In 1979,
a robotic spacecraft flew by the planet Jupiter.
There it found an uncharted body the size of our moon.
On that small world,
it observed something almost unbelievable.
We live on an active planet.
The rock itself is alive.
The Earth spits out hot lava, creating new land.
It is a sight we can only watch in awe.
I find it overwhelming to look inside a volcano
and see hot lava oozing out.
It's like the lifeblood of the planet.
For a geologist like Jim Head,
the Earth is an inspiration to look further afield.
What does it look like on the other planets?
'Would there be hot lava? Volcanoes?
'Would we see ocean basins?
'We had absolutely no clue.'
The history of this planet is written in its rocks.
Over hundreds of millions of years,
mountain ranges have risen up,
entire continents have drifted apart.
Is this how other worlds would be?
Earth's celestial partner, the Moon,
could hardly be more different.
Nothing has happened on this dusty world
for billions of years.
Its surface shows only the scars of countless asteroid impacts.
The story of the other planets
must also be locked on their surfaces.
But they are so far away, they are just dots of light.
Astronomers gazed at those dots,
but only on one of them could they make out anything
that looked remotely like the landforms of the Earth.
It was the red disc of Mars.
Over a century ago,
an Italian astronomer, Giovanni Schiaparelli,
began to chart the dark and light regions of Mars.
His maps were the best we had until space probes came along.
In 1964, NASA launched Mariner 4.
Its mission was to fly past Mars
and try to send back close-up pictures.
The mission was a technical success,
but the fuzzy images showed nothing interesting -
just craters like those on the Moon.
Astronomers were convinced
the probe had looked in the wrong place.
Brad Smith had spent years gazing at the Red Planet.
He thought he knew where the interesting terrain was.
For the next mission,
NASA asked him to guide them to it.
'I had been observing Mars for quite some time'
through ground-based telescopes,
and had noticed certain regions that were very changeable.
They changed with the seasons and had colours.
'We thought those would be particularly interesting.
'I set up the targeting so that Mariner 6 and 7
'would look at these particular areas during the approach.'
As Mariners 6 and 7 raced towards Mars,
they saw what appeared to be mountains,
dark plains, deep canyons.
But when the close-ups came back,
Brad and NASA were once again disappointed.
'Unfortunately, the surface was very heavily cratered.
'There were dark and light areas,
'but it was still a lot like looking at the Moon.'
Convinced that there had to be more than just craters on Mars,
NASA went back again.
Only this time the element of chance had been removed.
Mariner 9 was not a quick fly-by.
It was designed to go into orbit around Mars,
and photograph every square foot of the planet.
But as the probe closed in,
a large dust storm began to stir.
'I was watching the planet,'
hoping it wouldn't be a bad one.
But within just a matter of days,
a brilliant yellow cloud developed.
'We knew it was a bad dust storm.
'It spread across the planet,
'covering up everything.'
Mariner sat out the storm.
Then it saw something poking up through the dust.
'We saw these four dark spots.'
We weren't quite sure what they were.
'One of our team members had pointed out there were things
'that looked like the tops of volcanoes.'
He suggested that in fact, these were volcanoes so high,
they were poking up through the dust.
As the dust receded,
the four spots showed themselves.
They were volcanoes.
Giant ones.
The biggest of them was christened Olympus Mons.
It was 15 miles high,
three times the size of Everest.
Mars was not just another moon.
It was a real world.
Geologists were eager to draw comparisons with the Earth.
Jim Head went straight to the biggest volcanoes he knew,
on the island of Hawaii.
I remember when Olympus Mons first came out of the clouds.
It was spectacular, this gigantic volcano.
I couldn't wait to get back to Hawaii
to get some sense of perspective and scale
from something I knew about.
'We're on the edge of one of the largest volcanoes on Earth.
'It's tiny compared to Olympus Mons.'
To travel from the central crater of Olympus Mons
to its outer edge is a journey of 300 miles.
In between, there's an area the size of France
covered with crumbling red lava.
But the volcanoes weren't all.
As the dust withdrew,
Mariner 9's cameras took thousands more pictures.
A whole new world was taking shape.
NASA called in a new type of scientist -
planetary geologists.
'When this thing began to shape up,'
and we saw these features, that was the first time
planet Mars had ever taken shape
in terms of knowing its physical features.
'None of that had ever been seen before.'
'It was a historical moment.'
You start, like with an orange,
peeling the peel off
and exposing the planet for what it is.
As Mariner's cameras tracked across the belly of Mars,
a giant fissure appeared.
'Each day, we get a new set of images.'
As they mosaiced, we said
"Look at that, where is it going?"
It stretched across roughly an eighth of the planet.
They had discovered the biggest geological feature ever seen.
'Vallis Marineris is 4,000 miles across
'and maybe 100 miles at its widest
'and as much as six miles deep.'
On the scale of the United States,
over there would be San Francisco,
and over here would be New York.
So it would span the United States.
This little canyon here
is the size of the Grand Canyon.
The Vallis Marineris probably cracked open
when the four giant volcanoes to its north
pushed up and stretched the very skin of the planet.
Martian geology is written on a scale
that dwarfs the Earth's.
Features like Vallis Marineris and Olympus Mons
had been steadily growing for billions of years.
It suggested the surface of the planet did not move,
unlike the drifting continents of the Earth.
Mariner 9 had gone in search of geological life
and had found it.
'It was a very active planet,
'a planet that had real geology going on,'
even in modern geologic history.
But was it all history?
Was Mars a graveyard of geological features,
or were the volcanoes still active?
To find out,
a probe had to land on the planet's surface.
Gerry Soffen was the mission scientist.
'The point of Viking was to be the first time'
you really explored the surface of the planet.
The Mariners had orbited or flown by the planet,
but the idea of getting a lander on the surface...
'It was as important as Columbus's voyage.'
I was terrified we wouldn't land successfully
because I had spent so much of my life
aimed at this great moment in history.
'15,480 feet per second.
'3,000 feet.'
On July 20th, 1976,
Viking dropped into Mars's upper atmosphere
and began its descent.
(Landing countdown )
'When the landing took place,'
it was like...it was two minutes ago.
'I can see every single face and the expectation.'
The signals from Viking took 18 minutes
to travel back to mission control.
There was nothing to do but wait.
'When the touchdown came, I just exploded inside.'
We have touchdown.
Yeah!
Looking good.
The idea of being there in history,
when the first landing on Mars took place, was thrilling.
And now the real waiting began.
What would the surface of Mars look like?
Was it going to be like some place in the US -
a desert, sandy, dusty?
'All we knew is it was going to be red.'
An hour after Viking landed,
the first black and white pictures began to creep back.
Line by line, the surface of Mars was revealed.
Then came colour pictures.
The surface was littered with rocks,
some of them dark and porous, clearly volcanic.
They must have been hurled there by volcanoes.
We knew there were volcanoes on Mars,
but to see a piece was astonishing.
But when had the pieces landed there?
Viking carried a device to find out
whether Mars was still active -
a seismometer to tell if the ground was shaking.
Viking listened and waited but felt nothing.
For all its spectacular volcanoes and canyons,
it seemed geological activity on Mars was a thing of the past.
While the Americans were putting all their efforts into Mars,
the Russians headed for Venus.
This world is almost as large as the Earth,
and geologists always thought it would be our twin.
But Venus was no easy target.
The surface was hidden by a thick blanket of cloud,
and below that serene exterior,
the conditions were hellish.
The pressure of the atmosphere
had already crushed three Soviet probes.
The Russians had found to their cost
that the surface temperature was nearly 500 degrees centigrade.
In 1975, they tried again,
and equipped their probe with a camera.
They hoped it would cling on long enough
to send back just one picture of the surface.
Mission chiefs didn't want anyone to know it might fail.
Seconds after landing,
signals showed Venera 9's systems were intact.
On the surface, the temperature was hotter than an oven.
Would the probe survive to send back the image?
The camera had captured a blurred view of some rocks.
It was the first-ever image of the surface of Venus.
But it was only a tantalising glimpse.
With their next probe,
the Russians hoped for something better.
On landing, all systems radioed back OK,
but there was no image of the surface.
Sasha Basilevsky was on the team
that tried to work out what had gone wrong.
We had a technical meeting and discussion,
and the chief designer at that time said,
"You know, I have an idea
"that we have landed in something sticky and viscous."
And a young, nasty voice said,
"Yes, sir, in the shit."
But Venera had not sunk.
The intense heat on the surface
had melted the lens cap on to the camera.
Three years later,
another pair of probes headed for Venus.
This time, they took beautiful pictures
of a lava-filled patch of ground.
But when the probe tried to sample Venusian rock,
that lens cap came back to haunt them.
'In Venera 14 as in Venera 13,'
they had a special device to measure electric
and mechanical properties of the surface.
This arm puts the device on the surface and measures.
'Venera 13 did it in a perfect way
'but it just got the cap.'
So, we measured the mechanical and electrical properties
of that thing they brought from Earth.
On the planet's surface, the probes could only survive
for an hour or two at most.
Another way had to be found to see through the clouds.
In 1989, NASA launched Magellan.
Magellan wouldn't take pictures
but would scan the planet with radar
to make out the contours of the surface,
cutting through the clouds as if they weren't there.
We were able to come around the globe every day many times
and build up a picture of the global geology of Venus.
Magellan began to send back reams of data -
and a new generation of geologists set to work on it.
'When the first image came back, I went in at 4 in the morning'
and looked at this first track
down the planet.
'To see the planet surface revealed in such detail,'
to say "I'm one of the first to ever look at this,"
'you felt like such an explorer.'
The first images showed
that Venus had many similarities to Earth.
'There were large mountain ranges,'
some of them almost similar to the Himalayas.
There were long faults on the planet
that looked similar to faults on the Earth.
'There were lots of very large volcanoes,
'some larger than on the Earth, others on a similar scale.'
But then an alien landscape emerged.
'There were these huge circular features.
'By huge, I mean 250-300 kilometres across.
'They were encircled by ridges.
'They were high, sort of mountainous.
'They tended to have volcanoes all over the surface.
'How could they have formed?
'We'd never seen anything like them.'
The 3-D images revealed giant blisters
that had oozed lava from every crack.
And the surface of Venus seemed to be cut with channels.
'They looked like long rivers,'
but with Venus' incredibly high surface temperatures,
there's no way water could have formed them.
'They had to have been formed by lava.
'Other volcanic features looked like pancakes.
'They had flat tops.'
Like someone had thrown pancakes out onto the surface.
'Other volcanoes looked like little squashed bugs.
'Everywhere you see some volcanic feature,
'a flow, a weird channel - it's dominated by volcanism.'
We weren't prepared for that at all.
'It's intriguing to look at this surface'
that you say should be like the Earth.
It's not. How did it get this way?
It's a puzzle.
That's what makes Venus so interesting.
But when had this volcanic surface formed?
Geologists tried to find out by counting impact craters.
'When we got the global picture,'
we looked for areas that had high or low density of craters,
indicating old and young ages.
Looking at the Moon, Mars or Mercury,
if an area has more craters, it's older.
Fewer craters, it's younger.
Amazingly, the craters were almost randomly distributed.
The whole planet can't be the same age -
it's the size of the Earth.
How can the entire surface form at the same time?
It didn't make sense.
What could have happened for a planet
to create all its surface at one time?
It was mystifying, nobody could be sure.
We hadn't seen anything like it.
Venus may have been catastrophically resurfaced
a few hundred million years ago.
The idea that Venus boiled over
in a planet-wide flood of lava
is still hotly debated.
But if it did, and has now cooled down,
will it ever erupt again?
It would be exciting to find an erupting volcano on Venus,
to say "Here's proof, Venus is still geologically active.
"It really is like the Earth.
"It's not a dead planet, it's still alive, active."
That's something I'd give a lot to be able to find.
Magellan scanned the planet for four years,
but found no fresh lava flows.
Venus may well be alive,
but there is still no sign of it.
If you orbited the Earth for a year,
you may not see any volcanic activity.
It's hard to tell if it's going -
like trying to find the smoking gun or the smoking volcano.
Despite being laid bare by Magellan,
Venus remains a planet shrouded in mystery.
Was Earth the only place
where geologists would find active volcanoes?
It was beginning to look that way.
The only other rocky planet is Mercury,
but it's a small world, barely bigger than our moon.
And its surface is just as cratered and dead.
No geological activity of any kind
has wiped Mercury's face clean.
Outside, it is baking hot,
but inside, stone cold.
There could be no geology on Jupiter
and the other giant planets -
they have no solid surface.
There is some solid rock out here.
On its way past Jupiter,
Voyager flew by the planet's moons.
No one expected interesting geology on these worlds.
We expected small objects the size of the Moon
to be lifeless, geologically,
and to be holding records
of the very early solar system,
the impact processes,
and fairly esoteric kinds of questions
that solar system geologists might be interested in.
Not something the public would care about.
'The first thing we ran into was Callisto.
'That was what we had thought these moons would look like.'
Callisto is dark and icy.
Like Mercury, its cratered surface
hasn't changed for billions of years.
The next moon, Ganymede, is the largest in the solar system,
but it too held few surprises.
Circling closest to the giant planet is Io,
a world about the size of our moon.
As we looked at it from a great distance,
we saw a lot of dark spots on the surface
'which we thought maybe were impact craters.'
Voyager took a few pictures, sailed past Io,
and the scientists focused on Jupiter itself.
Meanwhile, one of the engineers busied herself
with some routine spacecraft maintenance.
I came in about 9 o'clock that morning to the navigation area.
The tape with the pictures
the spacecraft had taken the day before was on my desk.
I put them on the computer and displayed them.
I could see the moon of Io was a crescent,
'as often our own moon is in the night sky.
'I enhanced the brightness,
'and there appeared beside Io a huge object
'that I couldn't recognise and could never have expected.
'It completely captured my attention.
'I wanted to know what that was.
'I asked myself "My goodness, what is that?"'
The answer that occurred to me first was
it looked like another moon peeking out behind Io.
But there was no other moon,
and no fault in the camera.
Linda Hyder concluded this object had to be part of Io.
'In fact, that was very hard to accept'
because the size of it was enormous with respect to Io.
'When I explored it,'
I found that this large, strange object was exactly coincident
and fell over a heart shape feature on Io.
'I had discovered a huge plume of a volcanic eruption
'rising 270 kilometres over the surface of Io
'and raining back down onto it.
'I had discovered the first-ever'
volcanic eruption ever seen on another world besides the Earth.
(Loud rumbling explosion )
'We didn't expect to find active volcanic eruptions'
throwing material from a volcanic vent
to an altitude of a couple of hundred miles,
300 kilometres above the surface.
It goes up with the velocity of a high-powered rifle.
Of course, it comes back down with the same velocity.
So, a healthy place to stand would not be the surface of Io.
The entire surface of Io is covered with volcanoes.
This moon is awash with multi-coloured lava flows.
But why is Io an active world?
It's only the size of our moon.
It should be cold inside.
Head of the Voyager camera team was Brad Smith.
Till now, he had concentrated on Jupiter's atmosphere.
Now Io drew his attention.
'Io is small.'
It doesn't have have enough of the radioactive materials
that heat up the rock as on the Earth.
So we didn't expect any volcanism on Io.
The explanation was to be found in our own moon.
And a NASA scientist in California had predicted it.
'The power of the Moon's gravity
'can move oceans on the Earth.'
Imagine what the power of Jupiter -
300 times the mass of the Earth - can have on Io.
'Io, as it circles Jupiter,
'approaches it closer at one point than at another.'
This changes the gravitational force from Jupiter
and results in a giant squeeze.
Physicist Ray Reynolds had realised
that friction from the constant wrenching by Jupiter
was heating up the interior of Io to incredible temperatures.
We plugged in the numbers into our equations,
and we come out with thousands of degrees
near the surface of this satellite.
This raised visions in our mind of volcanoes going off.
But nobody was prepared for the ferocity
of the sulphur-spewing volcanoes that Voyager found.
'Io is just enormously volcanically active,'
more active than the Earth
and any other body in the solar system.
Nothing even comes close to it.
Its surface is completely covered with volcanic debris.
Finally, geologists had found a world that was alive
and changing before their eyes.
Draw a map of Io,
and it will be obsolete the next day.
But Io wasn't the only surprise
Voyager found among Jupiter's moons.
Next to it was the bright disc of Europa.
'Europa was surprisingly smooth.
'There was little or no topography on it at all.'
Scaled down, it'd be as smooth as a billiard ball.
Why there was no topography, we could only guess.
Close up, the surface was baffling.
Looking at Europa, we see a startling lack of craters.
We also see there are large linear features
'that look like cracks on the surface.'
Reynolds thought the cracks might be a result
of the constant squeezing from Jupiter's gravity,
just like at Io.
And there was a more intriguing possibility.
Europa's surface is made of ice,
and that would be much easier to melt than Io's solid rock.
Our calculations indicated there was a good possibility
that there could be an ocean beneath a thin ice layer.
Voyager's images were too crude
to prove or disprove the idea of a subterranean ocean.
But 20 years later, another spacecraft, Galileo,
is back for a closer look.
Close up,
Europa's surface looks like a crazy paving of ice.
There are giant icebergs and a network of cracks
where it seems that hot water has welled up from below
and then frozen instantly in place,
like lava from a volcanic fissure on Earth.
'Instead of hot rock coming out, it's liquid water.'
But the principle is the same.
There's a fluid down underneath the crust.
'From time to time,
'this pushes up and flows onto the surface
'in the way hot lava flows onto the surface of the Earth.'
No actual eruption of ice has yet been spotted,
but Europa has opened up the possibility
of completely new kinds of geological activity.
As space probes ventured further out,
they found other frozen worlds.
Among the moons of Saturn and Uranus,
there were signs of strange geological events.
But they all seemed to have occurred aeons ago.
Then, a decade after its encounter with Jupiter,
Voyager met Triton, moon of Neptune.
As we got closer to Triton,
it became evident it was a very tiny object.
It was covered with bright material.
'That made it very cold -
'the coldest thing we've encountered so far
'throughout the solar system.'
Triton is so cold
that even its thin atmosphere of nitrogen
freezes into a solid ice cap every winter.
To expect geologic activity
on such a surface would be insane.
But when Voyager got close,
it saw there were dark streaks all over the fresh ice cap.
The markings had to be sitting on top of that ice.
They had to be laid down there in the recent past.
That meant something had to make them an active process.
Larry Soderblom spent two months
looking at pictures of Triton before he spotted it.
He flickered images taken from different angles
to give a 3-D view of the surface.
Suddenly, he saw something
that seemed to stand up from the ice.
'One afternoon,
'we were stunned to find active geysers
'shooting up above the Triton ice cap.'
Geysers - fountains of material,
rising five, ten kilometres - miles above the surface.
Even in this dark corner of the solar system,
the faint heat of the Sun penetrates Triton's ice cap.
It warms liquid nitrogen trapped beneath the surface
to a point where it bursts out and rises up into space
before turning a right angle in Triton's high altitude winds.
'It's as if Triton was the last sentence
'in the message from the Voyager mission:'
that no matter where you go in the universe,
expect the unexpectable.
'It changed our whole concept of what volcanism is.'
Our classical concept before Voyager
was volcanism was hot rock
coming out of the interior of a planet.
But Voyager showed us
there are other materials as well that produce volcanism.
'On Io, we saw molten sulphur - sulphur dioxide.
'On Europa, water is an important element,
'and on Triton, liquid nitrogen may be the fluid
'involved in volcanism.'
After so many amazing discoveries
across the solar system,
geologists are now returning
to the Earth's neighbouring planets.
In 1997, NASA returned to Mars
with a new orbiting spacecraft,
that's photographing the surface in incredible detail.
Mars Global Surveyor can pick out individual boulders
and can see places where they have rolled down into gullies.
Was it wind or small tremors?
If there is any activity on Mars,
Global Surveyor should find it.
'What we now see on Mars is like a shot from a helicopter
'compared with a shot from a space shuttle.'
The whole sense of Mars
is a little bit different
when you get down to the detail we can see.
'We're just beginning to get into that
'and get a sense of what's going on on Mars.'
Soon, we will have a map of Mars
that's as detailed as those of the Earth.
Now, when I come out to Hawaii,
my whole perspective has changed.
I walk around on the surface here,
and begin to see things that are the same scale as on Mars.
It's exciting. It has brought it together.
Almost like I'm walking around on Mars.
This is how it's worked with all the planets.
They were alien objects you had little knowledge about.
But particularly from the geology, you see old friends -
volcanoes, lava flows and sand dunes.
Then, little by little, you transport yourself there.
I dream about being on Mars. It's amazing.
One day soon, a probe may also return to Venus,
and perhaps survive for long enough in its hostile atmosphere
to observe a volcano erupting.
We know now
that the Earth is just one of a family of active worlds.
But geologists will never grasp
the true nature of another planet
until they have touched the stuff
that comes from deep within it.
The grand finale is still to come.
If all goes to plan,
the Galileo probe will end its mission and the millennium
with one of the most breathtaking stunts
ever performed.
With its fuel spent
and suffering critical radiation damage,
the dying spacecraft will dive headlong
into a plume of fiery ash from an erupting volcano on Io,
sampling with its last gasp the geologist's Holy Grail:
the inside of another world.