NOVA

ZACHARY QUINTO: Nearest to the young sun, four worlds form, each endowed with ingredients for life.

SUZANNE SMREKAR: There are a lot of places in the solar system where environments could be habitable.

QUINTO: Four stories... With surprising beginnings.

(craft thuds, thunder rumbles) LARRY NITTLER: This was quite a surprise and immediately told us that some of the older ideas of how Mercury formed could not be correct.

(thunder claps) LYNNAE QUICK: Venus may have once had global oceans, but as the sun became warmer and warmer, the surface temperatures on Venus heated up.

(thunder rumbling) ANJALI TRIPATHI: The orbits of the planets and their current locations have changed over the history of the solar system.

So where things are today doesn't mean that's necessarily where they formed.

They could have and probably did move from different locations to where they are now.

QUINTO: Their destinies diverging, until only one remains hospitable.

DAVID GRINSPOON: Life as we know it depends on liquid water.

Only Earth has had that ability to retain liquid water over its long history.

That is what's remarkable about our planet.

QUINTO: But for how long?

JONATHAN LUNINE: We have to realize that as stable as we think the sun is and as permanent as we think the habitability of the Earth is, it's not that way.

QUINTO: As our sun destroys our planet, a mission to an outer world reveals a new hope.

GRINSPOON: I believe that we live in a very fertile universe.

And of course we won't know until we make that discovery.

But if I had to bet right now, based on what we do know, I would place money on the idea that, um, this universe is full of a great number of habitable worlds.

QUINTO: "The Planets: Inner Worlds."

Right now, on "NOVA."

Major funding for "NOVA" is provided by the following: ("The Void" by Muse playing) They'll say no one can see us That we're estranged and all alone They believe nothing can reach us And pull us out of the boundless gloom They're wrong They're wrong They're wrong (music slows and ends) QUINTO: For the first few million years after the sun's birth, there are no worlds to see it rise.

Just a vast cloud of dust and gas... Left over after the sun's formation.

Then, over tens of millions of years, gravity draws this debris together... To form the first planetary embryos... That become the four rocky worlds closest to the sun, their dramatic stories dominated by the changing fortunes of their parent star... As each in its turn has become more or less hospitable.

Today, scorched Mercury orbits too close to a pitiless sun.

Further out, overheated Venus... (thunder claps) Choked by a thick atmosphere.

And farthest: Mars... A frozen, desert world.

Of the four, only on one... Has something extraordinary emerged.

Earth is a tremendously special place, with all of the just-right, finely tuned ingredients for life.

GRINSPOON: Life as we know depends on liquid water.

Only Earth has had that ability to retain liquid water over its long history.

That is what's remarkable about our planet.

(seagulls squawking) SMREKAR: Earth is the only planet that we know of that has life.

It appears to be unique in the solar system today, but we only are seeing a small snapshot of time and even space.

There are a lot of places in the solar system where environments either have been, or in the future could be, habitable.

QUINTO: And yet, today the sun shines on only one habitable world.

Why ours and no others?

As we've left the blue planet and explored our sister worlds, we've discovered something surprising.

Our rocky neighbors were not always so hostile.

Somehow, they too may have once supported near Earth-like conditions.

So what happened?

Most surprising is Mercury.

A world of mystery.

(wind blowing) The least explored of the rocky planets-- until recently out of reach because of the enormous difficulty of navigating into orbit around a planet so close to the sun.

MAN: Five, four, three-- main engines start-- two, one, and zero, and liftoff of Messenger on NASA's mission to Mercury.

A planetary enigma in our inner solar system.

(engines roaring) Now going through the sound barrier.

QUINTO: Following a direct route to Mercury would be impractical.

A spacecraft would arrive with so much velocity that it would need to haul a prohibitive amount of heavy fuel in order to slow down enough to enter orbit.

MAN: We just had spacecraft separation.

(device beeping) MAN: Ground-lit solids have jettisoned.

QUINTO: So Messenger controls its trajectory by stepping from one planet to the next, using gravity to slow itself, spiraling inwards toward its target.

Even so, Messenger still approaches Mercury moving so fast that it is forced to fly past the planet three times...

Slowing on each pass... Until, after nearly seven years of flawless navigation, it arrives safely in orbit... At last able to begin its true mission: to map Mercury's surface.

NANCY CHABOT: That first image ever acquired from orbit around Mercury was amazing.

When it finally came in, it looked perfect, it looked exactly like we expected it to be.

For me, that was the real moment when I realized that we had successfully made it into orbit, and everything was working.

QUINTO: This pioneering voyage has led to a completely new idea of how Mercury may have formed.

Mercury is the most cratered planet in the solar system... With many puzzling features that hint at a violent past, like its eccentric orbit and bizarrely slow rotation.

CHABOT: Mercury has a very unusual orbit.

If you were on the surface of the planet, you would actually have to go around the sun twice to get one full Mercury solar day.

QUINTO: Also highly unusual is the planet's disproportionately large core.

CHABOT: Mercury is basically a big ball of metal surrounded, like, a tiny bit of rock.

How do you end up with a planet that has so much metal inside and then almost none on its surface?

That was a huge mystery.

CHABOT: And the major question is, "Why?

"Why is it like this, when the rest of the planets aren't?"

QUINTO: Mercury's odd core and strange orbit make it unlike any other planet.

But most perplexing is its chemistry.

NITTLER: So very soon after we got into orbit, we started to get chemical data back from the surface.

And we immediately got some surprises.

MAN (on radio): Zero-six forward... QUINTO: The tiny probe detects volatile chemical elements in concentrations that no one had thought possible this close to the sun.

NITTLER: These are elements that go into rocks, but that evaporate at relatively low temperatures.

The volatile elements on Mercury are surprising.

A lot of the formation theories predicted that you shouldn't have that on the planet closest to the sun.

NITTLER: Immediately told us that some of the older ideas of how Mercury formed could not be correct.

QUINTO: What Messenger discovered about Mercury could suggest a new twist in the story of the solar system.

Just a few million years after its formation, the young Mercury is still seething with the heat of its violent birth.

Slowly, a crust forms, rich with volatile elements.

If it had been close to the sun, these elements would have evaporated before the rock hardened.

So what could explain why they are so abundant on Mercury today?

The orbits of the planets and their current locations have changed over the history of the solar system.

So where things are today doesn't mean that's necessarily where they formed.

They could have and probably did move from different locations to where they are now.

NITTLER: One possible thing is that Mercury didn't form where it is today, it formed much closer to the other planets, maybe even outside of Venus or Earth, or somewhere in between there.

QUINTO: If Mercury had formed near Earth and remained at a more comfortable distance from the sun... Its destiny could have been very different.

But it wasn't to be.

So what turned it into the strange, small, sun-scorched world we see today?

TRIPATHI: We don't know the full story.

What we have right now are theories, and as we have new observations, we are trying to change everything to fit those observations.

(Messenger rumbling) QUINTO: Messenger's evidence of the volatiles on Mercury's surface-- and the unusual size of its core-- suggest an interesting new theory.

It's possible that Mercury's life began as much as 100 million miles further away from the sun than it lies today... In the region of space where the young Earth is also forming, a region with scores of planetary embryos all fighting for position.

In the chaos, it's possible that something large pushes Mercury off orbit and in towards the sun.

Mercury brushes against another planetary embryo.

Just this glancing blow could be enough to pulverize much of its crust and mantle... Leaving the material behind... Perhaps to become part of the early Venus.

Mercury, now little more than a metallic planetary core, continues towards the sun... but escapes total solar annihilation when it drops into the peculiar elongated orbit we see to this day.

With its astonishing revelations, Messenger is helping to completely rewrite the theories of Mercury's formation.

GRINSPOON: Before we sent the Messenger mission to Mercury, I think we had a very simplistic idea of what it was going to be like.

And Mercury turns out to be a more complex place with a more interesting and complex history than we had previously imagined.

But, of course, all things have to come to an end, and once we were out of fuel, we could no longer burn our engines to keep from crashing into Mercury.

MAN: End sequencers, chief launch detector on.

MAN 2: Confirming your... QUINTO: After four years of observation, the fuel Messenger uses to correct its orbit finally runs out.

(people talking on radios in background) (craft crashes) QUINTO: And Messenger adds yet another crater to this tiny world, where any prospects for life were scorched away when it was thrown too close to the sun.

But distance doesn't necessarily protect a planet from the sun's power.

30 million miles beyond Mercury lies a world that looks to be far more Earth-like, shrouded by an unbroken blanket of cloud, an invitation to the imagination.

LYNN ROTHSCHILD: Looks so close, and so well-known even to the ancients, but so mysterious because we couldn't see through to the surface.

GRINSPOON: Venus has been mysterious to human beings for a long time.

Early astronomers figured out that Venus has an atmosphere, and they figured out that the reason it's so bright in our sky is because it's completely covered with clouds.

Science-fiction writers were very captivated with the idea that it was, you know, this swappy, swampy tropical jungle.

Is it another Earth?

Is it an Earthly paradise, even, just sort of hidden beneath that, that envelope?

QUINTO: We were so sure of Venus's habitability... That the first missions were designed for a splash landing.

(man speaking Russian) QUINTO: Throughout the '60s and '70s, the Soviet Venera program sends multiple missions to Venus.

(man speaking Russian) QUINTO: Many fail.

(beeping, rocket roaring) (man speaking Russian) QUINTO: But with each attempt, they learn a little more of the extreme conditions on the planet.

(man speaking Russian) QUINTO: After 20 years of trying, Venera 13 begins its perilous descent.

(wind whipping against craft) (wind whistling) The craft is built to withstand pressures that would crush a car in seconds and temperatures that would melt lead.

(wind blowing) (craft slams onto surface, thunder claps) On the first of March 1982, the Soviets take the first color photograph of the Venusian surface.

(wind blowing, thunder rumbling) For 127 minutes, the probe collects data.

(thunder rumbling) Far from being a benign ocean world, Venus is a vision of hell... (thunder rumbling) Where no life can survive.

(wind blowing) But when scientists analyze the atmosphere, they find Venus was once far more welcoming.

SMREKAR: Although there's very little water in the atmosphere of Venus today, and certainly none on the, on the surface, we have evidence that it was once wetter than it is today.

The reason we can tell that Venus has lost a lot of water is from the isotopic composition of hydrogen measured in its atmosphere.

GRINSPOON: That is a clue that most of the water escaped long ago.

So Venus, when it was young, would have probably had enough water so that it most likely had liquid-water oceans.

QUINTO: Far from the hell it is today, the young Venus is in fact very similar to Earth.

At just the right distance from the sun for something wonderful to happen.

(thunder rumbling) (thunder rumbling) (rain pattering) The heavens open.

(thunder rumbling, rain falling) (water gurgling) Liquid water floods the surface.

Rivers run into shallow seas.

Venus becomes an ocean world, rich with the potential for life.

GRINSPOON: As far as we know, when Venus was a young planet, it was very much like Earth, with liquid-water oceans and a temperate climate, the kind of place where life could have formed and thrived.

ROTHSCHILD: Say life did arise on Venus, which is at least plausible, because it did on Earth during that time in similar conditions.

What happened to it?

QUINTO: Long ago, something happens to cause Venus's hope for life to die: a change in its parent star.

SMREKAR: The fate of a planet's habitability is tied up in the history of the sun-- where the planet is in relation to the sun is key, but also the, the age of that star.

GRINSPOON: It's very well understood that a star like the sun becomes brighter as it ages.

As the young sun warmed up, the radiation balance started to shift.

QUICK: Venus once may have had global oceans, but as the sun became warmer and warmer, the surface temperatures on Venus heated up.

QUINTO: Gradually, over two billion years, as the nuclear reactions at the sun's heart heat it up, the sun grows brighter.

(cracking) Its increased energy output causes temperatures on Venus to rise, evaporating more and more water, turning it into vapor in the air.

ROTHSCHILD: You've got the surface heating up, you've got water going into steam.

Steam's a greenhouse gas, the temperature goes up, more water turns into steam.

GRINSPOON: You can see where this is going-- the hotter it gets, the more that water evaporates, the more water vapor in the air, the more that radiation is blocked, and the more it heats up.

It's a positive feedback, it runs away.

QUINTO: Rain evaporates long before reaching the ground.

(wind blowing) Venus's clouds grow thicker by the day, until its face is forever lost to space.

What was happening beneath them is revealed in 1990...

When NASA's Magellan probe begins to explore the planet from above.

Equipped with cloud-penetrating radar, Magellan is able to peer through the thick atmosphere.

And what it discovers is a world covered in vast volcanic lava plains, fields of small lava domes, and in places, some of the largest shield volcanoes seen anywhere in the solar system.

Venus is the volcanic capital of the solar system, with more extinct volcanoes scattered across its surface than any other planet.

And we think many of these volcanoes were active at the time Venus's atmosphere was being warmed by the sun.

(rumbling) GRINSPOON: Early on, the atmosphere of Venus was being fed volcanic gases, and a lot of those volcanic gases are greenhouse gases.

What's coming out of volcanoes today on Earth?

It's CO2, it's water vapor, it's methane.

Those are all really strong greenhouse gases.

So that early atmosphere of Venus was presumably also being pumped full of these volcanic gases.

QUINTO: Venus reaches a tipping point... And a runaway greenhouse effect takes hold.

(water burbling) Its oceans and all hope for life vaporize.

SMREKAR: Once it lost those oceans, the CO2 built up and built up over time and the oceans were no longer present to suck out the CO2 out of the atmosphere.

So at that point, the runaway greenhouse just took over, causing Venus to be the, you know, sulfur hell that it is today.

(wind blowing) QUINTO: Venus's moment in the sun sets, its cracked surface now even hotter than Mercury, the hottest of all the planets.

As the sun's brightness increases, the effects are felt across all the terrestrial planets.

Mars, much smaller and further out than Venus, has its moment in the sun, too.

With an atmosphere rich in greenhouse gases, its rivers and seas flow freely across the surface for hundreds of millions of years.

But Mars, being much smaller than Venus and with weaker gravity, has a harder time holding on to its original atmosphere.

The molten iron core in its center is too small to retain its heat.

It cools and solidifies, shutting off the smaller world's protective magnetic field.

Its intense aurora fades, leaving it exposed to a bombardment of high-energy particles from the warming sun-- the solar wind-- stripping away its atmosphere, leaving little to protect its water from simply evaporating into space... The tiny traces left behind...

Frozen in patches across the planet-- Mars's chance for life to develop extinguished by its parent star.

(wind blowing) TRIPATHI: The fate of the planets is intricately tied with that of the star that they orbit.

So for our own solar system, the sun dictates our fate.

QUINTO: As our sun has changed, so has the potential for life on our neighboring planets.

Their history shows that habitability is a delicate balance... That doesn't always last.

There is only one planet so far that has retained its water and habitability-- and that's Earth.

GRINSPOON: What's remarkable about Earth is the stability of those conditions, that Earth has been able to maintain oceanic conditions at its surface throughout its entire history, through billions of years, and that's what's facilitated the very rich biological evolution of Earth.

Earth is a very special place.

It's the only place in the universe where we know definitively that there is life.

For that to happen, you need not just liquid water appearing on a planet, but liquid water staying on a planet.

And that's the magic of Earth.

QUINTO: Thanks to the size and geology of our planet, the atmosphere has remained stable enough for billions of years, protecting the precious water that has enabled complex life to evolve.

(birds chirping) (snorts) Life has woven itself into the fabric of the planet... To shape the continents and the oceans.

So that now life itself maintains the very atmosphere that protects our fragile ecosystems.

But as the sun ages, this delicate balance cannot last.

TRIPATHI: All of the planets are changing, including our own Earth, and as it evolves, life will have to change with it.

We have to realize that as stable as we think the sun is, and as permanent as we think the habitability of the Earth is, it's not that way.

SMREKAR: Right now, we're at the period where Earth is getting just the right amount of radiation.

But, over time, that radiation from the sun is going to increase.

There will come a time that it will be so hot that we will have a situation like poor Venus had, where we could well have a runaway greenhouse.

QUINTO: The aging sun continues to grow hotter.

Temperatures on Earth rise... (thunder booms) Upsetting weather patterns, raising great storms across the planet, and devastating droughts.

As plants around the world die out, oxygen levels plummet.

Around a billion years from now, the age of complex life on Earth will finally draw to a close.

Earth will ultimately be like Venus and Mars, just mostly CO2.

There'll be nothing else, really, in the atmosphere.

And it will remain in that state as a sort of hot oven planet until, ultimately, the sun, when it uses up its hydrogen and goes into a different phase, will become what we call a red giant.

SMREKAR: You know, as we've seen in other stars throughout our galaxy, throughout the universe, our sun will continue to get hotter and hotter over time.

At that point, it will expand greatly to the point where it will nearly engulf the Earth entirely.

And at that point, the planets will, will lose their atmospheres and they'll be just sort of swept away.

(wind howling) QUINTO: As the sun exhausts its hydrogen fuel, the sun's outer edge inflates.

It enters a red giant phase, expanding millions of miles out into space.

Mercury is the first to be engulfed.

Then Venus's fate is sealed.

(rumbling) Some models predict that Earth may barely escape the fiery fate of its neighbors... Hanging on beyond the edge of the dying star with Mars.

But the long era of the four terrestrial planets will be over...

The lives lived on one of them nothing more than a distant memory.

As the sun approaches its end, the habitable zone will move outwards, and any hope for life will move with it into the outer edges of the solar system.

GRINSPOON: There are places farther out from the sun where life could exist, and, in fact, where the potential for life could increase-- places that may wake up to their biological potential late in the sun's lifetime.

QUINTO: Out here is the realm of the gas giants.

So the places where life may find a home would not be on the planets themselves, but on the terrestrial moons that orbit them.

ROTHSCHILD: For example, Enceladus, which is a little icy moon around Saturn.

Or Europa, which is an icy moon around Jupiter, or even Titan, which is a moon of Saturn.

GRINSPOON: Saturn's moon Titan, in particular, interesting possibility, an exciting possibility for a... for a different kind of life in a different kind of habitable zone.

QUINTO: In 1997, we set off to investigate this potential future home for life.

MAN: T minus five, four.

three, two, one... And liftoff of the Cassini spacecraft on a billion-mile trek to Saturn.

We have cleared the tower, T plus 20 seconds.

All systems are go.

QUINTO: Cassini heads to the cold, distant reaches of the solar system, past Jupiter... Its destination nearly a billion miles away.

Its mission: to orbit Saturn and investigate its icy rings.

(launches) From here, it deploys a tiny probe to explore Titan, a planet-sized moon even bigger than Mercury.

Surrounded by a thick, hazy atmosphere, its surface has long remained a mystery.

CARRIE ANDERSON: The Cassini orbiter contained the Huygens probe, designed to descend through Titan's atmosphere and land on the surface, the first landing of anything in the outer solar system.

So this is just ground-breaking history.

QUINTO: During its descent, the Huygens camera sends back these first astonishing glimpses of this distant moon.

ANDERSON: We saw bright highlands, but they had dark drainage channels.

When you're looking at it, so breathtaking to see this active surface.

It opened up our eyes to this magnificent and amazing very Earth-like world.

(probe thuds) QUINTO: Incredibly, the probe makes a soft landing and sends back our first look at the surface of Titan.

It's surprisingly similar to the landscapes on Earth.

Soon after Huygens landed, Carrie Anderson began analyzing the data sent back from this distant world.

ANDERSON: When I look at this image that the Huygens probe took when it was on Titan's surface, I'm struck by the incredible similarities that we have on Earth in the formation of the rocks in flood plains and river plains on Earth.

Just like here, liquid water is flowing, we know that similar processes are going on and making pebbles just like this, but the difference is the composition is water ice so hard, frozen so solid because of the low temperatures.

But we have the similar processes forming rocks like this on Earth and also forming rock-solid ice pebbles on Titan.

QUINTO: The discovery of smooth, rock-like ice pebbles suggests flowing liquid.

But at minus-300 degrees Fahrenheit, there can be no liquid water on Titan.

So what is flowing out here in the cold?

The instruments pick up significant amounts of methane, a flammable gas on Earth.

But the relatively high atmospheric pressure and cold temperatures at the surface of Titan mean that methane can exist as a liquid.

Titan could be wet not with water, but with liquid methane.

ANDERSON: So what we think is that the methane probably rained out in some storms very similar to storms on Earth.

The shape of these very rounded-looking stones most likely was due to probably liquid methane pushing these ice-like rocks, raining down these channels, and emptying into this open floodplain.

QUINTO: Huygens survives for just a few hours, but doesn't detect anything like enough liquid methane to carve out riverbeds.

But the probe's mother ship, Cassini, remains active in orbit around Saturn.

A year after Huygens landed, Cassini passes high above Titan's poles and sees something truly spectacular... Our first glimpse of liquid pooling on another world.

Ontario Lacus, a lake whipped by winds that erode the shoreline.

Ligeia Mare, where we have seen bubbles rising from the depths.

And Kraken Mare, almost five times larger than our own Lake Superior.

In all, Cassini discovers hundreds of methane lakes.

Few scientists think life can exist on the surface of Titan today; it is too cold.

But because of the presence of crucial elements, it might be a very different story if Titan were to warm up.

GRINSPOON: Titan today has a surface that's full of these really juicy organic compounds, but it may simply be too cold and therefore too dry for anything biologically interesting to be happening with them.

However, in the far future, when Earth and the inner solar system become uninhabitable, we can imagine what might happen to Titan then.

QUINTO: In the light of the aging, expanding sun, when the far reaches of the solar system receive more solar energy, Titan will begin to warm.

(trickling) Its mountains of ice may shrink and melt.

(water flowing) And the frozen water they contain may replace the liquid methane as it evaporates away.

The mountains could become oceans.

ROTHSCHILD: So you have this mixing, then, of the hydrocarbons and liquid water, and so you have this wonderful sort of nirvana for some period of time, where you have everything that you need for life.

GRINSPOON: So even if Titan doesn't have life today, and I think that's something that we still have to search for, there's every reason to imagine that at some point in the future, it could be a great place for biology.

QUINTO: In a strange twist of fate, at the end of the life of our star, one of the last water worlds in the solar system will be born.

Titan will have a final brief moment in the sun.

The story of our solar system shows us that habitability isn't a permanent feature.

Our solar system is a dynamic place.

And our star has its own life cycle... ...creating zones of habitability that ebb and flow throughout its lifetime.

Far from being unique to Earth, there is hope for life throughout our solar system... And, many believe, beyond.

TRIPATHI: One of the most amazing discoveries of the past couple of decades is that when you look up at the stars at night, almost all of those stars host their own solar systems, and so we know that within our own galaxy, there are hundreds of billions of planets.

And with so many planets out there, you can't help but think that some of them have the right conditions for life.

GRINSPOON: I believe that we live in a very fertile universe.

And of course we won't know until we make that discovery, but if I had to bet right now, based on what we do know, I would place money on the idea that this universe is full of a great number of habitable worlds.

ROTHSCHILD: It's a really remarkable time to be alive and to be in this business.

And when I say "be in this business," everyone's in this business, because I have yet to meet anyone who isn't interested in that question, "Are we alone?"

ZACHARY QUINTO: A frozen, desert world...

But once blue with water... and perhaps, more?

JOHN GROTZINGER: What everybody wants to know is whether or not Mars once had life.

(explosion echoes) QUINTO: And if it still survives today.

JENNIFER EIGENBRODE: Finding the organic matter is the clue to searching for life.

QUINTO: "The Planets: Mars."

Next time on "NOVA."

I'm losing sight of our reality