Narrator: In the universe,
everything seems to orbit something.
Planets orbit stars, and moons orbit planets.
Some moons are volcanic, but the volcanoes are ice.
Others are awash with great oceans.
There may be more habitable moons in our galaxy
than there are habitable planets.
Narrator: Moons tell the unknown stories of our solar system
and show us how it all works.
In our own solar system, there are just eight planets.
But orbiting six of those planets are moons...
...lots and lots of moons... more than 300 of them.
Each one is different...
...each one a world all its own.
Dr. McKay: Well, when we look out on our solar system,
we see a lot of planets.
But even more than planets, we see moons.
And in many ways, they're more interesting
than the planets that they go around.
We have moons that are airless and apparently dead, like ours.
Then, out in the outer solar system,
we have moons with oceans inside them
and moons with atmospheres around them.
I'm for moons. You can keep the planets.
Narrator: The biggest eruptions...
...the coldest temperatures...
...and the largest oceans in the solar system...
they're all on moons.
There are moons with ice volcanoes.
There are moons with lakes of methane and methane rainfall,
smog clouds...
...moons that are so volcanically active
that they keep remaking their surface...
Moons with all kinds of plumes shooting off into space...
really a much wider range of environments
than we ever could have imagined.
Dr. McKay: Often, when I'm describing
to the general public, or even to my fellow scientists,
these moons of Saturn and Jupiter,
I call them "worlds" because they really do have
the complexity and mystery of a whole world.
Narrator: Jupiter and Saturn have over 60 moons each.
These giant gas planets and their moons
are like mini solar systems,
and each moon has a distinct personality.
Lapetus, a two-toned moon in black and white.
Titan, with a dense, orange atmosphere.
And icy Enceladus, blasting ice geysers 200 miles into space.
Each moon is unique.
But they all have one thing in common.
All moons are natural satellites,
held in place by gravity.
But moons do more than just go around planets.
They help stabilize the planets in their orbits
and keep the machinery of the solar system
running smoothly.
Dr. Grinspoon: The diversity of moons
is an interesting combination of predictable laws of science
and then complete randomness of just things smashing together
and the chips kind of falling where they did
in a way that you could never predict.
Narrator: Planets and moons begin the same way.
Once a star turns on,
there's a lot of dust and gas left over.
Slowly, the dust particles clump together, forming rocks.
The rocks smash into each other and form boulders.
Slowly, the objects get bigger and bigger.
The process is called accretion.
Dr. Durda: One can think of it as forming a snowball
and rolling it down a hill.
As it rolls down the hill,
it collects and gathers up yet more snow,
which makes it roll faster and harder.
And so that process of runaway accretion
actually happens
in the formation of the planets
and in the formation of moons, as well.
Narrator: It sounds simple enough,
but nobody knew for sure how it worked until 2003.
On the International Space Station,
astronaut Don Pettit was experimenting in zero gravity.
He put grains of salt and sugar inside a plastic baggie.
Instead of floating apart, they began to clump together.
This is how both planets and moons build up.
But instead of taking shape around stars,
most big moons take shape around planets.
If the same process makes them all,
what makes all of them so different from each other?
Take two of Jupiter's moons, Callisto and Ganymede...
...two very different moons,
each born from the same debris when Jupiter was still young.
Ganymede formed close to Jupiter,
where there was lots of debris.
Because there was so much material,
it came together quickly... in about 10,000 years...
and it was hot.
The heat separated the ice from the rock.
You can still see it in Ganymede's distinct landscape.
Dr. Hendrix: The primary factor that affects
why moons are the way they are today
is energy...
how much energy was put into them
as heat during accretion
and how much energy has been lost.
All of those factors go into telling us
why moons behave the way they do
and why they look the way they do today.
Narrator: Callisto's surface tells a different story.
It formed much farther out,
where there was less debris and less heat.
It took longer and cooled faster.
Unlike Ganymede, Callisto's surface is uniform.
Rock and ice never separated.
Where a moon forms can also mean the difference
between survival and destruction.
Get too close,
and a planet's gravity will rip a moon to shreds.
Scientists believe this is what happened to many moons
when Jupiter was young.
Dr. Durda: And it's very likely that Jupiter had
an entire conveyor belt of large moons
that were wanting to form,
only to be swallowed up by the planet itself.
The large moons we see today
are only the last ones that were able to stabilize
right at the end of that process,
stop their death spiral,
and survive into the position we see today.
Narrator: But Jupiter keeps trying to eat them.
The gravity of the giant planet
reaches out and pulls hard on the orbiting moons.
It transforms them from lifeless balls of rock
into strange and dramatic worlds.
Narrator: Jupiter is the largest planet
in our solar system.
It has 63 moons.
The four largest are called the Galilean moons,
named after the astronomer Galileo,
who discovered them in 1610.
They show how gravity controls
both what moons look like
and how they behave.
The first of the Galilean moons, lo,
orbits closest to the planet,
just 260,000 miles above Jupiter.
That's about the same distance as our Moon is from Earth.
But unlike our Moon,
the surface of lo has no impact craters.
Scientists realized that meant the surface was new.
But how could that be?
Every time you look at lo,
with a spacecraft or even with a telescope,
it's a little bit different.
So the geology on lo changes
like the weather on other planets.
It's that active.
Narrator: When NASA first sent probes to fly past lo,
they were shocked.
They saw dozens of active volcanoes.
This is footage of an erupting supervolcano on lo,
blasting 200 miles into space.
Everyone had the same question...
how could there be active volcanoes on a moon?
The answer was simple... gravity.
Jupiter's gravity is so huge
that it reaches out and crunches the moon.
And it's not just Jupiter's gravity pulling on lo.
Other nearby moons also pull on it as they pass by.
So the core of the moon
is being worked back and forth all the time.
It's called tidal friction
and generates extreme heat in lo's core.
Dr. Durda: Almost like bending a wire coat hanger until it breaks.
And you feel the inside of the coat hanger there...
it feels rather warm.
That tidal friction... that internal friction...
heats the interior of lo until it's become,
actually, one of the most volcanically active worlds
in the solar system.
Narrator: The constant pushing and pulling
generates temperatures thousands of degrees high
inside lo.
It blasts out in gigantic eruptions of lava.
Io is the prime example
of tidal forces and gravitational interactions
in the solar system.
It is constantly being pulled by Jupiter,
and it's constantly getting pulled
by the other moons, as well.
And so, as a result,
there's a tremendous amount of heat created.
Narrator: The floods of erupting lava
constantly resurface lo,
which is why there are no visible impact craters
on this moon.
Gravity also heats lo's neighbor, Europa.
Europa's orbit is farther away from Jupiter,
so it's much colder.
Instead of lava, the surface of Europa is ice.
The lowest recorded temperature in Antarctica
is minus-128 degrees.
Europa's surface is twice as cold.
But underneath all the ice,
there may be an ocean of water
heated by the same tidal friction
that makes lo volcanic.
Dr. Hendrix: Europa has a subsurface ocean,
almost certainly.
And that subsurface ocean is in contact with the rocky mantle,
which provides heat
and also provides, probably,
appropriate nutrients to sustain life.
Narrator: Someday we'll send a probe
to explore beneath the ice on Europa.
And maybe we'll discover life-forms living there
in warm European oceans.
Out beyond lo and Europa are nearly 60 more moons.
They orbit much further away from Jupiter,
where the effects of the giant planet's gravity
are much weaker.
Out here, it's too weak
to generate tidal friction and heat the moons.
So these remote worlds
are cold and barren...
But not featureless.
They bear the scars of countless collisions,
and scientists believe it was collisions that created
the most extraordinary moon system of them all.
Narrator: The planet with the most unusual moon system
is Saturn.
It's spread out over more than 200,000 miles.
Technically, there are more than a billion moons.
That's right... a billion moons.
And all together, they make up Saturn's rings.
A moon can be a hunk of rock or ice no bigger than a pebble,
as long as it orbits a planet.
The rings of Saturn are made
of countless pieces of rock and ice.
They go from the size of a pebble
up to the size of a city.
We don't refer to all the ring particles
that can get to be as big as 10 or 20 meters across.
We don't refer to them as individual moons.
But when we find a body
that is maybe a kilometer or two across,
then you can start talking about it as a moon or a moonlet.
Narrator: Saturn's rings
are one of the oldest mysteries of astronomy.
Where did they come from?
To try and find out,
NASA sent the Cassini probe on a 12-year mission
to study Saturn, its rings, and its moons.
Dr. Porco: We took, with Cassini,
probably the most beautiful picture that's ever been taken,
and I'm not the only one who has said this.
Cassini was in the shadow of Saturn, cast by the Sun,
and so you don't see the Sun.
You see the backlit planet of Saturn and its beautiful rings.
You see the refracted image of the Sun
poking out from the side of Saturn.
And nestled in all of that splendor
is this small little dot.
Narrator: That tiny dot is not a moon.
That is the distant planet Earth,
nearly a billion miles away.
Most of what we know about Saturn,
of its rings and moons, comes from Cassini.
Before Cassini, we thought there were only eight rings.
Today we can see over 30.
Dr. Porco: What we have found at Saturn
has been just literally an embarrassment of riches.
We're seeing something that we had seen before,
but now we're seeing it with a level of detail and clarity
that was just mind-blowing.
Narrator: Scientists used to think
the rings were made of the icy leftovers
after Saturn was formed about 4 billion years ago.
But anything that old
should be covered with cosmic dust, and dirty.
So why does Saturn's rings
appear bright and clean, almost new?
To get the answer,
Mission Control maneuvered Cassini close to the rings.
The probe saw that all the ice pieces in the rings
are constantly colliding and breaking up.
And each collision exposes new surfaces
that are clean and polished.
This is what astronomers think happened.
When Saturn was young,
it had no rings, just lots of moons.
At some point, an icy comet
zoomed in from deep space
and smashed into one of those moons.
The comet broke up into billions of pieces.
The impact also pushed the moon closer to Saturn,
where the planet's enormous gravity broke it up.
Now debris from the moon and ice from the comet mixed.
Gradually, Saturn's gravity
pulled all those fragments into rings around it.
The story of moons is the story of gravity.
Gravity holds them in orbit.
It heats up their insides and shapes their surfaces.
In the end, it controls everything about moons,
even their survival and destruction.
Gravity can even create new moons
by kidnapping asteroids, comets, and even whole planets.
Narrator: We know that gravity makes moons.
The standard way is to assemble them
from debris left over when planets are formed.
But gravity makes moons a second way, too.
It captures them.
Imagine a wandering comet or asteroid.
Somehow it gets knocked off course.
It wanders too close to a planet.
Gravity acts like a science-fiction tractor beam
and grabs it.
Not quite enough gravity, and it escapes.
Too much gravity, and it collides with the planet.
Just enough, and the comet or asteroid
goes into orbit around the planet
and becomes a new moon.
Mars has two tiny moons, named Phobos and Deimos.
Both are captured asteroids.
One is pushing outward as it circles the planet
and will eventually break free
and continue on its journey through space.
The other is circling inwards,
a little closer to Mars all the time.
Eventually, it'll smash into it.
This is Cruithne.
It's an asteroid, really, just three miles across.
But it's sometimes described as Earth's second moon.
Dr. Durda: With the little object Cruithne,
which was discovered back in 1986,
we start to get into this realm of...
of what does it mean to be a moon.
Narrator: Only a few thousand years ago,
Cruithne was an ordinary asteroid,
orbiting the Sun like billions of others.
But eventually, it wobbled
out of its orbit in the Asteroid Belt
and got snagged by Earth's gravity.
But then Cruithne did something unusual.
Instead of orbiting around the Earth,
like a normal moon,
Cruithne began to follow behind it.
And so one might call it a sort of a moon of the Earth...
not exactly, though, because that object is on...
you know, it's on its own independent orbit
around the Sun, not the Earth.
Narrator: Sometimes asteroids capture their own moons.
In 1993, the Galileo spacecraft
flew past the asteroid Ida
and found something nobody expected...
a tiny half-mile-wide moon.
Dr. Durda: The fact that we saw a satellite
around only the second asteroid
ever to be encountered with a spacecraft
immediately tells us
that moons around asteroids must be incredibly common.
Narrator: Not all captured moons are small.
The mother of all captured moons is Triton.
It orbits the planet Neptune, and it is big...
about 1,700 miles in diameter.
But Triton is a moon with an unusual story.
Triton was a very puzzling problem
for planetary scientists,
because our traditional view
would tend to make all the moons orbit
in the same direction that the planet itself spins.
In the case of Triton around Neptune,
it's the other way around.
Neptune is spinning this way.
Triton is orbiting around in the opposite direction.
Narrator: This means it didn't form like most moons,
out of the debris left over from the birth of the planet,
or it would orbit in the same direction.
So something wasn't right.
Dr. Grinspoon: Triton is huge, and its orbit is funny.
It's anomalous.
It does not seem as though it formed
as a part of the Neptune system.
It seems much more like a captured planet.
Narrator: Scientists now think Triton
was once a dwarf planet, like Pluto.
And a giant planet like Neptune certainly has enough gravity
to capture a moon the size of Triton.
Dr. Hendrix: Triton was almost certainly formed
way out in the outer solar system
and then at some point was captured by Neptune.
Perhaps Triton, early on, had its own moon,
they both were captured,
and then that moon was destroyed during the capture process.
Narrator: But Triton is in danger.
Neptune is dragging it closer and closer.
Eventually, it will get too close,
and Neptune's immense gravity will tear it apart.
Triton the moon will be reborn
as a ring system around the planet.
But what about our Moon?
How did it get there?
Was it captured?
The truth is even more extraordinary.
It was born in extreme violence.
Narrator: Our Moon, like a lot of moons,
is rocky, barren, and pockmarked with craters.
But in one way, our Moon is unique in the solar system.
For a long time,
astronomers thought the Moon formed
from debris left over from the birth of the Earth.
But researchers in the 1960s
came up with a radically different idea.
They suggested it came from a giant impact.
Dr. Hartmann: When we first had the idea
of forming the Moon from a giant impact,
that was not a terribly popular idea.
And I actually did have good science friends... colleagues...
coming to me, saying, you know, we really have to exhaust
all the slow evolutionary theories
before we start talking about cataclysms.
Narrator: The evidence Bill Hartmann needed
was on the Moon itself.
And the proof had to wait
until Apollo astronauts finally went there in 1969.
They brought back hundreds of pounds of Moon rocks.
Scientists analyzed the rocks and were amazed.
They were identical to rocks in the Earth's crust,
and they'd been superheated.
So, how did pieces of the Earth's crust
become superhot and wind up on the Moon?
Hartmann was pretty sure he knew.
Dr. Hartmann: This whole idea was that the Earth forms.
Now you hit it with something.
You blow all this light, rocky material off the top.
That material goes into orbit and makes the Moon.
The Moon's just made out of rocky debris.
Narrator: Lmagine our chaotic solar system
4.5 billion years ago.
The young Earth is just one
of a hundred or so new planets orbiting the Sun.
One of them is a Mars-sized planet called Theia,
and it's on a collision course with Earth.
They smash into each other
at many thousands of miles an hour.
Theia is destroyed, and Earth barely survives.
The impact blasts billions of tons of debris into space.
The Earth's gravity pulls it into orbit around the planet.
Now these hunks of leftover Earth
clump together and form our Moon.
That's the theory, anyway. But how do you test it for real?
Here at NASA's Vertical Gun Range,
they're re-creating that ancient collision in a lab.
This 30-foot-long gun fires a tiny projectile
at 18,000 miles an hour.
The projectile is Theia.
This ball represents the Earth.
By changing the angle of Theia's impact,
the team can figure out how precise
the ancient collision had to be in order to make the Moon.
In the first shot,
Theia hits the top of the Earth with a glancing blow.
So, here's the Earth, if you will, suspended in space.
And now it's gotten hit.
So, now we see the planet ejecta
is being ripped out of the Earth
and is forming this giant impact basin.
And if this really were the Earth,
this basin would be thousands of kilometers...
thousands of miles... across.
Narrator: In this simulation,
Theia only skims off the surface of the planet,
and very little debris is thrown out into space...
not nearly enough to build our Moon.
[Indistinct conversation]
[Buzzer]
[Crash]
The second shot is a head-on collision.
Dr. Schultz: Ka-pow!
That's the end of planet Earth. It's gone.
Some of the debris is gonna go out of the solar system.
Some of the debris will reaccrete
to form small planetesimals within the solar system.
Narrator: There's no Earth left,
so there's no gravity
to gather the debris and form the Moon.
Now the gun is set to just the right angle...
halfway between a glancing blow and a direct hit.
So we'll see what happens if the Earth barely survives.
Oh, oh, gorgeous! Oh, my gosh!
Ka-pow!
Now we have the entire part of the Earth
being ripped apart,
but the vapor plume is... oh, my gosh.
Aw, geez!
That is gorgeous.
But this was the beginning... the beginning of our Moon.
Narrator: The experiment shows
that Theia could have smashed into the Earth
and formed the Moon.
But the collision had to be just right.
And lucky for us, it was.
Today, the Moon orbits 250,000 miles from Earth.
But when it first formed,
the Moon orbited just 15,000 miles
above the Earth's surface.
Dr. Schultz: 500 million years after the Moon formed,
if we looked up in the sky,
the Moon would have comprised a tremendous portion of the sky.
It would have been enormous,
because the Moon would have been much closer.
Narrator: Back then, the Earth was rotating so fast,
a day lasted just six hours.
But the Moon was so close, its gravity acted like a brake.
It slowed our planet down
until a day now lasts 24 hours.
The Moon's gravity also created giant tides
that surged across the planet,
churning up the seas, mixing minerals and nutrients.
This created the primordial soup
from which the first forms of life arose.
Without our Moon, life on Earth may never have happened.
And there may be other moons with a link to life, as well.
Moons may be the great biology experiments of the universe...
the true laboratories of life itself.
Narrator: Moons are full of surprises.
There are moons with giant volcanoes,
moons with vast oceans sealed under thick ice.
And now we know a few are rich in organic compounds.
In the right combination, they might even support life.
Dr. McKay: In our solar system, the biological window
through which we can understand the rest of the universe
may be through these moons of the outer solar system.
That may be where we find our second genesis,
and that second genesis
is really our first deep understanding
of the biological nature of the universe.
Narrator: At first glance, moons don't look ideal for life.
Take Enceladus.
It's a shiny ball of ice, 300 miles across,
orbiting Saturn.
It's the brightest object in the solar system.
It reflects 100% of the light that hits it,
so it's superbright,
and that's because it's water ice.
Narrator: In 2005, the Cassini probe
spotted ice volcanoes erupting from the surface of Enceladus.
That meant there had to be heat under all that ice...
heat that created oceans of water.
And where there's water, there's the possibility of life.
So, this is Beehive Geyser here in Yellowstone,
and it is shooting water vapor and water
about 150 feet into the sky.
And it's pretty incredible.
So, now imagine if you're on the surface of Enceladus.
You would see geysers that look a lot like this,
and they are shooting ice grains and water vapor into space
thousands of times higher than this geyser here.
Narrator: The ice volcanoes are powered by gravity.
Here's how.
Saturn's gravity works on the core of the moon,
heating it up.
The underground water expands
and forces its way up through cracks in the surface ice
and blasts out into space as ice crystals.
These are some of the most spectacular eruptions
in our solar system.
They make Beehive Geyser look like a squirt gun.
From the ice in the volcanoes,
scientists have detected salt and simple organic compounds.
That means the water under the ice
is not only warm but full of nutrients.
Sound familiar?
Heat, water, and nutrients...
that's how life on Earth began.
Dr. Porco: We realize you could have all the things
that we associate with oceans on the Earth
going on inside a moon.
It's the discovery of a lifetime.
Narrator: Saturn's Enceladus has an ocean.
So does Jupiter's Europa.
But these aren't the only moons where life could emerge.
Saturn has another moon... Titan...
with an even greater potential for life.
In 2005, Cassini sent a probe, called Huygens,
on a one-way mission to Titan.
For just 31/2 hours,
Huygens transmitted live pictures
from the hostile surface, nearly a billion miles away.
Then the battery died.
Dr. Porco: It was just incredible.
This was the first time humans had ever touched this moon
with something of our own making.
It was just an event
that should have been celebrated the world over.
We should have had ticker-tape parades
in every major city across the U.S. And Europe
to celebrate this.
It was that history-making and that astonishing.
Narrator: Raindrops on Titan
are twice as big as raindrops on Earth.
But the rain isn't water.
It's methane.
[Thunder crashes]
On Earth, methane is a gas,
but on Titan, it's a liquid because the moon is so cold.
Dr. McKay: There may be methane icebergs.
There are certainly methane lakes and rivers,
and there's methane rain and methane clouds
and maybe bugs swimming in methane.
Narrator: Bugs living in liquid methane
may sound unbelievable.
But scientists have discovered
that Enceladus, Europa, and Titan
are all covered with a substance called tholin.
Tholin contains the chemical building blocks
for life to begin.
So could life emerge on any or all of these moons?
We can't get our hands on the tholin from the moons,
so Chris McKay makes it in the lab.
He zaps a mixture of gases found on Titan with electricity.
What he gets is a reddish-brown mud.
So, this is what we make... tholin,
this sort of nonbiological organic material.
It's produced by chemical energy
put into simple molecules, like methane and nitrogen,
and here we got it.
And that's the material we see on Titan.
We see evidence for something like this on Enceladus.
We see it on the surface
of many of the moons in the outer solar system.
This is nature's recipe
for making the stuff that life eventually emerges from.
Narrator: Somewhere in the outer reaches of our solar system,
on some remote moon,
life may have already emerged.
But it probably won't be life as we know it.
Life 2.0 doesn't necessarily have to have
the same genetics as life 1.0, right?
In fact, the more different it is, the more interesting it is.
Narrator: Whether it's the same or very different,
the discovery of life on the moons of our solar system
will change the way we look at the universe.
Dr. Porco: I think that, should we ever find
that life had originated
not once but twice in our solar system,
then you... you can easily dismiss any arguments
that say that life is unique to the Earth.
Narrator: Moons are small,
but they're still diverse and dynamic worlds.
They help us understand how the universe works.
They're essential cogs in the cosmic machine.
Without any moons,
our solar system would be a very different place.
Without our Moon, life may never have evolved on Earth.
And who knows...