All language subtitles for NOVA.S51E10.Solar.System.Strange.Worlds.1080p.WEB.h264-BAE_track3_[eng]

NARRATOR:
Our solar system
is home to eight planets,

near-perfect spheres

spinning through the darkness
of space.

But the more we explore,

the more weird and wonderful
worlds we discover.

ANJALI TRIPATHI:
Our solar system is filled

with these strange worlds
that defy all expectations.

NAOMI ROWE-GURNEY:
Patchwork worlds
that look like they've been

stuck together
like Frankenstein monsters.

AMY BARR MLINAR:
The images were shocking.

Nobody expected
to see enormous ice cliffs

on a moon
in the outer solar system.

NARRATOR:
Worlds that don't seem to play
by the rules.

HAKEEM OLUSEYI:
There is so much
strange activity

out there in the solar system
that we don't understand.

Sometimes it does feel like
being a detective,

trying to find out
why it's there, what it's doing.

NARRATOR:
Each discovery

offers new clues
to how our solar system works.

A lot of the weirdness
that we find

has something to do
with gravity.

NARRATOR:
But other forces
are also at play.

LYNNAE QUICK:
Each planet and each moon
in our solar system

are examples of how
the rules of physics

can play out differently.

GEOFFREY COLLINS:
And sometimes
we don't understand

which rules can be broken.

NARRATOR:
"Solar System: Strange Worlds."

Right now on "NOVA."

♪ ♪

♪ ♪

NARRATOR:
On the edge of the solar system
is the Kuiper Belt.

As we travel farther
from the warmth of the sun,

we find a world like no other.

Haumea was
a really exciting discovery

because it's
a Kuiper Belt object

that doesn't seem to follow
any of the rules.

NARRATOR:
One of the most basic rules

is that gravity usually shapes
planetary bodies like this

into spheres.

The force of gravity
is always pulling mass

toward a central point.

A sphere is the shape
that packs the most material

closest to the center
as possible.

NARRATOR:
But Haumea's shape
is a little harder to define.

Haumea is shaped
sort of like a football.

It's a bit of a stretched egg.

COLLINS:
Haumea looks like
a rounded pebble

that you would find
on the beach.

Haumea is definitely one of
the strangest worlds out there.

NARRATOR:
And the solar system
has even more worlds

that defy our expectations.

COLLINS:
As we explore the solar system,

we find more and more
bizarre objects out there.

Oh man, I just love
everything weird,

and what's weirder
than outer space?

No two worlds are the same,

and there's always
something new to find.

NARRATOR:
Oddly-shaped worlds,

moons that look
like they've been torn apart

and strange water worlds.

TRIPATHI:
How did we get
so many different

and unique worlds
in our solar system?

That's the million-dollar
question.

♪ ♪

(eerie static whirring)

♪ ♪

NARRATOR:
From a cloud of gas and dust...

...gravity, the great sculptor
of our universe,

fashioned our star

and the planets and moons
around it...

...creating the solar system.

And gravity has continued

to shape these myriad worlds
ever since.

But how?

What exactly is gravity?

Michele DOUGHERTY:
It depends who you ask.

If you ask Newton, he would say
that gravity is a force

that helps pull things together.

It's what's keeping me seated
on the Earth at the moment.

QUICK:
It builds planets,
stars and galaxies

by pulling together the dust
and the gas and the rocks

that make them up.

NARRATOR:
But Einstein saw it
a little differently.

DOUGHERTY:
If you asked Einstein, he said

it wasn't a force but that it
curved space and time.

Very simply put, space and time
were linked to him,

as if they were a fabric.

So any kind of object
with mass, uh, would, um,

kind of bend that fabric and,
um, things would fall into it.

NARRATOR:
At the center
of our solar system

is the most massive object
in it, our sun,

curving the fabric
of space-time around it.

The planets follow
this curvature,

creating their orbits.

The sun binds everything
in the solar system together

within its gravity,
and without that,

uh, there wouldn't be anything.

There wouldn't be
a solar system,

and we wouldn't exist, either.

NARRATOR:
But gravity is not alone

when it comes to shaping
our solar system.

The solar system would be
a really boring place

if it was only gravity that was
acting on all of these things.

It would mean that everything
was spherical and, uh, the same.

QUICK:
We see such a variety of shapes

because gravity
is not the only force at play.

TRIPATHI:
Even though it's counter to
what we think about

when we talk about
the planets and gravity,

it's actually the weakest force
in day-to-day life.

PROUDFOOT:
So, although gravity
likes making spherical planets,

sometimes rocks are just strong
enough to resist gravity.

We don't live
in a universe of marbles

because you have to have
enough mass

for gravity to pull everything
into the spherical shape.

NARRATOR:
So how much mass
does gravity need

to overcome the strength of rock

and make planets spherical?

James Dottin
is a planetary scientist

who studies rocks to understand
the evolution of planets.

DOTTIN:
Gravity is directly
proportional to mass.

The more mass an object has,

the stronger
the gravitational force.

So, in order to iron out

all the lumps and bumps

of rocky objects in
our solar system,

it requires a lot of
gravitational force.

to overcome
the strength of rock,

it must reach a critical size.

We think that, in order for them
to form into a sphere,

they need to have a radius
of about 200 miles,

so that they're massive enough
for gravitational forces

to be strong enough
to form them into a sphere.

It's called the potato radius
because objects that are smaller

end up looking like potatoes.

NARRATOR:
A rocky world with a radius
under 200 miles...

...will tend to be oddly-shaped.

While everything with a radius
larger than about 200 miles

is a sphere.

That is, almost everything.

Haumea, out in the depths
of the solar system,

is a rule breaker.

This is a world
about 1,300 miles long,

1,000 miles wide,

and less than 700 miles high
pole to pole...

...and it isn't alone.

Two icy moons in a thin ring
of rock and ice orbit Haumea,

making an unexpected
and odd system...

...the first like it
ever discovered.

Haumea was such
an exciting discovery

because it's large enough
to be round,

but for some strange reason,
it's shaped like a football.

NARRATOR:
So, if its odd shape

is not due to its size,
then what is it?

There is a clue we can see
at work here on Earth,

if you know how to look.

We've left this camera out
all morning,

fixed on a single point
in the sky.

NARRATOR:
In this case, the sun,

and that helps to visualize
the rotation of the planet.

DOTTIN:
Wow. I mean, how cool is that?

Now that's no camera trick,

that's literally
the Earth rotating

at about 1,000 miles per hour.

And although that's superfast,
I can't feel it,

and that's because
everything around me

is rotating with the Earth.

NARRATOR:
But even if we cannot feel
Earth's rotation

we can still feel
the forces created

by a rapidly spinning object.

MLINAR:
So a merry-go-round
on a playground

is this big spinning disc,
it has handles,

you can hold onto it,
you get on,

your friends start to spin
the merry-go-round faster

and faster and faster,
eventually, it goes so fast

that you can't hang on anymore
and you kind of fly off.

So when that happens,

you're experiencing
centrifugal force.

NARRATOR:
And it turns out, Haumea is
spinning incredibly quickly.

This world is spinning so fast,

it experiences an entire day
and night in under four hours.

It's the fastest known rotating
object in the Kuiper Belt.

So if this is
our model of Haumea,

gravity is acting
to try and make it

into a sphere,

but because Haumea is
spinning so quickly,

it actually means that
centrifugal forces

can make it propel away
from itself.

And you'll notice that it starts
to become more egg-shaped

as it spins.

Oh, cool,
it's really egg-shaped now.

I'm gonna turn it off
before it kills us.

(laughs)

The immensely fast rotation
of Haumea spinning around is

what explains the shape that we
see as a stretched out oval

as opposed
to a perfectly round sphere.

MLINAR:
It's just been

forced to deform

into this completely
football, egg shape.

It has no choice,
it has to be that shape.

NARRATOR:
And Haumea's spin may also
be responsible

for the formation
of the two icy moons

orbiting this strange world.

PROUDFOOT:
So most people generally think

that Haumea was formed
in a giant collision.

That impact probably got Haumea
spinning really, really fast.

When something spins too fast,
centrifugal force beats gravity

and things can actually become
detached from the body.

So if we go back to the analogy

of kids riding
on a merry-go-round,

these would be the kids that
fell off the merry-go-round

when it was going too fast.

So that's one way of making tiny
little icy moons around Haumea.

NARRATOR:
Haumea and its moons formed

in an ongoing battle

with gravity pulling
the world together...

...and its spin
pushing it apart.

The battle between
these two forces--

gravity and spin--
creates a truly strange world.

♪ ♪

But it isn't the only oddball
in our solar system.

Travel in
from the Kuiper Belt...

...past the ice giants...

...and past Saturn....

...and we discover what happens

if we dial up
a planet's gravity.

This is a world so enormous

you could fit
all the other planets inside it

with room to spare.

And such a gargantuan planet
has massive moons

that also feel the effects
of Jupiter's pull.

Since 2016,

NASA's Juno spacecraft
has been exploring Jupiter

and its many moons...

...including one unlike
any other in the solar system.

SCHENK:
Ganymede is really big.

It's about 3,000 miles across.

It's almost as big as
the planet Mars.

It's really big.

NARRATOR:
But it's not just
the size of the moon

that makes Ganymede unusual.

DOUGHERTY:
The surface of Ganymede looks

weird in that it's got lots of

craters on the surface.

It's got grooves and cracks

on the surface.

COLLINS:
The surface is mostly ice,

in some places
there's a thin layer

of rocky dust on top of the ice.

And you might see some icy
mountains in the background.

NARRATOR:
And above this icy surface,

Juno witnessed
strange ribbons of light,

An aurora

dancing above
the poles of the moon.

A spectacular light show

that has helped reveal
something even stranger

about this world.

We think that Ganymede
might have a secret

hiding beneath the surface.

NARRATOR:
The aurora above the surface

helps us peer beneath it.

O'DONOGHUE:
The aurora of Ganymede
are produced when

electrically charged
particles are

flowing down
magnetic field lines

and they're hitting
the atmosphere,

which is made of oxygen

and they're causing it to glow

in green and red lights.

SCHENK:
Ganymede's magnetic field
is a lot like Earth's.

It's generated
by a liquid magnetic

iron core.

If you stood
on the surface of Ganymede

with a compass
and you looked at it,

the needle of the compass
would point to the north pole

of the magnetic field,
just like it does on the Earth,

it is the only moon
that you can do it on

because it's the only moon
in our solar system

that has
an internal magnetic field.

NARRATOR:
And Ganymede's aurora

rocked back and forth
across the moon.

QUICK:
Because Jupiter
also has a magnetic field

and Ganymede sits within
Jupiter's magnetic field,

it should cause
Ganymede's aurora

to rock back and forth.

NARRATOR:
But when scientists used

the Hubble Space Telescope
to look

at Ganymede's aurora,

something didn't add up.

DOUGHERTY:
The images of the aurora
at Ganymede

showed that they weren't
rocking back and forth

as much as we expected them to.

Because the motion
of Ganymede's aurora

don't match
scientific predictions,

we think there must be
something else there

that's affecting them.

NARRATOR:
If there were a second
magnetic field being generated

within Ganymede,

that would interfere
with the aurora,

reducing the rocking.

But the only way
to generate that extra field

would be if another layer,

besides the molten core,
were conducting electricity.

There has to be something else.

SCHENK:
That something else

turns out to be a liquid layer,

an ocean underneath the surface.

Ganymede's internal ocean
is damping down

the oscillation that we see.

QUICK:
It's extremely cool

that we can tell
that there's an ocean

beneath Ganymede's surface,

despite never having
a lander there.

NARRATOR:
Scientists estimate

Ganymede has a global ocean.

60 miles deep,

hidden beneath around 95 miles
of rock-hard ice.

DOUGHERTY:
It's pretty mind-blowing,

if you think about it.

This moon
out in the outer solar system,

which is much smaller
than the Earth,

could potentially have
more water within it

than we have in our own oceans
on the Earth.

NARRATOR:
Of all the water worlds
in the solar system,

Ganymede's ocean is the largest.

DOUGHERTY:
One of the questions
I always ask myself

is how does an icy moon like
Ganymede get this huge ocean.

NARRATOR:
Strange gouges
on the surface of Ganymede

hint at a fascinating theory.

These are impact craters,

not individual ones
like we see on most other worlds

but a long chain of them.

To understand how these form,

we have to look back to
the monster living next door...

...Jupiter.

Veronica Bray Durfey

is a planetary scientist
who studies impact craters...

...on the surface of Ganymede.

DURFEY:
A lot of planetary science
these days is,

you know, I-I wait
for the pictures to come back

from spacecraft
that have been to the planets.

But there's something
a lot more personal

about getting it
through a telescope.

And to-to see all of
the Galilean moons out tonight,

that's always extra special.

This pinprick of light
just on the edge of Jupiter,

that's Ganymede.

It's the biggest
of the Galilean moons.

It's the biggest moon
in the solar system.

It's bigger
than the planet Mercury.

NARRATOR:
And Ganymede's location,

orbiting Jupiter,

may play a part in how
the moon got its hidden ocean.

Any objects that have mass

will have a force of attraction
between them,

and that's gravity.

The larger the mass,

the larger
the gravitational attraction

between the two objects.

Because Jupiter's so massive,

it has a really massive
gravitational pull.

So this means that it attracts

a lot of bodies
of the solar system towards it.

If an asteroid or a comet
gets close enough,

it can feel the pull of Jupiter.

COLLINS:
So if you're drawn in
toward Jupiter

by its gravity and you don't

quite hit Jupiter,
but you get very close...

Jupiter's gravity is so strong

that it will start to pull

bodies apart.

SCHENK:
We've actually seen the process.

This was back in 1993

when astronomers
observed a comet

broken up after a close passage
of Jupiter.

And it was called
Shoemaker-Levy 9

after the astronomers
who discovered it.

COLLINS:
It had been disrupted

by Jupiter's gravity into...

a string of objects.

And looking at its orbit,
they realized

that it was going to come back

a year later
and actually hit Jupiter.

NARRATOR:
Watching this series
of comet fragments explode

as they hit
the dark side of Jupiter

provided scientists with clues

as to how these
strange crater chains

were formed on Ganymede.

SCHENK:
So on Ganymede we observed
these chains of craters,

all nicely lined up in a row,

evenly spaced, very peculiar.

So when we saw the chain
of cometary fragments

that make up Shoemaker-Levy 9

and we saw that in 1993,

it suddenly
occurred to us,

that same set
of cometary fragments,

if it the moon on the way out,

would form a crater chain
just like this.

NARRATOR:
Ferocious, high energy impacts

create these incredible
chains of craters.

But Jupiter's gravity means
that so much more

has hit Ganymede than just
these torn apart objects.

And it is
these violent collisions

that may help explain
the moon's vast hidden ocean.

COLLINS:
The early history of the solar
system was a very chaotic place.

There were
more asteroids and comets

flying around,
impacts were just

a much more common occurrence.

Nothing was spared the chaos

of the early solar system.

NARRATOR:
As Jupiter drew in countless
asteroids and comets

with its immense gravity...

...Ganymede was caught
in the crossfire.

Each impact delivers
huge amounts of heat

and energy to the early moon.

DURFEY:
And this allowed
Ganymede to heat up

and some of its components
to become molten.

And once you
have that molten mix,

you're going
to get differentiation.

NARRATOR:
Differentiation

is where gravity
organizes material

based on its density.

DURFEY:
We can visualize
this differentiation.

So if this oil
is our low density material...

...we can add

a higher density material.

This sand.

NARRATOR:
Shaking the jar lets us imagine

what Ganymede would've been like
at the beginning.

A mixture of high
and low-density materials.

DURFEY:
And then over time

the gravitational pull

will help this separate out.

And so in Ganymede's case,

that is the high-density metals

falling towards its core

and the low-density ices

remaining at its surface.

Differentiation takes
millions and millions of years.

But this will not take
that long.

NARRATOR:
But gravity had
one more trick to play.

DURFEY:
As the dense material

heads towards
the core of Ganymede,

it flows past
the less dense material,

and this creates heat
through friction,

keeping Ganymede
molten for longer

and making its differentiation

almost a self-sustaining system.

NARRATOR:
And this continued
until Ganymede's interior

separated out
into different layers.

MLINAR:
So we know that Ganymede got
hot enough to melt completely.

And not just to separate the ice

from the rock,

but actually to separate

the metal from the rock

inside of Ganymede, as well.

NARRATOR:
And over hundreds
of millions of years

the moon started to cool.

MLINAR:
What will happen is that the ice

deep inside Ganymede starts

to freeze from below,
but it also freezes from above,

and then you get left
with this layer

of salty water
that just won't freeze.

NARRATOR:
But this is just one theory
about Ganymede's ocean

and how it got the heat to form.

When I get asked the question
will we ever know

exactly what happened
at Ganymede,

and the answer's
probably no,

we will be able to come up
with suggestions

as to what might've happened.

Um, but we'll probably
never know it completely.

But to me that's
part of what makes it

so interesting to study.

Because there are
always new ideas.

There are always new things
that we can measure.

Always new techniques
that we can try.

NARRATOR:
Leaving this hidden
ocean world behind,

with its bizarre surface...

...we head out
away from the sun...

...past Saturn...

...to see what strange things
can happen

when you pair a massive world

with a tiny one.

Uranus is pretty odd
to begin with.

The entire planet
knocked onto its side,

likely by some massive impact
in the past.

But that's nothing
compared to how weird

one of its moons is.

GROUND CONTROLLER:
Four, three, two,

one.

We have ignition
and we have lift off.

NARRATOR:
On August 20, 1977,

spacecraft Voyager 2

was launched
to explore the outer planets

of the solar system.

The Voyager mission was
really exciting, it was...

a rare mission
of first exploration.

NARRATOR:
And after more than
eight years...

...Voyager 2 reached Uranus.

It was the first,
and remains the only spacecraft

to visit this planet
and its moons.

COLLINS:
And we'll never have that

experience again
of seeing those places

for the first time.

NARRATOR:
As it flew past
Miranda's south pole...

Voyager 2's cameras
captured images of one

of the most astonishing surfaces

in the entire solar system.

MLINAR:
I think the images of Miranda

were shocking
when they came back.

SCHENK:
Because we weren't
really expecting much.

We were expecting it
to be cold and dead.

Not much happening.

And yet there's signs
of some kind of activity inside.

It looks like somebody

was making an art project.

Like somebody ripped up

pictures of two different moons

and glued them
back together again.

It didn't look real to me
the first time I saw it.

NARRATOR:
Scientists spotted
plunging canyons.

With cliffs
taller than Mount Everest.

And ridged patches
that punctuate the surface.

All on a moon
only 293 miles in diameter.

Less than the width of Arizona.

MLINAR:
It raised a lot of questions

as to what's going on,
on those small icy moons?

SCHENK:
It was a real resetting event.

It told us that, you know,
small bodies can be

very interesting
and dynamic, too.

And we had to sort of go back

and understand
why that was the case.

NARRATOR:
The moon's size offers a clue.

MLINAR:
So Miranda is much,
much smaller than the Earth.

So the surface gravity
on Miranda is about...

1/100th that on the surface
of the Earth.

NARRATOR:
And that means
the cliffs can soar

to unbelievable heights.

QUICK:
Miranda's tallest cliff
is pretty high

compared to its overall size.

The tallest cliff is
six miles high.

That's equivalent
to 2%

of Miranda's diameter.

160 miles high.

QUICK:
The reason that Earth's
tallest mountain

is only
five and a half miles tall

is because Earth is
more massive and because it's

more massive,
it has much stronger gravity,

and gravity won't allow

mountains or cliffs to grow
that tall on Earth.

NARRATOR:
Like other worlds,

Miranda suffers
the occasional meteorite strike.

But because
of its weak gravity...

...the results play out
in slow motion.

With the debris taking
over eight minutes

to fall the height
of its tallest cliffs.

Compare that to Earth,

where the same drop would take
less than a minute.

COLLINS:
Jumping on Miranda
would be a lot of fun.

You could jump
really high in the air

because gravity is so low.

In fact, just trying to walk
normally would be difficult.

NARRATOR:
There are countless
other small, icy worlds

also with weak gravity,

but we've only seen strange
patches like this on Miranda.

So where did this
weird patchwork surface

come from in the first place?

All scientists have to go on
are those images...

...grabbed in 1986
as Voyager 2 streaked past.

SCHENK:
One of the keys
to understanding Miranda

is to recognize that there
is actually order

to this apparently
chaotic picture.

COLLINS:
You've got this ancient,
cratered terrain over here

and then these patches
like here, here and up here

that we call coronae.

NARRATOR:
Where regions of Miranda's
surface have been remade.

COLLINS:
And inside that patch,

you see are these ridges
and troughs

that are like stretch marks
on the surfaces.

NARRATOR:
It looks as if the surface
has been ripped apart...

...suggesting these scars
were formed by internal forces.

COLLINS:
If you had some kind of

warm material inside Miranda,

it's less dense
and it starts rising up

uh, in a big blob.

And as it comes up
toward the surface,

it pushes the material
out of its way...

...ripping the surface apart.

QUICK:
It cracks and it fractures open

and we're left with a corona.

NARRATOR:
This left hundreds of miles
of canyons...

...where the surface cracked
open along fault lines...

...creating this
incredible landscape.

I think since the Voyager images
came back

people have been wondering

how a body so small
could be so active.

Small bodies
lose their heat rapidly,

so we were expecting it to

not have any
real geologic history.

COLLINS:
So the big question is,

where did the heat come from

to drive the creation
of this bizarre landscape?

NARRATOR:
For that, we have to look
to Miranda's history.

Our best theory involves
the moon's giant parent planet,

Uranus...

...and another quirk of gravity.

QUICK:
Billions of years ago,

Miranda had
a more eccentric orbit.

And what that means is that
when Miranda orbited Uranus,

it wasn't a perfect circle,

it was more in the shape
of an ellipse.

So, when Miranda

is very, very close to Uranus,

the gravity from Uranus

sort of deforms it
into more like an egg shape.

And then when Miranda's
farther away, it's more round.

And that stretching
and squeezing

causes a lot of friction
on the inside

and that friction
results in heat,

which we call tidal heating.

NARRATOR:
And it's this tidal heating

that drives Miranda's geology

and forms the coronae.

SCHENK:
Not only is
the gravity responsible

for bringing
the material together

that created Miranda
in the first place,

but because of
its gravitational interaction

with Uranus, it's also
responsible for the energy

that remade Miranda later on.

NARRATOR:
Today, Miranda has
a nearly circular orbit.

And scientists think that heat
is mostly gone.

QUICK:
Because Miranda's so small,

it would've been difficult
for it to hold onto

its heat for a long time.

But the Uranus system
is not very well explored.

There are a lot of things
we don't understand

about Uranus and its moons.

And we should prepare
to be surprised

next time we go back.

NARRATOR:
Though now frozen,

Miranda reveals how small worlds

can be shaped through gravity
by larger ones.

But traveling inwards towards
the sun, we see evidence

that small worlds can also
leave a big mark of their own.

On one of the most spectacular
structures in the solar system.

Glimmering rings
of rock and ice...

...they are one of the hallmarks
of our cosmic neighborhood.

And when examined more closely,

show signs of
remarkable organization.

Lumps of ice and rock
spread out in a thin disc,

split into hundreds
of repeating tracks and gaps

that look like
grooves on a record.

Looping for hundreds
of thousands of miles

through space.

Saturn's rings are
amazingly complex.

And the more we zoom into them,

the more complex seem to be.

It is one of the wonders
of the solar system.

NARRATOR:
Scientists think the rings
may have first formed

when a moon strayed
too close to Saturn...

...and was pulled apart
by its gravity...

...creating a jumble
of trillions

of individual fragments
of rock and ice.

So what turns such chaos

into the ordered beauty
we see today?

NASA's Cassini spacecraft

gave us the best view
of the rings we've ever had.

OLUSEYI:
The photographs from Cassini

gave us Saturn's rings
at all angles.

we saw Cassini look through them
toward the sun.

Just the spectacular beauty
of them.

Just mind-blowing.

NARRATOR:
And lurking among the loops
of rock and ice...

Cassini imaged one
of the most startling moons

in the entire Saturn system.

EL MOUTAMID:
So, Pan is this
weird, tiny object.

It is only 17 miles across.

And it looks, for me,
like a walnut.

And, uh, it looks like it has
a dusting of material around it

that could easily break off
if you were to touch it.

NARRATOR:
Despite its small size,
Pan has a big impact

on the structure of the rings.

BROOKS:
Pan is a great example of how
gravitational interactions

can shape Saturn's rings

and create the gaps that we see.

NARRATOR:
Pan orbits inside a wide track
within Saturn's rings

called the Encke Gap.

EL MOUTAMID:
We think Pan
created the Encke Gap

by cleaning the orbit
and by accumulating

all this ring material,
dust and ice, around it.

O'DONOGHUE:
This ring material has settled

specifically on Pan's equator

again and again and again
and that meant

there is this large,
uh, ridge around Pan itself.

OLUSEYI:
It looks like an empanada

because so much water and ice
from the rings

have built up
around its equator.

NARRATOR:
For millions of years,

Pan has been nibbling away,

clearing particles
out of its orbit...

...and creating this pathway...

...only that can't be
the full story.

Pan is just 17 miles across,

yet it orbits
within the Encke Gap

that's 200 miles wide...

...far broader
than Pan could clear

through simply snacking alone.

So the big question is,
how can a small moon like Pan

carve out such a huge gap
in Saturn's rings?

In addition to sweeping up
ring particles,

Pan also managed to open up
the Encke Gap

by pushing away the particles
on either side of the gap

through gravitational
interactions.

NARRATOR:
This turns out to be a quirk
of orbital physics.

If a particle of ice
gets close to Pan,

the moon's gravity
gives it a tug,

speeding the particle up
or slowing it down.

That moves it to a new orbit,

clearing a path
through the rings.

And Pan is not alone.

The Cassini spacecraft
also spotted tiny Daphnis...

...just five miles across,

clearing its own gap
in the rings.

Even tiny, odd worlds can create
structures of staggering scale.

But not all the gaps
have moons embedded in them,

including one of the biggest,

the massive Cassini Division,

almost 3,000 miles wide.

So how did these gaps
in the rings form

without a moon inside
clearing the way?

James O'Donoghue
is a planetary scientist

who studies Saturn
and its rings.

O'DONOGHUE:
Saturn's rings
are an amazing example

of the most beautiful
and complex patterns

being produced by a single event

occurring over time
again and again.

We see these patterns occurring
all across nature

in various forms,
as we can see here,

with ripples running
across these sand dunes.

The ripples form when the wind

is gliding over the surface
of the sand dunes,

and it's lifting up
small pieces of sand

and bouncing them
along the surface.

NARRATOR:
When the bouncing grains
hit the surface,

they kick up more grains.

And as this process repeats,

the ripples form.

And just like wind
creates structure

in these sandy dunes,

over 700 million miles away,

in the Saturn system,

regular, repeating
gravitational interactions

form the structures
of the rings.

O'DONOGHUE:
This is Saturn and its rings--

it's not to scale--
and we also have

a selection of moons.

Here is Pan
and here is Pandora,

and we also have
the moon Mimas.

And we also have Titan,

which is 50% larger
than our moon,

which is much further out.

Saturn has over 140 moons.

We only show four here
because it would be too busy,

and they interact

with each other gravitationally,

in a really complex way,

and it weirdly leads
to a lot of order.

NARRATOR:
One of these moons, Mimas,

which has more
than a passing resemblance

to the Death Star
from "Star Wars,"

creates the Cassini Division,

despite being around
40,000 miles away from it.

And it does this
all thanks to gravity.

O'DONOGHUE:
The Cassini Division
is the biggest gap in the rings,

and it's produced by
a gravitational interaction

between Mimas
and the ring particles

around about here.

NARRATOR:
Mimas is in a two to one
orbital resonance

with the ring particles
of rock and ice

that would be found
in the Cassini Division.

O'DONOGHUE:
And what that means is

that Mimas, for every one orbit

that it makes around the planet,

the ring particle makes two.

And because these
are in resonance,

every time Mimas

and the ring particle meet,

they actually meet at the same
point in space around Saturn,

and Mimas implants
a gravitational tug

onto the ring particle,
which changes its orbit.

NARRATOR:
And Mimas has this
gravitational relationship,

not with just
a single ring particle,

but all the ring particles
in the same orbit.

Each time the moon
and the ring particles align,

Mimas's gravity tugs at
the fragments of ice and rock,

like an invisible hand...

...opening up the giant gap.

And there are more moons
sitting outside the main rings

creating structures within them.

EL MOUTAMID:
It is fascinating that,

even if the moons are far away
from the ring,

they still have an impact
on the ring,

and this is the magic
of gravity.

Gravity is the main force,

that it is shaping
everything in universe,

including the Saturnian system.

NARRATOR:
The orbital dance
of Saturn's moons

create the constantly changing

and dynamic pattern
within the rings.

One we are lucky to see.

Saturn's rings allow us
to see gravity at work,

constantly shaping
our solar system.

But leave these
beautiful patterns behind...

...and we see
how a planet's size

and the influence of gravity

can have astonishing
consequences for life.

More than half a billion miles
closer to the sun...

...through the asteroid belt,

rubble left over

when gravity failed
to pull a planet together,

and we reach
the inner rocky worlds

where we find perhaps
the most bizarre world of all.

A true outlier
unlike anything else.

Our solar system's
beautiful blue marble.

TRIPATHI:
We're living in this

amazing period of
the Earth's history

when we have liquid water in
the form of oceans

on the surface
of our planet,

not only the solar system,

but the thousands
of other planets

we've discovered to date.

NARRATOR:
The fact that Earth
has oceans on the surface

turns out to be, again,

thanks in part to gravity,

which pulls down
on the atmosphere.

ROWE-GURNEY:
So our atmosphere is made up
of lots of gasses,

and that gas exerts a pressure

on the surface of the Earth,

and that pressure stops water

from evaporating into space.

NARRATOR:
But if Earth were smaller,

it might have been
a different story.

TRIPATHI:
If it was much smaller,
it wouldn't have enough mass,

and, therefore, enough gravity
to hold on to an atmosphere.

We're lucky to live
on a rocky planet

that is large enough to keep
its atmosphere in place.

NARRATOR:
With little to no atmosphere

to press down on the oceans...

...water would boil
at much lower temperatures,

and Earth would become
a desolate, barren ball.

ROWE-GURNEY:
So, without the atmosphere,

Earth wouldn't have life on it,

uh, and we wouldn't have
the ability to breathe,

um, there wouldn't be oceans,

uh, and forests and trees.

Uh, we wouldn't have anything

like the Earth
that we know today.

NARRATOR:
Earth's size has helped shape
its destiny.

Too small,

and Earth could've been
a misshapen potato

with no atmosphere.

Too big,

about ten times
its current mass,

it could've grown
to become a gas giant...

...with little hope
for life as we know it.

It turns out,

life can run riot across
the surface of the planet...

...because Earth
is the right size

with just the right amount
of gravity.

The same force that has
helped shape all the other

radically different,

wonderfully strange worlds
out there.

The more we go out and visit
our solar system in detail,

the more we discover things
we've never seen before.

SCHENK:
We're not entirely sure
why we see so many

different sizes and shapes

and complexity
of planetary bodies,

uh, but we think that gravity

has a very strong role
to play in it.

Without gravity, the universe
would be a pretty boring place.

It's gravity that assembles

the materials of the universe

into the large structures
that we see.

TRIPATHI:
Gravity is the backdrop

that's setting the stage
for other forces to get to work.

ROWE-GURNEY:
We need to study these strange
worlds in the solar system

because, without them,
we wouldn't understand

how all of these forces
come together to create them.

NARRATOR:
But our solar system
only contains

a fraction of the
strange worlds out there.

DOUGHERTY:
We talk a lot about strange
worlds in our solar system,

but there are certainly
stranger worlds out there

that we haven't found yet.

NARRATOR:
And scientists

will never stop looking
for new, weird worlds.

I don't think I'm ever gonna
get bored of strange worlds.

There's so much out there
to explore and discover.

It's what gets me
out of bed every day.

So, the strangeness
is only just beginning.

♪ ♪

♪ ♪

♪ ♪

♪ ♪

♪ ♪

♪ ♪

♪ ♪

♪ ♪