In July 1976, NASA's Viking 1 spacecraft
attempted a monumental test.
'For hundreds of years, people have been asking the question,
'since planets were first discovered,'
"Is there's life on Mars?"
It's been asked throughout history.
'This was the first time we'd look at the possibility of life,
'other than life on Earth.
'Can you imagine what it was like when the team and myself'
knew that we had a signal and we'd soon get an answer?
There is one thing that sets this planet apart from the rest.
The Earth is alive.
But is this the only cradle for life
in an otherwise barren universe?
It is perhaps the greatest question of the age.
'The fundamental question is: are we alone?
'Is life on Earth the only life there is?'
When I look at the sky
and see all the planets and stars out there,
we wonder, "Is there life there?"
All we have is the example on Earth.
Even if we found the simplest little bug on another world,
different from all the bugs here,
that would tell us that in two places there's life.
From two, it's clear the universe must be full of life.
If there is life, there's still no sign of it.
For decades, radio dishes have scanned the skies,
listening for a message from across the galaxy.
So far, they have heard nothing.
But with the coming of the space age,
we could, at last, go and look for alien life.
In 1962, America sent Mariner 2 to Venus.
Venus showed every sign of being Earth's heavenly twin.
But its surface was hidden by thick cloud.
What lay below was an enticing mystery.
Some imagined there would be a steaming swamp,
a misty haven teeming with life.
Others thought that its clouds were caused
by a planet-wide dust storm on a parched desert world.
Mariner 2 was built to find out.
It took with it the hopes of the first space biologist,
Dr Carl Sagan.
Many theories of the Venus environment have been suggested.
However, new information eliminates some theories.
Measurements with radio telescopes show
that there's a region on Venus
where temperatures are greater than 600 degrees Fahrenheit.
It is just possible that the hot region exists
at a high altitude, in the ionosphere of Venus.
The surface temperature could then be Earth-like,
and life could exist there.
But Mariner 2 found the surface of Venus
was a searing 450 degrees centigrade.
Life was inconceivable here.
Where else was there to look?
Mercury is just a naked ball of rock, baked by the sun.
Jupiter seemed a better prospect.
Some biologists imagined lifeforms
floating in its clouds like hot-air balloons.
But when the first probe got there,
it found the conditions were atrocious.
The clouds were made of super-heated ammonia.
They could never support life.
Emissaries to the planets were sending depressing news.
Space was a desolate place.
But there was one world that still held our fascination.
Humans had dreamed of going to Mars for over a century.
Boris Chertok was first drawn to it in 1924,
when he saw the film "Aelita",
the story of a beautiful Martian princess.
As a boy, Chertok dreamt of finding life on Mars.
Those dreams were thanks largely to one visionary astronomer.
In 1894 a Bostonian travelled to Flagstaff, Arizona,
and set up an observatory.
Percival Lowell had heard from European astronomers
that Mars was criss-crossed with a series of straight lines.
He'd heard them called "canals".
He was so taken with the idea
of a canal-building civilisation on Mars
that he began a systematic study of the planet,
charting the features of these supposed waterways.
The maps he made were the best for a century.
When the first space mission to Mars was ready, it was 1961.
Boris Chertok had grown up
to be second-in-command of the Soviet space programme.
As it flew by Mars,
the spacecraft would look for life on the surface.
But before launch, Chertok decided to double-check
that the life-detecting device was working properly.
The truth about life on Mars
remained elusive for another 15 years.
And then came Viking.
It was the most ambitious robotic space mission
NASA had ever undertaken.
It was the summer of 1976.
America was on a bicentennial high,
and a spacecraft was on its way to test for life on Mars.
We had waited our whole career for this magic moment,
when we would actually test for life on Mars.
Can you imagine what it's like to be the first scientist
who'll ask the question, even if you get no answer?
'Once Viking was on the ground, you can imagine the excitement.'
Everybody with their fingers crossed,
and hoping to get some kind of result out of this.
For seven days,
Viking treated the world
to picture after stunning picture of the Red Planet.
They showed Mars as a frigid desert,
where the temperature hovered around 100 degrees below zero.
This was not the sort of place where life ought to flourish.
But Viking was built to detect microscopic life
lurking in Martian soil.
On the eighth day,
a scoop reached out to grab a sample of red earth,
and the test was under way.
'The light went on
'and told us "We're incubating, it's working now...
'"Any moment now you'll start getting data."'
Inside Viking, nutrients were added to the soil.
To everyone's amazement, there was an instant response.
Gas came pouring off.
This gas was just what bacteria on Earth produce.
Every single point was an emotional moment.
"Look, it's going up a little bit! A little bit more!"
Then, when it stops... we couldn't believe it.
"Hey, the curve has stopped moving!"
As the team pored over the results,
they could hardly believe their eyes.
The sample had done just what was expected
if there were organisms in it.
Viking seemed to be saying there was life on Mars.
'We never slept. Never thought about anything.'
Our lives stopped. Our children stopped. Everything stopped.
Bank bills were left unpaid,
while we focused on the events going on.
Before they announced their discovery,
mission scientists wanted to make sure
the reaction from the soil was really caused by microbes.
So Viking performed another test,
one that scanned the soil for organic chemicals,
the raw materials from which all life is made.
'All of us were so certain
'there'd be organic material on Mars.
'When it said "There isn't any,"'
we said, "Try somewhere else. It must be a sterile sample."
We moved rocks, dug down, we did everything we could.
'We did the tests again and again. All came out negative.'
With that one result, hopes of life on Mars were dashed.
There were no microbes.
That initial test result
was probably due to a corrosive chemical in the soil,
created by intense ultra-violet radiation from the sun.
Nothing could live in Martian soil.
Viking was over...
and it had taken all our dreams of Mars with it.
'Viking was sent to Mars with a particular hypothesis,
'to test if there were microbes living in the soil.
'And it turns out there weren't.'
And so the sense was that Mars had come up barren,
and no one was thinking
about other environments on other sorts of planets.
But life on Earth had its own surprise.
In 1978, a scientific submarine
dived to the bottom of the Pacific Ocean,
and found something completely unexpected.
'People took vehicles like the Alvin deep submersible
'to hydrothermal vents on the East Pacific rise.
'They found fantastic environments with tubeworms,
'clams, crabs, surviving off the Earth's geothermal heat.'
It took a while to sink in,
but it means there are ways to support life,
beside sunlight at the surface.
That opens up a new range of habitability.
Since then, the limits of life on Earth have been pushed back
further than anyone imagined possible.
NASA scientist Chris McKay has chosen to study life
in one of the world's harshest environments -
California's Death Valley.
'Here in the salt flats of Death Valley...
'Not a place you'd expect to find life,'
but since Viking, we've found life in many unexpected places.
Underneath the salt crust here, just under the surface,
a layer of algae.
These organisms are deep enough in the salt to access moisture,
but close enough to the surface to get light to photosynthesise.
What we found, looking at life in very harsh environments -
dry, cold, hot environments -
is that wherever there's some mechanism trapping water,
life can flourish.
That's the key. If an alien civilisation said,
"What kind of life do you have?" my answer would be,
"It's water-based life. What do you have?"
If water was the key,
the search for life in the solar system became one for water.
In 1979, a spacecraft called Voyager ventured out to Jupiter.
It would study the giant world and its planet-sized moons,
Ganymede, Callisto, Europa and Io.
'What we knew about the satellites of Jupiter
'before we got there was very little.'
These things went from four points of light
that were understood only marginally better
than Galileo had understood them when he discovered them,
to entire worlds you could map and study in 48 hours.
It was an amazing experience.
Among the moons of Jupiter, they found a jewel.
Europa was smooth,
covered in a layer of ice.
But all over it was an intricate web of cracks.
What could they be?
'You can make mistakes in this business by saying,'
"I think I know what's going on, because it looks familiar."
But it did not escape our attention
that those fractures looked like sea ice.
Could there be an ocean hiding below Europa's icy skin?
Beneath the cracked surface, could life exist,
just as in the Earth's oceans?
'We know that life is resourceful enough
'to use many sorts of energy.
'Energy sources are widespread - sunlight at the surface
'or geothermal energy from within.'
That was what we realised we had potential to have on Europa.
NASA plans to go back to Europa to find an underground ocean.
Stephen Squyres hopes to design a mission to dive into it.
First you would land...
You would then have to get down through the ice.
So you would have a probe with a heat source in it.
And it would melt its way down.
'Once in the water, it's got to transform from a melting probe
'to a swimming probe.
'Then, when you get to the bottom,'
you'll be looking for all kinds of stuff.
I'd want to take pictures, so you need a light source...
'It's gonna be, if we do it,'
one of the most challenging...
planetary missions we've ever done.
'It's going for one of the most important questions out there.'
Could life have arisen on Europa?
It's hard to guess
until more is known about how life began on Earth.
How easily did life emerge?
Was it a miracle never to be repeated?
To answer that, you must travel back in time
to the beginning of the Earth's history.
'Life must have arisen'
after formation of the planet, about 4.5 billion years ago.
But how soon and how quickly after?
This is what we try to answer
from studying the geological record of the oldest rocks.
Steve Mojzsis is a geologist turned fossil hunter.
To probe the earliest secrets of the planet,
he travelled to Greenland,
where a rare outcrop of ancient rock survives almost unscathed
from when the Earth was young.
In those rocks, he hoped to find microscopic traces
of the most ancient lifeforms imaginable.
During a field trip in 1995,
he gathered a haul of sedimentary rocks,
minerals formed at the bottom of the world's first ocean.
'The rocks that are the oldest sediments of all
'have been through everything you can do to a rock,
'without quite melting it.'
That's a good one.
'They've been regurgitated, ground up, heated, crushed...'
When he broke open the rocks, he found tiny mineral grains.
He suspected they might be charred remnants
of primitive microbes.
'It's a preserved time capsule, this little mineral.'
Mojzsis put some grains into an ion probe,
which deciphers the precise make-up and age of minerals.
If the lumps were once living things,
they'd have a specific chemical fingerprint.
We found that it was spot on.
A veritable stamp...of life... that's unique to it.
But the real surprise was the age.
The ion probe showed
that the fossils were 4 billion years old.
'As far back as we can go,'
3.9 billion years ago,
there is evidence of life,
that life did exist here.
'And it existed soon after the Earth formed.
'That became something of a surprise to us.'
It had been thought that Earth was uninhabitable
for nearly a billion years.
Now it seemed life sprouted as soon as Earth came into being.
'As long as life appeared here so quickly,'
then perhaps it's a cosmic imperative
that life should appear
as a chemical consequence of the evolution of a planet.
Life took hold here,
not in the temperate cradle of today's Earth,
but when the planet was a steaming cauldron.
If life began so easily,
then perhaps the Earth isn't so special.
If life began when the Earth was an alien world,
what happened billions of years ago on the other planets?
Venus will never tell its secrets.
Its boiling atmosphere has turned any evidence to vapour.
But the barren, cratered surface of Mars
is a four-billion-year-old relic.
Trying to decipher Mars' history
became an obsession for some astronomers.
'It's interesting, all the personalities
'that have emerged in the course of Martian history on Earth...'
Percival Lowell built this extraordinary telescope
and dominated the scene of his day,
and had this great vision.
'We fault him for calling them canals,
'but he understood the importance of water,
'H20, to living things.
'It was an important mark in history.'
Lowell saw his canals as traces of an ancient civilisation,
on a planet starved of water.
In 1976, when the Viking landers were proving
that the surface of Mars was an arid desert,
the orbiters were photographing the planet
from pole to pole in unprecedented detail.
They saw not canals, but something just as exciting.
Snaking across the southern hemisphere
was a network of eroded channels,
flood plains and river valleys.
Billions of years ago,
this must have been a world of rivers and oceans.
Mars too was a place where life could have begun.
In 1969, Apollo 12 made man's second trip to the Moon,
and changed all ideas of life in the solar system.
'Roger. All dressed up and no place to go.'
'Oh, we're going some place.
'It's getting bigger all the time.'
'Roger, we copy that, 12.'
'The challenge of Apollo 12 was a pinpoint landing
'at a crater that contained Surveyor III.'
Surveyor III had landed on the Moon in 1967 -
one of the robotic trailblazers for the Apollo astronauts.
Now they were going to pay it a visit.
'We're ready.'
'We didn't expect anything else.'
We didn't train for anything else.
You better believe it.
One thing they wanted us to do, if successful,
was to bring pieces of the Surveyor back
for the engineers to examine what it meant for this structure
to stay 33 months on the Moon,
and to have total radiation from the sun for half that time;
to be in the vacuum of space, which is almost total,
realise the temperature changes from -250 to +250,
practically a 500-degree temperature spread.
They wanted to see what the effect was on those parts.
First they had to find that crater.
'OK, at 19,000ft. I got a horizon out there.
'Craters too, but I don't know where I am yet.'
'We were three days into the lunar day,
'with the sun 15 or 20 degrees behind our back,
'when we came in to land.'
We had this crater pattern picked out.
I pitched over at 7,500 feet above the lunar surface,
where I could see.
I thought I'd pick out this crater pattern...
'and it just wasn't there.
'There were 10,000 craters.
'But Al gave me the number to look down,
'and I did that and the thing popped out,
'and we were headed right for the crater.'
'That crater's right where it's meant to be!'
'They're beautiful.'
'Hey, you're really manoeuvring around.
'Coming down at four. You're looking good.
'50ft, coming down. Watch for the dust.
'40, coming down. Watch the left.
'You got plenty of gas, plenty of gas. Hang in there.
'Coming down at three, he's got it made.'
'24ft... Contact.'
'Roger. Copy contact.'
(Indistinct exchange over radio )
'Command override off.'
Once we landed, I knew it was the right crater pattern,
but I hadn't seen the Surveyor.
'So I went down the ladder,
'and the first thing I wanted to do was look at the crater,
'and there was the Surveyor sitting there.
'That made my day. I knew it was all downhill from there.'
'There's Surveyor. Yes, sir!'
'Is that luck, Pete?'
And so Pete Conrad went for a stroll
and, with the help of bolt cutters,
snipped the camera off the defunct robotic ship.
'We're ready to start getting a TV camera.
- 'Give a big smile. - OK.'
'So they got the television camera off the Surveyor.
'When they opened it up in the lunar receiving lab,'
apparently, when it was assembled, three years before,
the worker must have had a cold and sneezed into the styrofoam,
and they found the spicules all dried in there.
But good microbiologists took that,
put it in their petri dishes
and did whatever you do with it,
and lo and behold, this bacteria came back to life.
'It took off like nothing had happened.'
Apollo 12 proved bacteria can survive the vacuum of space.
Life is tougher than imagined. It is almost indestructible.
Microbes had made a journey from one world to another...
and survived.
'Apollo 12, Houston.'
Could life have taken a trip like this before?
The idea that meteorites might carry life
from one planet to another had existed for decades.
It was considered crazy.
But there were some unusual meteorites around.
No one knew where they were from.
Then two scientists proposed another seemingly crazy idea.
'Twenty years ago, at a bar in Houston,'
a colleague and I discussed this problem,
and came to a rather startling conclusion.
'That is, that these particular samples were rocks from Mars,'
that were knocked off in a large impact
and had arrived on Earth.
We were graduate students at Harvard,
and didn't realise the resistance the idea would enjoy,
and, in fact, that the idea was considered impossible
by the scientific community.
But the more of these we consumed,
the more plausible it seemed.
For several years, the idea kicked around.
Then a NASA scientist took a close look at one of the rocks.
'Did you think this idea was crazy when you heard it?'
I didn't think it was crazy.
The idea of meteorites from Mars was developing for years.
I was a non-believer, but willing to be converted,
if evidence came along.
This is a small piece of the meteorite.
I guess these black areas are like what you analysed?
Yes, this black inclusion on top is similar,
although the one we measured was larger, more spherical.
'The greater surprise was the nature of the evidence...'
When Don Bogard analysed a grain of glass in the rock,
he found it contained gas.
To his amazement,
it exactly matched the gas sampled by the Viking landers.
This rock had to have come from Mars.
I think no one anticipated you'd get evidence for that process
in such an unlikely way as going into a piece of glass
and showing a bit of the Martian atmosphere was trapped in it.
We now know that pieces of Mars have rained on Earth
for billions of years.
Could life have once travelled with them?
'If life was present on Earth at the end of its formation,'
the impacts then would've been more numerous than today.
Material would constantly be being knocked off the planets,
and exchanging between the planets.
They could've carried organisms from one planet to another.
The planets would not have been biologically isolated.
Material going back and forth
would have been like swapping spit between the planets.
In 1996, some scientists thought they had found proof
that life had travelled to Earth from Mars.
One Martian meteorite seemed to have remnants of microbes
that were thought to be Martian.
That turned out not to be the case.
The truth about Martian life won't come so easily.
If life did exist on Mars,
it only had a billion years to evolve,
before Mars lost its atmosphere and became too cold and dry.
On Earth, there is one place close to how Mars must have been
when any life there would have died out -
the dry valleys of Antarctica.
'This is how Mars must look.'
The Antarctic desert is cold and dry.
Mars is also, only colder and drier than Antarctica.
'In the high mountains, it is absolutely dry
'and on the surface is lifeless.'
But biologist Imre Friedmann
thought there was one refuge for life here -
the type he believed was the most advanced organism
that could have existed on Mars.
'We thought life exists more inside rocks,'
where micro-organisms can find their protected habitat,
rather than soil, which is more exposed
to the extremes of the environment.
In 1976, Friedmann found sandstone rocks
with strangely mottled surfaces.
And when he broke them open,
there were the signs of life he was looking for.
'You see under the surface a continuous green layer
'of photosynthetic micro-organisms.'
From the outside the rock looks dead, it is brown.
But one millimetre below the surface, it's green.
These rocks are not brown, but green.
The microbes cling to life,
because even when it's freezing outside,
water can form in droplets inside the rock.
'These micro-organisms have a very hard life,
'because most of the time they are hard frozen.
'Only a few hours a year, they are coming to life.
'They have water, the temperature and the light,
'that they can photosynthesise and live actively.'
These microbes are the toughest form of life on this planet.
They could have survived on Mars three or four billion years ago.
These organisms live at the limit of existence;
so to say, at the precipice.
If conditions deteriorate only a little bit, they die,
and the result is extinction.
Perhaps the last life on Mars left its traces
locked inside rocks on the planet's surface.
We're on the threshold of a new search for life on Mars.
This time it will be different from Viking.
'Our whole attitude towards life and a planet
'is different now than back in 1976.
'We've gone through a revolution in thinking.'
We're looking at what could be a Mars that had life,
but doesn't have life any more.
Is Mars a world full of fossils?
NASA has committed itself to finding out.
Beginning with Mars Pathfinder in 1997,
it has embarked on a decade of missions
to scour the planet for signs of ancient life.
The rover that will go to Mars in 2003
will have a drill and a microscope
to help it hunt for fossils.
Stephen Squyres is designing the mission.
'This is totally different from Viking.
'That objective was to test the idea
'that there are today on Mars microbes living in the soil.'
That's not what this is about.
We're studying what the environment was like long ago,
whether there was life there back then,
that today would be present only in fossils, some remnant form.
'The evidence is in the rocks.
'That's why the mission is strongly focused on rocks.
'Instead of scooping in the soil and doing experiments there,'
what we'll do is find the best rocks, take samples,
bring them back to laboratories on Earth.
There are many places on Mars where fossils might be found -
dry lake beds, river valleys, deep canyons.
These are the places where the hunt is now on.
We see places like this on Mars - dried lake beds,
with deposits on the surface that could contain organisms
that lived in salt crust like this billions of years ago.
Imagine if you lived on Mars then.
This would be a great place to be preserved,
because we might be going there and pulling out those remains.
'Suppose we find evidence that life came into being.'
If that's the case, literally half the planet, I think,
is covered with four-billion-year-old rocks.
So, there's a chance that the record of that process,
of life coming into being from non-living material,
is still there to be read in the Martian geologic record.
So, maybe by going to Mars
we can understand better where we came from.
The search for alien life is a quest to understand our origins.
Did life begin on Earth?
Did it travel here from Mars,
or even from a more distant world?
The answers could lie clear across the solar system.
'20, 30 years ago, our view of life was bounded by Earth.'
We conceived of life originating on Earth,
having written its full history on Earth.
Now we realise Earth is not an island.
It's connected to other planets.
'We realise that Earth lives in a neighbourhood,
'and that neighbourhood influences the life here.'