Nova (1974–…): Season 46, Episode 12 - The Planets: Inner Worlds - full transcript
The rocky planets Mercury, Venus, Earth and Mars all have similar origins, but only one supports life. Was it always this way?
Nearest to the young sun,
four worlds form,
each endowed with ingredients
for life.
There are a lot of places
in the solar system
where environments
could be habitable.
Four stories...
With surprising beginnings.
This was quite a surprise
and immediately told us
that some of the older ideas
of how Mercury formed
could not be correct.
Venus may have once had
global oceans,
but as the sun became warmer
and warmer,
the surface temperatures
on Venus heated up.
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.
Their destinies diverging,
until only one remains
hospitable.
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.
But for how long?
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.
As our sun destroys our planet,
a mission to an outer world
reveals a new hope.
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.
"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
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...
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.
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.
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.
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.
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.
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.
Now going through
the sound barrier.
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.
We just had
spacecraft separation.
Ground-lit solids
have jettisoned.
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.
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.
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.
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.
Also highly unusual
is the planet's
disproportionately large core.
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.
And the major question is,
"Why?
"Why is it like this,
when the rest of the planets
aren't?"
Mercury's odd core
and strange orbit
make it unlike any other planet.
But most perplexing
is its chemistry.
So very soon
after we got into orbit,
we started to get chemical data
back from the surface.
And we immediately
got some surprises.
Zero-six forward...
The tiny probe detects
volatile chemical elements
in concentrations
that no one had thought possible
this close to the sun.
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.
Immediately told us
that some of the older ideas
of how Mercury formed
could not be correct.
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.
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.
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?
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'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.
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.
End sequencers,
chief launch detector on.
Confirming your...
After four years of observation,
the fuel Messenger uses
to correct its orbit
finally runs out.
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.
Looks so close,
and so well-known
even to the ancients,
but so mysterious
because we couldn't see through
to the surface.
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?
We were so sure
of Venus's habitability...
That the first missions
were designed
for a splash landing.
Throughout the '60s and '70s,
the Soviet Venera program sends
multiple missions to Venus.
Many fail.
But with each attempt,
they learn a little more
of the extreme conditions
on the planet.
After 20 years of trying,
Venera 13 begins
its perilous descent.
The craft is built
to withstand pressures
that would crush a car
in seconds
and temperatures
that would melt lead.
On the first of March 1982,
the Soviets take
the first color photograph
of the Venusian surface.
For 127 minutes,
the probe collects data.
Far from being
a benign ocean world,
Venus is a vision of hell...
Where no life can survive.
But when scientists analyze
the atmosphere,
they find Venus was once
far more welcoming.
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.
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.
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.
The heavens open.
Liquid water floods the surface.
Rivers run into shallow seas.
Venus becomes an ocean world,
rich with the potential
for life.
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.
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?
Long ago, something happens
to cause Venus's hope for life
to die:
a change in its parent star.
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.
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.
Venus once may have had
global oceans,
but as the sun became warmer
and warmer,
the surface temperatures
on Venus heated up.
Gradually,
over two billion years,
as the nuclear reactions
at the sun's heart
heat it up,
the sun grows brighter.
Its increased energy output
causes temperatures on Venus
to rise,
evaporating more and more water,
turning it into vapor
in the air.
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.
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.
Rain evaporates
long before reaching the ground.
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.
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.
Venus reaches a tipping point...
And a runaway greenhouse effect
takes hold.
Its oceans and all hope for life
vaporize.
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.
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.
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.
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.
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.
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.
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.
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.
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.
The aging sun continues
to grow hotter.
Temperatures on Earth rise...
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.
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.
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.
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.
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.
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.
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.
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.
In 1997, we set off
to investigate
this potential future home
for life.
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.
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.
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.
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.
During its descent,
the Huygens camera sends back
these first astonishing glimpses
of this distant moon.
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.
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.
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.
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.
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.
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.
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.
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.
Its mountains of ice
may shrink and melt.
And the frozen water
they contain
may replace the liquid methane
as it evaporates away.
The mountains
could become oceans.
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.
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.
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.
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.
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.
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?"
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A frozen, desert world...
But once blue with water...
and perhaps, more?
What everybody wants to know
is whether or not
Mars once had life.
And if it still survives today.
Finding the organic matter is
the clue to searching for life.
"The Planets: Mars."
Next time on "NOVA."
I'm losing sight of our reality
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four worlds form,
each endowed with ingredients
for life.
There are a lot of places
in the solar system
where environments
could be habitable.
Four stories...
With surprising beginnings.
This was quite a surprise
and immediately told us
that some of the older ideas
of how Mercury formed
could not be correct.
Venus may have once had
global oceans,
but as the sun became warmer
and warmer,
the surface temperatures
on Venus heated up.
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.
Their destinies diverging,
until only one remains
hospitable.
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.
But for how long?
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.
As our sun destroys our planet,
a mission to an outer world
reveals a new hope.
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.
"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
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...
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.
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.
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.
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.
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.
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.
Now going through
the sound barrier.
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.
We just had
spacecraft separation.
Ground-lit solids
have jettisoned.
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.
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.
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.
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.
Also highly unusual
is the planet's
disproportionately large core.
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.
And the major question is,
"Why?
"Why is it like this,
when the rest of the planets
aren't?"
Mercury's odd core
and strange orbit
make it unlike any other planet.
But most perplexing
is its chemistry.
So very soon
after we got into orbit,
we started to get chemical data
back from the surface.
And we immediately
got some surprises.
Zero-six forward...
The tiny probe detects
volatile chemical elements
in concentrations
that no one had thought possible
this close to the sun.
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.
Immediately told us
that some of the older ideas
of how Mercury formed
could not be correct.
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.
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.
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?
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'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.
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.
End sequencers,
chief launch detector on.
Confirming your...
After four years of observation,
the fuel Messenger uses
to correct its orbit
finally runs out.
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.
Looks so close,
and so well-known
even to the ancients,
but so mysterious
because we couldn't see through
to the surface.
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?
We were so sure
of Venus's habitability...
That the first missions
were designed
for a splash landing.
Throughout the '60s and '70s,
the Soviet Venera program sends
multiple missions to Venus.
Many fail.
But with each attempt,
they learn a little more
of the extreme conditions
on the planet.
After 20 years of trying,
Venera 13 begins
its perilous descent.
The craft is built
to withstand pressures
that would crush a car
in seconds
and temperatures
that would melt lead.
On the first of March 1982,
the Soviets take
the first color photograph
of the Venusian surface.
For 127 minutes,
the probe collects data.
Far from being
a benign ocean world,
Venus is a vision of hell...
Where no life can survive.
But when scientists analyze
the atmosphere,
they find Venus was once
far more welcoming.
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.
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.
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.
The heavens open.
Liquid water floods the surface.
Rivers run into shallow seas.
Venus becomes an ocean world,
rich with the potential
for life.
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.
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?
Long ago, something happens
to cause Venus's hope for life
to die:
a change in its parent star.
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.
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.
Venus once may have had
global oceans,
but as the sun became warmer
and warmer,
the surface temperatures
on Venus heated up.
Gradually,
over two billion years,
as the nuclear reactions
at the sun's heart
heat it up,
the sun grows brighter.
Its increased energy output
causes temperatures on Venus
to rise,
evaporating more and more water,
turning it into vapor
in the air.
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.
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.
Rain evaporates
long before reaching the ground.
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.
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.
Venus reaches a tipping point...
And a runaway greenhouse effect
takes hold.
Its oceans and all hope for life
vaporize.
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.
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.
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.
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.
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.
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.
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.
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.
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.
The aging sun continues
to grow hotter.
Temperatures on Earth rise...
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.
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.
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.
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.
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.
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.
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.
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.
In 1997, we set off
to investigate
this potential future home
for life.
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.
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.
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.
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.
During its descent,
the Huygens camera sends back
these first astonishing glimpses
of this distant moon.
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.
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.
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.
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.
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.
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.
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.
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.
Its mountains of ice
may shrink and melt.
And the frozen water
they contain
may replace the liquid methane
as it evaporates away.
The mountains
could become oceans.
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.
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.
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.
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.
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.
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?"
Major funding for "NOVA"
is provided by the following:
A frozen, desert world...
But once blue with water...
and perhaps, more?
What everybody wants to know
is whether or not
Mars once had life.
And if it still survives today.
Finding the organic matter is
the clue to searching for life.
"The Planets: Mars."
Next time on "NOVA."
I'm losing sight of our reality
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