Original subtitles

The oceans define the earth.

They're crucial to life.

In fact, without the oceans, there would be no life.

We once thought they were unique to our planet.

But we were wrong.

We've recently discovered oceans all over our solar system

and they're very similar to our own.

Imagine this at the bottom of Enceladus' Ocean.

Now scientists are going on an epic journey in search of new life

in places that never seemed possible.

Life has got this amazing ability to, you know, just keep surprising us.

I want to get data back from a probe and be able to say,

"It's life, Jim, but not as we know it."

NASA are even planning to dive to the depths of a strange,

distant ocean, with a remarkable submarine.

That first picture... Are you kidding?

That first picture on the surface of a sea, on another planet

in our solar system changes the world.

The hunt for oceans in space marks the dawn of a new era

in the search for alien life.

Nearly two centuries ago, Charles Darwin set out on a journey

across the world's oceans to uncover the secrets of life.

What he came to understand was that the answer to the mystery

of where we came from lay beneath the hull of his ship, the Beagle.

As he filled his notebooks with beautiful sketches of the birds

and animals he came across, he began to formulate an idea that

life might have actually started in water.

Darwin's important for the whole story of the evolution of life

and natural selection, where we all came from,

how life ultimately started, as well.

A lot of that goes back to Darwin.

He had ideas, not very well publicised ideas, not

in the Origin Of Species, but on how life started in a small, warm pond.

So Darwin had put his finger on the importance of water

in the origin and evolution of life very early on.

Water is so essential that it's dictated where scientists

look in the search for life in our solar system.

Life needs water. You look at all life forms on earth.

The one requirement they all have in common is water.

An ocean may be a good place to incubate life and,

not surprisingly, an ocean has got what life needs to survive.

Everywhere where we look on earth, whether it's frozen

or boiling hot, wherever we find water, we find life.

Water is our working fluid.

You're mostly made up of water, I'm mostly made up of water.

The search criteria were simple - to find life,

first find a liquid ocean.

Only beyond earth, there didn't appear to be any in our solar system.

There used to be the idea of the Goldilocks Zone,

where everything was "just right" for water to be in the liquid

state on the surface of a planet and earth was slap-bang in it.

Venus was too close to the sun,

too hot really for liquid water on the surface.

Mars, thought to be a little bit too far away.

But is finding liquid water and life on Mars impossible?

We've been sending evermore complex

and sophisticated spacecraft to the Red Planet for decades

and we now know more about it than we ever did.

Unfortunately, all the scientific evidence gathered so far

points to Mars being dry, cold and seemingly lifeless.

But has it always been?

It's a question that's intrigued scientists

and astronomers, like Geronimo Villanueva, for years.

Ironically, the search for evidence of an ancient Martian ocean

is being conducted from one of the driest places on earth -

the Atacama Desert in Chile.

So there is a strong relationship between Mars and Atacama,

because Mars is a very dry place

and Atacama is one of the driest places on the planet.

Actually, the relative humidity measured by Curiosity Rover on Mars

is practically the same as we are right now, here on this desert.

Fittingly, it's that lack of water that makes the Atacama

the perfect place to build one of the biggest

telescopes in the world,

because water in the atmosphere here would

drastically limit the telescope's ability to find water anywhere else.

Water and many other things like organics are what we're

looking for, so we come to a place which is devoid of those things,

like a desert, so we don't get the contamination from those

things when we observe through the atmosphere.

So when you come to a place like this,

you're trying to look through the water in our own atmosphere.

What's immediately obvious to anyone with even an ordinary telescope,

is that there IS water on Mars.

But today, it's frozen solid at the poles.

Yet the Martian landscape looks strangely as though

it was carved and shaped by liquid water.

Planets show all this morphology, geomorphology, driven by water,

a huge amount of water, so the estimates of how much

was on the planet vary a lot, because we didn't know.

I mean, we see all this carving, all these big valleys,

and so how much water was there is a big question.

Answering that question was pretty much impossible

until scientists got lucky in 1984 in another desert.

This time in the coldest place on earth - Antarctica.

Here they found a remarkable meteorite.

Analysis confirmed it was Martian in origin and that they had

discovered the key that would unlock the mystery of Mars's watery past.

So once you identify when in the history of our solar system,

where it came, then you say, "OK, this rock is dated there and it comes from Mars."

So you have a good reference point in time and in place of that rock.

Careful analysis revealed that this meteorite was 4.5 billion years old.

The meteorite also carried crucial chemical information -

an isotopic signature fixed by the amount of water on Mars

4.5 billion years ago.

On its own, this signature was worthless, but by measuring

the amount of water on Mars today, then comparing the signatures

of recent rocks against the ancient meteorite, all would be revealed.

And that's where the huge telescope comes in.

It's so powerful it can detect water molecules on the surface of the planet.

You can actually see the molecules in every...

Above a volcano in Mars, above a valley, you can

actually map those molecules from here.

It's really astonishing.

Armed with a precise measurement of the amount of water on Mars today,

Geronimo was able to make an astonishing calculation.

We extrapolated back in time and we inferred that there was almost

seven times more water than there is right now.

What happened? Mars, topographically speaking, has very low plains

in the north and a very high altitude place on the south.

So if you throw water, it will tend to flow into the lower topography,

which is going to be the northern plains.

So one of the things we did is,

OK, so, we had this volume of water and so what do we do with this?

So one trick was we said, OK, just throw it on the planet

and let's see where it falls.

And they just did, like, you know, follow the rivers and everything,

and it formed an ocean on the northern plains of the planet.

4.5 billion years ago, the Martian ocean

covered 19% of the planet and was as deep as the Mediterranean.

In fact, NASA's planetary models reveal a Mars at its warmest,

complete with an earth-like atmosphere.

If you were in an alien spacecraft randomly coming to earth,

the chances are better than even that you're going to land

up in water, so bring a boat.

And it's the same on early Mars, and that's a fundamental point,

that Mars was a water world.

It would have been better to characterise it as a water world,

whereas now, of course, it's a desert world.

But it's that water world that's interesting.

That's the world that may have had life

and that's the world we want to investigate.

It even had waves!

The reduced gravity on Mars meant that these waves

would have been twice as tall as those on Earth -

a surfer's paradise,

but, according to Nasa scientists,

most of the time

you'd have to be pretty tough to catch a Martian wave.

If we think back to early Mars,

we would expect it to be an Earth-like environment -

if it had water

and a thicker atmosphere, and was warmer.

The one big difference, I think,

would be that it would be more like the Arctic Ocean.

It would be an ice-choked,

ice-covered ocean.

So, if you imagine standing on the north shore of Greenland

looking out at the ice packs moving,

I think you'd get a good imagination

of what early Mars might have looked like.

Would the surfers like it better or less?

It depends really on the wet suit,

because it's going to be very cold on early Mars,

so you've got great waves, but you're inside a wet suit to survive it.

Not very many people surf in the Arctic Ocean

and this could be part of the explanation.

It may have been cold.

Mars is much further away from the sun than the Earth,

but four and a half billion years ago,

life on Mars would have been technically possible.

This is the time when Mars was the most habitable time.

When the planet was formed, actually,

planet Earth and Mars were similar in some aspects.

It had a thicker atmosphere,

maybe there was a big ocean there,

so habitability of the two planets were similar

and, interestingly, the time that we think this ocean was there

was a time that life started in our planet.

So, you know, if the conditions were favourable for life here -

to start life, you know -

what could be the conditions on the planet Mars?

Sadly, however habitable that early ocean was,

it didn't last.

Scientists think that the early Martian atmosphere

was vulnerable to solar radiation and,

over the course of one and a half billion years,

it evaporated away

leaving just 13% frozen at the poles.

But if Martian life was theoretically possible

in that ocean millions of years ago,

is it possible that anything could have survived until now?

I think the possibility of finding life on Mars now

traces directly to the possibility

of finding liquid water on Mars

that's relatively fresh.

And finding water has been a large part

of the Curiosity rover's mission.

Curiosity has been trundling around Mars since 2012

and the images it's been sending back have been stunning.

Sequences, like this blue sunset,

are starting to change our understanding of the planet,

but it's the pictures from the Mars reconnaissance orbiter

of a region called the Newton crater

that are helping to shed new light

on the amount of liquid water left on Mars.

This is a time-lapse sequence

showing streaks on the crater wall -

apparently growing and getting darker.

Scientists think that they might be caused by water.

These small amounts of water -

compared to an ocean on Earth or even an ocean on early Mars -

they're insignificant.

But as an indicator of Mars still being active

and still having liquid phases,

and maybe a hint of bigger and better things elsewhere,

then I think it's very important.

What appears to be happening

is that the moisture in the soil

is evaporating during the relative warmth of the day

and condensing back at night when it's colder.

So, Mars still has a heartbeat.

It's a faint one if we measure its heartbeat

in terms of the presence of water.

At one time, it was huge,

it was an ocean

and now there's just a faint glimmer of it.

The problem is that these small amounts of water

are exceptionally salty.

The Curiosity rover has identified,

in the Martian soil,

a salt called calcium perchlorate.

It's this salt that absorbs the Martian dew

as it condenses onto the cold surface each day.

The salt also lowers the water's freezing point,

keeping it a liquid -

even at sub-zero temperatures.

But it also makes these faint traces of brine

so concentrated

they'd be toxic to conventional life forms.

So, could they support life on Mars?

There may be clues in the saltiest parts of the Earth,

like the Bonneville Salt Flats in Utah.

It's famous for land-speed records,

but it's fascinating for astrobiologists

because the salty surface here

not only mimics that found on Mars,

it contains life.

Even though this looks dead,

we could probably take some of these crystals right here

and get bacteria to grow.

I know it seems ridiculous,

but, you know, as a microbiologist

one of the things that

we've come to appreciate is

if there's any liquid water present,

you're typically going to find life.

So, life has got this amazing ability to, you know,

just keep surprising us.

Unfortunately, Mars is way colder

than the Bonneville Salt Flats.

The average temperature of minus 50 degrees Celsius

is a huge challenge for anything living

on the surface of the red planet...

..and it's partly to do with the angle of its axis.

Earth spins on an axis of 23 degrees,

which should make the planet unstable -

but it isn't.

Earth's axis is stabilised by the moon -

sort of like an outrigger,

a gravitational outrigger that keeps the Earth stable.

Mars doesn't have a large moon and so...

And it's also closer to Jupiter.

As a result, its axis wobbles significantly.

Much, much more than Earth's.

More than double the wobble of Earth's.

Over 100,000 years,

Mars' tilt wobbles by as much as ten degrees,

causing huge climate change.

Similar but more extreme

than the Earth's ice ages.

At the peaks of that cycle,

the surface of Mars is briefly warm enough to support life -

but to survive 100,000 years of cold between these peaks

would demand a strategy of extreme hibernation.

But for micro-organisms, this strategy of living when it's warm

and then sleeping when it's freezing cold is a good one.

Those organisms can be frozen and thawed without any damage at all.

Every once in a while, when the tilt is right,

you get a few thousand years of time to have a go at it,

and then you go back to deep-freeze sleep.

That all sounds fine in theory,

but could any living thing possibly hibernate for up to 100,000 years?

The answer lies in the salt.

The salt crystals form in cubes and,

as they form,

you'll have pockets of liquid

that become entrapped

as the solid salt is forming,

and the micro-organisms that are present become trapped

in those fluid inclusions,

those little pockets of fluid.

How long, then, could a single bacteria survive

trapped in a salt crystal?

Melanie took a crystal

dated at 97,000 years old

and drilled into its core.

She extracted the fluid,

placed it in a nutrient-rich dish

and walked away.

When she came back a week later,

something astonishing had happened.

97,000-year-old bacteria

were flourishing in the dish.

It was pretty amazing, you know,

to be able to have

such strong evidence.

I mean, taking that fluid inclusion up

and using it to inoculate media, you know,

and then having something to grow -

that's pretty...

Pretty powerful stuff.

But how could something survive

for nearly 100,000 years

trapped in a salt crystal?

Only the basic metabolisms

would be still functional,

so these organisms are probably

just expending enough energy

to keep maybe their DNA repaired,

and that's probably about it.

So, right here on earth,

these bacteria have developed a hibernation strategy

extreme enough to cope with the length of the Martian ice age

but, even at its warmest,

Mars is much, much colder

than the Bonneville Salt Flats.

Extreme endurance alone wouldn't be enough.

So, is there any life form capable of hibernating through extreme cold?

This doesn't look a very likely place to answer that question,

but biologists Carl Johansson and Byron Adams

aren't here to drink in the obvious beauty

of the Bridal Veil Falls in Utah.

What we want to try and target is that base there,

where the upper falls are kind of falling down.

Just right below the main part of the fall,

you can see all the moss beds that are in there.

That's all pretty good stuff. You get in there.

That's nice and slick.

THEY LAUGH

All right, let's go. All right, man.

They're looking for a creature with an unusual ability -

one that might prove crucial

in the search for alien life.

Unsurprisingly, it loves water,

and there's plenty of that here.

Now, this looks good.

Here's a good way around this way, I think.

It's a good spot.

Watch your step, man. It's slippery, bro.

Yeah, this looks really good here, man.

Yo, Carl!

Bag me, bro.

This creature is so small that it's almost impossible to see

with the naked eye.

Being small doesn't mean it's insignificant,

it just means they have to collect lots of very damp moss

to make sure they wrangle one.

Bag 'em and tag 'em.

It's got her.

Dude, I'm taking it right here, bro.

It's, like, raining on me.

I know it. That's why I wasn't there.

It's only when they get back to their lab,

at Brigham Young University,

that they can see what they've got.

So, you remember the samples that we just collected up at the waterfall?

We brought them back to the lab here

and we put them in some dishes,

and I'm picking the animals out of those dishes

and putting them onto a slide,

and then I'm going to hand this slide to Carl

so that he can put it under the microscope,

and then we'll be able to get a better look at them.

So, when we look at the slide that Byron brought us

and we start looking through,

we can see some movement, right here, of an animal.

This is the tardigrade.

Tardigrade means this

Latin name slow-stepper.

"Tardi" means slow,

and "grade" refers to foot.

You can start to see,

he's got long thin filaments

coming off his body

and some actual...

What almost look like horns

coming off his head that he uses in feeding.

Tardigrades are aquatic,

so you'd expect them to die

if they weren't in water,

but they have a very special ability.

As that sample jar starts to dry out,

as that specimen starts to dry out,

what's cool about these guys is

they can survive that extreme desiccation,

drying down to like a crispy little booger.

It's called a tun.

They roll up into a special...

A tight ball, essentially -

they're like a roly-poly bug almost -

and then go through a series of radical chemical changes

in the cells in their bodies

to deal with this loss of water.

It looks like it's dead,

but, when they add water,

it springs back to life.

It's not really dead, because

when we add more water to them -

when environmental conditions are good again -

they can come right back alive.

It's very energetically costly for them.

They can't go back and forth and back and forth,

but they can survive some really extreme conditions

and what happens is,

as their environment starts to dry out,

in order to survive that,

they actively pump all the water out of their bodies

and out of the cells.

And so the genes that are being expressed

for normal cellular processes shut down

and they completely change the way

they express their DNA.

They've got one operating system,

their genes that operate to put them into and maintain them in a tun,

and then they switch operating systems when they're, you know,

carrying out life's activities -

when they're eating and moving around, and mating,

and all those kinds of things.

It's almost like two complete life operating systems.

But drying out and thriving in a temperate lab

is completely different from surviving

on the chilly surface of Mars.

The coldest place on Earth

that's in any way comparable to the red planet

is the Antarctic.

Tardigrades have been found here,

but can they be reanimated?

I've got some animals that have been frozen here

since the last field season in Antarctica,

so we extracted them from soils in Antarctica,

shipped them back here frozen solid

and they've been frozen solid here at

at least minus 60 since 2012.

So, this is the sample that we pulled out of that freezer

and it's thawed out now.

What I'm going to do now is I'm going to have a look at it.

So...

Holy moley!

It's mind-blowing, dude.

It's basically the same community that I saw

when I collected them in Antarctica.

We put them in a tube,

froze them,

shipped them.

Four or five years later, we want to study them, right?

Pull them out, we thawed them out

and now what I'm seeing now

looks almost exactly like

what I saw when I was looking at them, like, fresh in Antarctica.

You know, there's a few of them that didn't survive the trip, right?

But, for the most part,

if you were to show me this, like,

double blind, fresh,

I would struggle to tell the difference

between the sample that I got live down there

versus one that's been in the freezer

for like four, five...

Who knows how long, how many years.

As well as surviving extreme cold,

tardigrades have another trick up their sleeve.

In 2007, the European Space Agency

sent a sample of tardigrades up to the International Space Station

for an astonishing experiment.

They took them into space

put them on a satellite,

opened up the door, sent them outside,

exposed them to extreme temperatures -

vacuum, hot, cold...

Huge radiation. And then, when they brought them back to Earth,

they did what you're seeing here.

They dumped some water on them to see if they actually reanimated.

INTERVIEWER: What happened? Voila!

They take the water up, man,

and they start... Right?

They swap out the molecules and... Like a machine, man.

You add the water to it, they take them up,

the cells start to do their thing again and they come back alive.

It always blows my...

Look, I'm an old, fat dude and I've looked at these 100 times,

thousands of times, millions maybe...

You're not that old. Well...

..and when I actually look at them under the microscope,

every single time, I'm like, "Dang, that's cool, man."

So, the remarkable tardigrade can survive the extremes of space

AND the killing cold of Antarctica -

conditions similar to modern-day Mars.

And, of course, life can also survive

for tens of thousands of years

locked away in a salt crystal.

So, there could possibly be life on Mars.

It used to have an ocean

and there might still be traces

of that ocean left today.

But what about the rest of our solar system?

From the early 1960s,

scientists have been sending probes out

into the furthest reaches of our solar system -

looking, in part, for liquid water...

..but everything appeared largely frozen, dry and lifeless.

Most of our solar system was colder than anywhere on Earth -

even the icy wastes of the high Atacama Desert.

But, in these remote mountains,

scientists have uncovered tantalising clues

that could help answer the question,

"Are Earth's rich and flourishing oceans unique or ubiquitous?"

And the Voyager probe launched by Nasa in 1977 pointed the way.

In 1980, it photographed a small moon of Saturn

called Enceladus.

It's tiny, about the same size as the UK -

and, at first, it looked insignificant.

Enceladus is this bizarre little moon

that the Voyager spacecraft

took a few snapshots of.

The surface could be seen to be cratered in the north -

a lot of craters on its icy surface.

Now, to a planetary scientist and astronomer,

that means old ice.

But in the south and, in particular,

down near the south pole,

what was seen was a fresh ice surface, very few craters.

If the ice was fresh,

then where had it come from?

Scientists had to wait for years before they got an answer,

and it was provided by the Cassini probe

which span past Enceladus in 2005.

And what Cassini saw shocked scientists.

Plumes of water vapour

pouring out from the surface

of the little moon's south pole.

So, when Cassini returned these images of the plumes,

the community just went nuts.

This was astounding to see these jets of water

erupting out of this bizarre little moon.

Enceladus is just 500 kilometres in diameter -

that's about the width of the United Kingdom.

And to see these jets erupting was phenomenal.

As Cassini got closer to Enceladus,

it revealed the plumes were spewing

not just from one crack,

but from four huge fractures in the ice.

Each of them was about 130 kilometres long,

two kilometres wide

and about 500 metres deep

with water vapour pouring out of them.

That amount of water could only mean one thing.

Enceladus had to have a liquid ocean

beneath its frozen surface...

..but this dark, subterranean ocean

would be lacking in one thing

that's crucial for life on Earth.

Life, as we know it,

needs not only liquid water,

it also requires the elemental building blocks for life -

the carbon, the hydrogen, the oxygen -

a smattering of the elements across the periodic table.

And life requires some form of energy.

On Earth, the energy for life comes primarily from the sun.

It's captured through the remarkable process of photosynthesis,

thanks to plant life - like this very primitive aquatic algae.

This stuff doesn't look like much.

People try and avoid it when they go into the sea,

but it's changed the world.

This is photosynthesis in action.

The cells that made this up

arose around about two billion years ago or thereabouts

and they cracked the trick

of using the energy of the sun

to split water and release oxygen,

and they're still about the major supplier of oxygen on the planet.

These things produce more oxygen than the rainforests.

It's remarkable.

It looks like slime,

but without this,

there wouldn't be any animals,

there wouldn't be any complex life on this planet.

This makes the world.

In the darkness of Enceladus' hidden oceans,

there could be no photosynthesis to capture the sun's energy -

yet the possibility of finding life there isn't entirely hopeless.

This is El Tatio.

It's a massive geyser field.

It sits 4,300 metres above sea level,

high in the Atacama Desert,

and it's a riot of hydrothermal activity.

And there's something bubbling up here

that makes the prospect of life on the distant moon of Enceladus

just a little more feasible.

But the clue is what we find here on Earth.

If we look alongside of this geyser,

we see these geyser pearls.

This is silica,

SiO2,

that has sintered out of this geyser water.

And the cosmic dust analyser

on the Cassini spacecraft

has captured grains like this -

except much, much smaller.

And the fact that those grains are found in the plume of Enceladus

leads us back to the water/rock interaction

where that silica in the plumes of Enceladus

could only be there

if the ocean of Enceladus

is cycling with an active, rocky,

potentially hot sea floor.

Cassini's measurements indicated that,

deep in the oceans of Enceladus,

a process very similar to the geysers of El Tatio

must be underway.

Imagine this at the bottom of Enceladus' ocean.

We have reasonably good evidence that the chemistry and,

in fact, some of the temperature of the water

that's coming out of these geysers, right now,

is comparable to the sea floor of Enceladus.

The bottom of Enceladus' ocean might look like this,

but it's cut off from the life-giving properties of the sun

by kilometres of ice.

So, does that make finding life impossible?

In the deepest abyss of our own oceans,

every bit as dark as those on Enceladus,

life was thought to be impossible

until a remarkable discovery,

just a few decades ago,

changed all that.

And so, in the late 1970s,

spring of 1977,

explorers went down to hydrothermal vents along the East Pacific rise.

Originally, they thought that

they might find some hot springs

at the bottom of the ocean.

They did not necessarily expect to find

a tremendous amount of biology -

but, lo and behold,

the hydrothermal vents,

despite being at incredible depths,

incredible pressures

and cut off from the energy of our parent star,

lo and behold, life was thriving.

And so, it may be that those kinds of eco-systems,

the kind of geology and chemistry

that underlies those eco-systems,

could also power life

within these ocean moons.

This huge abundance of life

was surviving and thriving

despite being totally cut off from life-giving sunlight.

Instead of photosynthesis,

it was powered by an entirely separate chemistry.

Here, we're bringing together the keystones

for life as we know it, the keystones for habitability.

We've got the water,

we've got the elements

and we've got a lot of energy.

Within that winning combination,

water plays a crucial - if very simple - role.

If you simply remove the water and have dry surfaces,

everything would remain stuck in its place on the surface

and there would be no movement to bring things together to react,

so I suppose water is the universal lubricant that makes things happen.

And the evidence for that can be found throughout

this seemingly inhospitable environment.

At the most basic level,

biology is a layer on geology.

Biology is harnessing some of the stored chemical energy

that exists in chemically-rich waters interacting with rocks.

And, right here, we've got a beautiful example

of exactly that kind of biology being a layer on geology.

Everything that you see here,

the red that you see,

those are microbes

utilising the rich chemistry of the geyser water.

The presence of these extreme life forms

thriving in almost alien chemistries

raises real hope for scientists -

not just in the search for life,

but in answering one of biology's most fundamental questions.

Is there a second independent origin of life elsewhere

within our own solar system?

And if there is,

then that tells us that life arises

wherever the conditions are right,

and we live in a biological universe.

If we don't find life within these worlds,

then that may be an indication

that the origin of life is hard

and that life is quite rare

within our solar system and beyond.

Both outcomes are equally profound.

Our solar system may be largely cold and inhospitable,

but, against all expectations,

we're now discovering it's also wet.

But just how soggy is it?

Is the ocean on Enceladus a freakish one-off?

Would it be the only moon with an ocean?

Or could there be other bodies out there

with as much water as the earth?

High on the list of possibilities

would have to be Ganymede,

orbiting around Jupiter.

This icy moon is the biggest in our whole solar system...

..but, initially, it didn't look that promising.

Back in the 1970s,

when we only had, like, grainy,

pixely images from the moon,

we knew it was icy,

the surface was icy,

but we had no idea what's inside.

What is going on? Does it have a magnetic field?

Does it have other things like we have on Earth?

So, it was just a ball of ice.

Nasa sent the Galileo probe

to take a closer look -

and, in 1996,

it found something completely unprecedented -

a magnetic field.

And this would ultimately lead to yet another watery discovery.

The Galileo mission was definitely a breakthrough in a way,

because it discovered Ganymede's magnetic fields,

and Ganymede was suddenly not only the largest moon,

but also the first moon we know of that has its own magnetic field,

interior magnetic field.

Intrigued, Nasa focused the huge power of the Hubble Telescope

on the surface of Ganymede.

In orbit around Earth,

this telescope has sent back amazing pictures of the universe

as well as our solar system.

And when it was pointed at Ganymede,

it revealed yet another first -

the moon had auroras encircling its north and south poles.

Because Ganymede has a magnetic field,

it can direct the charged particles

from Jupiter's magnetosphere

and they get directed towards the poles of Ganymede -

and so what that produces is aurora.

So, just like we have the Northern Lights and Southern Lights of Earth,

in Ganymede's case, the energetic particles

are hitting the really tenuous atmosphere which Ganymede has,

and that actually causes aurora on Ganymede.

But the auroras on Ganymede

held another surprise.

Astronomers had correctly predicted

they would rock like a seesaw

as the moon orbited Jupiter,

tugged by the magnetic pull of that giant planet.

They'd calculated the rocking

to reach a full six degrees,

but the reality was very different.

What we saw is that it was always rocked by only two degrees -

so not six -

but it seems like a small difference,

but it is significant

so we see it's only rocking by two degrees,

and so there must be an effect

that suppresses this rocking.

This apparently trivial detail

led scientists to a thrilling conclusion.

The only possible explanation

for this suppressed rocking of the aurora

is basically magnetic induction in a liquid...

In a salty, liquid global ocean inside Ganymede.

They'd discovered an immense ocean

calculated to be 100 kilometres deep -

ten times deeper than any ocean on Earth -

and it encircles the whole moon.

In Ganymede's ocean,

there's more water than the whole of the Earth,

but it lies under 150 kilometres of ice.

It's still, even for me, really hard to imagine these worlds.

I mean, I see images of Ganymede

and four of the moons every day,

and I have a really good idea of what they look like,

but it's still most exciting

when you look through a, like, small telescope

and see, like, a bright dot next to Jupiter

and then you know that the moon really exists.

And knowing now that this bright dot I see in the telescope,

next to Jupiter, does have an ocean is really exciting.

Each new ocean discovered gives a boost to the chances

of finding life in the solar system.

In 2022,

the European Space Agency will send a probe

to peer beneath the icy surface of Ganymede

in the hope of revealing some of the secrets

hidden in the icy depths of that huge ocean.

But is there only one way to cook up life?

Could you make it from a different set of ingredients?

Science fiction writers have speculated wildly

about alternative life forms -

but, in the cold, hard world of science,

we only have proof of life as we know it.

But if an ocean really is critical,

does it have to be an ocean of water?

That's a question that drives Nasa's Chris McKay.

What I'm really interested in finding is

what I call a second genesis of life.

Organisms that are clearly not related to any life on Earth.

All life on Earth is related to itself, forms a single tree.

You can call that Life One.

What I'm looking for is Life Two -

something that's not related.

It doesn't have to be profoundly different,

but it has to be different enough

that we can say with very high confidence

that they are not related to us.

We do not have a common ancestor.

Where such a life form could feasibly emerge

was anyone's guess -

until, in 2005, the world's attention turned to Titan,

the biggest of the moons which orbit around Saturn.

At that time, all we knew of it

was that it looked gassy, orange and lifeless.

We knew that Titan was a fuzz ball from telescopes.

Before a spacecraft ever went to Titan,

just looking at Titan with a telescope,

we could tell that it had a thick atmosphere.

We didn't know the composition of the atmosphere

or the temperature of it, but we knew it had a thick atmosphere.

But then, in 2005,

the Cassini-Huygens probe span by,

revealing a surface that was unexpectedly Earth-like.

It was dotted with huge lakes

bearing an uncanny geographical similarity

to the Great Lakes of North America.

From a physical point of view,

the presence of liquid

creates all these other similarities,

and so we realised that liquid on Earth, liquid on Titan -

we're going to expect a lot of commonality, and we see it.

So, visually, when we look at these images of the lakes,

we see reflections of what we see in aeroplanes

when we look down as we fly over the Great Lakes.

There was one crucial difference, though.

These weren't lakes of water,

they were lakes of methane -

and, at minus 180 degrees Celsius,

they're too cold for any life form

with an Earth-like chemistry.

I would contend that we don't understand

the role of temperature directly in life.

Now, on Earth, of course,

we're used to living in a high-temperature liquid

at high temperature. We're in the fast lane.

We metabolise very rapidly

because we're living at high temperature.

While on Titan,

the liquid there is cold,

the temperatures are cold.

If there's life there, it's obviously in the slow lane.

It's metabolising very slowly

but, so what? What's the rush?

There's not an absolute tempo

that life must keep to.

But is that possible?

Can you have life using methane rather than water?

With this in mind, scientists at the picturesque and very watery

Cornell University, in New York,

are trying to establish whether methane-based life

is even theoretically possible.

They took the chemical ingredients that exist on Titan

and mixed them up.

Not in a test tube,

but inside a computer.

The computer built a three-dimensional membrane -

the outside wall of a cell.

Except this alien membrane functions in methane, not water.

It's not life yet, it's just a house.

But the very first thing that you have to do is

you have to have somewhere to shelter,

and a membrane is a way of keeping the outside to the outside.

A small step,

but this was ground-breaking science.

For the first time, it opened up the possibility

that there could be a second tree of life.

We tend to think that life would look like us.

You just have to look at the Star Trek movies.

All the aliens kind of look like insects

and things that we already know,

but why not be something completely different?

Something that we can't imagine,

but something perfectly suited to the conditions that are on Titan?

But if this extraordinary computer model's right, how would we know?

At the moment, we can't physically search for life on Titan,

but that doesn't mean there wouldn't be other telltale signs

that we can detect.

If we look at carbon dioxide,

just out in the field, down the road -

during winter, it rises

and during summer, it drops.

And that's because plants take it up to make leaves.

They pull in the carbon dioxide, it drops,

they make leaves.

In the fall, those leaves fall, decompose,

the carbon dioxide comes back up.

So, there's a seasonal phase in carbon dioxide

that's directly due to biological activity at the surface

consuming, and then releasing, that carbon dioxide.

We're pretty sure there's no vegetation on Titan,

but what could be the equivalent

of the fluctuations of carbon dioxide

that would indicate that something

was alive on the distant moon?

And the answer, we think, is hydrogen.

Organisms on Titan would derive their energy

by reacting hydrogen with various other organic compounds

and so, if there was life on Titan,

that life should represent a strong sink -

a strong loss -

of hydrogen at the surface.

And that loss of hydrogen at the surface

would have an effect on the hydrogen distribution.

So, we've said that the way to detect life on Titan

is to look at the distribution of hydrogen.

If there's no life,

the distribution will just be flat, uninteresting,

but if there is life, and the life is growing vigorously,

it will eat out the lower part of that hydrogen concentration.

There will be a depletion in hydrogen near the surface.

In 2005,

astronomers finally had an opportunity to test this hypothesis

when the Cassini spacecraft sent down a probe called Huygens

to land on Titan.

The pictures the probe sent back were stunning.

Unfortunately, there were no obvious signs of life...

..but Huygens was doing more than taking images of Titan -

it was making detailed measurements of the mysterious atmosphere.

As it turned out, the most important were the readings it took

of hydrogen levels as it floated down from space to the surface.

As the probe landed, scientists noticed something remarkable -

the hydrogen levels dropped abruptly.

When I heard about this result,

for a couple of minutes, I was ecstatic, thinking,

"Oh, my God, this is just textbook science -

"prediction, confirmation and a Nobel Prize comes next," right?

But reality set in soon after

as I looked at the paper in detail

and considered how easy it is

to jump to the answer you want.

It's really a question of excluding other possibilities.

On its own, Huygens' sensational measurement was inconclusive.

What they needed was verification.

So, Nasa put together a team of their best

and brightest engineers

to design a spacecraft capable of exploring

the unique and technically challenging oceans

of this liquid world.

And, after a number of false starts and dead ends,

they came up with this -

a submarine.

I was reading 20,000 Leagues Under The Sea

and thought, you know,

"Titan has this wonderful group of seas. What's underneath there?"

If we don't look there,

we really haven't seen what's going on in Titan.

So, we came up with a fairly long submarine.

As you see from terrestrial submarines,

they're usually about 10:1 on dimensions,

length to the diameter,

and the reason for this is, it really reduces your drag.

We are obviously a little power-limited.

We have a lot of communications to do.

We have four thrusters in the back, here,

which use electrical energy - so we went with a very long submarine.

If you can get below the surface of the sea

and get all the way down to the bottom in certain areas,

and actually touch the silt that's on the bottom and sample it,

and learn what that's made of,

it'll tell you so much about the environment that you're in.

But if you have a boat that just drives on the surface,

figuring out how to get a probe all the way down to the bottom,

get that sample all the way back up to the surface and sample it,

it really becomes an intractable problem.

There's so many things that can go wrong doing that.

And, instead, we said, "If we can encapsulate everything together

"in a submarine, then we could go right down

"and do that sampling and come all the way back up to the surface."

And so the submarine allows us to explore the atmosphere,

the wind, the waves,

to sound with a sonar to the bottom,

to measure the topography,

to see what the contours of the bottom look like

and then to go down and actually touch the silt

that's been settling there for thousands and thousands of years.

But sailing a large, one-tonne sub

around Titan's super-cold, methane-rich seas

isn't without its problems.

Fortunately, Nasa has the technology

to replicate conditions on the freezing moon,

and this is it.

Inside this huge tank,

scientists can safely and accurately mix up

the highly volatile cocktail of chemicals

that make up the atmosphere of the huge moon.

As we design and build the craft,

we can basically use this facility

to test problems or issues

that come up for the submarine,

so we can use this facility

to basically create the seas of Titan,

the coldness of Titan, the pressures of Titan.

They have discovered that one of the biggest problems

of Titan's methane seas

is that they're rich in nitrogen,

and that could make it very difficult to sail the sub around.

There could be so much nitrogen dissolved in the sea that,

when the propellers turn on our jets,

it might just make a lot of bubbles

and not be able to push against the liquid.

So we're doing analysis now

and we hope to do some testing in the near future that shows us

what happens if you spin a propeller in liquid methane

and liquid ethane with lots of nitrogen dissolved in it,

and can you get any thrust out or not?

This is a really important question to answer.

There's other ways to propel the submarine if that doesn't work,

but the design that we came up with

helps us get to that simple place,

in terms of space operations.

The sub will be packed full of scientific instruments

and bristling with cameras,

but there's one thing the scientists feel

will make the mission more than anything else.

That first picture, are you kidding?

That first picture from a submarine,

from anybody's submarine,

on the surface of a sea on another planet in our solar system,

changes the world.

I mean, that's something that none of us have ever seen before.

That is true discovery.

That is why we do any of this

and that would be awesome.

That first picture alone would make this entire mission worth it.

No scientist is saying that the cameras of the Titan sub

will definitely ping back pictures of living organisms,

but they believe sending a sub to this strange moon

gives them the best chance of finding a new form of life.

I grew up when Star Trek

was just coming out,

and it was an inspiration to me,

but the key moment was when I realised

that the job I wanted was not Kirk's job,

but Spock's job.

He's the one with the tricorder.

He's the one that's detecting life

and my favourite saying is,

"It's life, Jim, but not as we know it."

That's what I want to be able to say.

I want to get data back from a probe -

Titan, Mars, Enceladus, wherever -

and be able to say, "It's life, Jim, but not as we know it."

Is it possible that we could see stuff

that hints really strongly at life?

It's possible. I mean, we might see things

that look like lichens or algae

growing on the rocks on the shore.

We might see massive stuff on the surface,

but we have no idea.

We used to think that the rest of our solar system

was frozen and dead,

but we now know that there are oceans of water and liquid

in places we never thought possible.

In 2015, the New Horizon mission to Pluto

ticked off the last of the great worlds

to be explored in the solar system,

but we're only at the beginning of the quest

to find the Holy Grail of space science -

life.

We're through with the age of discovery.

We've discovered all the planets, we know what's there.

We've got a rough map of them all

and a rough understanding of how they work.

The next question -

the question that I think should motivate and guide planetary science

for the next 20 years - is,

"Is there any life in these various and diverse oceans?"

Nearly two centuries ago,

Charles Darwin set out on a voyage of discovery

that changed the world.

Perhaps Nasa's Titan submarine

will be a modern counterpart

to Darwin's ship, the Beagle -

and, in the search for a new form of life,

will boldly go where no-one has gone before.