Space's Deepest Secrets

The moon... Earth's nearest neighbor,

but one of the most mysterious objects in the solar system.

If you actually want to look at one of the greatest mysteries

of the universe right now, just look up at the moon.

For decades, scientists were sure

they'd solved many of the moon's deepest secrets.

But now, new evidence suggests that nearly everything

we thought we knew about the moon could be wrong.

We might need to completely rethink

our understanding of the formation of the moon.

Imagine if you could have looked up at the night sky

and saw two moons.

That would've been lit!

Why is our moon so big?

Why does it look so strange?

And why is the nearside

so totally different from the far side?

Today, pioneering technology could finally reveal answers.

Z is preparing to fire.

This is the inside story

of the race to unravel

the mysteries of our moon.

captions paid for by discovery communications

The moon...

Since mankind first looked up to the sky,

it's been there, looking back.

It remains the only place beyond our home planet

where humans have set foot.

That's one small step for a man,

one giant leap for mankind.

Without it, life as we know it probably wouldn't exist.

The moon has been the earth's companion for 4.5 billion years.

It drives the motions of the tides,

and the motions of the tides

are intricately linked to the cycle of life on earth.

The moon is vital for life on earth.

The moon is there

in the sense of a satellite of planet earth,

but it's there, also, in a sense of allowing life as we know it

to have the features that we experience.

As important as the moon is,

scientists now know that it's also extremely weird.

The first clue is its surprising size.

If there were an alien astronomer

from another solar system,

I think they would come to our planet

and look up at the moon with astonishment.

There's something about our moon

that makes it incredibly strange.

It's just too darn big.

Of all the major planets in the solar system,

earth has the biggest moon

relative to the size of the planet,

and that's very, very strange.

Large moons are rare amongst the rocky planets

in our solar system.

Mercury and Venus have no moons at all.

Mars has two tiny moons, deimos and phobos,

many hundreds of times smaller than their host planet.

It's thought they were once asteroids

that Mars ensnared with its gravitational pull.

But earth's moon is a freak.

Over 2,100 miles across,

weighing in at 77 million trillion tons,

it's more than a quarter the size of earth,

far too big to be a captured asteroid.

How did we get this giant cosmic companion?

Scientists think that the moon must have been formed

by an event of extraordinary violence.

When you look at the surface of the moon,

one thing you can see is that it's been hit a lot,

but maybe it was created from the biggest hit.

For over 30 years, one theory for the nature

of this violent birth has ruled supreme.

It's called the giant-impact hypothesis.

The main idea of how the moon formed

was that something came in, smacked the earth,

bits flew everywhere, and they eventually recondensed,

forming the moon as we see it today.

4.5 billion years ago,

when earth is still a scorched ball of hot rock,

another young planet called theia

comes too close for comfort.

As it strikes the edge of the earth at an angle,

traveling at more than 26,000 miles per hour,

theia rips off the top of the earth.

The debris forms a ring...

and over time,

the fragments fuse together into a ball of scorching rock

that eventually becomes our moon.

Scientists want to investigate

if this established theory is correct.

If the giant-impact hypothesis is true,

then we should be able to find evidence.

So they revisit clues

first gathered over 45 years ago.

In the 1960s and '70s,

Apollo astronauts collected over 830 pounds of moon rocks.

That Boulder's gonna roll.

Man, that is hard.

Just don't stub your toe.

Scientists think that these rocks

could hold the answer...

It's orange!

As to whether the conventional theory

of how the moon formed is right.

Edward young is a geochemist.

He's got his hands on a precious sample of moon rock.

We get samples like this from Johnson space center.

We request a milligram or less.

That's all we need, really, to make an analysis.

This particular sample comes from the Apollo 16 mission.

It was brought back by the astronauts.

If the conventional theory is correct,

and theia collided with the earth in a glancing blow,

then the moon should be mostly made out of theia.

It means that moon rock should be chemically different

than earth rock.

Ed designs experiments to test this prediction.

In his lab, ed has the largest and highest resolution

mass spectrometer of its kind in the world.

The machine analyzes the oxygen the rocks contain.

Every solar system body

has its fingerprint of molecular weight of oxygen.

Theia should've been different from earth,

and if theia is mostly in the moon,

we should expect to see the moon having a slightly different

molecular weight of oxygen than the earth.

Ed's colleague uses a laser

to prepare a sample for analysis.

The laser melts the rock

and causes it to react with a chemical

within the chamber, releasing oxygen gas.

What most people don't think about is that rock

is mostly oxygen. It's 90%, by volume, oxygen,

and it's about half, by weight, oxygen.

And so when we heat the sample

and melt the sample with the laser beam,

that leaves us, when we're all done,

with just oxygen remaining as oxygen gas,

and the oxygen gas is what we analyze.

The oxygen acts as a unique identifier,

the chemical fingerprint of the rock.

Ed compares the chemical fingerprint of the moon rocks

to earth rocks.

The big question... Are they different as predicted?

Well, in this plot, we're looking at our results

for samples from the earth and samples from the moon.

And on this axis of this plot,

we're seeing the real fingerprint of the oxygen

that makes up these bodies.

If the collision that formed the moon left more

theia in the moon than it did in the earth,

we would expect there to be an offset,

and what we're seeing on this plot

is that there's no resolvable difference

between the terrestrial samples from the earth

and the lunar samples.

The result is surprising.

The moon and earth rocks

have virtually identical chemical fingerprints.

It flies in the face of the conventional theory

of how the moon formed.

You have this object, theia,

that impacts with this other object, earth,

and then in the end, you have two objects,

the earth and the moon,

that are now combinations of earth and theia.

The problem is, is that we can't find evidence of theia.

It's like stumbling upon

a site of a massive collision

between what you know is two cars,

but you can't find the other car.

So is the entire theia hypothesis

just plain wrong, or does the standard theory

just need reworking for the earth and the moon

to end up with virtually identical chemical fingerprints?

One scientist might just have the answer.

Nothing comes close to realizing

that you've found something

that no one has ever thought of before.

And she's ready to put a radical new idea to the test.

The moon...

One of the strangest objects in the solar system.

Astronomers are in a race to unlock the secrets

of its mysterious origin.

They are convinced the conventional model

for how the moon formed is flawed.

But is it completely wrong?

Astrophysicist Sarah Stewart believes that the basic idea

of the giant-impact hypothesis is still correct,

but with one crucial difference.

Instead of hitting the earth in a glancing blow,

theia could've slammed right into it.

The energy of the collision partially vaporizes

the two planets,

mixing them together to create a superheated cloud

of liquefied rock and gas called a synesthesia.

As the seething mass cools,

droplets of molten rock fuse together