The Planets (2019–…): Season 1, Episode 1 - A Moment in the Sun - The Terrestrial Planets - full transcript
The four inner planets were born the same, yet have lived radically different lives. Why?
The night sky is ablaze with
stars...
..hundreds of billions in our
galaxy alone...
..many larger, brighter
and more majestic than our sun.
On the scale of galaxies and stars,
the planets of our solar system
are little more than grains of sand
caught momentarily
in the light of the sun.
But on those motes of dust,
for over four billion years,
great stories have played
out unseen.
Stories of worlds born...
..and worlds lost.
Planets forged amongst the calm...
..and the chaos.
Their destinies more entwined than
we ever imagined.
We know this...
..because in the last few decades...
..we've sent spacecraft to all
seven...
..of the worlds beyond our own.
These are the stories
that they return to Earth,
the stories of the planets.
For the first few million years
after the sun formed,
there were no planets to see it
rise...
..just clouds of dust and the gas...
..the leftovers from the birth
of the sun.
Over tens of millions of years,
the dust began to stick together...
..and form the first rocks.
Eventually, gravity assembled
the rocks
to create planetary embryos...
..that in time formed the four
closest planets to the sun.
Today, Mercury is the closest of
all,
enduring the sun's full glare.
Further out...
..lies Venus...
..chocked by a thick atmosphere.
Then Venus's neighbour, Earth.
And farthest of all, Mars...
..a cold desert world.
Together, they formed the only rocky
so-called terrestrial planets
in the solar system.
And of the four...
..one is unique.
Just look at this...
..and listen to it.
This is what a planet looks
and sounds like
after four billion years of
evolution by natural selection.
There is nowhere else in the solar
system
that looks and sounds like this,
which is interesting,
when you think about it,
because all the planets and moons
and are made out of the same stuff -
they're carbon, nitrogen, oxygen,
iron.
All those atoms were present in the
cloud that collapsed
to form the solar system,
four-and-a-half billion years ago.
And yet Earth appears
to be exceptional -
a lone living planet in an otherwise
desolate solar system.
So what is it that makes
this place so special?
Is it fate? Is it chance?
These are important questions
because Earth is the only place
we know of
where the most complex
phenomena in the universe exists,
the thing that brings meaning
to the universe - life.
Earth is a special world
in our solar system
and perhaps even for thousands of
light years beyond.
Our world certainly has unique
properties.
It's the right size and distance
from the sun
to have retained an atmosphere
that's protected its oceans
of life-giving water for billions of
years.
But as we've left the blue
planet,
and explored our sister worlds...
..we've discovered that each
appears to have had a moment
when it enjoyed almost
Earth-like conditions.
Every one of our rocky neighbours
has a story of what might have been.
Mercury is a small, tortured
world.
More than any other planet,
it's endured the unflinching glare
of the sun for billions of years.
Mercury is a world of mystery
and apparent contradictions.
It's in quite an elliptical orbit,
which means it can be as far away
from the sun as 70 million km,
but as close as 46 million.
That means that temperatures
at midday
can rise to 430 degrees
Celsius on the surface,
but, at night, because it's a small
planet and it's got no atmosphere,
temperatures fall to minus 170
degrees.
It's also locked into what's called
the spin-orbit resonance,
which means the planet spins
precisely three times on its axis
for every two orbits
and that in turn means that its day
is twice as long as its year.
That means that I could walk
over the surface like this
about 2mph
and keep the sun at the same point
in the sky.
I could stroll in eternal twilight.
Mercury is the least explored
of the inner rocky worlds...
..because getting to a planet in
such a strange oval-shaped orbit,
so close to the sun...
..is a tremendous challenge.
Five, four, three,
main engine start,
two, one and zero.
And lift off of Messenger on Nasa's
mission to Mercury...
..a planetary enigma in our
inner solar system.
Now going through the sound
barrier.
A direct route to Mercury is
impractical.
Now going through the period of
maximum dynamic pressure.
A spacecraft would arrive with so
much speed
that it would need vast amounts of
fuel to slow down
and enter orbit around Mercury.
We just had spacecraft separation.
So Messenger controlled
its trajectory
by stepping from one planet to the
next,
using gravity to slow itself,
spiralling inwards towards its
target.
Even so, Messenger approached
Mercury at such high speed
that it was forced to fly past the
planet three times...
..slowing on each pass...
..until, after almost seven years
of flawless navigation,
it arrived safely in orbit.
Messenger set about its mission
to map Mercury's surface...
..and began revealing the secrets
of the most cratered planet
in the solar system in exquisite
new detail.
Messenger was able to do much
more
than just take images of Mercury's
surface.
By tracking radio signals emitted
by the spacecraft,
we're able to see very slight
changes in the orbital path
around Mercury, as seen from Earth,
and that allows us to map out
Mercury's gravitational field.
There are also instruments
that allow us
to see how the planet wobbles around
as it spins on its axis.
And putting all these measurements
together
allows us to take a cross section
through the planet,
to see what it's made of.
And when we do that,
we find something very strange.
Mercury's core extends out about
85%
from the centre of the planet
to the surface.
It's almost entirely an exposed
planetary core.
It's as if the rocks of the surface
were smashed away
and removed at some
point in its past.
And there was more.
The tiny probe began detecting
chemical elements in concentrations
that no-one had thought possible
this close to the sun.
The discovery of relatively
large concentrations of elements
like sulphur and potassium on
Mercury's surface
was a huge surprise.
If you think back to the time
when the planets were forming,
you don't expect high concentrations
of those elements close to the sun,
where Mercury orbits today, because
they're so-called volatile elements.
They boil away easily,
so you only find high concentrations
further out,
in the colder reaches of
the solar system.
So Mercury is an enigma
and discoveries like these have
forced us to completely rethink
our theories about the formation of
the planet.
Just a few million years
after its formation,
Mercury was still seething with the
heat of its violent birth.
Slowly, it cooled and a crust
formed.
Over time, the crust became enriched
in the volatile elements that were
escaping Mercury's interior.
But this could only happen
if Mercury started out
not in the position we see it
today...
..but much further out.
We now think Mercury was born
perhaps 170 million km
further away,
close to the orbit of Mars...
..a place where,
if it had stayed,
its destiny could have been very
different.
But it wasn't to be.
The young planetary embryo was
ripped from its promising position
long before it could mature.
Today, it's hard to imagine
the planets
in orbits other than the ones we see
in the night sky.
They feel eternal, permanent.
It's natural to think of the solar
system
as a piece of celestial clockwork,
almost like a Swiss watch.
So if we knew where all the planets
were at some point in time,
let's say today, then we could
imagine calculating
exactly where they're going to be at
any point in time.
Now, that is true if there's only
one planet and one star.
So imagine that's the sun
and this is Mercury.
Now, we know the gravitational force
between Mercury and the sun.
And, indeed, if that's all there is,
then we can calculate its orbit
around the sun
with essentially
infinite precision.
But add in one more planet,
let's say Jupiter over there.
Now there's a gravitational force
between all three of these objects
and it turns out that,
even in principle,
it is not possible to calculate
exactly
where they're all going to
be in the future
or where they were at some
point in the past.
This means that any uncertainty,
even of a few metres
in our knowledge of the position of
the planets,
can lead to radically
different predictions.
And that's because the system
itself, the orbits of the planets,
are not stable over very long
timescales.
So planets don't necessarily remain
in the same orbits forever.
And the evidence we've gathered
from the volatiles on Mercury's
surface,
and the unusual size of its core,
suggests that this may have been
what happened.
If Mercury began its life
170 million km further away
from the sun...
..then it would have been in a
region of space
where the young Mars was also
forming.
This region was full of scores
of planetary embryos,
all fighting for position.
Amongst the chaos, something
large kicked Mercury inwards
towards the sun.
Mercury collided with another
embryo.
A glancing blow saw much
of its crust
and mantle lost to space.
Much of this material remained
behind,
perhaps helping to form
the early Venus.
If this theory is correct,
then Mercury,
now little more than a planetary
core, continued towards the sun
and ended up in the peculiar
elliptical orbit we see today.
The idea that Mercury's outer layers
were stripped away
in some violent collision many
billions of years ago
is a superficially attractive one,
but the theory does have problems.
Any collision violent enough to do
that heats up the planet
and that boils away the volatiles.
So you have to think of
a very specific kind of collision,
or perhaps even multiple
more delicate collisions,
in order to fit the data.
So I think it's fair to say
that the precise nature
of Mercury's formation is still one
of the great unsolved mysteries
in planetary science.
After four years of observation,
and its discoveries that hint
at Mercury's turbulent past,
Messenger finally ran out of fuel...
..and added yet another crater
to this tiny world
that just perhaps could've had
a different story to tell.
50 million km beyond Mercury,
shrouded by an unbroken
blanket of cloud...
..lies a world which,
at first sight,
has the potential to be far more
Earth-like.
See that bright point of light out
there in the evening sky?
That's Venus.
It's so bright because it's quite
a large planet,
about the same size as the Earth.
It's not too far away.
But, in particular,
because it's shrouded in highly
reflective clouds.
That's the frustrating but also
tantalising thing about Venus.
Even through a big telescope,
when you look at it,
it is featureless.
You never see the surface.
And that meant that,
even until the 1950s,
astronomers speculated that it might
be a living world,
with jungles and forests
and rivers and oceans.
So much so, in fact, that when
we first sent a spacecraft to land
on the surface of Venus,
we prepared for a splash landing.
Throughout the 1960s and '70s,
the Soviet's Venera programme sent
multiple missions to explore Venus.
Many failed...
..but, with each attempt,
we learned a little more
of the extreme conditions on the
planet.
After 20 years of trying, Venera 13
began its perilous descent.
The craft was prepared to withstand
pressures
that could crush a car in seconds...
..in temperatures that would melt
lead.
On March 1st 1982...
..the Soviets took the first
full colour picture...
..of the Venusian surface.
Even under the most extreme
of conditions,
the probe sent its precious data
home to Earth...
..until, 127 minutes after
touchdown, it finally succumbed.
Far from a benign ocean world...
..Venus is a vision of hell...
..where no life can survive.
So where did it all go wrong
for Venus?
Well, that is a good question
and it's an important one.
It's been said that we won't fully
understand the Earth
until we understand Venus
and that's because the planets
are so similar.
Venus is the same size as the Earth.
It's the same composition,
as far as we know.
And although it's closer to the sun,
it's not as close as Mercury.
So why is it that one world
remained heaven
whilst the other became hell?
Counterintuitively, the surface
temperatures today on Venus
are hotter than those on Mercury...
..and the story of Venus's climate
is further complicated
by the fact that,
over the lifetime of the planet,
the sun itself has been evolving.
As the sun gets older, the star
burns hotter and hotter and hotter.
That means that, in the past,
when the sun was younger,
it must have been cooler.
It's called the faint young sun
and that has a big impact
on the planets.
At the time, when life was just
about beginning on the Earth,
three-and-a-half to four billion
years ago,
the sun was fainter and
that means that Venus was cooler.
In fact, temperatures on Venus
at that time
would have been like a pleasant
spring day here on Earth.
Within a few million years
of its formation,
the surface of Venus had cooled.
The planet now found itself
at just the right distance
from the faint young sun for Venus
to experience a sight
familiar to us here on Earth.
The heavens opened.
Great torrents flooded the surface.
Rivers of water flowed.
Venus became an ocean world.
The planet's atmosphere allowed
it to hold on to the oceans...
..by acting as a blanket, keeping
the surface temperate...
..thanks to the greenhouse effect.
The greenhouse effect is pretty
simple physics.
The gases like carbon dioxide, water
vapour and the planetary atmosphere
are transparent to visible light
and it's obvious because there's
a source of visible light,
the sun, and I can see it.
So that radiation falls onto the
surface of the planet
and it heats it up.
The rocks then re-radiate that
out into the atmosphere again,
but, this time, not as visible
light, but as infrared,
which my eyes can't see.
Now, carbon dioxide and water vapour
absorb infrared,
and so they trap that energy and the
planet heats up.
Now, that's not necessarily
a bad thing.
The Earth would be at an average
temperature
of around minus 18 degrees Celsius
without the greenhouse effect,
but there is a thin line between
heating the planet up and frying it.
Gradually, over two billion years,
the young sun grew brighter.
Temperatures began to rise,
lifting more and more water vapour
into the atmosphere.
The greenhouse effect grew
more intense.
Rain evaporated long before reaching
the ground.
Venus had reached a tipping point.
A runaway greenhouse effect
had taken hold.
Venus's moment in the sun was over.
Its cracked surface today
is even hotter than Mercury's,
making Venus the hottest
of all the planets.
As the young sun's brightness
continued to increase,
the effects were felt across all the
terrestrial planets.
Mars, much further out than Venus,
enjoyed its moment in the sun too.
With an atmosphere rich
in greenhouse gases,
rivers flowed across its surface for
hundreds of millions of years.
But Mars, being smaller than Venus,
couldn't hold on to its atmosphere.
Much of its water evaporated and...
..and escaped into space...
..leaving only small traces behind,
frozen in patches across the
planet...
..where missions continue to search
for the first signs of
extraterrestrial life.
There's a crater on Mars called
the Hellas Basin,
which is 1,500 km across and 9km
deep.
That means you could put Everest
on the floor
and the summit would
not reach the rim.
The air pressure is so high
down there
that liquid water can exist.
So I suppose it's not impossible
to imagine microbes
coming up from deep below the
surface to bask in the midday sun
before disappearing back
down below again
to survive the cold
of the Martian night.
But if life does exist out
there,
it will certainly only
be simple life.
There will be nothing anywhere
near as complex as you or me,
or even this plant.
The story of the solar system is,
in a sense,
a story of instability and constant
change,
at least for the inner rocky
worlds.
Mercury has changed its position
radically.
Its orbit now takes it close to the
searing heat of the sun.
Venus probably had water on its
surface
for around two billion years before
it became hotter than Mercury.
And Mars lost its oceans and rivers
perhaps three-and-a-half billion
years ago.
But unique amongst those worlds is
Earth
because it's remained pretty
much like this,
liquid water on the surface,
for four billion years,
and that has allowed complex carbon
chemistry to develop.
Today, our planet is dominated
by life.
It's in every nook and cranny.
I mean, look at this place.
This is a volcano in the middle
of the Atlantic Ocean
and it is literally teeming
with life.
And think about all the chance
events that had to happen
over four billion years
just to produce the little creatures
in this rock pool.
Life has woven itself into
the fabric of the planet.
It's an integral part of every
continent and every ocean.
It plays a crucial role
in maintaining the balance
of our atmosphere...
..that keeps our planet temperate.
Of all the terrestrial planets,
Earth has enjoyed the longest
moment of them all...
..but it can't last.
Earth will ultimately follow the
fate of the other rocky planets
because, even though we don't feel
it day-to-day,
the sun's ageing process is
relentless.
We can say with confidence what's
going to happen to the sun
towards the end of its life,
partly because we understand physics
and the nuclear physics of what
happens inside the cores of stars,
but also because the life cycle of
stars
is written across the night sky.
Take that bright star there,
for example.
It's called Arcturus.
It's around the mass of the sun,
perhaps a little bit heavier,
but it's between six and eight
billion years old,
perhaps three
billion years older than the sun -
and it is now a red giant star.
It's exhausted the hydrogen fuelled
in its core,
and it's swollen up and cooled.
And that is what we think
will happen to the sun
in about five billion years' time.
As the sun exhausts its hydrogen
fuel in the core,
its outer edge will inflate.
It will enter a red giant phase,
expanding millions of kilometres
out into space.
Mercury will be the first
to be engulfed.
Then Venus's fate will be sealed.
Earth may just escape the fiery
fate of its neighbours...
..hanging on with Mars beyond
the edge of the dying star.
The era of the four terrestrial
planets will be over.
The lives lived on the surface
of one of them
nothing more than a distant memory.
But that's not quite the end
of the story.
Right at the end of the sun's life,
something wonderful will happen.
A collection of icy worlds that have
lain dormant
for the entire history of the solar
system will awake.
These are the worlds that orbit
the outer planets,
the moons of Jupiter and Saturn.
These distant worlds that circle
the outer gas giants
will begin to warm...
..like Saturn's moon Enceladus...
..or Jupiter's moon Europa.
Amongst all these moons,
there is one above all others
that we think perhaps has the best
chance
of becoming a place that we'd
recognise.
Way out in the cold, distant
reaches of the solar system...
..past Jupiter...
..around the icy-ringed planet
Saturn,
orbits a gem...
And the Cassini spacecraft
is on its way to Saturn.
..Titan...
..a planet-sized moon bigger
than Mercury...
..surrounded by a thick atmosphere
of nitrogen and methane...
..with a surface that has long
remained a mystery.
The Huygens probe was our
first chance to explore
beneath the clouds...
..and its camera sent back these
first glimpses of the distant moon.
Wonderfully, the craft made a soft
landing
and continued to beam back
what it saw.
This is a remarkable photograph and,
as is always the case in science,
the more you know about it,
the more wonderful it gets.
This is a photograph from
the surface of the moon
orbiting around a planet over a
billion kilometres away,
so we've got a camera down onto the
surface of a world
in the frozen far reaches
of the solar system.
What you see here is something
that looks like a flood plain,
or a riverbed,
very much like this, actually.
And we can say that this is
a riverbed or a flood plain
because these rocks on the
surface...
..look like this.
They've been smoothed and eroded
by flowing liquid.
We know these are in fact
boulders of frozen water.
They're frozen solid because
the temperature on the surface
of this moon is minus 180
degrees Celsius.
That raises an interesting question.
If it's so cold...
..then what was the flowing liquid?
Huygens detected significant
amounts of methane...
..a flammable gas on Earth.
But the relatively high atmospheric
pressure
and cold temperatures at the surface
of Titan
means that this methane
exists as a liquid.
Titan could be wet...
..not with water, but with liquid
methane...
..driving rock-like chunks of ice
down mountain channels
and out into open flood plains.
Huygens survived for just a few
hours,
but didn't detect any trace
of liquid methane
at its landing site.
But the probe's mother ship Cassini
remained in orbit around Saturn.
A year after Huygens landed,
Cassini again flew high
above Titan's North Pole
and discovered something seen
nowhere else in the solar system
beyond Earth.
Liquid pooling into not just one,
but scores of great lakes.
Cassini discovered lakes...
..of liquid methane.
Earth has a strange cold twin.
In some ways, you could just imagine
floating in a boat on those lakes
and it'll look something like this,
except this would be liquid
methane gas
and those mountains there would be
mountains of frozen water ice
as hard as rock.
What's also fascinating, and in fact
tantalising,
is that Titan has a complex
chemistry
and that chemistry is carbon
chemistry - the chemistry of life.
So we found molecules like hydrogen
cyanide,
which are the building
blocks of amino acids.
We found molecules called vinyl
cyanides,
which chemists and biologists
speculate
could form some sort of
cell membranes.
And so all the ingredients
for life are present on Titan.
Now, very few scientists think
there will be life on Titan today.
It is, after all, minus 180
degrees Celsius at the surface,
but, because of the presence
of all those ingredients,
it might be a very different story
if you warm Titan up.
In the light
of the old expanding sun...
..the far reaches of the solar
system
will receive more solar energy.
Titan's atmosphere will begin
to warm.
Mountains of ice will shrink
and melt as temperatures rise...
..the frozen water they contain
replacing the liquid methane.
Mountains will become oceans
of water.
In a strange twist of fate,
at the end the life of the sun...
..the solar system's last
ocean world
will wake up to its own biological
possibilities.
This distant moon will enjoy
its brief moment in the sun.
It's easy to think of habitability
as a permanent feature
of a world's defining
characteristic, if you like.
So the Earth is a living planet
because it's in the Goldilocks zone
around the sun - not too close
and not too far away.
But things are more complicated
than that.
Solar systems are dynamic places.
Planetary orbits can change
and stars can vary in brightness,
so planets that were once
heaven can become hell.
We now understand that the Earth
has been a fortunate world,
an oasis of calm in an ever-changing
solar system,
that's maintained a stable climate,
perhaps against the odds,
for the four billion years it took
complex living things to evolve.
We don't know how many planets
like Earth there are
out there amongst the stars,
places where the ingredients of
solar systems
have assembled themselves into
structures
that can dream of other worlds,
but we have to take the possibility
very seriously
that there might be few.
And that would make Earth, and us,
extremely rare and precious.
Mercury is the most enigmatic of all
the planets and difficult to study.
The real problem is that it's really
hot.
It's the planet closest to the sun
and obviously that makes it
challenging.
So you have to protect the
spacecraft
from the heat from the sun, and the
way Messenger addressed this
was to put the whole spacecraft
behind a giant ceramic sunshade.
It had this sunshade.
If it didn't face the sun,
it would have melted the
instruments, literally.
The other major challenge is, you
have to protect a spacecraft
from the heat reflected
from the planet
and the way we dealt with that
was to be in an extremely elliptical
orbit,
where we flew in very close over the
North Pole and took observations.
The instruments would heat up and
then we'd fly like 10,000km
farther out from the planet...
..while we cooled off,
and, in this way,
heat up, cool down and kept
everything below
the danger temperatures, where
instruments could be damaged.
I was in charge of the camera team.
I don't think I slept much
of the night before.
I was really anxious to get
that first image back.
When that image came back,
we just started pointing
at all the features
that had never been seen before and
saying, "Look at this, look at
that."
Over time, the close-up flights
of Mercury's North Pole
allowed the team to peer deep into
the shadows
of one particular crater.
We realised that we could actually
design a way
to take a picture using a very long
exposure
to see inside these dark craters.
Messenger was designed with
instruments
that could specifically look at
their reflectance
and also measure hydrogen.
So we saw, "OK, there's a lot
of hydrogen there,"
and there was like this fantastic
case built for these ice deposits.
Incredibly, Messenger detected
hundreds of billions
of tonnes of frozen water ice...
..scattered in the permanent
shadows of the polar craters.
The fact that water ice can survive
for a very long period of time
is a reflection of the fact
that the Mercury is rotating
almost perfectly straight up, so
that there are craters near the pool
that are so deep and completely
shaded
from sunlight ever hitting them.
So ice could be stable in those
polar regions
that are permanently shadowed for
billions of years.
Messenger had confirmed
the existence
of an essential ingredient for
life...
..on the surface at the closest
planet to the sun.
During Messenger's final orbits,
all the fuel was depleted -
there was nothing that we could do -
and every orbit just brought
it slightly closer to the planet.
One of our engineers realised
that we still had some helium
on board the spacecraft
and, if you blew helium out the
back it would work like fuel,
and that managed to extend us
for a few extra weeks.
But, of course, all things
have to come to an end.
The last day,
many of us watched the signal
as it went behind the planet and
never came back
and we knew it had crashed
and it left us with a real
bittersweet feeling,
because we were happy
at the success,
but, of course, sad that it was
over.
We all took so much pride
in this amazing spacecraft
that lasted way longer than any
of us had planned for it to last
and that had told us so much
about Mercury
and really changed the way that we
looked at this planet.
Touchdown on the red planet...
..to uncover...
..the story of Mars.
Journey through our solar system
with this free poster
produced by the Open University
and discover more about its planets
and moons.
Order your free copy by calling...
Or go to...
..and follow the links
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stars...
..hundreds of billions in our
galaxy alone...
..many larger, brighter
and more majestic than our sun.
On the scale of galaxies and stars,
the planets of our solar system
are little more than grains of sand
caught momentarily
in the light of the sun.
But on those motes of dust,
for over four billion years,
great stories have played
out unseen.
Stories of worlds born...
..and worlds lost.
Planets forged amongst the calm...
..and the chaos.
Their destinies more entwined than
we ever imagined.
We know this...
..because in the last few decades...
..we've sent spacecraft to all
seven...
..of the worlds beyond our own.
These are the stories
that they return to Earth,
the stories of the planets.
For the first few million years
after the sun formed,
there were no planets to see it
rise...
..just clouds of dust and the gas...
..the leftovers from the birth
of the sun.
Over tens of millions of years,
the dust began to stick together...
..and form the first rocks.
Eventually, gravity assembled
the rocks
to create planetary embryos...
..that in time formed the four
closest planets to the sun.
Today, Mercury is the closest of
all,
enduring the sun's full glare.
Further out...
..lies Venus...
..chocked by a thick atmosphere.
Then Venus's neighbour, Earth.
And farthest of all, Mars...
..a cold desert world.
Together, they formed the only rocky
so-called terrestrial planets
in the solar system.
And of the four...
..one is unique.
Just look at this...
..and listen to it.
This is what a planet looks
and sounds like
after four billion years of
evolution by natural selection.
There is nowhere else in the solar
system
that looks and sounds like this,
which is interesting,
when you think about it,
because all the planets and moons
and are made out of the same stuff -
they're carbon, nitrogen, oxygen,
iron.
All those atoms were present in the
cloud that collapsed
to form the solar system,
four-and-a-half billion years ago.
And yet Earth appears
to be exceptional -
a lone living planet in an otherwise
desolate solar system.
So what is it that makes
this place so special?
Is it fate? Is it chance?
These are important questions
because Earth is the only place
we know of
where the most complex
phenomena in the universe exists,
the thing that brings meaning
to the universe - life.
Earth is a special world
in our solar system
and perhaps even for thousands of
light years beyond.
Our world certainly has unique
properties.
It's the right size and distance
from the sun
to have retained an atmosphere
that's protected its oceans
of life-giving water for billions of
years.
But as we've left the blue
planet,
and explored our sister worlds...
..we've discovered that each
appears to have had a moment
when it enjoyed almost
Earth-like conditions.
Every one of our rocky neighbours
has a story of what might have been.
Mercury is a small, tortured
world.
More than any other planet,
it's endured the unflinching glare
of the sun for billions of years.
Mercury is a world of mystery
and apparent contradictions.
It's in quite an elliptical orbit,
which means it can be as far away
from the sun as 70 million km,
but as close as 46 million.
That means that temperatures
at midday
can rise to 430 degrees
Celsius on the surface,
but, at night, because it's a small
planet and it's got no atmosphere,
temperatures fall to minus 170
degrees.
It's also locked into what's called
the spin-orbit resonance,
which means the planet spins
precisely three times on its axis
for every two orbits
and that in turn means that its day
is twice as long as its year.
That means that I could walk
over the surface like this
about 2mph
and keep the sun at the same point
in the sky.
I could stroll in eternal twilight.
Mercury is the least explored
of the inner rocky worlds...
..because getting to a planet in
such a strange oval-shaped orbit,
so close to the sun...
..is a tremendous challenge.
Five, four, three,
main engine start,
two, one and zero.
And lift off of Messenger on Nasa's
mission to Mercury...
..a planetary enigma in our
inner solar system.
Now going through the sound
barrier.
A direct route to Mercury is
impractical.
Now going through the period of
maximum dynamic pressure.
A spacecraft would arrive with so
much speed
that it would need vast amounts of
fuel to slow down
and enter orbit around Mercury.
We just had spacecraft separation.
So Messenger controlled
its trajectory
by stepping from one planet to the
next,
using gravity to slow itself,
spiralling inwards towards its
target.
Even so, Messenger approached
Mercury at such high speed
that it was forced to fly past the
planet three times...
..slowing on each pass...
..until, after almost seven years
of flawless navigation,
it arrived safely in orbit.
Messenger set about its mission
to map Mercury's surface...
..and began revealing the secrets
of the most cratered planet
in the solar system in exquisite
new detail.
Messenger was able to do much
more
than just take images of Mercury's
surface.
By tracking radio signals emitted
by the spacecraft,
we're able to see very slight
changes in the orbital path
around Mercury, as seen from Earth,
and that allows us to map out
Mercury's gravitational field.
There are also instruments
that allow us
to see how the planet wobbles around
as it spins on its axis.
And putting all these measurements
together
allows us to take a cross section
through the planet,
to see what it's made of.
And when we do that,
we find something very strange.
Mercury's core extends out about
85%
from the centre of the planet
to the surface.
It's almost entirely an exposed
planetary core.
It's as if the rocks of the surface
were smashed away
and removed at some
point in its past.
And there was more.
The tiny probe began detecting
chemical elements in concentrations
that no-one had thought possible
this close to the sun.
The discovery of relatively
large concentrations of elements
like sulphur and potassium on
Mercury's surface
was a huge surprise.
If you think back to the time
when the planets were forming,
you don't expect high concentrations
of those elements close to the sun,
where Mercury orbits today, because
they're so-called volatile elements.
They boil away easily,
so you only find high concentrations
further out,
in the colder reaches of
the solar system.
So Mercury is an enigma
and discoveries like these have
forced us to completely rethink
our theories about the formation of
the planet.
Just a few million years
after its formation,
Mercury was still seething with the
heat of its violent birth.
Slowly, it cooled and a crust
formed.
Over time, the crust became enriched
in the volatile elements that were
escaping Mercury's interior.
But this could only happen
if Mercury started out
not in the position we see it
today...
..but much further out.
We now think Mercury was born
perhaps 170 million km
further away,
close to the orbit of Mars...
..a place where,
if it had stayed,
its destiny could have been very
different.
But it wasn't to be.
The young planetary embryo was
ripped from its promising position
long before it could mature.
Today, it's hard to imagine
the planets
in orbits other than the ones we see
in the night sky.
They feel eternal, permanent.
It's natural to think of the solar
system
as a piece of celestial clockwork,
almost like a Swiss watch.
So if we knew where all the planets
were at some point in time,
let's say today, then we could
imagine calculating
exactly where they're going to be at
any point in time.
Now, that is true if there's only
one planet and one star.
So imagine that's the sun
and this is Mercury.
Now, we know the gravitational force
between Mercury and the sun.
And, indeed, if that's all there is,
then we can calculate its orbit
around the sun
with essentially
infinite precision.
But add in one more planet,
let's say Jupiter over there.
Now there's a gravitational force
between all three of these objects
and it turns out that,
even in principle,
it is not possible to calculate
exactly
where they're all going to
be in the future
or where they were at some
point in the past.
This means that any uncertainty,
even of a few metres
in our knowledge of the position of
the planets,
can lead to radically
different predictions.
And that's because the system
itself, the orbits of the planets,
are not stable over very long
timescales.
So planets don't necessarily remain
in the same orbits forever.
And the evidence we've gathered
from the volatiles on Mercury's
surface,
and the unusual size of its core,
suggests that this may have been
what happened.
If Mercury began its life
170 million km further away
from the sun...
..then it would have been in a
region of space
where the young Mars was also
forming.
This region was full of scores
of planetary embryos,
all fighting for position.
Amongst the chaos, something
large kicked Mercury inwards
towards the sun.
Mercury collided with another
embryo.
A glancing blow saw much
of its crust
and mantle lost to space.
Much of this material remained
behind,
perhaps helping to form
the early Venus.
If this theory is correct,
then Mercury,
now little more than a planetary
core, continued towards the sun
and ended up in the peculiar
elliptical orbit we see today.
The idea that Mercury's outer layers
were stripped away
in some violent collision many
billions of years ago
is a superficially attractive one,
but the theory does have problems.
Any collision violent enough to do
that heats up the planet
and that boils away the volatiles.
So you have to think of
a very specific kind of collision,
or perhaps even multiple
more delicate collisions,
in order to fit the data.
So I think it's fair to say
that the precise nature
of Mercury's formation is still one
of the great unsolved mysteries
in planetary science.
After four years of observation,
and its discoveries that hint
at Mercury's turbulent past,
Messenger finally ran out of fuel...
..and added yet another crater
to this tiny world
that just perhaps could've had
a different story to tell.
50 million km beyond Mercury,
shrouded by an unbroken
blanket of cloud...
..lies a world which,
at first sight,
has the potential to be far more
Earth-like.
See that bright point of light out
there in the evening sky?
That's Venus.
It's so bright because it's quite
a large planet,
about the same size as the Earth.
It's not too far away.
But, in particular,
because it's shrouded in highly
reflective clouds.
That's the frustrating but also
tantalising thing about Venus.
Even through a big telescope,
when you look at it,
it is featureless.
You never see the surface.
And that meant that,
even until the 1950s,
astronomers speculated that it might
be a living world,
with jungles and forests
and rivers and oceans.
So much so, in fact, that when
we first sent a spacecraft to land
on the surface of Venus,
we prepared for a splash landing.
Throughout the 1960s and '70s,
the Soviet's Venera programme sent
multiple missions to explore Venus.
Many failed...
..but, with each attempt,
we learned a little more
of the extreme conditions on the
planet.
After 20 years of trying, Venera 13
began its perilous descent.
The craft was prepared to withstand
pressures
that could crush a car in seconds...
..in temperatures that would melt
lead.
On March 1st 1982...
..the Soviets took the first
full colour picture...
..of the Venusian surface.
Even under the most extreme
of conditions,
the probe sent its precious data
home to Earth...
..until, 127 minutes after
touchdown, it finally succumbed.
Far from a benign ocean world...
..Venus is a vision of hell...
..where no life can survive.
So where did it all go wrong
for Venus?
Well, that is a good question
and it's an important one.
It's been said that we won't fully
understand the Earth
until we understand Venus
and that's because the planets
are so similar.
Venus is the same size as the Earth.
It's the same composition,
as far as we know.
And although it's closer to the sun,
it's not as close as Mercury.
So why is it that one world
remained heaven
whilst the other became hell?
Counterintuitively, the surface
temperatures today on Venus
are hotter than those on Mercury...
..and the story of Venus's climate
is further complicated
by the fact that,
over the lifetime of the planet,
the sun itself has been evolving.
As the sun gets older, the star
burns hotter and hotter and hotter.
That means that, in the past,
when the sun was younger,
it must have been cooler.
It's called the faint young sun
and that has a big impact
on the planets.
At the time, when life was just
about beginning on the Earth,
three-and-a-half to four billion
years ago,
the sun was fainter and
that means that Venus was cooler.
In fact, temperatures on Venus
at that time
would have been like a pleasant
spring day here on Earth.
Within a few million years
of its formation,
the surface of Venus had cooled.
The planet now found itself
at just the right distance
from the faint young sun for Venus
to experience a sight
familiar to us here on Earth.
The heavens opened.
Great torrents flooded the surface.
Rivers of water flowed.
Venus became an ocean world.
The planet's atmosphere allowed
it to hold on to the oceans...
..by acting as a blanket, keeping
the surface temperate...
..thanks to the greenhouse effect.
The greenhouse effect is pretty
simple physics.
The gases like carbon dioxide, water
vapour and the planetary atmosphere
are transparent to visible light
and it's obvious because there's
a source of visible light,
the sun, and I can see it.
So that radiation falls onto the
surface of the planet
and it heats it up.
The rocks then re-radiate that
out into the atmosphere again,
but, this time, not as visible
light, but as infrared,
which my eyes can't see.
Now, carbon dioxide and water vapour
absorb infrared,
and so they trap that energy and the
planet heats up.
Now, that's not necessarily
a bad thing.
The Earth would be at an average
temperature
of around minus 18 degrees Celsius
without the greenhouse effect,
but there is a thin line between
heating the planet up and frying it.
Gradually, over two billion years,
the young sun grew brighter.
Temperatures began to rise,
lifting more and more water vapour
into the atmosphere.
The greenhouse effect grew
more intense.
Rain evaporated long before reaching
the ground.
Venus had reached a tipping point.
A runaway greenhouse effect
had taken hold.
Venus's moment in the sun was over.
Its cracked surface today
is even hotter than Mercury's,
making Venus the hottest
of all the planets.
As the young sun's brightness
continued to increase,
the effects were felt across all the
terrestrial planets.
Mars, much further out than Venus,
enjoyed its moment in the sun too.
With an atmosphere rich
in greenhouse gases,
rivers flowed across its surface for
hundreds of millions of years.
But Mars, being smaller than Venus,
couldn't hold on to its atmosphere.
Much of its water evaporated and...
..and escaped into space...
..leaving only small traces behind,
frozen in patches across the
planet...
..where missions continue to search
for the first signs of
extraterrestrial life.
There's a crater on Mars called
the Hellas Basin,
which is 1,500 km across and 9km
deep.
That means you could put Everest
on the floor
and the summit would
not reach the rim.
The air pressure is so high
down there
that liquid water can exist.
So I suppose it's not impossible
to imagine microbes
coming up from deep below the
surface to bask in the midday sun
before disappearing back
down below again
to survive the cold
of the Martian night.
But if life does exist out
there,
it will certainly only
be simple life.
There will be nothing anywhere
near as complex as you or me,
or even this plant.
The story of the solar system is,
in a sense,
a story of instability and constant
change,
at least for the inner rocky
worlds.
Mercury has changed its position
radically.
Its orbit now takes it close to the
searing heat of the sun.
Venus probably had water on its
surface
for around two billion years before
it became hotter than Mercury.
And Mars lost its oceans and rivers
perhaps three-and-a-half billion
years ago.
But unique amongst those worlds is
Earth
because it's remained pretty
much like this,
liquid water on the surface,
for four billion years,
and that has allowed complex carbon
chemistry to develop.
Today, our planet is dominated
by life.
It's in every nook and cranny.
I mean, look at this place.
This is a volcano in the middle
of the Atlantic Ocean
and it is literally teeming
with life.
And think about all the chance
events that had to happen
over four billion years
just to produce the little creatures
in this rock pool.
Life has woven itself into
the fabric of the planet.
It's an integral part of every
continent and every ocean.
It plays a crucial role
in maintaining the balance
of our atmosphere...
..that keeps our planet temperate.
Of all the terrestrial planets,
Earth has enjoyed the longest
moment of them all...
..but it can't last.
Earth will ultimately follow the
fate of the other rocky planets
because, even though we don't feel
it day-to-day,
the sun's ageing process is
relentless.
We can say with confidence what's
going to happen to the sun
towards the end of its life,
partly because we understand physics
and the nuclear physics of what
happens inside the cores of stars,
but also because the life cycle of
stars
is written across the night sky.
Take that bright star there,
for example.
It's called Arcturus.
It's around the mass of the sun,
perhaps a little bit heavier,
but it's between six and eight
billion years old,
perhaps three
billion years older than the sun -
and it is now a red giant star.
It's exhausted the hydrogen fuelled
in its core,
and it's swollen up and cooled.
And that is what we think
will happen to the sun
in about five billion years' time.
As the sun exhausts its hydrogen
fuel in the core,
its outer edge will inflate.
It will enter a red giant phase,
expanding millions of kilometres
out into space.
Mercury will be the first
to be engulfed.
Then Venus's fate will be sealed.
Earth may just escape the fiery
fate of its neighbours...
..hanging on with Mars beyond
the edge of the dying star.
The era of the four terrestrial
planets will be over.
The lives lived on the surface
of one of them
nothing more than a distant memory.
But that's not quite the end
of the story.
Right at the end of the sun's life,
something wonderful will happen.
A collection of icy worlds that have
lain dormant
for the entire history of the solar
system will awake.
These are the worlds that orbit
the outer planets,
the moons of Jupiter and Saturn.
These distant worlds that circle
the outer gas giants
will begin to warm...
..like Saturn's moon Enceladus...
..or Jupiter's moon Europa.
Amongst all these moons,
there is one above all others
that we think perhaps has the best
chance
of becoming a place that we'd
recognise.
Way out in the cold, distant
reaches of the solar system...
..past Jupiter...
..around the icy-ringed planet
Saturn,
orbits a gem...
And the Cassini spacecraft
is on its way to Saturn.
..Titan...
..a planet-sized moon bigger
than Mercury...
..surrounded by a thick atmosphere
of nitrogen and methane...
..with a surface that has long
remained a mystery.
The Huygens probe was our
first chance to explore
beneath the clouds...
..and its camera sent back these
first glimpses of the distant moon.
Wonderfully, the craft made a soft
landing
and continued to beam back
what it saw.
This is a remarkable photograph and,
as is always the case in science,
the more you know about it,
the more wonderful it gets.
This is a photograph from
the surface of the moon
orbiting around a planet over a
billion kilometres away,
so we've got a camera down onto the
surface of a world
in the frozen far reaches
of the solar system.
What you see here is something
that looks like a flood plain,
or a riverbed,
very much like this, actually.
And we can say that this is
a riverbed or a flood plain
because these rocks on the
surface...
..look like this.
They've been smoothed and eroded
by flowing liquid.
We know these are in fact
boulders of frozen water.
They're frozen solid because
the temperature on the surface
of this moon is minus 180
degrees Celsius.
That raises an interesting question.
If it's so cold...
..then what was the flowing liquid?
Huygens detected significant
amounts of methane...
..a flammable gas on Earth.
But the relatively high atmospheric
pressure
and cold temperatures at the surface
of Titan
means that this methane
exists as a liquid.
Titan could be wet...
..not with water, but with liquid
methane...
..driving rock-like chunks of ice
down mountain channels
and out into open flood plains.
Huygens survived for just a few
hours,
but didn't detect any trace
of liquid methane
at its landing site.
But the probe's mother ship Cassini
remained in orbit around Saturn.
A year after Huygens landed,
Cassini again flew high
above Titan's North Pole
and discovered something seen
nowhere else in the solar system
beyond Earth.
Liquid pooling into not just one,
but scores of great lakes.
Cassini discovered lakes...
..of liquid methane.
Earth has a strange cold twin.
In some ways, you could just imagine
floating in a boat on those lakes
and it'll look something like this,
except this would be liquid
methane gas
and those mountains there would be
mountains of frozen water ice
as hard as rock.
What's also fascinating, and in fact
tantalising,
is that Titan has a complex
chemistry
and that chemistry is carbon
chemistry - the chemistry of life.
So we found molecules like hydrogen
cyanide,
which are the building
blocks of amino acids.
We found molecules called vinyl
cyanides,
which chemists and biologists
speculate
could form some sort of
cell membranes.
And so all the ingredients
for life are present on Titan.
Now, very few scientists think
there will be life on Titan today.
It is, after all, minus 180
degrees Celsius at the surface,
but, because of the presence
of all those ingredients,
it might be a very different story
if you warm Titan up.
In the light
of the old expanding sun...
..the far reaches of the solar
system
will receive more solar energy.
Titan's atmosphere will begin
to warm.
Mountains of ice will shrink
and melt as temperatures rise...
..the frozen water they contain
replacing the liquid methane.
Mountains will become oceans
of water.
In a strange twist of fate,
at the end the life of the sun...
..the solar system's last
ocean world
will wake up to its own biological
possibilities.
This distant moon will enjoy
its brief moment in the sun.
It's easy to think of habitability
as a permanent feature
of a world's defining
characteristic, if you like.
So the Earth is a living planet
because it's in the Goldilocks zone
around the sun - not too close
and not too far away.
But things are more complicated
than that.
Solar systems are dynamic places.
Planetary orbits can change
and stars can vary in brightness,
so planets that were once
heaven can become hell.
We now understand that the Earth
has been a fortunate world,
an oasis of calm in an ever-changing
solar system,
that's maintained a stable climate,
perhaps against the odds,
for the four billion years it took
complex living things to evolve.
We don't know how many planets
like Earth there are
out there amongst the stars,
places where the ingredients of
solar systems
have assembled themselves into
structures
that can dream of other worlds,
but we have to take the possibility
very seriously
that there might be few.
And that would make Earth, and us,
extremely rare and precious.
Mercury is the most enigmatic of all
the planets and difficult to study.
The real problem is that it's really
hot.
It's the planet closest to the sun
and obviously that makes it
challenging.
So you have to protect the
spacecraft
from the heat from the sun, and the
way Messenger addressed this
was to put the whole spacecraft
behind a giant ceramic sunshade.
It had this sunshade.
If it didn't face the sun,
it would have melted the
instruments, literally.
The other major challenge is, you
have to protect a spacecraft
from the heat reflected
from the planet
and the way we dealt with that
was to be in an extremely elliptical
orbit,
where we flew in very close over the
North Pole and took observations.
The instruments would heat up and
then we'd fly like 10,000km
farther out from the planet...
..while we cooled off,
and, in this way,
heat up, cool down and kept
everything below
the danger temperatures, where
instruments could be damaged.
I was in charge of the camera team.
I don't think I slept much
of the night before.
I was really anxious to get
that first image back.
When that image came back,
we just started pointing
at all the features
that had never been seen before and
saying, "Look at this, look at
that."
Over time, the close-up flights
of Mercury's North Pole
allowed the team to peer deep into
the shadows
of one particular crater.
We realised that we could actually
design a way
to take a picture using a very long
exposure
to see inside these dark craters.
Messenger was designed with
instruments
that could specifically look at
their reflectance
and also measure hydrogen.
So we saw, "OK, there's a lot
of hydrogen there,"
and there was like this fantastic
case built for these ice deposits.
Incredibly, Messenger detected
hundreds of billions
of tonnes of frozen water ice...
..scattered in the permanent
shadows of the polar craters.
The fact that water ice can survive
for a very long period of time
is a reflection of the fact
that the Mercury is rotating
almost perfectly straight up, so
that there are craters near the pool
that are so deep and completely
shaded
from sunlight ever hitting them.
So ice could be stable in those
polar regions
that are permanently shadowed for
billions of years.
Messenger had confirmed
the existence
of an essential ingredient for
life...
..on the surface at the closest
planet to the sun.
During Messenger's final orbits,
all the fuel was depleted -
there was nothing that we could do -
and every orbit just brought
it slightly closer to the planet.
One of our engineers realised
that we still had some helium
on board the spacecraft
and, if you blew helium out the
back it would work like fuel,
and that managed to extend us
for a few extra weeks.
But, of course, all things
have to come to an end.
The last day,
many of us watched the signal
as it went behind the planet and
never came back
and we knew it had crashed
and it left us with a real
bittersweet feeling,
because we were happy
at the success,
but, of course, sad that it was
over.
We all took so much pride
in this amazing spacecraft
that lasted way longer than any
of us had planned for it to last
and that had told us so much
about Mercury
and really changed the way that we
looked at this planet.
Touchdown on the red planet...
..to uncover...
..the story of Mars.
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