The Solar System-2.
When Galileo first looked up at Saturn, through his primitive telescope in 1610,
he thought that the planet had ears.
He thought that Saturn had two giant moons orbiting it.
To his dismay he found that the moons seemed to disappear after a while.
What Galileo was seeing, were the rings of Saturn disappearing as Saturn and Earth orbited the Sun,
shifting our perspective, so that the rings were edge on to us.
Saturn does have moons, about 62 of them.
They hold the key to understanding why the rings have such an intricate structure.
Saturn is like a mini Solar System.
Dione is typical of Saturn’s icy moons.
It looks similar to our own moon, but its composition is very different.
It is about 2/3rds water.
The surface temperature is minus 190 degrees.
At these temperatures the water surfaces behaves like solid rock.
Iapetus is the third largest moon of Saturn, and also one of the most mysterious.
One half of the moon is clean ice, and the other is black dusty deposits.
It is now believed that half the surface is covered in a layer of carbon.
These dark deposits are thought to have come from a initial Metroid impact.
Saturn’s largest moon is Titan.
This giant moon is bigger than the planet Mercury.
It is almost as big as Mars, and is the second largest moon in the Solar System.
It is the only moon that we know of, that has a fully developed atmosphere.
The atmosphere is 4 times as dense as our atmosphere.
It is rich in organic molecules.
It may have a chemistry very similar to that of primordial Earth, before life began.
The moon Hyperion has a battered surface, and has the texture of a sponge.
One theory is that it is a comet, that drifted in from the distant icy reaches of the Solar System,
and was captured by Saturn’s gravity.
Saturn’s moons are a collection of diverse and fascinating objects.
They are the driving force behind the beauty and structure of the rings.
The icy moon of Enceladus was discovered by William Herschel in 1789.
Herschel was trained as a musician, and composed 24 symphonies.
However it is his inventiveness as an astronomer, that assured him of a place in history.
Herschel’s crowning achievement was the discovery of Uranus in 1781.
Despite believing that every planet was inhabited, including the Sun,
Herschel was a brilliant telescope builder, innovator and observer of the night sky.
His sister Caroline discovered several comets and nebulae.
His son John became a famous astronomer.
In 1789, Herschel built the famous 12 metre telescope,
which was the largest in the world at that time.
On the very first night he became the first person to see Saturn’s 6th largest moon, Enceladus.
Enceladus is very tiny.
For 200 years we only knew that it was made of water ice, its orbit,
and estimations of its mass and volume.
An intriguing property of Enceladus, was that it is the most reflective object in the Solar System.
Its icy surface reflects almost all the sunlight that strikes it.
It is over a billion kilometres away from Earth, and only 500 kilometres across.
The extraordinary secrets of Enceladus remained a mystery for over 2 centuries.
Our first proper glimpse of Enceladus came in 1981,
when the voyager spacecraft passed within 87,000 kilometres of the moon.
Buried deep within the E ring of Saturn, the images it took of Enceladus,
reveal something that no one was expecting.
This ancient moon had a smooth surface.
Virtually every moon in the Solar System is riddled with craters, from the impact of astroids.
It is impossible for any place in the Solar System,
to have escaped the heavy bombardment of debris from space over billions of years.
So the surface of Enceladus must be young.
Its terrain must be constantly regenerating.
So, this frozen moon must be geologically active.
Only after seeing the images from Cassini, have we began to understand,
the strange and wonderful truth, behind the smooth surface of Enceladus.
Its heavily crated northern hemisphere looks like any other icy moon.
But its sudden hemisphere is smooth and crater free.
It is carved by canyons and riven by cracks.
It looks remarkably similar to the geology of Earth, but carved in ice rather than rock.
Some extraordinary features have been found over the south pole.
A high resolution image from Cassini shows four parallel trenches over 130 kilometres long,
and forty kilometres apart, and possibly a few hundred metres deep.
It was nicknamed ‘The Tiger Stripes’.
These features look like the tectonic fault lines found on Earth.
But Earth’s geology is powered by the powerful heat source, of its molten core.
The heat is leftover from Earth’s formation 4.5 billion years ago.
It is also partly due to the slow decay of heavy radioactive elements in its core.
But a tiny moon like Enceladus should have lost its meagre supplies of heat long ago,
to the cold of space, and should surely be geologically dead.
Iceland sits on the dividing line between two continents.
This great divide is one of the best places on Earth,
to explore the mysterious Tiger Stripes of Enceladus.
In Iceland you can see the inner workings of Earth exposed.
The North American plate, is the land mass that forms the Western half of the North Atlantic,
and the United States, Canada and parts of Siberia.
It is slowly moving west.
The Eurasian plates, comprising Europe and Northern Asia, is drifting east.
Standing at the boundary, you can see the result of the inexorable drift of the continents,
ripping apart the surface of the Earth, and creating a plane of new crust,
between the two towering cliff tops, formed from molten lava,
pushing up from deep beneath the surface.
Something similar may be happening on Enceladus.
A rift valley could be created on Enceladus, sculpted not from molten rock, but from ice.
In 2005, Cassini flew directly over the south pole of Enceladus, at a distance of 175 kilometres.
Using a infrared spectrometer it discovered direct evidence of geological activity beneath the surface.
The thermal reading showed hotspots under the Tiger Stripes.
The average surface temperature of Enceladus is 75 Kelvin, but around the stripes it was 130 Kelvin.
This was a startling discovery.
There is more heat coming out of the southern polar cap of Enceladus,
then from the equatorial regions.
It is like more heat coming out of Antartica than the Equator.
The greatest revelation came when Cassini photographed Enceladus,
just as the Sun was setting behind it.
The backlit images revealed giant fountains erupting from the Tiger Stripes,
Volcanoes blasting out ice instead of rock.
Until recently, Enceladus was thought to be a small, frozen barren lump of rock and ice.
But these fountains of ice erupting thousands of kilometres into space,
revealed that is something incredibly interesting going on beneath its surface.
An analogy is the Great Geysir in Iceland.
Its a towering fountain of boiling water and steam.
It gave its name to the phenomena wherever it is found on Earth.
It is currently dormant.
It is a few metres away from Strokkur Geyser, which erupts every two minutes.
Geyser’s are the Earthly phenomena most like the ice fountains of Enceladus.
Geyser on Earth require 3 things.
A ready source of water.
An intense source of heat, and the right geological plumbing.
If the geysers on Enceladus are based on a similar mechanism, this raises an intriguing possibility.
There must be a source of liquid water beneath the surface.
Maybe a small lake or perhaps even an ocean that feeds the explosive volcanoes of ice.
Yet Enceladus is a billion kilometres away from the Sun, in the cold outer regions of the Solar System.
It is far too small to have retained any meaningful source of heat in its core.
So, where does the heat come from?
On Earth, the geyser’s are driven by the same primordial heat source,
that powers the drift of the continents.
But Enceladus is so tiny that its core should be frozen solid.
Enceladus must probably be getting its heat from its peculiar orbit around Saturn.
Enceladus moves around Saturn in an elliptical orbit.
This means that during each orbit Enceladus moves closer and then further away from Saturn.
This eccentric orbit has a profound effect on Enceladus,
changing the gravitational force exerted on the moon during every turn.
As the difference in forces between the near and far sides of the moon changes,
it literally flexes the moon as it travels around Saturn.
This distorts its shape creating vast amounts of friction deep within.
Friction causes heat, and it is thought that the interior of Enceladus,
is heated just enough to melt a small underground ocean of water.
As this water meets the vacuum of space, it immediately vaporises and explodes out of the surface,
creating a new wonder of the Solar System.
On Earth geyser’s erupt every few minutes almost, blasting boiling water 20 metres into the air.
In Enceladus the plumes are thought to be erupting constantly.
Bursting through the surface at 1300 kilometres an hour, they rise thousand of kilometres into space.
The water vapour in the plumes then freezes into tiny ice crystals.
Some of it falls back on Enceladus’ surface, giving it its reflective icy sheen.
But the rest keeps going all the way around Saturn.
The ice fountains are creating one of Saturn’s rings.
The whole E-ring is made from pieces of Enceladus.
Enceladus is not the only moon that shapes the rings.
Saturn’s other moons also play a crucial role in creating these beautiful patterns,
but they do so indirectly.
Gravity as Newton published in 1687, is a simple force to describe.
Einstein completely rewrote our understanding of gravity, in his theory of General Relativity,
published in 1915.
His superior and more accurate theory is not necessary for understanding,
the clockwork of the Solar System.
Mercury is an exception, because it orbit is so close to the massive Sun,
that relativistic effects are important.
Gravity provides an attractive force that is proportional to the masses of objects,
and falls away with the square of the distance between them.
In Einstein’s theory, we should say energy as well, but we do not need this subtleties here.
Saturn’s moons gravitationally attracts the tiny particles of ice in the rings towards them.
If you double the distance between the ice particles and the moon,
the force of gravity drops by a factor of four.
Imagine the complexity of the gravitational tugs on the ice particles in the rings,
as they orbit around Saturn.
They are constantly being pulled by Saturn’s vast phalanx of orbiting moons,
in an intricate complex dance.
It is this complex gravitational field created by Saturn and its moons,
that gives rise to the structure in the rings.
The gravitational mishmash is present around any planet that has moons.
Saturn is special because there is a sheet of orbiting dust and ice, sprinkled through out the system,
allowing us to see gravity in action.
When we sprinkle iron fillings on a sheet of paper over a bar magnet,
we can see them form into a beautiful pattern, that reveals the hidden magnetic field.
This is exactly what happens around Saturn.
Except in Saturn, it is the gravitational field that is being revealed by billions of tiny sprinkles of ice.
The easiest structure in the rings to explain, is the biggest.
It is the gap called the Cassini division, between the rings A and B.
This huge swathe of the rings is devoid of ice particles, because of the influence of the moon Mimas,
the Death Star moon, which orbits well outside the rings.
It is a general feature of orbits, that further away from the planet the orbit is,
the longer it takes to complete a single orbit.
All the rings are inside the orbit of Mimas.
So all the particles orbits Saturn faster and constantly overtake Mimas.
When they hit to the closest approach to Mimas, the gravitational force exerted by the moon,
on the particles will be at its greatest, disturbing their orbit a little bit.
For most of the ring particles, this extra little kick happens at quite random points during the orbit,
and the overall effect cancels out.
There are some particles in the rings whose orbits have an interesting relationship with Mimas.
They go around Saturn twice for every single orbit of Mimas.
They get close to the moon on one of every two of their orbits.
By the time they have gone around Saturn twice, Mimas would have returned to the same position.
We say that the particles and Mimas are in orbital resonance.
This means they get periodic gravitational kicks on a regular basis, and their orbits are disturbed.
This causes them to move out of that orbit.
This is the case for any particle that wanders into the Cassini division.
The gap is the place in a space where the ring particles orbits would be resonant with Mimas.
It is thought that much of the structure in Saturn’s rings is down to resonances,
between the ring particles and one or more of the planet’s moons.
Understanding how Saturn’s moons shape the rings can shed light on the events,
that shaped the early Solar System.
It seems that between 4.1 and 3.8 billion years ago, our moon came under an extraordinary attack.
It was bombarded in a meteorite storm that transformed and shaped the surface we see today.
This showering of debris should also have affected Earth and other inner planets.
Many scientists now believe this is evidence of an incredibly violent period,
in our Solar Systems’ history.
It is known as the late Heavy Bombardment.
In 1971, the Apollo 15 mission landed on the moon in an area known as the Mare Imbrium.
The crew used a Lunar buggy to collect over 77 kilograms of Lunar rock samples.
Mare Imbrium is a vast basin, created by a colossal impact early in its history.
Its smooth surface was created when the then Volcanically active moon,
flooded the impact crater with lava, creating the flat surface we see today.
Many of the rock samples collected by Apollo 15, 16 and 17 missions, where impact melt rocks.
It was created by the extreme conditions of direct meteorite impacts.
Samples collected all over the moon have been dated using radioactive dating techniques.
A significant number of these rocks seem to have been created by impacts,
that took place in a relatively short time frame, during the late heavy bombardment.
This period also affected Earth and the other inner planets.
Earth would have been peppered with thousands of impacts.
Many impact craters were 1000 to 1500 kilometres in diameter.
The cause of the bombardment may lie exactly the same phenomena that shaped,
the structure and complexity of Saturn’s rings - orbital resonance.
It is now thought that billions of years ago, the two giants of the Solar System,
Jupiter and Saturn entered a resonance.
If planets enter a resonance, the orbits of the planets are changed.
When planets start to fly around, the Solar System becomes an incredibly turbulent and violent place.
It is now thought that Saturn, Uranus and Neptune formed much closer to the Sun,
then they are today.
Their orbits drifted slowly for hundreds of millions of years,
until Jupiter and Saturn entered a resonance.
Once every cycle, the two planets aligned in the same spot, creating a gravitational surge,
that played havoc with the orbits of all other planets.
Saturn, Uranus and Neptune all migrated outwards.
It plunged the Solar System into violently unstable era, triggering the Late Heavy Bombardment,
in particular.
Neptune was catapulted outwards and smashed into the ring of icy material,
in the outer Solar System, randomly scattered them into orbits that crisscrossed it.
For a hundred million years, a rain of comets ploughed through the Solar System.
It peppered the planets and created many of the craters we see on the planets and moons today.
One defining characteristic of Earth maybe a direct result of the thousands of comet impacts,
that battered the Earth around 3.6 billion years ago.
Comets have a different composition to asteroids.
This becomes visible when they venture into the inner Solar System,
close enough to be warmed by the Sun.
As they absorb the Sun’s heat they display a tail, and atmosphere,
as the ingredient they have in abundance, water, evaporates into space.
It seems that a significant amount of the water in Earth’s oceans was delivered,
by the impacts of water rich comets, and other objects, during the Late Heavy Bombardment.
Before this Earth may have been relatively barren rock.
After the advent of oceans, it would have become the crucible for life.
The water rich world we see today, may have been shaped by violent resonances,
generated by the gas giants, Jupiter and Saturn.
The Solar System is an ordered structure, formed from the chaos of the primordial dust cloud.
The Solar System is a violent, inhospitable place.
On every planet and moon, we have encountered extremes.
We have discovered fiercest heat and bitterest cold.
We have seen landscapes sculpted by overwhelming pressure.
We have witnessed storms the size of planets.
Amongst all these hostile wonders sits our Earth,
an oasis of calm amidst the violence of the Solar System.
What separates us from the extremes, is a thin, flimsy envelope of gas.
It is due to the atmosphere we have, that we have air to breathe, water to drink,
and the landscape that surrounds us.
The Earth would not be a wonderfully diverse place, without the atmosphere, the thin blue line.
It acts as a soothing blanket that traps the warmth of the Sun,
yet protects us from the harshness of its radiation.
The oxygen, water and carbon dioxide in the atmosphere,
plays a fundamental role in the survival of millions of species, living on Earth.
When perfectly balanced, a world as beautiful as the Earth can evolve beneath the clouds.
The slightest changes can lead to alien and violent worlds.
There are planets in our Solar System, that have been transformed into hellish worlds,
by nothing more than the gases in their atmosphere.
An atmosphere can choke a planet to death.
They are also powerful enough to shape their surfaces.
There are planets which are giant balls of churning gas,
where storms three times the size of the Earth rage for hundreds of years.
All atmospheres in the Solar System are unique.
But the ingredients and forces that shape them are universal.
At the heart of each is a fundamental force of nature that holds the Solar System together: gravity.
Gravity is by far the weakest known force in the Universe.
It is easy to pickup a rock, though there is a whole planet Earth, pulling the rock down.
It is incredibly weak, but important because it’s the only force there is to hold an atmosphere,
to the surface of Earth.
In 1687, Newton described it as a force by which objects with mass are attracted to each other.
This is an approximation.
Einstein provided a more sophisticated view of gravity in the General Theory of Relativity in 1915.
For our purpose, Newton’s simpler view is sufficient.
Compared to the other three fundamental forces of nature, the strong nuclear force,
the weak nuclear force, and electromagnetism - gravity is the weakest.
Yet when combined with these forces, it creates the conditions for the stars to ignite,
holds the planets and the moons in their orbits, and binds the galaxies together.
Gravitational force between two masses, is equal to universal gravitational constant G,
multiplied by the mass of each object, and divided by the square of the distance between them.
It’s a great example of how the power of simple mathematics can be.
The most distant planet from our Sun, was predicted by Newton’s law of gravity,
long before it was directly observed by the human eye.
Over four billion kilometres from Earth, the blue planet of Neptune is one of the coldest places,
in the Solar System.
It is the only planet entirely invisible to the naked eye.
The real clue to Neptune’s existence, came not from the observations of this planet,
but from the observations of its nearest neighbour, Uranus.
Uranus was discovered in 1781 by William Herschel.
In 1821, the first astronomical tables of Uranus’ orbit was published by the astronomer Bouvard.
Using Newton’s laws of gravitation, the tables provided accurate predictions,
for the future positions of the planet as it orbited the Sun.
However, the path of Uranus did not agree with the path that Newton’s law predicted.
Some scientists thought that something was disturbing the orbit of Uranus.
They thought this perturbation was caused by an as yet undiscovered planet.
In 1845-46 the astronomer’s Verrier and Adams independently calculated,
the mass and position of such a new planet.
Their collective work led to the precise prediction of the existence of a giant planet,
orbiting outside Uranus.
The prediction was confirmed by the astronomer Galle,
who made the first telescopic observation of Neptune in 1846.
The discovery of Neptune is an example of the predictive power of the laws of physics,
and the universal influence of gravity.
In this case it described the massive force between two giant planets orbiting the Sun.
It can also explain the tenuous connection that joins a planet with its atmosphere.
Our atmosphere, the thin fragile layer of gas, that surrounds Earth,
is held to our planet by nothing more than gravity.
Just as two planets exert a force on one and another,
so there is a force between each atom in our atmosphere, and the Earth.
This incredibly weak force binds the atmosphere to Earth.
The more massive the planet, the greater the gravitational force,
binding the atoms in the atmosphere to the surface.
Fortunately the Earth has enough mass to keep a tight grip on the heavier gas molecules,
that make up our atmosphere.
Nitrogen, the odourless tasteless gas, constitutes 78% of our atmosphere.
Nitrogen is extremely stable, and reacts very little,
making it an incredibly long lived gas in the atmosphere.
Oxygen constitutes 21% by volume, of the atmosphere.
The inert gas argon constitutes 1%.
The rest of the atmosphere consists of trace gases, such as carbon dioxide, neon,
nitrous oxide and methane, which comprise just .039% of the Earth’s atmosphere.