We live on a world of wonders,
a place of astonishing beauty and complexity.
We have vast oceans and incredible weather,
giant mountains and breathtaking landscapes.
If you think that this is all there is
that our planet exists in magnificent isolation, then you're wrong.
As a physicist, I am fascinated
by how the laws of nature that shaped all this
also shape the worlds beyond our home planet.
I think we're living through the greatest age of discovery
our civilization has known.
We've voyaged to the farthest reaches of the solar system.
We've photographed strange new worlds,
stood in unfamiliar landscapes,
tasted alien air.
But what makes the wonders of the solar system
even more astonishing is that
it all started as nothing more than a chaotic cloud of gas and dust.
It was from that cloud that everything in the solar system formed.
All this order.
The Sun, the rotating planets, me,
coalesced from a collapsing cloud of dust.
In this film, we 'l! Discover how the solar system
made the journey from chaos into order.
And see how that cloud gave rise
to the solar system 's most beautiful wonder,
the majestic rings of Saturn.
We'll discover how Saturn 's amazingly varied moons
govern the intricate patterns of the rings.
And how another wonder, recently discovered on one of those moons,
is changing our ideas about the nature of the outer solar system.
It's cool.
There it is!
We 'l! Witness the fundamental forces
that control the universe.
It's beginning to come, the end of the world.
And see how those forces were unleashed
to create the beautifully ordered solar system we live in.
It's only now that we're beginning to understand the origins of that order
and that has implications for our understanding
of the entire solar system and ultimately of why we are here.
This is the Great Mosque in the city of Kairoaun in Tunisia
and this mosque is the fourth holiest place in Islam.
And so, for the last 14 centuries,
the relentless passing of the days has been celebrated by prayers
before dawn, at sunrise, at noon, at sunset and in the evening.
The calls to prayer mark out the passing of time
as the Sun travels across the sky.
But it's not the Sun that's moving.
What we are really observing
is the movement of the Earth through space.
This is the ball of rock we live on.
It carries us through cycles of night and day
as it turns on its axis every 24 hours.
A year is the time it takes to orbit the Sun-
And we have seasons
because the Earth's axis is tilted by 23 degrees.
T o see how that works we need to speed time up
so a year passes in just 10 seconds.
At this pace, we can see how
the southern and then the northern hemispheres
are angled towards the warmth of the Sun,
taking us through yearly cycles of summer and winter.
All the rhythms of our lives are governed
by how the Earth travels through space.
And in Tunisia, in April, it's springtime.
This is the seasonal flower market in Kairouan.
And it's only here for two months of the year
because that's when these flowers are in flower.
And it's a beautiful example
of how the structure, the clockwork of the solar system
affects things here on Earth in the most unexpected of ways,
because if our Earth's axis wasn't tilted by 23 degrees
then there wouldn't be any seasons.
And if there weren't any seasons,
then seasonal flowers wouldn't have evolved
and there wouldn't be a flower market.
But it's not just the Earth,
the whole solar system is full of rhythms.
Each planet orbits the Sun at its own distinctive tempo.
Mercury is the fastest.
Closest to the sun, it reaches speeds of 200,000 kilometers an hour
as it completes its orbits in just 88 days.
Venus rotates so slowly
that it takes longer to spin on its axis than it does to go around the Sun,
so that, on Venus, a day is longer than a year.
Further out, the planets orbit more and more slowly.
Jupiter, the largest planet, takes 72 Earth years to complete each orbit.
And at the very furthest reaches of the solar system,
four and half billion kilometers from the Sun,
Neptune travels so slowly
that it hasn't completed a single orbit
since it was discovered in 1846.
The solar system is driven by these rhythms,
so regular that the whole thing could be run by clockwork.
It seems extraordinary that such a well-ordered system
could have come into being spontaneously.
But it is, in fact, a great example
of the beauty and symmetry that lies at the heart of the universe.
I want to explain how that order emerged from the chaos of space
because understanding that will help us understand
the origins and formation of the solar system
and the beauty of its wonders.
These are the Atlas mountains in North Africa.
According to Roman legend they held the heavens above the Earth.
And they are one of the finest places to come to view the stars.
From a place like this, it's easy to appreciate the profound effect
that the night sky would have had on our ancestors.
You know, from a modern perspective, astronomy can seem remote and arcane
because we've lost our connection with the night sky.
From a city, you just don't see a sky look anything like this.
From the darkness of the Atlas mountains,
it's really, truly majestic.
So, for our ancestors the connection with the night sky
would have been incredibly intimate.
They looked into the skies to understand their place in creation
and the movement of the stars told them one thing.
They were at the center of the universe.
Up there is Polaris, the North Star,
and it's almost exactly aligned with the Earth's spin axis,
which means that as the Earth rotates,
all the stars rotate through the sky around that point.
So it looks for all of the world
as if the Earth is at the center of the universe
and the stars rotate around it.
And that's, of course, what the ancients thought for thousands of years.
And why not? Because it's obvious
but wrong.
T o understand the Earth's real position in the solar system,
we need to look at the one set of bodies that doesn't behave
as predictably as the stars.
The Greeks named them "planets" or wandering stars,
and we've kept the name "planet" to describe them.
This is Mars photographed once a week over a period of months.
Rather than traveling in a straight line
across the background of the stars,
it occasionally changes direction and loops back on itself I.
It's very hard to explain these "retrograde loops"
if the Earth is at the center of the universe.
Understanding the retrograde motion of Mars didn't come easy.
That's why it took over 2,000 years to work out.
But I'm gonna explain it using a stick and some rocks.
The key thing is that the Earth is not at the center of the solar system.
The Sun is.
And the Earth and Mars go around it
in almost circular orbits.
So when Mars is viewed from the Earth,
then it's seen on the sky, in fact, on the constellations of the zodiac.
So as Mars orbits around and the Earth orbits around,
then from that position Mars will look like it's there from the sky.
Mars moves and the Earth moves in that position.
Mars moves in that direction across the sky
and again, in that position, Mars will be here.
So, you see, it's moving in a straight line across the sky.
What happens when the Earth overtakes Mars?
Then, look at the line of sight.
Mars has moved back to there. It's reversed its direction.
And it continues to do that
until the Earth gets round to somewhere like there.
Mars is here and then you see
the line of sight means that it's started moving that way again.
So Mars has executed that strange looping motion on the sky
because the Earth overtook Mars on the inside.
And that's why the retrograde motion happens.
Simple.
Understanding the retrograde loops
was one of the major achievements of early astronomy.
It created the concept of the solar system
and allowed us to build the first accurate maps of the planets
and their orbits around the Sun.
Once you have this picture of a solar system running like clockwork,
the Sun surrounded by the orbiting planets,
then you might start asking questions like,
"Why is the solar system so ordered?"
And "How did that order come into existence?"
Well, a clue lies in those sweeping, circular motions of the planets.
T o understand how the solar system came into being,
we need to understand the physical principles
that govern the whole universe.
But, of course, the same laws of physics also control the world we inhabit,
so to discover how the solar system started
we don't need to look out into space or back in time,
we just need to look around us.
You know, one of the remarkable things about the laws of nature
is that they're universal.
And that means that the same laws that describe
the formation of the solar system
can also describe the most mundane things here on Earth.
Like the motion of water as it drains from a sink.
These spinning spirals are seen all across the universe.
We see them everywhere because the laws of physics are the same everywhere.
I've come here to Oklahoma to see how those laws can unleash forces
that drive some of the most powerful and destructive phenomena
in the atmosphere of our planet, tornadoes.
Oklahoma is in the middle of what is known as Tornado Alley.
No, no, no, get way over.
Oh, my God.
Yep, it's huge.
Oh, wow. Big, big, big.
Look at that thing. Look at it spin.
Every year hundreds of twisters tear across the landscape.
They're incredibly dangerous and destructive
and their key feature is that same spinning spiral.
Don Giuliani is a professional storm chaser.
He's going help me try to get close to a tornado.
That, little developing storm there, is this.
It's a severe thunderstorm that is capable of producing a tornado.
Anywhere inside that purple area
it's possible that the tornado could be there or is moving in that direction.
And what would happen if, in this car,
we deliberately or not went straight through it?
Well, it would probably pick up our car
and toss it a quarter of a mile through the air,
and crush it into a little ball and...
I'm gonna just do a U-turn.
Bizarre as it sounds,
the processes that drive these vast storm systems
are the same as would have been seen five billion years ago
at the start of the solar system.
Everything that we know and see around us was formed
from a nebula, a giant cloud of gas and dust.
Drifting across light years of space,
that cloud remained unchanged for millions of years.
But then something happened that caused it to coalesce
into the solar system we have today.
It's thought that a supernova, the explosive death of a nearby star,
sent shockwaves through the nebula.
This caused a clump to form in the heart of the cloud.
Because it was more dense, its gravitational pull was stronger
and it started to pull in more and more gas.
Soon, the whole cloud was collapsing and crucially, it began to spin.
It's a feature of all things that spin
that, if they contract, they must also rotate faster.
It's a universal principle called the conservation of angular momentum.
Just pull off anywhere here. Yeah.
It's this that leads to those spinning spirals
and it applies equally well to the early solar system
and storms like these.
As the giant thunderheads build they suck up hot air and contract
and like the cloud that built the solar system,
when they contract, they spin faster and faster.
On Earth, in storms like this,
the conservation of angular momentum means that you get
well, in the most extreme case, tornadoes,
you get very rapidly rotating columns of air
where the wind speeds can rise to 300, 400,
even sometimes 500 kilometers an hour.
It's a very similar process to the process that occurred early on
in the formation of the solar system.
When a collapsing cloud of dust got, for some reason, a little bit of spin
and as that dust cloud collapsed,
the spin rate has to speed up and speed up.
That's the kind of thing you can see in storms like this
when tornadoes are formed.
The spin of the big storm system
can become concentrated and speeded up in, well, a tornado.
You get these immense wind speeds,
although the wind speed isn't too gentle now.
And it looks pretty wild up there, I've got to say.
In fact, you'll find I've never...
Look at that! I have rarely seen such dramatic clouds.
And, apparently, we've got about five minutes before the end of the world.
And, so we have to get back in the car.
There's thunder and lightning and there was a report earlier of,
from this storm, of hail the size of baseballs,
and I don't want one of those on my head.
Let's go. Get back to the car.
Let's go. It's beginning to come.
The end of the world!
This storm never developed into a tornado,
but when they do you can really see
the conservation of angular momentum in action.
There it is. A huge tornado. Look at that!
As the storm contracts, its core rotates faster and faster
until a column of violently rotating air descends from the cloud.
Man, look at that funnel!
The awesome spinning power of tornadoes
has incredibly destructive effects,
but it's this same phenomenon that is responsible for creating
the stability of the solar system.
Because it was the conservation of angular momentum
that stopped the early solar system collapsing completely.
Oh, my God! That's gonna be violent.
While gravity caused the nebula to contract,
its conserved spin gave rise to a force
that balanced the inward pull of gravity and allowed a stable disc to form.
When the Sun ignited, it [it up this spinning disc.
And within the disc the planets formed
all orbiting the Sun in their regular clockwork patterns.
In just a few hundred million years,
the cloud had collapsed to form a star system,
our solar system.
The Sun, surrounded by planets.
The journey from chaos into order had begun.
And there's no better place to see the results of that journey
than in what I think is one of the wonders of the solar system.
Of all the solar system's wonders there's a place we can go
where the processes that built the solar system
are still in action today.
A place of outstanding beauty and complexity.
A place that has entranced astronomers for centuries,
the planet Saturn.
This is NASA's jet propulsion laboratory
and I've known about this place or, at least, known its address,
Oak Grove Drive, Pasadena, California,
since I was very small, 'cause I wrote to them in 1975
to ask for pictures from the circle of Mars taken by Viking
and they sent them.
But, today, this is the control center for Cassini,
which is our one and today only spacecraft in orbit around Saturn.
Cassini was launched in 1997.
It is the largest, most sophisticated spacecraft
ever sent to the outer solar system.
Its purpose is to study Saturn and its rings.
And since 2004, it has been sending back the most amazing pictures.
They revealed that the rings are impossibly intricate,
made up of thousands upon thousands of individual bands and gaps.
The whole system is surrounded by a network of moons.
Part of Cassini's mission is to discover how the rings came to be like this,
how all this incredible structure was created.
Because, strange as it seems,
these beautiful patterns are as close as we can get to the disc
that formed the solar system.
And that is why the Saturnian system has so much to tell us.
I mean, I like to think of it as like a miniature solar system
because the moons are the equivalent of the planets
and Saturn's the equivalent of the Sun.
Carl Murray has spent a lifetime studying Saturn's rings.
In the rings, we're learning something about our own origins, if you like.
Because the physical processes that go on in the rings,
and their interaction with the small moons that are around,
are probably similar to what went on in the early solar system
after the planets formed and there is still a ring or debris
left over from the formation of the planets.
Ah! So if you looked at the solar system four and a half billion years ago,
the Sun at the center,
you'd have a disc of dust not unlike Saturn's ring system.
That's right.
But if we can't understand a disc of material that's in our own backyard,
what chance do we have of understanding a disc that's long since disappeared,
which is the one out of which the solar system formed.
So it's the same processes.
But we've got this incredible opportunity
with Cassini to observe things happening in front of our eyes.
Using the data from Cassini,
we are able to recreate Saturn's rings in incredible detail.
We can journey from the vast scale of the disc
to the minute structure of individual ringlets.
All the rings are in motion, orbiting Saturn at immense speeds.
Like the planets orbiting the Sun,
the rings nearest Saturn are the fastest,
traveling at over 80,000 kilometers an hour.
And while the rings appear solid, casting shadows onto the planet,
they are also incredibly delicate.
The main disc of the rings is over 100,000 kilometers across,
but as little as three metres thick.
Saturn's rings are undoubtedly beautiful.
And when you see those magnificent pictures from Cassini,
it's almost impossible to imagine that that level of intricacy and beauty
and symmetry could have emerged spontaneously,
but emerge spontaneously it did.
And for that reason alone,
Saturn's rings are one of my wonders of the solar system.
But there's more than that.
Because in studying the origin and evolution of Saturn's rings,
we've begun to gain valuable insights
into the origins and evolutions of our own solar system.
To try to understand the true nature of Saturn's rings,
I've come to this glacial lagoon in Iceland.
You know, there are two things,
the boat driver told me about these icebergs.
One, is that they can come up from the bottom of the seabed
without any warning at all,
fly up to the surface, tip the boat over and then you die.
Secondly, if you take some of it and take it home,
it's absolutely brilliant in whisky.
Because the water is pure, over one thousand years old,
no pollutants in it, and it makes whisky taste superb.
So it's either death or whisky.
That's my kind of pond.
But I'm really here because the structure of the rings
is remarkably similar to the way these icebergs float in the lagoon.
Because, despite appearances, the rings aren't solid.
Each ring is made up of hundreds of ringlets
and each ringlet is made up of billions of separate pieces.
Caught within the grasp of Saturn's gravity,
the ring particles independently orbit
around the planet in an impossibly thin layer.
Thanks. See you in a minute.
- Yea h, hopefully. - Yeah.
But the similarity doesn't end with the layout.
It also lies in what the rings and the icebergs are made of
and that explains why the rings are so bright.
Well, this is why we can see Saturn's rings from Earth.
Because this is what they're made of.
They're made of beautiful, pure water ice,
sparkling in the sunlight.
Billions of these pieces a billion kilometers away from Earth.
Most of the pieces are, well, smaller than that.
Less than a centimeter. Many are micron-sized ice crystals.
Some are as big as this iceberg. Some are as big as houses.
Some can be over a kilometer across.
Now, imagine sitting on one.
Imagine if this were a piece of Saturn's rings.
What a view.
This is the closest we can get to Saturn's rings on Earth
and the view would be remarkably similar.
Billions of chunks of ice shining brightly as they catch the sunlight.
And the reason the rings shine so brightly
is that, like the icebergs, the rings are constantly changing.
As the ring particles orbit Saturn,
they 're continually crashing into each other
and collecting into giant clusters
that are endlessly forming and breaking apart.
As they collide, the particles shatter,
exposing bright new faces of ice that catch the sunlight.
It's because of this constant recycling
that the rings are able to stay as bright and shiny
as they were when they formed.
For me, one of the most remarkable things about Saturn's rings
is their dynamism, their constant renewal.
It's because of that dynamism that we can see them at all.
That's why they're clean. That's why they reflect sunlight
and we can see them from Earth.
It's, I suppose, a bit like a city
that people come and go, buildings get torn down and rebuilt.
But the city always remains the same. So it is with Saturn's rings.
They're different today than they were a thousand years ago.
They'll be different in a hundred or a thousand years' time.
But that structure and that beauty,
that magnificence will always remain.
Saturn's rings are magnificent for more than just their beauty.
Because by looking at the rings,
we can begin to understand our own origins.
And the key to understanding the rings can be found orbiting around them.
That is absolutely incredible.
I can see the rings are completely end-on.
I mean, when you see that,
I've looked at the sky, I've looked at Saturn hundreds of times,
but I've never seen it through a telescope like this.
And you really get a feeling it's a planet.
I know what Galileo thought when he said that the planet had ears.
He didn't have one of these telescopes though.
And I can see one, two, three...
Four, five moons around the planet.
This is incredibly beautiful.
Seen like this it's easy to appreciate how Saturn is like a mini solar system
with the moons orbiting like planets around the Sun.
From Earth, we can only see a few of the larger moons.
But, in total, Saturn has more than 60 moons
and seen close up, they are a weird and wonderful bunch.
Dione is typical of Saturn's icy moons.
It looks similar to our moon, but its composition is very different.
It's about two-thirds water,
but the surface temperature is minus 190 degrees Celsius.
And, at those temperatures, the surface behaves pretty much like solid rock.
Iapetus is known as the "Yin and Yang" moon.
One half clean ice, the other coated in black, dusty deposits.
The giant moon Titan is bigger than the planet Mercury.
But the unique thing about Titan is this atmosphere,
which is four times as dense as the Earth's.
It's rich in organic molecules and it's thought that the chemistry
is very similar to that of the primordial Earth before life began.
And Hyperion is a moon unlike any other.
It's not even round and its battered surface has the texture of a sponge.
And one theory for that
is that it's actually a captured comet that has drifted in
from the distant reaches of the solar system
and been captured by Saturn's gravity.
But the moons of Saturn aren't just a celestial freak show.
They're the driving force behind the beauty and structure of the rings.
The most remarkable of them is hidden in of one of the outer rings.
Buried in the heart of the E-ring is a moon that is rapidly becoming
one of the most fascinating places in the solar system,
Enceladus.
Enceladus has long been an astronomical curiosity
because it's the most reflective object in the solar system.
But we've known little about it
because Enceladus is tiny, only 400 kilometers across
and over a billion kilometers away.
It's only now we have these amazing images from Cassini
that we can see just how strange it is.
Its heavily cratered northern hemisphere looks like any other icy moon.
But the southern hemisphere tells a very different story.
It's almost completely free from craters
which means that the surface is probably newly formed.
It's scarred by canyons and riven by cracks.
It all looks remarkably similar to the geology of Earth,
but carved in ice rather than rock.
And right over the South Pole are the Tiger Stripes.
Four parallel trenches
over 130 kilometers long and possibly hundreds of metres deep.
They look just like tectonic fault lines.
This is what tectonic faults look like on Earth.
This is the continental divide in Iceland
where the American and European plates are spreading apart.
The cliffs at the edge of the plates
look out over a plain of new crust formed from molten lava
pushing up from the center of the Earth.
Carolyn Porco, head of the Cassini Imaging Team,
thinks that something similar may be happening on Enceladus.
It is one of the most unique places in all the solar system.
And you can tell that just by looking at it.
And we think that it's possible
that there is something similar to what's happening right here
where you might get slushy ice,
viscous ice, that comes up through the cracks, okay,
and creates more surface ice the way you get more crust created right here,
pushing things out to the side
and it's buckling by the time it gets to what is now the mountains.
Oh, so it really is similar to Iceland
actually where you're getting lava
welling up from the surface and creating new land.
So, in the same way, you've got ice. Possibly.
We think. In fact, it gives an indication of just how
this whole system down there may be working.
The next clue that something was happening under the surface
came when Cassini flew directly over the South Pole.
Thermal readings showed hotspots under the Tiger Stripes.
For some reason, the stripes were much hotter than the rest of the moon.
Cassini has found the unthinkable.
It's found that this southern tip of Enceladus is excessively warm.
There's more heat coming out of the south polar cap, if you will,
of Enceladus than is coming out of the equatorial regions.
It would be like saying there's more heat coming out of Antarctica
than the Equator on Earth.
Then, one day in November 2005,
Cassini photographed Enceladus just as the Sun was setting behind it.
What it saw became one of the most remarkable discoveries
ever made in the outer solar system.
The backlit images revealed giant fountains erupting from the South Pole.
Volcanoes blasting out ice instead of rock.
Those images blew everybody away.
I mean, that was like game over, you know.
Here you have these dozen or more narrow jets
and they just look ghostly and fantastic.
Just a few miles away from the continental divide
is an area that can help us understand the ice fountains.
This is one of Earth's hotspots
where the volcanic heat of the planet's core
bubbles up to just below the surface.
Until a few years ago, Enceladus was thought to be an unremarkable world,
a small, frozen, barren lump of rock and ice.
But those fountains of ice
erupting thousands of kilometers out into space
mean that there's something
incredibly interesting going on beneath its surface.
It's here we find the earthly phenomenon most like the ice fountains,
geysers.
They're formed when underground pockets of water
suddenly boil and explode into the air.
Geysers on Earth require three things.
They require a ready source of water.
They require an intense source of heat just below the surface
and they need just the right geological plumbing.
So, if the geysers on Enceladus are similar,
then that raises the intriguing possibility
that there's an ocean of liquid water beneath the surface of the moon.
And it raises a very interesting question.
Because Enceladus is far too small
to have retained any meaningful source of heat in its core.
So, where does that heat come from?
On Earth, the geysers are driven by the intense temperatures
inside the planet, hot enough to melt rock and power volcanoes.
But Enceladus is so tiny that its core should be frozen solid.
Enceladus must be getting its heat from somewhere else
and it's thought that it might come from its peculiar orbit around Saturn.
So, the next thing to investigate was whether or not
you could have, what we call, tidal forces flex Enceladus.
And that simply arises because the orbit of Enceladus is eccentric,
meaning it's elliptical, out of round.
And as it encircles Saturn in its orbit,
it gets close to Saturn and then far away, close and far away
and the gravitational pull changes as it moves in its orbit.
So, that means the body's flexing,
and if it's flexing, it means it's undergoing friction inside.
This is a major process for injecting energy that turns into heat
into a body like Enceladus.
As Enceladus orbits,
Saturn's gravity actually distorts the shape of the moon.
It's thought that this heats the interior of the moon just enough
to melt a small underground ocean of water.
As it contacts the vacuum of space,
that water vaporizes and explodes out of the moon,
creating this wonder of the solar system.
These geysers are incredibly impressive natural phenomena,
but they pale into insignificance
compared to the ice fountains of Enceladus.
It's cool.
That is so cool.
While this geyser erupts every few minutes,
blasting boiling water 20 metres into the air,
on Enceladus, the plumes are thought to be erupting constantly
and for them, the sky is the limit.
Bursting through the surface at over 1,300 kilometers an hour,
they soar up into space for thousands of kilometers.
They must be one of the most impressive sights in the solar system.
Any liquid water instantly freezes into tiny ice crystals.
Some of it falls back onto the surface,
giving the moon its reflective icy sheen.
But the rest keeps going all the way around Saturn.
The ice fountains are creating one of Saturn's rings as we watch.
The whole E-ring is made from pieces of Enceladus.
But Enceladus is not the only moon that shapes the rings.
Saturn's other moons also play a crucial role
in creating these beautiful patterns
and they do so in mysterious ways.
The Sahara Desert may seem an unlikely place to come
to explain Saturn's rings.
But the behaviour of the sand in the desert
can help us understand how the moons formed the patterns in the rings.
At first sight, the Sahara Desert seems an immensely chaotic place.
Just billions of grains of sand being blown
randomly around by the desert winds,
but actually look a little bit closer
and you start to see an immense amount of order.
There are sand dunes as far as the eye can see.
Then a remarkable thing is that,
the angles of the front of all the sand dunes are exactly the same.
In the Sahara, the emergence of that order is driven
by the desert winds blowing always in the same direction,
day after day, year after year, moving the sand around.
In the Saturnian system,
the order and beauty and intricacy of the rings
is driven, obviously not by wind, but by a different force,
the force of gravity.
As the moons orbit Saturn
their gravitational influence sweeps through the rings.
In these amazing images,
we can actually watch the moons as they work.
We can see gravity in action.
As the moons pass close to the rings,
their gravitational pull tugs the ring particles towards them,
distorting the shape of the rings.
The F-ring, one of the outer rings, is twisted into a spiral shape
by two moons, Prometheus and Pandora.
In this video taken by Cassini,
you can see how Prometheus drags plumes of material away
as it passes close to the rings.
These short-range gravitational effects account
for many of the patterns in the rings.
But sometimes the moons can exert their pull over much greater distances
and the way they do so reveals the subtlety
with which gravity can work.
Well, here's a model of the Saturnian system.
With Saturn in the middle and the magnificent ring system
going around the outside.
And the first thing you notice when you look at the rings
is a huge gap called the Cassini division.
Now what could possibly have caused that?
Well, it's all down to one of Saturn's moons called Mimas,
which orbits well outside the ring system.
And how could something that far outside the rings
have any influence at all on the particles inside the rings?
Well, it's all down to a phenomena called orbital resonance.
Now the particles in the Cassini division
have an interesting relationship with the moon, Mimas.
Because they orbit around Saturn twice for every single orbit of Mimas.
And that has an interesting consequence.
Imagine there's a particle inside the Cassini division.
Then every second year, for this particle,
it meets up with Mimas. They end up in the same place in space.
And that means that this particle will get a kick or a tug
from Mimas' gravity on a regular basis.
Every second year. Bang, bang, bang.
And that alters the orbit of anything that's in the Cassini division
and actually has the effect of throwing it out,
of clearing a gap in the rings.
And, in fact, much of the complex and beautiful structure
of Saturn's rings is down to these orbital resonances.
Not only with Mimas,
but with one or more of the 61 known moons of Saturn
that orbit outside and, indeed, some inside the rings.
And for me, that's part of the wonder of Saturn's rings.
Their beauty is such a good illustration
of how gravity can carve order out of chaos.
But more than that, understanding how Saturn's moons
shaped the rings can shed light on the events
that shaped the early solar system,
events that helped create the world we live in.
Resonance can be much more than a delicate sculptor.
Because it's not only small moons that can enter orbital resonance.
It's now thought that billions of years ago
the two giants of the solar system, Jupiter and Saturn, entered a resonance
and that unleashed forces that could move entire planets.
And that made the solar system
an incredibly turbulent and violent place.
The solar system is full of craters.
The record of a long history of cataclysmic impacts.
But there was one period 3.6 billion years ago
when the whole solar system was turned inside out
by the same forces of orbital resonance that shaped Saturn's rings.
We now believe that the giant planets
formed much closer to the Sun than they are today.
Their orbits drifted for hundreds of millions of years
until Jupiter and Saturn fell into a resonant pattern.
Once every cycle the two planets aligned in exactly the same spot,
creating a gravitational surge
that played havoc with the orbits of all the planets.
Neptune was catapulted outwards
and smashed into the ring of comets surrounding the solar system
with dramatic consequences.
For a hundred million years,
the solar system turned into a shooting gallery
as a rain of comets ploughed through it.
Millions of comets were scattered in all directions,
peppering the planets.
It was called the Late Heavy Bombardment.
It created many of the craters we see throughout the solar system today.
It left scars all over our moon.
And it had a lasting impact on the Earth as well.
The only impact craters we see on Earth today,
like this one in Arizona, were made much more recently.
But they reveal the scale of these impacts.
And today, impacts like this are relatively rare,
although they will happen again.
But during the Late Heavy Bombardment,
the Earth was hit by thousands of objects
with sizes far in excess of the object that made this crater,
and the environment was changed radically and dramatically.
But those changes weren't necessarily catastrophic
because it's now thought that a significant amount
of the water in the Earth's oceans was delivered
by the impacts of water-rich comets and other objects
during the Late Heavy Bombardment.
And that means that impacts could have played a key role
in the development of life on Earth.
Before the Late Heavy Bombardment, the Earth was a barren rock.
Afterwards, it supported the oceans that would become the crucible for life.
Without the water delivered in the Late Heavy Bombardment,
life on Earth may never have evolved.
It's quite a thought that all this
may have been caused by the violent resonances
generated by the orbiting planets.
The story of the solar system
is the story of the creation of order out of chaos.
The planets and their moons were created by the same universal laws.
The delicate interaction between gravity and angular momentum
that led to the spinning patterns we see around us today.
Ultimately, that journey,
created the finest example of those forces in action.
Because, in creating the solar system,
those forces that sculpted order out of chaos
also created the best and most beautiful laboratory
for studying how the solar system works,
Saturn's rings.
It's often the case in science,
that answers to the most profound questions
can come from the most unexpected of places.
Saturn's rings were initially studied because of their beauty.
But understanding their formation and evolution
has led to a deep understanding
of how form and beauty and order can emerge from violence and chaos.
And that understanding can be spread across the entire solar system.
And remember that you and me are part of the solar system.
You and me are ordered structures
formed from the chaos of the primordial dust cloud
4.5 billion years ago.
And that is one of the wonders of the solar system.