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.