Lift off of Messenger on NASA'S mission to Mercury.
MUSIC: The Void by Muse
# They'll say no-one can see us
# That we're estranged and all alone
# They believe nothing can reach us
# And pull us out of
# The boundless gloom
# They're wrong
# They're wrong. #
Our planetary neighbour, Mars, is a cold, barren rock.
Its rusted surface covered in parched sand.
But, beneath the dust,
the planet bears the scars of a former life.
Billions of years ago, Mars was just like Earth.
A world with a thick atmosphere
that supported oceans of water.
But, today, that world is gone.
Mars lies dead,
while the Earth thrives.
Why the two planets had such different fates is a mystery
that we've only just begun to answer.
You see that pale red point of light in the sky,
just there?
That's Mars.
Through a small telescope, it appears almost Earth-like.
Our sister world - polar ice caps and dark surface
markings that 19th-century astronomers thought were vegetation,
even canals bringing meltwater down from the poles
to arid equatorial cities.
Across the depths of space,
the inhabitants watched us "with envious eyes", wrote HG Wells.
We now know that there are no eyes looking back at us.
Mars is a frozen, arid desert world.
But a fleet of spacecraft have revealed
that it hasn't always been that way.
Mariner 4 was successfully launched on time
for its historic 228-day journey to Mars.
Picture information started to come in on July 15th, 1965.
A revelation comparable to Galileo's first view
of the moon through a telescope.
During its brief flyby,
Mariner 4 gave us our first close-up glimpses of Mars.
When Mariner 9 was placed into an orbit around Mars,
it saw a planet blanketed by a gigantic dust storm.
In nearly a year of operation,
they transmit more than 7,000 photographs.
From orbit, Mariner 9 photographed 80% of the Martian surface.
First of all, there are two eyes, not only in colour but also
in stereo, and in the infrared part of the spectrum.
It has a sense of touch, it has a sense of hearing,
but by far the most important feature of the lander is its brain.
The Viking programme took us down to the ground
for the first time...
Touchdown, we have touched down.
..and revealed Mars...
Perfect set-down.
..like never before.
There is the first piece of information coming in.
Oh! Oh!
The data gathered over the last 50 years has allowed us to create
detailed maps of the Martian surface...
..and begin to piece together its past.
Maps of Mars are like storybooks.
You can read the history of the planet
written across its surface, and the reason for that is that there's
virtually no erosion, hasn't been for billions of years,
so the scars of events that happened even four billion years ago
can still be seen.
This is a type of map called an elevation map.
The colours correspond to difference in heights on the surface,
so blue means low
and red and whites are high.
Now, this region here, which is much higher on average than the rest
of Mars, is called Tharsis and it's covered in volcanoes,
including the largest volcano in the solar system, Olympus Mons.
At the other side of Tharsis is the great Valles Marineris,
the Mariner Valley, and it is a canyon that dwarfs
anything we see on Earth.
On the opposite side of the planet is an impact basin called Hellas.
The height difference from the crater rim
to the crater floor is 9km.
That means you could fit Everest in the middle of there
and look down on its summit.
And the region surrounding the basin reveals Mars' former life.
The Hellas basin is punched into the oldest-surviving terrain on Mars.
It's called Noachis Terra
or The Land Of Noah.
And that's a wonderfully evocative name because its surface is sculpted
by flowing water.
All across the earliest Martian surface, we've glimpsed traces
of what appear to have been lakes and rivers.
And so a new generation of spacecraft has been sent to Mars,
to investigate the existence of water...
..and what happened to the planet for it all to disappear.
Led by the most audacious Mars mission ever attempted...
INDISTINCT RADIO COMMUNICATION
We have two-way Doppler and orbit around the planet Mars.
..to land a one-tonne rover on the Martian surface.
Its final descent has become known as the "seven minutes of terror".
Curiosity touched down in Gale crater,
a 150-kilometre-wide impact basin,
thought to have been home to an ancient lake.
The rover is a $2.5 billion mobile chemistry lab...
..designed to take samples of the Martian surface
and analyse its composition.
As it explored the crater, Curiosity saw pebbles polished
and rounded by running water
in what had once been rivers and streams.
Then, 61 days after landing, Curiosity identified the perfect
spot to begin its primary mission.
In a sandy area of the crater called the Rocknest,
the rover took its first scoops of Martian soil.
Chemical analysis of the fine, dusty sand revealed
something quite unexpected.
Even though the surface of Mars appears completely dry,
2% of the soil is still made up of water.
Curiosity had found evidence of just how wet a planet
ancient Mars had been.
Mars was a water world.
Rains fell,
rivers ran,
and, in the northern hemisphere,
water collected in a vast sea
that covered a fifth of the Martian surface.
The Red Planet was once blue.
All the evidence suggests that there were large bodies
of standing water on Mars around 4 billion years ago,
and the atmospheric pressure was at least that of Earth today,
perhaps even higher.
Temperatures were around 25 degrees, so I could have sat on Mars
all those years ago, admittedly with a mask to breathe,
because there was very little oxygen, but I could have sat there
and looked out over a view like that.
So, you don't have to imagine what Mars was like in the past.
You can experience it.
It was pretty much like this.
But, within a billion years,
all Mars' lakes and seas had disappeared.
In our solar system, only one blue planet survives...
..Mars' sister, Earth.
70% of our planet's surface is covered by ocean.
Under the waves, a million species thrive.
While on land, the rains support Earth's delicate ecosystems...
..providing a home for an abundance of life.
But it hasn't always been this way.
The early Earth was unrecognisable from the planet we know today.
Its atmosphere thick with carbon dioxide.
And its oceans acidic.
Four billions years ago, Earth was a troubled, toxic world...
..while Mars was flourishing.
But both planets were about to be engulfed
by a cataclysm from space.
To understand what happened,
we have to look beyond our own world.
You can't read the deep history of the Earth by looking
at its surface because our planet is a geologically active world.
The surface is constantly being reshaped by volcanic activity,
weathering, and the actions of the oceans, but we have a companion,
the moon, which has been inactive for many billions of years,
and so the history of events that happened in this region
of the solar system is written all over its surface.
The most distinctive feature of the moon's surface
are its craters - it is literally covered in a record of impacts
from space, and that allows us to estimate the relative ages
of different parts of the moon.
Quite simply, if there are more craters,
then that piece of the moon must be older.
There's been more time for the impacts to build up.
But we can do better than just measure the relative ages
because we have rocks, the moon rocks brought back
by the Apollo astronauts.
We can estimate the ages of rocks very precisely by looking
at the rates of decay of radioactive elements inside them.
They're like little stopwatches that start ticking the moment
the rocks are formed, in this case by the impacts from space.
So, the moon rocks allow us to tie the number of craters
in a particular region of the moon
to an absolute age measured by the rocks.
And this doesn't just allow us to date impacts on the lunar surface.
It means that craters can be used
to read the histories of worlds across the solar system.
Including Mars.
When we gathered all the data, we discovered something surprising.
There was a peak in the crater formation rate, about 3.8
to 3.9 billion years ago,
which signified a period of intense violence in the solar system,
and that is called the Late Heavy Bombardment.
Countless asteroids fragmented in Mars' atmosphere,
raining havoc across the planet.
It's estimated that 53 tonnes of rock
fell on every square metre of Mars.
Over a third of the planet's surface was obliterated...
..and Mars was pushed to the brink of death.
Whilst the evidence from the surface of the moon tells us
that the Late Heavy Bombardment happened, it doesn't tell us why.
For that, we have to resort to computer models of the evolution
of the solar system, and, when we do that,
they point the finger at Neptune.
It's thought that Neptune migrated outwards into the Kuiper belt...
..a region of icy, rocky objects
orbiting at the edge of the solar system.
The resulting gravitational interactions disrupted those orbits
and sent many of the objects inwards to the inner solar system,
and that may have been the cause of the Late Heavy Bombardment.
Earth also suffered the onslaught,
and, for tens of millions of years,
the fortunes of the two sister worlds hung in the balance.
But, just when conditions appeared at their least promising,
Earth's most precious characteristic emerged.
Life.
There is good evidence that life was present on Earth
around 3.8 billion years ago, and discounting the - I think -
remote possibility that life began elsewhere
in the solar system and was transported to the Earth
on meteorites or comets,
that means that life must have begun here.
So, somewhere on this planet there was a transition
from geochemistry - the chemistry of Earth,
to biochemistry - the chemistry of life.
And whilst the precise details of how that transition occurred
remain a mystery, it's thought that in warm volcanic pools
or deep sea hydrothermal vents, conditions were right
for the chemical building blocks of life to form spontaneously.
And that means that if similar conditions
were to be found elsewhere in the solar system,
it might be possible that life began there too.
Ignition, and lift off of the Atlas V rocket with MRO.
Surveying for the deepest insights
into the mysterious evolution of Mars.
So, in 2005, NASA embarked on a mission to look
for those same environments on Mars.
For more than a decade, the Mars Reconnaissance Orbiter
has been our eyes on the Red Planet...
..sending back more data
than all the other Mars missions combined.
MRO has made more than 60,000 orbits,
mapping over 99% of the planet's surface.
Its high-resolution cameras have revealed Mars as never before,
discovering polar avalanches,
shifting sand dunes...
..and what could be seasonal flows of sand or even liquid meltwater.
Then, in 2017, MRO turned its gaze
to one of the Red Planet's oldest features,
the Eridania Basin.
3.8 billion years ago, the basin was a vast sea...
..holding ten times more water
than the Great Lakes of North America.
And it was here that MRO found the evidence it was looking for.
400-metre-thick deposits of minerals that, on Earth,
form in deep sea hydrothermal vents.
In the Eridania Basin, MRO revealed that conditions on Mars
had once been ripe for the emergence of life.
We won't know for sure whether life began or even perhaps still exists
on Mars until we go there and find physical evidence -
so, microbes buried deep below the soil in oases of liquid water,
or maybe microbe fossils - but what we do know is that,
when life began here on Earth, 3.8 billion years ago,
the conditions on Mars were very similar.
There were seas, there was volcanic activity,
there were even hydrothermal vent systems on the floors of its oceans.
So, it is at least possible that Earth is not the only world
in the solar system where life began.
The habitable conditions during what's known
as Mars' Noachian era persisted for hundreds of millions of years.
But then, prospects for life on the Red Planet changed dramatically.
Around 3.5 billion years ago, the Noachian era drew to a close
and Mars entered a more frozen, arid phase, known as the Hesperian.
The water that flowed freely over the surface during the age of Noah
became locked away in giant reservoirs of ice.
But, around the same time, Mars became more volcanically active,
and the volcanic eruptions and sub-surface lava flows
occasionally melted the ice, leading to catastrophic flooding.
They must have been some of the most spectacular sights
in the history of the solar system.
As molten rock pushed upwards through the crust,
meltwater poured out onto the surface.
It raged down from the southern highlands...
..until, in a place known as Echus Casma, it plunged
over cliffs 4km high...
..creating the largest waterfall
the solar system has ever seen.
Echus Casma would have been like no waterfall ever seen on Earth.
350 cubic kilometres of water flowed over it.
That's like a cube 70km by 70km by 70km.
It all entered into a canyon 10km wide
and 100km long, and that happened in a few weeks.
Once the flood subsided, the water disappeared...
..leaving the evidence of the falls etched into the face of the planet.
We don't know precisely why the climate of Mars changed
from warm and wet to cold and arid.
We're talking about events that happened
three and a half billion years ago on a planet hundreds of millions
of kilometres away, so it is a hard problem.
But we do strongly suspect that changes happening
on the planet's surface were driven
at least in part by changes in the planet's interior.
Deep within Mars' core,
something was causing the planet to die...
..and the evidence can be found in Mars' atmosphere.
T-minus ten, nine, eight, seven, six,
five, four, three,
two, one.
Main engine start, ignition, and lift-off
of the Atlas V with MAVEN,
looking for clues about the evolution of Mars
through its atmosphere.
In September 2014, NASA'S MAVEN probe made its final
approach to the Red Planet.
Its mission - to understand what drove the planet's
dramatic climate change.
MAVEN is equipped with an array of instruments designed to measure
the behaviour of the atoms and molecules in Mars' atmosphere.
The spacecraft circles Mars in an elliptical orbit...
..allowing it to measure the full profile
of the planet's upper atmosphere.
At its lowest point,
it's just 150km above the surface.
At its highest, a little over 6,000 kilometres.
And it was at the very top of Mars' atmosphere that MAVEN found
the key to the mystery of what happened to Mars.
Detailed measurements revealed gas is being lost
from the Martian atmosphere,
escaping to space
at a rate of about two kilograms every second.
Over time, it's thought this gradual stripping away of Mars' atmosphere
has slowly thinned the insulating layer surrounding the planet...
..causing surface temperatures to plummet.
But what was it that caused Mars to lose its atmosphere
while Earth clung on to hers?
150 million kilometres away in that direction is the setting sun,
a giant nuclear fusion reactor.
You can fit one million Earths inside it.
Now, the surface temperature
is only around 6,000 degrees Celsius,
but the sun's atmosphere, known as its corona,
is at one million degrees.
And that means it's in the form of what's known as a plasma, a soup
of electrically charged particles.
Some of those particles are moving around so fast that they can escape,
and they stream away in what's known as the solar wind.
They reach the Earth travelling at a few hundred kilometres per second.
And, if we weren't protected, they would strip away our atmosphere.
And when the sun dips below the horizon...
..there are times when that protective force field is revealed.
Just look at that!
I mean, there is the aurora.
It's the laws of nature, all of them, written across the sky.
Electrically-charged particles have been driven away from the sun,
ultimately from nuclear fusion reactions in the core of a star.
They're crossing the solar system, hitting the Earth's magnetic field,
stretching it out on the dark side of the planet.
The field then snaps back like an elastic band,
accelerating all of those charged particles up and down
the field lines to the poles, which is here in the skies
over Iceland, and they hit nitrogen
and oxygen molecules in the atmosphere.
And you're seeing quantum mechanics - they're exciting the
molecules so that they emit light in characteristic colours.
And, if you think about it, this is the only time
that we really see the Earth's magnetic field.
It's one of the reasons why life on Earth
has been able to persist for four billion years.
In a sense, that's the reason that you exist.
It's Earth's magnetic field that protects our atmosphere
from the ravages of the solar wind,
and that protective shield has its origins deep
in the planet's interior.
Thousands of kilometres down below my feet,
actually below your feet now, is the Earth's outer core,
which is a seething mass of molten iron.
Convection currents cause the molten iron to rise,
and then the Earth's rotation causes it to spiral around.
Now, a spiralling, circling flow
of an electrically conducting liquid is a dynamo.
A dynamo generates a magnetic field and the Earth's field rises up,
not just to the surface here, but out into space,
forming our protective shield.
And that is what you see there.
And just like Earth,
ancient Mars was also shielded from the sun.
Aurora once danced above its poles...
..keeping guard over the Martian atmosphere and seas below.
But between 3.5 and 4 billion years ago,
Mars' dynamo switched off.
The aurora surrounding the poles slowly faded away
as the magnetic field diminished...
..allowing the atmosphere to be stripped away
by the solar wind.
Without protection, seas evaporated, the surface froze,
and Mars was transformed.
At the same time, the fortunes of Mars' sister world
were about to take a very different turn.
For the next billion years or so, Earth was indistinguishable
from those landscapes of early Mars -
barren continents surrounded by ocean.
But in Earth's oceans, life was beginning to transform the planet.
Primitive algae started to neutralise the ocean's acidity
and replace the dense red fog of Earth's methane-rich
atmosphere with oxygen.
Around 600 million years ago, that oxygen-rich atmosphere allowed
complex life to evolve in the oceans, colonise the land,
and ultimately produce this almost-infinitely rich living world
today, of which we are a part.
While Mars died, Earth flourished.
To understand why the two sisters had such different destinies,
you have to go right back
to the time the planets were forming.
When Mars and Earth were born,
the solar system was a chaotic vortex of gas and rock.
Material clumped together and grew, only to be smashed apart.
Over time, some of the objects became large enough to survive
at least the smaller impacts, and continued to grow,
including the embryonic planets Earth and Mars.
But there was one crucial difference between the young planets.
Mars formed in a region of the solar system
with considerably less rocky material.
And that had a profound impact on the planet's growth.
Mars is a significantly smaller world - it's about half the diameter
of the Earth, and that makes all the difference.
Although the details are not yet fully understood,
it seems clear that Mars' smaller size meant that its dynamo switched
off many billions of years ago.
Being smaller meant Mars' core cooled more quickly than Earth's.
And this is certainly part of the reason why Mars
lost its magnetic field.
Even though the planet is further away from the sun than we are,
that meant that the solar wind stripped away its atmosphere
and Mars died.
So, even though Earth and Mars are so similar in so many ways,
the difference in position and size in the solar system
led to very different fates.
Long ago, two sister worlds were born.
In childhood, Mars was warm and wet...
..whilst the Earth was inhospitable and toxic.
Both young planets survived the violence
of the Late Heavy Bombardment,
emerging as mature worlds,
primed with all the ingredients for life.
But deep inside, the smaller of the two was dying.
Mars' seas dried up.
And as the planet's interior cooled, one by one her fires went out.
Olympus Mons, the largest volcano in the solar system,
last erupted around 25 million years ago.
As the lava turned to stone,
Mars was frozen in time.
And so, today, her surface lies rusted and gathering dust.
But that might not be the end of Mars' story.
Because the next generation of spacecraft are already on their way.
NASA Orion - currently in advanced testing.
ESA ExoMars -
a fleet of spacecraft designed to search for signs of life.
And the most ambitious private space mission ever conceived.
A launch vehicle developed to take humans to the surface of Mars.
Mars is, in a sense, a failed world,
a faded ember etched with the memories of a more enticing past,
but there may have been, and may still be, life on Mars.
And the discovery of a second genesis in our solar system
would have profound philosophical, scientific and cultural consequences
because it would mean there is a sense of inevitability
about the origin of life,
and that would mean that the universe
is most likely teeming with life - that we are not alone.
But equally importantly, I think, is the role that a planet
with a history like Mars could play in our future.
Mars is rich in resources, it has vast reservoirs of frozen
water below the surface, and minerals -
iron, nitrogen, carbon, oxygen - all the things
you need to support a civilisation.
And that's why I think that, in my lifetime,
there will be Martians, but the Martians will be us.
We will go to Mars and make it our home,
and that old red world will become our first step
beyond the cradle, and out to the stars.
Mars really captures
our imagination,
partly because it's so close.
I think people are really interested in Mars because it actually
is so similar to Earth.
It's close by, it's easy to travel there with robots
and space missions, and so we've done a lot of exploration.
And, every time you go and look, you discover something new.
NASA Curiosity launched on the 26th of November, 2011.
But the biggest obstacle facing the mission team
wasn't leaving the Earth.
Mars has a unique set of challenges
compared to other places we go with spacecraft.
Mars has an atmosphere but it's thin, so it's not enough
to really slow you down,
but it is enough to actually burn you up as you're trying to land.
Curiosity reached the top of the Martian atmosphere,
travelling at 20,000km per hour.
Curiosity is a big rover. It weighs a metric ton,
and so landing that required every trick in the book
of how we've learned to land on Mars with previous missions.
To land safely, the rover had to be slowed
to less than 4km per hour.
You end up arriving at Mars going really fast,
so you actually have to slow down,
and we do that using a heat shield,
which burns off a lot of energy and creates a lot of heat,
so you have to absorb that somehow and not damage the spacecraft.
Then a parachute comes out.
The biggest parachute we've ever used in a planetary mission.
And that even doesn't slow Curiosity down enough,
because Mars' atmosphere is quite thin, so then rockets carry
the spacecraft and guide the spacecraft to the surface.
There's nothing you can do at that point to ensure its success
or prevent its crashing...
..and yet you've invested so much in the outcome.
All I could do was sort of curl up in a ball and wait for the
green light that Curiosity was safely on Mars.
Seven years and $2.5 billion in the making,
Curiosity finally touched down
at 6.32 Universal Time, on the 6th of August, 2012.
I was sitting in the control room watching the engineers,
who were actually monitoring the signals coming in from Curiosity,
and so they were reading out the data that they were getting
and they detected the wheels touching the soil.
Then a few seconds went by when cables had to be cut
and the rocket jet pack had to fly away.
And, only then, they understood that Curiosity was safe
on the ground, and the whole room just erupted in celebration.
Since it landed, Curiosity has been exploring Gale Crater
for more than six years.
Curiosity is a roving laboratory.
We actually collect samples by scooping it or by drilling,
or just by sucking in some of the atmospheric gas.
And it's that type of data that allows us to pick apart
the story that those things hold.
In 2015, we made our first identification of organic molecules
that we think were coming from the Martian materials.
And that is a turning point for us.
What we found in those rocks
is what we expected of natural organic matter.
It's what you would expect to find on Earth.
Finding the organic matter is the clue to searching for life.
What everybody wants to know is whether or not Mars
once had life, and the short answer is - we don't know.
The somewhat longer answer is -
we see all the signs of materials that could have supported life.
We have evidence for lots of water early on.
We see the nutrients, we see carbon, we see oxygen,
we see nitrogen, we see phosphorus,
we see all the stuff that life needs
in order to reproduce and survive as simple microorganisms.
For me personally, I find it might actually
be more surprising if we never found evidence of life on Mars.
Everything we've found suggests that Mars was such a friendly,
supportive place for life in its early history,
and there should be a lot of planets like that around other stars,
and lots of life in the universe.
So, maybe we're getting to the point where it'll be more surprising
if we never find other life.
And so, thanks to Curiosity's discoveries, the latest wave
of spacecraft might finally answer the question -
has there ever been life on Mars?
Next time...
..we enter the realm of the gas giants...
..to discover how the largest and oldest of the planets
sculpted the entire solar system.
Jupiter, the godfather.
Journey through our solar system with this free poster produced
by the Open University, and discover more about its planets and moons.
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