earth had formed,
a huge planetesimal was still roaming the solar system.
This massive rock, about the size of Mars,
slammed into our planet.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] The energy of that impact was so great,
it melted both the planetesimal and Earth's outer layers.
The two fused together, forming a new, larger Earth.
At the same time, this enormous collision
ejected into orbit vast amounts of molten rock.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF]
This debris eventually coalesced to form the Moon.
When Hartmann first went public with this idea in 1974,
it was considered scientific heresy.
HARTMANN: So here we come in saying
the Moon formed out of this gigantic catastrophe
that blew off part of the earth's mantle.
No one wanted to hear that.
No one wanted to, uh... start thinking
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] about that kind of model.
All of us were taught as junior geology students
that all processes in geology are slow--
one sand grain at a time-- erosion and so on.
And people would actually come to us and say,
you know, we really shouldn't consider that model
until we've exhausted all other models.
TYSON: Ten years passed before anyone would take the idea seriously,
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] and that was only after hundreds of computer simulations
showed that the Moon could have formed from a giant impact.
Today, Hartmann's big idea is almost universally accepted.
HARTMANN: So it's been
a long, slow process, and it's been really fun
to see, you know, a little idea that you had a long time ago
suddenly blossom forth as a... as a leading theory.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] TYSON: It was 16 minutes past midnight--
50 million years after our planet was born--
and the Moon had arrived.
But the repercussions of this disaster
were just beginning to be felt.
The Moon started out
about 200,000 miles closer to Earth than it is today
and appeared many times larger in the sky.
Earth was spinning much faster,
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] making each day just six hours long.
And with the Moon so close,
its gravitational pull on Earth was enormous.
Earth's surface rose and fell up to 200 feet
during the cycle of the Moon's phases.
Over time, Earth's rotation slowed down
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] as the Moon drifted away--
a process that continues even today.
HARTMANN: The idea of being able to measure
the movement of the Moon away from the earth
has always been a challenge.
And so when the astronauts went to the Moon,
one of the things they did is they carried out this big device
which was a reflector-- a retro-reflector--
that would beam a laser beam
back in the direction that it came.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] TYSON: On Earth, astronomers installed a laser so strong,
it could target the reflectors.
In 1969, they made their first measurement
of the time it took for the laser beam to reach the Moon,
hit the reflector and bounce back to Earth--
a round trip of about 2 1/2 seconds.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] HARTMANN: Doing this year after year after year,
we've actually been able to confirm
that the Moon is moving slowly away.
We not only get very exact information
on the orbit of the Moon,
but we can actually see the orbit change.
TYSON: Now about 240,000 miles from Earth,
the Moon is moving away at a rate of 1 1/2 inches every year.
The collision that created the Moon
was also a major stroke of luck for Earth.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF]
That impact was so immense
that it forced Earth's axis to tilt in relation to the Sun,
causing the familiar seasons.
And without the stabilizing influence of the Moon,
Earth would wobble dramatically about its axis.
Today, the planet would experience
wild climate swings.
But when did a planet that looks like the earth we know
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] begin to take shape?
Earth's hot molten surface
took at least a billion years after the Moon was created
to cool and form a thick skin, its crust...
or so scientists believed.
( thunder crashing )
But no one knew for certain
because Earth is such a geologically restless place
that none of the original crust survives today.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] ( explosion )
Yet startling new evidence is causing a major rethinking
of when Earth's crust first formed.
The clues to this mystery are embedded within these rocks
<http://www.pbs.org/nova/origins/earth.html>[type:PROGRAM][name:the origins game][39E0] in Western Australia.
Here, geologists have extracted tiny crystals called zircons.
About the size of sand grains,
zircons are nearly as tough as diamonds.
These relics of the early earth
formed when molten rock cooled into solid crust.
So the age of the zircon gives you the age of the crust itself.
And it was here that geologist Simon Wilde hit pay dirt
when he found one crystal so old
he's convinced it was formed in the earth's original crust.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF]
WILDE: When we look at the chemistry in detail
from the zircons in this rock,
we find that it's consistent
with having grown in a piece of continental crust.
TYSON: Radioactive dating shows
that the oldest of the zircons Simon Wilde found in these hills
is 4.4 billion years old,
suggesting that Earth might have cooled and created a crust
long before the Moon formed.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] WILDE: We don't know, of course,
whether the continental areas were extensive
or whether they were just small, little islands of material,
but certainly what we do know is that there was continental crust
at 4.4 billion years ago.
TYSON: This was just 150 million years after Earth was born,
not a billion years, as previously thought.
But that led to another mystery.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] Once Earth was cool enough to form solid ground,
water could collect on its surface.
So when did that happen?
Geologists, including Stephen Mojzsis,
think the answer may lie in these same tiny zircon crystals.
Zircons are extremely rare,
so to find just a few crystals, Mojzsis had to pulverize
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] and sift through hundreds of pounds of ancient rocks.
And analysis of the chemical composition of the crystals
revealed that the oldest zircons
contained a high concentration of a curious ingredient.
It was a type of oxygen called oxygen-18,
an isotope that could only be present in large quantities
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] if the zircon crystals had grown in water.
The news that water might have been present
so early in Earth's history was a bombshell.
MAN: Not only was there crust present,
which came as a surprise to most of us,
it looks like, from some of the zircons,
that that crust interacted with large volumes of liquid water.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] TYSON: The idea that water settled on Earth's surface so soon
is controversial, but if true, it suggests a planet
much more like today's than anyone had ever imagined.
MOJZSIS: By 200 million years after the formation of the earth,
you can imagine a landscape of islands and small continents
bathed by a primitive ocean.
TYSON: The time was only ten minutes to 1:00 in the morning.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] The Moon existed and so did a planet
with not just land but water.
Liquid water is the key to life.
Every living thing requires it to survive.
And eventually, water would cover
nearly three-quarters of the earth's surface.
In fact, all the world's oceans contain
nearly 100 million trillion gallons of it.
That's an almost incomprehensible amount.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] So where did it all come from?
How would Earth have ended up
with such vast quantities of the stuff?
Well, strange as it sounds,
these great oceans may have been there
from the very beginning... just hidden away.
One key to the riddle was volcanoes,
which, throughout Earth's infancy,
pumped huge amounts of steam into the atmosphere.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF]
Then, as Earth cooled, that steam condensed into rain.
Drop by drop, water collected in low-lying areas.
STEVENSON: There is nothing mysterious or surprising about this.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] The earth does it right now.
The main gas that comes out of Hawaiian volcanoes
is water, steam.
So this is happening all the time.
TYSON: But some scientists argue
it would take far too long to create such vast oceans
by volcanic out-gassing.
Instead, Earth may have had some help.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] ( whoosh, then explosion )
The water in our oceans might have come from outer space,
delivered to the surface by massive ice-bearing comets.
The evidence for these ancient impacts
is impossible to find today
since the original surface of our planet
has long since been eroded or destroyed.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] But there's one place that preserves a record of impacts
from that early era... our moon.
Every one of those craters
was a meteorite explosion at some time.
There's a nice one with fractures.
Another bigger crater out here, small craters...
TYSON: The Moon's surface is littered with craters,
some of them hundreds of miles across.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] In fact, the Moon was ravaged
by more than a million major impacts in its early years.
Since Earth is much more massive,
its gravitational pull would have attracted
even more debris,
resulting in possibly tens of millions of impacts.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] HARTMANN: We all hear about the impact 65 million years ago
that wiped out the dinosaurs.
And you're getting that kind of impact
something like once a month on the early earth.
But this rain of debris
left over from the formation of the solar system
continues for several hundred million years.
TYSON: And in this cosmic debris field,
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] comets, containing huge amounts of dust and ice,
would have been plentiful.
Like dirty snowballs the size of mountains,
roughly half their mass was water.
One NASA scientist, Michael Mumma, wonders
if these comets were the source of the water in Earth's oceans.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF]
MUMMA: One possibility is that Earth's water
was delivered by the impact of bodies from beyond the earth.
These would naturally be the comets,
which are rich in water.
The proof in that would be
to measure the composition of the cometary water
and to compare that
with the composition of water in our oceans.
TYSON: But studying comets is a tricky business.
In the last 20 years,
just a handful have passed close enough to examine in detail,
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] including one in 1997 called comet Hale-Bopp.
MUMMA: A comet like Hale-Bopp
would deliver about ten percent of the water
needed to fill one of the Great Lakes.
This is a lot of water.
Of course the oceans are much larger,
and so we need many more comets to fill the oceans.
But we're fortunate.
We had many such comets in the early solar system,
so we have every reason to believe
that it was cometary delivery
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] that brought water to the early earth.
TYSON: Mumma thinks that the heat of an impact
would have evaporated the ice within a comet,
creating storm clouds over vast areas of the planet.
These clouds produced a deluge of hot, possibly acidic, rain
that continued for millions of years.
( thunder crashing )
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF]
At first the rain would have formed lakes and rivers,
and eventually water would cover almost the entire globe.
But there's a problem with this theory.
Earth's oceans contain a mixture of normal water, H2O,
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] and a much smaller amount of a more exotic kind, known as HDO,
or heavy water, which contains an extra neutron.
In the comets analyzed so far,
the proportions of these two kinds of water
don't match the composition of water in our oceans.
MUMMA: They have twice the amount of heavy water
that we see in Earth's oceans,
so if they were the comets that delivered Earth's oceans,
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] they wouldn't fit the bill, basically.
They don't have the right properties.
TYSON: But Mumma hasn't given up.
The comets already studied
come from the outer reaches of the solar system.
And he thinks comets originating closer to the Sun
might be different.
Formed at higher temperatures,
these comets could have a lower proportion of heavy water,
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] more closely matching our oceans.
And tonight, Mumma hopes to test this idea
by getting a firsthand look at one of these elusive comets.
MUMMA: If its chemistry is different
and if the heavy water to light water
is like that on Earth,
it would be the first proof positive,
or the smoking-gun evidence, that comets did in fact
deliver water to the early earth.
( conversing quietly )
TYSON: But first, the team has to hunt down the comet.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF]
As soon as he has acquired it,
we should see an image of it on this screen.
There it is, all right, yes, sir, right there.
You can see the elongated material
flowing outward from the nucleus.
Joel, that looks excellent.
TYSON: With the comet in the crosshairs of their telescope,
they can home in on the kind of water it's carrying.
Bring up the spectrograph.
Yeah.
Moment of truth's here.
Ah, yep.
MUMMA: People often ask, "How can you measure water
in an object that is 100 million miles away?"
We do this by a method called spectroscopy.
It's a little bit like taking fingerprints.
The little ridges on your fingers
look different for every person.
And it is the same way the light that is emitted
by a given molecular compound is different--
<http://www.pbs.org/nova/origins/spectra.html>[type:PROGRAM][name:decoding cosmic spectra][E080] it emits at different wavelengths.
MUMMA: Dark spot.
I think we just need more.
TYSON: But it turns out this comet is a very dirty snowball indeed.
There's so much dust on the surface
that it can't reflect enough light
for the team to find out what kind of water is on board.
MUMMA: It did not brighten as expected;
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] this was a bit of a disappointment.
Comets are quite fickle, you know, they're unpredictable.
In some ways they're like cats.
They both have tails and they both do what they want to.
TYSON: But with astronomers finding two or three comets a year
from the inner part of the solar system,
Mumma could soon have another chance
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] to test his controversial ideas
about the origin of Earth's oceans.
MUMMA: One of the key things that every scientist keeps in mind
is you should never fall in love with your theory;
so it's an idea, it's a hypothesis,
it fits all the known facts, but it has not yet been proven.
And we must be willing to give it up and modify it
if it is not proven.
But we will learn something in doing so.
STEVENSON: It's still possible that comets played a role.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] In fact, it's hard to imagine that they played no role.
But it seems more likely and more physically sensible
to look closer to home for the source of the water.
TYSON: Besieged by volcanoes and battered by impacts...
Earth endured its most extreme punishment in its early years.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF]
It was beaten, bombarded, mangled and melted,
all in just the first hour
of our 24-hour history of the planet.
The young Earth was still very different
from the planet we know today.
It was a hostile and forbidding place...
with an atmosphere full of poisonous gases.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF]
Yet somehow these harsh conditions set the scene
for a crucial phase of Earth's development: the origin of life.
MOJZSIS: Very little is left behind
from the earth's earliest time period,
but what is left behind has revealed to us
a planet much more complicated than we ever thought,
with different rock types, liquid water present
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] and the kind of planet that we might expect life to emerge on.
Do we know if life was around 4.3 billion years ago?
Who can say?
We can say, however, that the template,
the ground underfoot was there.
Could life have been present?
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF] Why not?
TYSON: But first, the once hellish Earth would have to undergo
another change as radical as any that had come before.
Catastrophe and cataclysm transformed the earth.
Now our planet would be ready
for the greatest drama of all time, the rise of life.
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Earth Is Born][63AF]
Next time on NOVA...
Life takes hold on Earth,
but where did it come from?
And how did it survive
in the hostile environment of early Earth?
MAN: To take hold,
life might have needed a safe haven, perhaps underground.
Astrophysicist Neil deGrasse Tyson takes you back in time
in search of the recipe for life on Earth,
on "Origins," a NOVA miniseries.
To order this program on VHS or DVD,
<http://www.pbs.org/nova/origins>[type:PROGRAM][name:NOVA: Origins: Back to the Beginning][D337] or the book,
Origins: Fourteen Billion Years of Cosmic Evolution,
please call 1-800-255-9424.
Captioned by Media Access Group at WGBH access.wgbh.org
NOVA IS A PRODUCTION OF WGBH BOSTON.