Cosmos (1980): Season 1, Episode 4 - Heaven and Hell - full transcript
Carl Sagan examines the nature of comets before turning to a close look at the planet Venus.
SAGAN: This is the age of
planetary exploration...
...when our ships have begun
to sail the heavens.
In those heavens, there are
some worlds much like hell.
Our planet is, in comparison,
much like a heaven.
But the gates of heaven and hell...
...are adjacent and unmarked.
The Earth is a lovely...
...and more or less placid place.
Things change, but slowly.
You can lead a full life and never
encounter a natural catastrophe...
...more violent than a storm.
And so we become complacent...
...relaxed...
...unconcerned.
But in the history of the solar
system and even in human history...
...there are clear records
of extraordinary...
...and devastating catastrophes.
We have now achieved
the dubious distinction...
...of making our own
major catastrophes...
...both intentional and inadvertent.
On the landscapes of other planets...
...where past records
are better preserved...
...there's abundant evidence
of major catastrophes.
It's all a matter of time scale.
An event which is improbable
in 100 years...
...may be inevitable
in 100 million.
But even on the Earth
in this century...
...there have been
bizarre natural events.
In remote central Siberia...
...there was a time
when the Tungus people...
...told strange tales
of a giant fireball...
...that split the sky
and shook the Earth.
They told of a blast
of searing wind...
...that knocked down
people and forests.
It happened, they said,
on a summer's morning...
...in the year 1908.
In the late 1920s...
...L.A. Kulik, a Soviet scientist...
...organized expeditions
to try and solve the mystery.
He built boats to penetrate
this trackless land:
Snowbound in winter...
...a swampy morass in summer.
Eyewitnesses told of
a ball of flame...
...larger than the sun...
...that had blazed across the sky
20 years before.
Kulik assumed a giant meteorite
had struck the Earth.
He expected to find an enormous
impact crater...
...and rare meteorite fragments...
...chipped off some distant asteroid.
However, at ground zero...
...Kulik found upright trees
stripped of their branches...
...but not a trace of the meteorite
or its impact crater.
He was deeply puzzled.
He thought there were meteorite
fragments buried in the swampy ground.
So he set about digging trenches
and pumping out the water.
But the expected meteoritic rock
and iron was not found.
Undaunted, Kulik went on
to make a thorough survey...
...despite the swarms of insects
and other hardships.
Because he discovered something that,
in his own words...
..."exceeded all tales of eyewitnesses
and my wildest expectations."
For more than 20 kilometers
in every direction from ground zero...
...the trees were flattened radially
outward like broken matchsticks.
There must've been
a powerful explosion...
...several kilometers
above the ground.
The pressure wave, spreading out
at the speed of sound...
...was reconstructed from barometric
records at weather stations...
...across Siberia, through Russia
and on into Western Europe.
Dust from the explosion reflected
so much sunlight back to Earth...
...that people could
read by it at night...
...in London, 10,000 kilometers away.
This really remarkable occurrence...
...is called the Tunguska Event.
But what was it?
Well, perhaps, some scientists
have suggested...
...it was a chunk of antimatter
from space...
...annihilated on contact with
the ordinary matter of the Earth...
...disappearing in a flash
of gamma rays.
But the radioactivity you'd expect
from matter-antimatter annihilation...
...is to be found nowhere
at the impact site.
Or, perhaps, other scientists
have suggested...
...it was a mini black hole
from space...
...which impacted the Earth
in Siberia...
...tunneled through
the solid body of Earth...
...and plunged out the other side.
But the records of atmospheric
shock waves give not a hint...
...of something booming out of
the North Atlantic later that day.
Or maybe, other people have speculated,
it was a spaceship...
...of some unimaginably advanced
extraterrestrial civilization...
...in desperate mechanical trouble...
...crashing in a remote region
of an obscure planet.
Well, if so, it's pretty startling
that at the impact site...
...there is not a piece,
not the tiniest transistor...
...of a crashed spacecraft.
More prosaically, perhaps it was
a large meteorite...
...or a small asteroid
which hit the Earth.
But even here,
there are no observable telltale...
...rocky or metallic fragments
of the sort...
...that you'd expect
from such an impact.
The key point of the Tunguska Event...
...is that there was a tremendous
explosion, a great shock wave...
...many trees burned,
an enormous forest fire...
...and yet, no crater in the ground.
There seems to be
only one explanation...
...which is consistent
with these facts.
And that explanation is this:
In 1908, a piece of a comet...
...hit the Earth.
No one saw it approach.
A small point of light
lost in the glare of the morning sun.
It had been drifting for centuries
through the inner solar system...
...like an iceberg in the ocean
of interplanetary space.
But this time, by accident...
...there was a planet in the way.
From the time and direction of
its approach, what hit the Earth...
...seems to have been a fragment
of a comet named Encke.
Hurtling at more than
100,000 kilometers an hour...
...it was a mountain of ice
about the size of a football field...
...and weighing almost a million tons.
There was no warning, until
it plunged into the atmosphere.
(COMET RUMBLES)
If such an explosion happened today...
...it might be thought,
in the panic of the moment...
...to be produced by a nuclear weapon.
Such a cometary impact and fireball...
...simulates all the effects
of a 15-megaton nuclear burst...
...including the mushroom cloud,
with one exception:
There would be no radiation.
So could a rare but natural event...
...the impact of a comet with Earth...
...trigger a nuclear war?
It's a strange scenario:
A small comet hits the Earth...
...as millions have during
Earth's history...
...and the response
of our civilization...
...is promptly to self-destruct.
Maybe it's unlikely,
but it might be a good idea...
...to understand comets
and collisions and catastrophes...
...a little bit better than we do.
Now, a comet, at least as far as
we understand them today...
...is made mostly of ice:
Water ice, maybe some ammonia ice...
...a little bit of methane ice.
So in striking
the Earth's atmosphere...
...a modest cometary fragment...
...will produce a great radiant
fireball and a mighty blast wave.
It'll burn trees and level forests...
...and make a sound
heard around the world.
But it need not make
a crater in the ground.
Why? Because the ices in the comet
are all melted in the impact.
And there's going to be very
few recognizable pieces of comet...
...left on the ground.
We humans like to think of
the heavens as stable...
...serene, unchanging.
But comets suddenly appear...
...and hang ominously in the sky,
night after night, for weeks.
So the idea developed that the comet
had to be there for a reason.
The reason was that comets were
predictions of disaster...
...that they foretold the deaths
of princes and the fall of kingdoms.
In 1066, for example...
...the Normans witnessed an apparition
or appearance of Halley's comet.
Since a comet must, they thought,
predict the fall of some kingdom...
...they promptly invaded England.
Here's King Harold of England
looking a little glum.
The events were noted
in the Bayeux tapestry...
...a kind of newspaper of the day.
Or, in the early 13th century...
...Giotto, one of the founders
of modern realistic painting...
...witnessed another apparition
of comet Halley...
...and inserted it into a nativity
he was painting.
A harbinger of a different
sort of change of kingdoms.
Around 1517...
...another great comet appeared.
This time it was seen in Mexico.
And the Aztec emperor, Moctezuma...
...maybe this is he...
...promptly executed his astrologers.
Why? They hadn't predicted the comet,
and they sure hadn't explained it.
Moctezuma was positive that the comet
foretold some dreadful disaster.
He became distant and gloomy...
...and in that way,
helped to set the stage...
...for the successful Spanish conquest
of Mexico under Cort?s.
In many cases, a superstitious
belief in comets...
...becomes a self-fulfilling prophecy.
Here are two quite different
representations...
...of the great comet of 1577:
This one pictured by the Turks...
...and this one by the Germans.
In 1705, Edmund Halley finally...
...figured out that the same
spectacular comet...
...was booming by the Earth
every 76 years, like clockwork.
That comet is now called,
appropriately, comet Halley.
And it's the same one that we talked
about before, the comet of 1066.
At that point, the subject began
to lose a little...
...of its burden of superstition,
but hardly all.
Public fear of comets survived.
Well, for example...
...look at this terribly
nasty comet of 1857...
...that some people figured
would splinter the Earth.
By 1910, Halley's comet
returned once more.
But this time, astronomers using
a new tool, the spectroscope...
...had discovered cyanogen gas
in the tail of a comet.
Now, cyanogen is a poison.
The Earth was to pass through
this poisonous tail.
The fact that the gas was
astonishingly, fabulously thin...
...reassured almost nobody.
For example, look at the headlines
in the Los Angeles Examiner...
...for May 9, 1910:
"Say, Has That Comet
'Cyanogened' You Yet?"
"Entire Human Race Due
For Free Gaseous Bath.
Expect High Jinks."
Or take this from the San Francisco
Chronicle, May 15, 1910:
"Comet Comes And Husband Reforms."
"Comet Parties Now Fad In New York."
Amazing stuff!
In 1910, people were holding
comet parties, not so much to...
...celebrate the end of the world
as to make merry before it happened.
There were entrepreneurs
who were hawking comet pills.
I think I'm gonna take one for later.
And there were those
who were selling...
...gas masks to protect
against the cyanogen.
And comet nuttiness
didn't stop in 1910.
Long before 1066,
humans marveled at comets.
Our generation is beginning
to understand them.
Mercury, Venus, Earth and Mars...
...are small planets made mostly
of rock and iron.
Farther out where it's colder,
are the giant planets...
...made mostly of gas.
But comets originate from
a great cloud beyond the planets...
...almost halfway to the nearest star.
Occasionally, one falls in...
...accelerated by the sun's gravity.
Because it's made mostly of ice,
the comet...
...evaporates as it approaches
the sun.
The vapor is blown back
by the solar wind...
...forming the cometary tail.
Then it's flung back
into outer darkness...
...its orbit so large...
...that it will not return
for millions of years.
These are the long-period comets.
For every one plunging close enough
to the sun to be discovered...
...there may be a billion others...
...slowly drifting
beyond Pluto's orbit.
Very rarely, a long-period comet is
captured in the inner solar system...
...becoming a short-period comet.
It passes near a major planet,
like Saturn.
The planet provides
a small gravitational tug...
...enough to deflect it
into a much smaller orbit.
Though few are captured this way,
those that are...
...become well-known because
they all return in short intervals.
Once trapped in the inner
solar system, among the planets...
...the chances of another
near-collision are increased.
Here, a second encounter
with Saturn...
...further reduces the comet's
orbital period to decades.
A comet may take 10,000 years
between close planetary encounters.
But in this computer study,
we've sped things up.
A third encounter,
this time with Jupiter...
...further reduces
the comet's orbital period.
Now the comet must approach the sun...
...and grow a tail every few years.
Since the dust and gas in the tail
are lost forever to space...
...the comet must slowly be eroding.
Pieces of it break off.
Sometimes, as we've seen,
they even strike the Earth.
In a few thousand years...
...if a short-period comet
hasn't hit a planet...
...it will have evaporated away
almost entirely...
...leaving sand-sized fragments,
which become meteors...
...and its core which, perhaps,
becomes an asteroid.
Suppose I were a pretty typical comet.
And what you would see
would be a kind of...
...tumbling snowball...
...spending most of my time out here
in the outer solar system.
I'd be a kilometer across.
I'd be living most of my days...
...in the gloom beyond Saturn,
orbiting the sun.
But once every century,
I would find myself...
...careening inward,
faster and faster...
...towards the inner solar system.
By the time I would cross
the orbit of Jupiter...
...on my way to the orbit of Mars...
...I'd be heating up because
I'd be getting closer to the sun.
I'd be evaporating a little bit.
Small pieces of dust and ice...
...would be blown behind me
by the solar wind...
...forming an incipient cometary tail.
On the scale of such
a solar system model...
...l, me, a cometary nucleus...
...would be smaller than a snowflake.
Although, when fully developed,
my tail would be longer...
...than the spacing
between the worlds.
Now, sooner or later...
...comets on these long, elliptical
trajectories around the sun...
...must collide with planets.
The Earth and the moon...
...must have been bombarded
by comets and asteroids...
...the debris from the early history
of the solar system.
In interplanetary space, there are
more small objects than large ones.
So there must be,
on a given planetary surface...
...many more impacts of small objects
than of large objects.
So a thing like the Tunguska impact
happens on the Earth...
...maybe every thousand years.
But the impact of a giant
cometary nucleus...
...like Halley's comet, let's say...
...happens only every
billion years or so.
Now, is there evidence
of past collisions?
When a large comet...
...or a large, rocky asteroid
hits a planet...
...it makes a bowl-shaped crater.
The well-preserved impact craters on
Earth were all formed fairly recently.
The older ones have been softened,
filled in or rubbed out...
...by running water
and mountain building.
Impacts make craters on other worlds
and about as often.
But when the air is thin...
...when water rarely flows,
when mountain building is feeble...
...the ancient craters are retained.
This is the case on the moon
and Mercury and Mars...
...our neighboring
terrestrial planets.
They huddle around the sun...
...their source of heat and light...
...a little bit like campers
around a fire.
They are about
4? billion years old.
And all bear witness
to an age long gone...
...of major collisions...
...which do not happen at that scale
and frequency anymore.
If we move out past...
...the terrestrial planets
beyond Mars...
...we find ourselves in a different
regime of the solar system...
...in the realm of Jupiter...
...and the other giant,
or Jovian planets.
These are great worlds
composed largely of the gases...
...hydrogen and helium,
some other stuff too.
When we look at the surface,
we do not see a solid surface...
...but only an occasional patch
of atmosphere...
...and a complex array
of multicolored clouds.
These are serious planets...
...not fragmentary little world-lets
like the Earth.
In fact, 1000 Earths would fit...
...in the volume of Jupiter.
If a comet or asteroid were to...
...accidentally impact Jupiter, it would
be very unlikely to leave a crater.
It might make a momentary hole
in the clouds, but that's it.
Nevertheless, we know
that the outer solar system...
...has been subject to
a many-billion-year history...
...of impact cratering.
Jupiter's moon Callisto is studded
with thousands of craters.
Clear evidence of ancient
collisions beyond Mars.
And there are craters
on other moons of Jupiter.
Most of the thousands of
large craters on our own moon...
...were excavated
billions of years ago.
But were any recorded
in historical times?
The odds against it
are about 1000-to-one.
(BELL RINGS)
Nevertheless, there's a possible
eyewitness account...
...of just such an event.
It was the Sunday before the
feast of Saint John the Baptist...
...in the summer of 1178.
The monks of Canterbury Cathedral had
completed their evening prayers...
...and were about to retire
for the night.
The scholarly brother, Gervase...
...returned to his cell to read...
...while some of the others...
...went outside to enjoy
the gentle June air.
(PLAYS FLUTE)
In the midst of their recreation...
...they chanced to witness
an astonishing sight:
A violent explosion on the moon.
This was a time...
...when the heavens were
thought to be changeless.
The moon, the stars and the planets
were deemed pure...
...because they followed
an unvarying celestial routine.
They were expected to behave
without unseemly disruptions...
...like monks in a monastery.
Was it wise to discuss such a vision?
In every time and culture...
...there are pressures to conform
to the prevailing prejudices.
But there are also,
in every place and epoch...
...those who value the truth,
who record the evidence faithfully.
Future generations are in their debt.
A fire on the moon.
Might it be some portent
of ill fortune?
Should the chronicler
of the monastery be told?
Was this event an apparition
of the evil one?
Gervase of Canterbury was
a historian...
...considered today a reliable
reporter of political...
...and cultural events of his time.
This is his account of the
eyewitness testimony he was given:
"Now there was a bright new moon...
...and as usual in that phase...
...its horns were tilted
toward the east.
And suddenly the upper horn
split in two.
From the midpoint of this division,
a flaming torch sprang up...
...spewing out
over a considerable distance...
...fire, hot coals and sparks.
After these transformations,"
Gervase continued...
..."the moon from horn to horn
that is along its whole length...
...took on a blackish appearance."
Gervase took depositions
from all the eyewitnesses.
He later wrote:
"The writer was given this report by
men who saw it with their own eyes...
...and are prepared to stake
their honor on an oath...
...that they have made no addition
or falsification."
Gervase committed the account
to paper...
...enabling astronomers
eight centuries later...
...to try and reconstruct
what really happened.
It may be that 200 years
before Chaucer...
...five monks saw an event
more wonderful...
...than many another celebrated
Canterbury tale.
If a small drifting mountain
were to hit the moon...
...it would set our satellite
swinging like a bell.
Eventually, the tremors would
die down, but not in a mere 800 years.
So is the moon still quivering
from that impact?
The Apollo astronauts emplaced arrays
of special mirrors on the moon.
Reflectors made by
French scientists...
...were also put on the moon
by Soviet Lunakhod vehicles.
When a laser beam from Earth strikes
a mirror and bounces back...
...the roundtrip travel time
can be measured.
At the McDonald Observatory
of the University of Texas...
...a laser beam is prepared for firing
at the reflectors on the moon...
...380,000 kilometers away.
By multiplying the travel time
by the speed of light...
...the distance to that spot
can be determined...
...to a precision of
7 to 10 centimeters:
The width of a hand.
When such measurements are
repeated over years...
...even an extremely slight wobble
in the moon's motion...
...can be determined.
The accuracy is phenomenal.
The error is much less...
...than one-millionth of a percent.
The moon, it turns out,
is gently swinging like a bell...
...just as if it had been
hit by an asteroid...
...less than 1000 years ago.
(RINGING)
So there may be physical evidence
in the age of space flight...
...for the account of the Canterbury
monks in the 12th century.
If 800 years ago
a big asteroid hit the moon...
...the crater should be
prominent today...
...still surrounded by bright rays...
...thin streamers of dust
spewed out by the impact.
In billions of years,
lunar rays are eroded...
...but not in hundreds.
And there is a recent ray crater
called Giordano Bruno...
...in the region of the moon
where an explosion was reported...
...in 1178.
The entire evolution of the moon...
...is a story of catastrophes.
4 1/2 billion years ago...
...the moon was accreting
from interplanetary boulders...
...and craters were forming
all over its surface.
The energy so released
helped melt the crust.
After most of this debris was swept up
by the moon, the surface cooled.
But about 3.9 billion years ago...
...a great asteroid impacted.
It generated an expanding shock wave
and re-melted some of the surface.
The resulting basin
was then flooded...
...probably by dark lava...
...producing one of
the dry seas on the moon.
More recent impacts excavated
craters with bright rays...
...named after Eratosthenes
and Copernicus.
The familiar features
of the man in the moon...
...are a chronicle of ancient impacts.
Most of the original asteroids
were swept up...
...in the making
of the moon and planets.
Many still orbit the sun
in the asteroid belt.
Some, themselves almost fractured
by gravity tides...
...and by impacts
with other asteroids...
...have been captured by planets:
Phobos around Mars, for example...
...or a close moon of Jupiter
called Amalthea.
Similar to the asteroid belt
are the rings of Saturn...
...composed of millions
of small, tumbling, icy moonlets.
Maybe the rings of Saturn
are a moon...
...which was prevented from
forming by the tides of Saturn.
Or maybe it's the remains
of a moon that wandered too close...
...and was torn apart by
the tides of Saturn.
It's certainly a lovely place.
Jupiter also has
a newly discovered ring system...
...which is invisible from the Earth.
Now, there is a curious argument...
...alleging major recent collisions
in the solar system...
...proposed by a psychiatrist...
...named Immanuel Velikovsky in 1950.
He suggested...
...that an object of planetary mass,
which he called a comet...
...was somehow produced
in the Jupiter system.
He doesn't say exactly
how it's produced...
...but maybe...
...it's spat out...
...of Jupiter.
Anyway, however it was made
some 3500 years ago, he imagines...
...it made repeated
close encounters with Mars...
...with the Earth-moon system...
...having as entertaining
biblical consequences...
...the parting of the Red Sea so that
Moses and the Israelites could...
...safely avoid the host of pharaoh...
...and the stopping of
the Earth's rotation when...
...Joshua commanded the sun
to stand still in Gibeon.
He also imagined that
there was extensive flooding...
...and the volcanoes all over
the Earth at that time.
Well, then after
a very complicated game...
...of interplanetary billiards
is completed...
...Velikovsky proposed
that this comet...
...entered into a stable,
almost perfectly circular orbit...
...becoming...
...the planet Venus...
...which he claimed
never existed until then.
Now, these ideas are
almost certainly wrong.
There's no objection
in astronomy to collisions.
We've seen collision fragments...
...and evidence throughout
the solar system.
The problem is with recent
and major collisions.
In any scale model like this...
...it's impossible to have both
the sizes of the planets...
...and the sizes of their orbits
to the same scale...
...because then the planets
would be too small to see.
If the planets were really
to scale in such a model...
...as grains of dust...
...it would then be entirely clear...
...that a comet entering
the inner solar system...
...would have a negligible chance
of colliding with a planet...
...in only a few thousand years.
Moreover...
...Venus is a rocky and metallic...
...hydrogen-poor world...
...whereas Jupiter, where Velikovsky
imagines it comes from...
...is made of almost nothing
but hydrogen.
There is no energy source in Jupiter
to eject planets or comets.
If one did enter
the inner solar system...
...there is no way it could stop
the Earth from rotating.
And if it could, there's no way
Earth could start rotating again...
...at anything like 24 hours a day.
There's no geological evidence
for flooding and volcanism...
...3500 years ago.
Babylonian astronomers
observed Venus...
...in its present stable orbit...
...before Velikovsky said it existed.
And so on.
There are many hypotheses
in science which are wrong.
That's all right. It's the aperture
to finding out what's right.
Science is a self-correcting process.
To be accepted,
new ideas must survive...
...the most rigorous standards
of evidence and scrutiny.
The worst aspect of
the Velikovsky affair is not...
...that many of his ideas were
wrong or silly...
...or in gross contradiction
to the facts.
Rather, the worst aspect is
that some scientists...
...attempted to suppress
Velikovsky's ideas.
The suppression of uncomfortable ideas
may be common in religion...
...or in politics,
but it is not the path to knowledge.
And there's no place for it
in the endeavor of science.
We do not know beforehand...
...where fundamental insights
will arise from...
...about our mysterious
and lovely solar system.
And the history of our study
of the solar system shows clearly...
...that accepted and conventional
ideas are often wrong...
...and that fundamental insights...
...can arise from
the most unexpected sources.
We've evolved on the planet Earth...
...and so we find it
a congenial place.
But just next door is Venus...
...until recently, enveloped
in mystery.
It has almost the same size
and mass as the Earth.
Might our sister world
be a balmy summer planet...
...a little warmer than the Earth
because it's closer to the sun?
Are there craters, volcanoes,
mountains, oceans, life?
The first to look at Venus through
a telescope was Galileo in 1609.
But all he could see
was a featureless disk.
As optical telescopes got bigger,
that's all anybody could see:
A disk with no details on it at all.
Venus evidently was covered
with an opaque layer...
...thick clouds concealing the surface.
For centuries, even the composition
of the clouds of Venus was unknown.
I mean, you could go outside, look up,
see Venus with the naked eye...
...observe sunlight reflected
from the clouds of Venus.
What were you looking at?
What were the clouds made of?
Nobody knew.
As a result, imagination ran riot.
The absence of anything you
could see on Venus...
...led some scientists and others
to deduce...
...that the surface was a swamp.
The argument, if we can dignify it
with such a phrase...
...went like this:
"I can't see a thing
on the surface of Venus."
"Why not?"
"Because it's covered with
a dense layer of clouds."
"What are clouds made of?"
"Water, of course. Therefore, Venus
must have a lot of water on it."
"Then the surface must be wet."
"If the surface is wet,
it's probably a swamp.
If there's a swamp, there's ferns.
If there's ferns...
...maybe there's even dinosaurs."
Observation:
You couldn't see a thing.
Conclusion: dinosaurs.
If just looking at Venus
was so unproductive...
...what else could you do?
The next clue came from
early work with that:
A glass prism.
An intense beam of ordinary white
light is passed through a narrow slit...
...and then through the prism.
The result is to spread
the white light out...
...into its constituent
rainbow of colors.
This rainbow pattern
is called a spectrum.
Think about it.
White light enters the prism...
...what comes out of the prism
is colored light.
Lots of colors.
Where did they come from?
They must've been hiding
in the white light.
White light must be
a mixture of many colors.
Here we see the spectrum
running from...
...violet, blue, green,
yellow, orange, to red.
Since we see these colors, we call
this the spectrum of visible light.
The sun emits lots of visible light.
The air is transparent to it.
So our eyes evolved
to work in visible light.
But there are many other frequencies
of light which our eyes can't detect.
Beyond the violet
is the ultraviolet.
It's just as real, but you need
instruments to detect it.
Beyond the ultraviolet are
the x-rays and then the gamma rays.
On the other side of visible light,
beyond the red...
...is the infrared,
again real, again invisible.
Beyond the infrared
are the radio waves.
Now, this entire range from
the gamma rays way over there...
...to the radio waves
all the way over here...
...are simply different
kinds of light.
They differ only in the frequency.
They're all useful, by the way,
in astronomy.
But because of the limitations
of our eyes...
...we have a prejudice,
a bias, a chauvinism...
...to this tiny rainbow band
of visible light.
Now, a spectrum can be used
in a simple and elegant way...
...to determine the chemical
composition of the atmosphere...
...of a planet or star.
Different atoms and molecules
absorb...
...different frequencies
or colors of light.
And those absorbed or missing
frequencies appear as black lines...
...in the spectrum of the light
we receive from the planet or star.
Each and every substance
has a characteristic fingerprint...
...a spectral signature...
...which permits it to be detected
over a great distance.
As a result, the gases
in the atmosphere of Venus...
...at a distance of
60 million kilometers...
...their composition's been determined
from the Earth.
It's amazing to me still, we can tell
what a thing is made out of...
...at an enormous distance away,
without ever touching it.
Our eyes can't see in the near
infrared part of the spectrum.
But our instruments can.
Here's the absorption pattern
of lots and lots of carbon dioxide:
Dark lines in characteristic patterns
at specific frequencies.
You'd detect a different set
of infrared lines...
...if, say, water vapor were present.
If Venus were really soaking wet,
then you could determine that...
...by finding the pattern
of water vapor in its atmosphere.
But around 1920, when this experiment
was first performed...
...the Venus atmosphere seemed to
have not a hint...
...not a smidgen, not a trace
of water vapor above the clouds.
And so instead of a swampy,
soaking wet surface...
...it was suggested that Venus
was bone-dry, a desert planet...
...with clouds composed
of fine silicate dust.
But later, spectroscopic
observations revealed...
...the characteristic
absorption lines...
...of an enormous amount
of carbon dioxide.
Scientists thought there must be lots
of carbon compounds on the surface...
...making this a planet
covered with petroleum.
Others agreed that the atmosphere was
dry but thought the surface was wet.
With all that CO 2,
it had to be carbonated water.
Venus, they thought, was covered
with a vast ocean of seltzer.
The first hint of the true situation
on Venus came...
...not from the visible, ultraviolet
or infrared part of the spectrum...
...but from over here
in the radio region.
We're used to the idea of
radio signals from intelligent life...
...or at least semi-intelligent life,
radio and television stations.
But there are all kinds of reasons
why natural objects emit radio waves.
One reason is that they're hot.
And when, in 1956...
...Venus was, for the first time,
observed by a radio telescope...
...the planet was discovered
to be emitting radio waves...
...as if it were at
an extremely high temperature.
But the real demonstration that Venus'
surface was astonishingly hot...
...came when the first spacecraft
penetrated the clouds of Venus...
...and slowly settled on the surface
of the nearest planet.
These were the unmanned spacecraft
of the Soviet Venera series.
In our spaceship of the imagination,
we retrace their course.
From a distance, our sister planet
seems serene and peaceful...
...its clouds motionless.
These clouds are near the top
of a great ocean of air...
...about 100 kilometers thick,
composed mainly of carbon dioxide.
There's some nitrogen, a little
water vapor and other gases...
...but only the merest trace
of hydrocarbons.
The clouds turn out to be,
not water...
...but a concentrated solution
of sulfuric acid.
Even in the high clouds...
...Venus is a thoroughly nasty place.
The clouds are stained yellow
by sulfur.
There are great lightning storms.
As we descend, there are
increasing amounts...
...of the noxious gas sulfur dioxide.
The pressures become so high
that early Venera spacecraft...
...were crushed like old tin cans...
...by the weight
of the surrounding atmosphere.
Beneath the clouds
in the dense, clear air...
...it's as bright as
on an overcast day on Earth.
But the atmosphere is so thick
that the ground seems to ripple...
...and distort.
The atmospheric pressure down here is
90 times that on Earth.
The temperature is 380 degrees
centigrade, 900 degrees Fahrenheit.
Hotter than the hottest
household oven.
This is a world
marked by searing heat...
...crushing pressures,
sulfurous gases...
...and a desolate, reddish landscape.
Far from the balmy paradise
imagined by some early scientists...
...Venus is the one place
in the solar system most like hell.
But today, as in ancient tradition...
...there are travelers who will dare
a visit to the underworld.
Venera 9 was the first spacecraft
in human history...
...to return a photograph
from the surface of Venus.
It found the rocks curiously eroded...
...perhaps by the corrosive gases...
...perhaps because
the temperature is so high...
...that the rocks are partly molten
and sluggishly flow.
The Soviet Venera spacecraft,
their electronics long ago fried...
...are slowly corroding
on the surface of Venus.
They are the first spaceships
from Earth...
...ever to land on another planet.
The reason Venus is like hell...
...seems to be what's called
the greenhouse effect.
Ordinary visible sunlight penetrates
the clouds and heats the surface.
But the dense atmosphere
blankets the surface...
...and prevents it from
cooling off to space.
An atmosphere 90 times
as dense as ours...
...made of carbon dioxide,
water vapor and other gases...
...lets in visible light
from the sun...
...but will not let out the infrared
light radiated by the surface.
The temperature rises...
...until the infrared radiation
trickling out to space...
...just balances the sunlight
reaching the surface.
The greenhouse effect can make
an Earth-like world...
...into a planetary inferno.
In this caldron, there's not likely
to be anything alive...
...even creatures
very different from us.
Organic and other conceivable
biological molecules...
...would simply fall to pieces.
The hell of Venus is
in stark contrast...
...with the comparative heaven
of its neighboring world...
...our little planetary home,
the Earth.
Here, the atmosphere is
90 times thinner.
Here, the carbon dioxide
and water vapor...
...make a modest greenhouse effect...
...which heats the ground
above the freezing point of water.
Without it, our oceans
would be frozen solid.
A little greenhouse effect
is a good thing.
But Venus is an ominous reminder...
...that on a world
rather like the Earth...
...things can go wrong.
There is no guarantee that our planet
will always be so hospitable.
To maintain this clement world...
...we must understand it
and appreciate it.
The Earth is a place to our eyes...
...more beautiful than
any other that we know.
But this beauty has been
sculpted by change:
Gentle, almost undetectable change...
...and sudden, violent change.
In the cosmos,
there is no refuge from change.
The Sphinx:
human head, lion's body...
...constructed more than
5500 years ago.
That face was once crisp
and cleanly rendered...
...like this paw I am standing on.
The paw has been buried
in the sand until recently...
...and protected from erosion.
The face is now muddled
and softened...
...because of thousands of years
of sandblasting in the desert...
...and a little rainfall.
In New York City, there is an obelisk
called Cleopatra's Needle...
...which comes from Egypt.
In only a little more than a century
in New York's Central Park...
...the inscriptions on that obelisk
have been almost totally obliterated.
Not by sand and water...
...but by smog
and industrial pollution.
A bit like the atmosphere of Venus.
Slow erosion wipes out information.
On the Earth...
...mountain ranges are destroyed
by erosion...
...in maybe tens of millions
of years...
...small impact craters in maybe
hundreds of thousands of years.
And the greatest artifacts
of human beings...
...in thousands
or tens of thousands of years.
In addition to such slow
and uniform processes...
...there are rare but sudden
catastrophes.
The Sphinx is missing a nose.
In an act of idle desecration,
some soldiers once shot it off.
If you wait long enough,
everything changes.
Slow, uniform processes,
unheralded events:
The sting of a sand grain...
...the fall of a drop of water...
...can, over the ages,
totally rework the landscape.
And rare, violent processes...
...exceptional events
that will not recur in a lifetime...
...also make major changes.
Both the insignificant
and the extraordinary...
...are the architects
of the natural world.
The destruction of trees
and grasslands...
...makes the surface
of the Earth brighter.
It reflects more sunlight
back to space and cools our planet.
After we discovered fire...
...we began to incinerate forests
intentionally...
...to clear the land
by a process called...
..."slash and burn" agriculture.
And today, forests and grasslands
are being destroyed...
...frivolously, carelessly
by humans who are...
...heedless of the beauty
of our cousins the trees...
...and ignorant of the possible
climatic catastrophes...
...which large-scale burning
of forests may bring.
(TREES BREAKING)
The indiscriminate destruction
of vegetation...
...may alter the global climate...
...in ways that no scientist
can yet predict.
It has already deadened
large patches...
...of the Earth's
life-supporting skin.
And yet, we ravage the Earth
at an accelerated pace...
...as if it belonged
to this one generation...
...as if it were ours
to do with as we please.
The Earth has mechanisms
to cleanse itself...
...to neutralize the toxic substances
in its system.
But these mechanisms work
only up to a point.
Beyond some critical threshold,
they break down.
The damage becomes irreversible.
Our generation must choose.
Which do we value more:
short-term profits...
...or the long-term habitability
of our planetary home?
The world is divided politically.
But ecologically
it is tightly interwoven.
There are no useless threads
in the fabric of the ecosystem.
If you cut any one of them,
you will unravel many others.
We have uncovered other worlds...
...with choking atmospheres
and deadly surfaces.
Shall we then re-create
these hells on Earth?
We have encountered desolate moons
and barren asteroids.
Shall we then scar and crater this
blue-green world in their likeness?
Natural catastrophes are rare.
But they come often enough.
We need not force the hand of nature.
If we ruin the Earth,
there is no place else to go.
This is not a disposable world.
And we are not yet able
to re-engineer other planets.
The cruelest desert on Earth...
...is far more hospitable
than any place on Mars.
The bright, sandy surface
and dusty atmosphere of Mars...
...reflect enough sunlight
back to space to cool the planet...
...freezing out all its water,
locking it in a perpetual ice age.
Human activities brighten
our landscape and our atmosphere.
Might this ultimately
make an ice age here?
At the same time, we are releasing
vast quantities of carbon dioxide...
...increasing the greenhouse effect.
The Earth need not
resemble Venus very closely...
...for it to become
barren and lifeless.
It may not take much
to destabilize the Earth's climate...
...to convert this heaven,
our only home in the cosmos...
...into a kind of hell.
The study of the global climate,
the sun's influence...
...the comparison of the Earth
with other worlds...
These are subjects in their
earliest stages of development.
They are funded poorly
and grudgingly.
Meanwhile, we continue to load the
Earth's atmosphere with materials...
...about whose long-term influence
we are almost entirely ignorant.
There are worlds that began with
as much apparent promise as Earth.
But something went wrong.
Knowing that worlds can die
alerts us to our danger.
If a visitor arrived from another
world, what account would we give...
...of our stewardship
of the planet Earth?
In the history of the solar system,
have worlds ever been destroyed?
Most of the moons in the outer
solar system have craters on them...
...made by cometary impacts.
Some have such
large craters though...
...that if the impacting comets
had been just a little bit bigger...
...the moons would have
been shattered.
What would the results of
such a collision look like?
(EXPLOSION)
Maybe a planetary ring.
The idea has been growing
that little worlds are...
...every now and then,
demolished by a cometary impact.
The fragments then slowly coalesce,
and a moon arises again...
...from its own ashes.
Some moons may have been destroyed
and reconstituted many times.
For our own world,
the peril is more subtle.
Since this series was
first broadcast...
...the dangers of the increasing
greenhouse effect...
...have become much more clear.
We burn fossil fuels, like coal
and gas and petroleum...
...putting more carbon dioxide
into the atmosphere...
...and thereby heating the Earth.
The hellish conditions on Venus
are a reminder that...
...this is serious business.
Computer models that
successfully explain...
...the climates of other planets...
...predict the deaths of forests...
...parched croplands,
the flooding of coastal cities...
...environmental refugees...
...widespread disasters
in the next century...
...unless we change our ways.
What do we have to do?
Four things.
One: much more efficient use
of fossil fuels.
Why not cars that get 70 miles
a gallon instead of 25?
Two: research and development
on safe alternative energy sources...
...especially solar power.
Three: reforestation
on a grand scale.
And four: helping to bring
the billion poorest people...
...on the planet
to self-sufficiency...
...which is the key step
in curbing world population growth.
Every one of these steps makes sense
apart from greenhouse warming.
No one has proposed
that the trouble with Venus is...
...that there once was Venusians
who drove fuel-inefficient cars.
But our nearest neighbor,
nevertheless, is a stark warning...
...on the possible fate
of an Earth-like world.
planetary exploration...
...when our ships have begun
to sail the heavens.
In those heavens, there are
some worlds much like hell.
Our planet is, in comparison,
much like a heaven.
But the gates of heaven and hell...
...are adjacent and unmarked.
The Earth is a lovely...
...and more or less placid place.
Things change, but slowly.
You can lead a full life and never
encounter a natural catastrophe...
...more violent than a storm.
And so we become complacent...
...relaxed...
...unconcerned.
But in the history of the solar
system and even in human history...
...there are clear records
of extraordinary...
...and devastating catastrophes.
We have now achieved
the dubious distinction...
...of making our own
major catastrophes...
...both intentional and inadvertent.
On the landscapes of other planets...
...where past records
are better preserved...
...there's abundant evidence
of major catastrophes.
It's all a matter of time scale.
An event which is improbable
in 100 years...
...may be inevitable
in 100 million.
But even on the Earth
in this century...
...there have been
bizarre natural events.
In remote central Siberia...
...there was a time
when the Tungus people...
...told strange tales
of a giant fireball...
...that split the sky
and shook the Earth.
They told of a blast
of searing wind...
...that knocked down
people and forests.
It happened, they said,
on a summer's morning...
...in the year 1908.
In the late 1920s...
...L.A. Kulik, a Soviet scientist...
...organized expeditions
to try and solve the mystery.
He built boats to penetrate
this trackless land:
Snowbound in winter...
...a swampy morass in summer.
Eyewitnesses told of
a ball of flame...
...larger than the sun...
...that had blazed across the sky
20 years before.
Kulik assumed a giant meteorite
had struck the Earth.
He expected to find an enormous
impact crater...
...and rare meteorite fragments...
...chipped off some distant asteroid.
However, at ground zero...
...Kulik found upright trees
stripped of their branches...
...but not a trace of the meteorite
or its impact crater.
He was deeply puzzled.
He thought there were meteorite
fragments buried in the swampy ground.
So he set about digging trenches
and pumping out the water.
But the expected meteoritic rock
and iron was not found.
Undaunted, Kulik went on
to make a thorough survey...
...despite the swarms of insects
and other hardships.
Because he discovered something that,
in his own words...
..."exceeded all tales of eyewitnesses
and my wildest expectations."
For more than 20 kilometers
in every direction from ground zero...
...the trees were flattened radially
outward like broken matchsticks.
There must've been
a powerful explosion...
...several kilometers
above the ground.
The pressure wave, spreading out
at the speed of sound...
...was reconstructed from barometric
records at weather stations...
...across Siberia, through Russia
and on into Western Europe.
Dust from the explosion reflected
so much sunlight back to Earth...
...that people could
read by it at night...
...in London, 10,000 kilometers away.
This really remarkable occurrence...
...is called the Tunguska Event.
But what was it?
Well, perhaps, some scientists
have suggested...
...it was a chunk of antimatter
from space...
...annihilated on contact with
the ordinary matter of the Earth...
...disappearing in a flash
of gamma rays.
But the radioactivity you'd expect
from matter-antimatter annihilation...
...is to be found nowhere
at the impact site.
Or, perhaps, other scientists
have suggested...
...it was a mini black hole
from space...
...which impacted the Earth
in Siberia...
...tunneled through
the solid body of Earth...
...and plunged out the other side.
But the records of atmospheric
shock waves give not a hint...
...of something booming out of
the North Atlantic later that day.
Or maybe, other people have speculated,
it was a spaceship...
...of some unimaginably advanced
extraterrestrial civilization...
...in desperate mechanical trouble...
...crashing in a remote region
of an obscure planet.
Well, if so, it's pretty startling
that at the impact site...
...there is not a piece,
not the tiniest transistor...
...of a crashed spacecraft.
More prosaically, perhaps it was
a large meteorite...
...or a small asteroid
which hit the Earth.
But even here,
there are no observable telltale...
...rocky or metallic fragments
of the sort...
...that you'd expect
from such an impact.
The key point of the Tunguska Event...
...is that there was a tremendous
explosion, a great shock wave...
...many trees burned,
an enormous forest fire...
...and yet, no crater in the ground.
There seems to be
only one explanation...
...which is consistent
with these facts.
And that explanation is this:
In 1908, a piece of a comet...
...hit the Earth.
No one saw it approach.
A small point of light
lost in the glare of the morning sun.
It had been drifting for centuries
through the inner solar system...
...like an iceberg in the ocean
of interplanetary space.
But this time, by accident...
...there was a planet in the way.
From the time and direction of
its approach, what hit the Earth...
...seems to have been a fragment
of a comet named Encke.
Hurtling at more than
100,000 kilometers an hour...
...it was a mountain of ice
about the size of a football field...
...and weighing almost a million tons.
There was no warning, until
it plunged into the atmosphere.
(COMET RUMBLES)
If such an explosion happened today...
...it might be thought,
in the panic of the moment...
...to be produced by a nuclear weapon.
Such a cometary impact and fireball...
...simulates all the effects
of a 15-megaton nuclear burst...
...including the mushroom cloud,
with one exception:
There would be no radiation.
So could a rare but natural event...
...the impact of a comet with Earth...
...trigger a nuclear war?
It's a strange scenario:
A small comet hits the Earth...
...as millions have during
Earth's history...
...and the response
of our civilization...
...is promptly to self-destruct.
Maybe it's unlikely,
but it might be a good idea...
...to understand comets
and collisions and catastrophes...
...a little bit better than we do.
Now, a comet, at least as far as
we understand them today...
...is made mostly of ice:
Water ice, maybe some ammonia ice...
...a little bit of methane ice.
So in striking
the Earth's atmosphere...
...a modest cometary fragment...
...will produce a great radiant
fireball and a mighty blast wave.
It'll burn trees and level forests...
...and make a sound
heard around the world.
But it need not make
a crater in the ground.
Why? Because the ices in the comet
are all melted in the impact.
And there's going to be very
few recognizable pieces of comet...
...left on the ground.
We humans like to think of
the heavens as stable...
...serene, unchanging.
But comets suddenly appear...
...and hang ominously in the sky,
night after night, for weeks.
So the idea developed that the comet
had to be there for a reason.
The reason was that comets were
predictions of disaster...
...that they foretold the deaths
of princes and the fall of kingdoms.
In 1066, for example...
...the Normans witnessed an apparition
or appearance of Halley's comet.
Since a comet must, they thought,
predict the fall of some kingdom...
...they promptly invaded England.
Here's King Harold of England
looking a little glum.
The events were noted
in the Bayeux tapestry...
...a kind of newspaper of the day.
Or, in the early 13th century...
...Giotto, one of the founders
of modern realistic painting...
...witnessed another apparition
of comet Halley...
...and inserted it into a nativity
he was painting.
A harbinger of a different
sort of change of kingdoms.
Around 1517...
...another great comet appeared.
This time it was seen in Mexico.
And the Aztec emperor, Moctezuma...
...maybe this is he...
...promptly executed his astrologers.
Why? They hadn't predicted the comet,
and they sure hadn't explained it.
Moctezuma was positive that the comet
foretold some dreadful disaster.
He became distant and gloomy...
...and in that way,
helped to set the stage...
...for the successful Spanish conquest
of Mexico under Cort?s.
In many cases, a superstitious
belief in comets...
...becomes a self-fulfilling prophecy.
Here are two quite different
representations...
...of the great comet of 1577:
This one pictured by the Turks...
...and this one by the Germans.
In 1705, Edmund Halley finally...
...figured out that the same
spectacular comet...
...was booming by the Earth
every 76 years, like clockwork.
That comet is now called,
appropriately, comet Halley.
And it's the same one that we talked
about before, the comet of 1066.
At that point, the subject began
to lose a little...
...of its burden of superstition,
but hardly all.
Public fear of comets survived.
Well, for example...
...look at this terribly
nasty comet of 1857...
...that some people figured
would splinter the Earth.
By 1910, Halley's comet
returned once more.
But this time, astronomers using
a new tool, the spectroscope...
...had discovered cyanogen gas
in the tail of a comet.
Now, cyanogen is a poison.
The Earth was to pass through
this poisonous tail.
The fact that the gas was
astonishingly, fabulously thin...
...reassured almost nobody.
For example, look at the headlines
in the Los Angeles Examiner...
...for May 9, 1910:
"Say, Has That Comet
'Cyanogened' You Yet?"
"Entire Human Race Due
For Free Gaseous Bath.
Expect High Jinks."
Or take this from the San Francisco
Chronicle, May 15, 1910:
"Comet Comes And Husband Reforms."
"Comet Parties Now Fad In New York."
Amazing stuff!
In 1910, people were holding
comet parties, not so much to...
...celebrate the end of the world
as to make merry before it happened.
There were entrepreneurs
who were hawking comet pills.
I think I'm gonna take one for later.
And there were those
who were selling...
...gas masks to protect
against the cyanogen.
And comet nuttiness
didn't stop in 1910.
Long before 1066,
humans marveled at comets.
Our generation is beginning
to understand them.
Mercury, Venus, Earth and Mars...
...are small planets made mostly
of rock and iron.
Farther out where it's colder,
are the giant planets...
...made mostly of gas.
But comets originate from
a great cloud beyond the planets...
...almost halfway to the nearest star.
Occasionally, one falls in...
...accelerated by the sun's gravity.
Because it's made mostly of ice,
the comet...
...evaporates as it approaches
the sun.
The vapor is blown back
by the solar wind...
...forming the cometary tail.
Then it's flung back
into outer darkness...
...its orbit so large...
...that it will not return
for millions of years.
These are the long-period comets.
For every one plunging close enough
to the sun to be discovered...
...there may be a billion others...
...slowly drifting
beyond Pluto's orbit.
Very rarely, a long-period comet is
captured in the inner solar system...
...becoming a short-period comet.
It passes near a major planet,
like Saturn.
The planet provides
a small gravitational tug...
...enough to deflect it
into a much smaller orbit.
Though few are captured this way,
those that are...
...become well-known because
they all return in short intervals.
Once trapped in the inner
solar system, among the planets...
...the chances of another
near-collision are increased.
Here, a second encounter
with Saturn...
...further reduces the comet's
orbital period to decades.
A comet may take 10,000 years
between close planetary encounters.
But in this computer study,
we've sped things up.
A third encounter,
this time with Jupiter...
...further reduces
the comet's orbital period.
Now the comet must approach the sun...
...and grow a tail every few years.
Since the dust and gas in the tail
are lost forever to space...
...the comet must slowly be eroding.
Pieces of it break off.
Sometimes, as we've seen,
they even strike the Earth.
In a few thousand years...
...if a short-period comet
hasn't hit a planet...
...it will have evaporated away
almost entirely...
...leaving sand-sized fragments,
which become meteors...
...and its core which, perhaps,
becomes an asteroid.
Suppose I were a pretty typical comet.
And what you would see
would be a kind of...
...tumbling snowball...
...spending most of my time out here
in the outer solar system.
I'd be a kilometer across.
I'd be living most of my days...
...in the gloom beyond Saturn,
orbiting the sun.
But once every century,
I would find myself...
...careening inward,
faster and faster...
...towards the inner solar system.
By the time I would cross
the orbit of Jupiter...
...on my way to the orbit of Mars...
...I'd be heating up because
I'd be getting closer to the sun.
I'd be evaporating a little bit.
Small pieces of dust and ice...
...would be blown behind me
by the solar wind...
...forming an incipient cometary tail.
On the scale of such
a solar system model...
...l, me, a cometary nucleus...
...would be smaller than a snowflake.
Although, when fully developed,
my tail would be longer...
...than the spacing
between the worlds.
Now, sooner or later...
...comets on these long, elliptical
trajectories around the sun...
...must collide with planets.
The Earth and the moon...
...must have been bombarded
by comets and asteroids...
...the debris from the early history
of the solar system.
In interplanetary space, there are
more small objects than large ones.
So there must be,
on a given planetary surface...
...many more impacts of small objects
than of large objects.
So a thing like the Tunguska impact
happens on the Earth...
...maybe every thousand years.
But the impact of a giant
cometary nucleus...
...like Halley's comet, let's say...
...happens only every
billion years or so.
Now, is there evidence
of past collisions?
When a large comet...
...or a large, rocky asteroid
hits a planet...
...it makes a bowl-shaped crater.
The well-preserved impact craters on
Earth were all formed fairly recently.
The older ones have been softened,
filled in or rubbed out...
...by running water
and mountain building.
Impacts make craters on other worlds
and about as often.
But when the air is thin...
...when water rarely flows,
when mountain building is feeble...
...the ancient craters are retained.
This is the case on the moon
and Mercury and Mars...
...our neighboring
terrestrial planets.
They huddle around the sun...
...their source of heat and light...
...a little bit like campers
around a fire.
They are about
4? billion years old.
And all bear witness
to an age long gone...
...of major collisions...
...which do not happen at that scale
and frequency anymore.
If we move out past...
...the terrestrial planets
beyond Mars...
...we find ourselves in a different
regime of the solar system...
...in the realm of Jupiter...
...and the other giant,
or Jovian planets.
These are great worlds
composed largely of the gases...
...hydrogen and helium,
some other stuff too.
When we look at the surface,
we do not see a solid surface...
...but only an occasional patch
of atmosphere...
...and a complex array
of multicolored clouds.
These are serious planets...
...not fragmentary little world-lets
like the Earth.
In fact, 1000 Earths would fit...
...in the volume of Jupiter.
If a comet or asteroid were to...
...accidentally impact Jupiter, it would
be very unlikely to leave a crater.
It might make a momentary hole
in the clouds, but that's it.
Nevertheless, we know
that the outer solar system...
...has been subject to
a many-billion-year history...
...of impact cratering.
Jupiter's moon Callisto is studded
with thousands of craters.
Clear evidence of ancient
collisions beyond Mars.
And there are craters
on other moons of Jupiter.
Most of the thousands of
large craters on our own moon...
...were excavated
billions of years ago.
But were any recorded
in historical times?
The odds against it
are about 1000-to-one.
(BELL RINGS)
Nevertheless, there's a possible
eyewitness account...
...of just such an event.
It was the Sunday before the
feast of Saint John the Baptist...
...in the summer of 1178.
The monks of Canterbury Cathedral had
completed their evening prayers...
...and were about to retire
for the night.
The scholarly brother, Gervase...
...returned to his cell to read...
...while some of the others...
...went outside to enjoy
the gentle June air.
(PLAYS FLUTE)
In the midst of their recreation...
...they chanced to witness
an astonishing sight:
A violent explosion on the moon.
This was a time...
...when the heavens were
thought to be changeless.
The moon, the stars and the planets
were deemed pure...
...because they followed
an unvarying celestial routine.
They were expected to behave
without unseemly disruptions...
...like monks in a monastery.
Was it wise to discuss such a vision?
In every time and culture...
...there are pressures to conform
to the prevailing prejudices.
But there are also,
in every place and epoch...
...those who value the truth,
who record the evidence faithfully.
Future generations are in their debt.
A fire on the moon.
Might it be some portent
of ill fortune?
Should the chronicler
of the monastery be told?
Was this event an apparition
of the evil one?
Gervase of Canterbury was
a historian...
...considered today a reliable
reporter of political...
...and cultural events of his time.
This is his account of the
eyewitness testimony he was given:
"Now there was a bright new moon...
...and as usual in that phase...
...its horns were tilted
toward the east.
And suddenly the upper horn
split in two.
From the midpoint of this division,
a flaming torch sprang up...
...spewing out
over a considerable distance...
...fire, hot coals and sparks.
After these transformations,"
Gervase continued...
..."the moon from horn to horn
that is along its whole length...
...took on a blackish appearance."
Gervase took depositions
from all the eyewitnesses.
He later wrote:
"The writer was given this report by
men who saw it with their own eyes...
...and are prepared to stake
their honor on an oath...
...that they have made no addition
or falsification."
Gervase committed the account
to paper...
...enabling astronomers
eight centuries later...
...to try and reconstruct
what really happened.
It may be that 200 years
before Chaucer...
...five monks saw an event
more wonderful...
...than many another celebrated
Canterbury tale.
If a small drifting mountain
were to hit the moon...
...it would set our satellite
swinging like a bell.
Eventually, the tremors would
die down, but not in a mere 800 years.
So is the moon still quivering
from that impact?
The Apollo astronauts emplaced arrays
of special mirrors on the moon.
Reflectors made by
French scientists...
...were also put on the moon
by Soviet Lunakhod vehicles.
When a laser beam from Earth strikes
a mirror and bounces back...
...the roundtrip travel time
can be measured.
At the McDonald Observatory
of the University of Texas...
...a laser beam is prepared for firing
at the reflectors on the moon...
...380,000 kilometers away.
By multiplying the travel time
by the speed of light...
...the distance to that spot
can be determined...
...to a precision of
7 to 10 centimeters:
The width of a hand.
When such measurements are
repeated over years...
...even an extremely slight wobble
in the moon's motion...
...can be determined.
The accuracy is phenomenal.
The error is much less...
...than one-millionth of a percent.
The moon, it turns out,
is gently swinging like a bell...
...just as if it had been
hit by an asteroid...
...less than 1000 years ago.
(RINGING)
So there may be physical evidence
in the age of space flight...
...for the account of the Canterbury
monks in the 12th century.
If 800 years ago
a big asteroid hit the moon...
...the crater should be
prominent today...
...still surrounded by bright rays...
...thin streamers of dust
spewed out by the impact.
In billions of years,
lunar rays are eroded...
...but not in hundreds.
And there is a recent ray crater
called Giordano Bruno...
...in the region of the moon
where an explosion was reported...
...in 1178.
The entire evolution of the moon...
...is a story of catastrophes.
4 1/2 billion years ago...
...the moon was accreting
from interplanetary boulders...
...and craters were forming
all over its surface.
The energy so released
helped melt the crust.
After most of this debris was swept up
by the moon, the surface cooled.
But about 3.9 billion years ago...
...a great asteroid impacted.
It generated an expanding shock wave
and re-melted some of the surface.
The resulting basin
was then flooded...
...probably by dark lava...
...producing one of
the dry seas on the moon.
More recent impacts excavated
craters with bright rays...
...named after Eratosthenes
and Copernicus.
The familiar features
of the man in the moon...
...are a chronicle of ancient impacts.
Most of the original asteroids
were swept up...
...in the making
of the moon and planets.
Many still orbit the sun
in the asteroid belt.
Some, themselves almost fractured
by gravity tides...
...and by impacts
with other asteroids...
...have been captured by planets:
Phobos around Mars, for example...
...or a close moon of Jupiter
called Amalthea.
Similar to the asteroid belt
are the rings of Saturn...
...composed of millions
of small, tumbling, icy moonlets.
Maybe the rings of Saturn
are a moon...
...which was prevented from
forming by the tides of Saturn.
Or maybe it's the remains
of a moon that wandered too close...
...and was torn apart by
the tides of Saturn.
It's certainly a lovely place.
Jupiter also has
a newly discovered ring system...
...which is invisible from the Earth.
Now, there is a curious argument...
...alleging major recent collisions
in the solar system...
...proposed by a psychiatrist...
...named Immanuel Velikovsky in 1950.
He suggested...
...that an object of planetary mass,
which he called a comet...
...was somehow produced
in the Jupiter system.
He doesn't say exactly
how it's produced...
...but maybe...
...it's spat out...
...of Jupiter.
Anyway, however it was made
some 3500 years ago, he imagines...
...it made repeated
close encounters with Mars...
...with the Earth-moon system...
...having as entertaining
biblical consequences...
...the parting of the Red Sea so that
Moses and the Israelites could...
...safely avoid the host of pharaoh...
...and the stopping of
the Earth's rotation when...
...Joshua commanded the sun
to stand still in Gibeon.
He also imagined that
there was extensive flooding...
...and the volcanoes all over
the Earth at that time.
Well, then after
a very complicated game...
...of interplanetary billiards
is completed...
...Velikovsky proposed
that this comet...
...entered into a stable,
almost perfectly circular orbit...
...becoming...
...the planet Venus...
...which he claimed
never existed until then.
Now, these ideas are
almost certainly wrong.
There's no objection
in astronomy to collisions.
We've seen collision fragments...
...and evidence throughout
the solar system.
The problem is with recent
and major collisions.
In any scale model like this...
...it's impossible to have both
the sizes of the planets...
...and the sizes of their orbits
to the same scale...
...because then the planets
would be too small to see.
If the planets were really
to scale in such a model...
...as grains of dust...
...it would then be entirely clear...
...that a comet entering
the inner solar system...
...would have a negligible chance
of colliding with a planet...
...in only a few thousand years.
Moreover...
...Venus is a rocky and metallic...
...hydrogen-poor world...
...whereas Jupiter, where Velikovsky
imagines it comes from...
...is made of almost nothing
but hydrogen.
There is no energy source in Jupiter
to eject planets or comets.
If one did enter
the inner solar system...
...there is no way it could stop
the Earth from rotating.
And if it could, there's no way
Earth could start rotating again...
...at anything like 24 hours a day.
There's no geological evidence
for flooding and volcanism...
...3500 years ago.
Babylonian astronomers
observed Venus...
...in its present stable orbit...
...before Velikovsky said it existed.
And so on.
There are many hypotheses
in science which are wrong.
That's all right. It's the aperture
to finding out what's right.
Science is a self-correcting process.
To be accepted,
new ideas must survive...
...the most rigorous standards
of evidence and scrutiny.
The worst aspect of
the Velikovsky affair is not...
...that many of his ideas were
wrong or silly...
...or in gross contradiction
to the facts.
Rather, the worst aspect is
that some scientists...
...attempted to suppress
Velikovsky's ideas.
The suppression of uncomfortable ideas
may be common in religion...
...or in politics,
but it is not the path to knowledge.
And there's no place for it
in the endeavor of science.
We do not know beforehand...
...where fundamental insights
will arise from...
...about our mysterious
and lovely solar system.
And the history of our study
of the solar system shows clearly...
...that accepted and conventional
ideas are often wrong...
...and that fundamental insights...
...can arise from
the most unexpected sources.
We've evolved on the planet Earth...
...and so we find it
a congenial place.
But just next door is Venus...
...until recently, enveloped
in mystery.
It has almost the same size
and mass as the Earth.
Might our sister world
be a balmy summer planet...
...a little warmer than the Earth
because it's closer to the sun?
Are there craters, volcanoes,
mountains, oceans, life?
The first to look at Venus through
a telescope was Galileo in 1609.
But all he could see
was a featureless disk.
As optical telescopes got bigger,
that's all anybody could see:
A disk with no details on it at all.
Venus evidently was covered
with an opaque layer...
...thick clouds concealing the surface.
For centuries, even the composition
of the clouds of Venus was unknown.
I mean, you could go outside, look up,
see Venus with the naked eye...
...observe sunlight reflected
from the clouds of Venus.
What were you looking at?
What were the clouds made of?
Nobody knew.
As a result, imagination ran riot.
The absence of anything you
could see on Venus...
...led some scientists and others
to deduce...
...that the surface was a swamp.
The argument, if we can dignify it
with such a phrase...
...went like this:
"I can't see a thing
on the surface of Venus."
"Why not?"
"Because it's covered with
a dense layer of clouds."
"What are clouds made of?"
"Water, of course. Therefore, Venus
must have a lot of water on it."
"Then the surface must be wet."
"If the surface is wet,
it's probably a swamp.
If there's a swamp, there's ferns.
If there's ferns...
...maybe there's even dinosaurs."
Observation:
You couldn't see a thing.
Conclusion: dinosaurs.
If just looking at Venus
was so unproductive...
...what else could you do?
The next clue came from
early work with that:
A glass prism.
An intense beam of ordinary white
light is passed through a narrow slit...
...and then through the prism.
The result is to spread
the white light out...
...into its constituent
rainbow of colors.
This rainbow pattern
is called a spectrum.
Think about it.
White light enters the prism...
...what comes out of the prism
is colored light.
Lots of colors.
Where did they come from?
They must've been hiding
in the white light.
White light must be
a mixture of many colors.
Here we see the spectrum
running from...
...violet, blue, green,
yellow, orange, to red.
Since we see these colors, we call
this the spectrum of visible light.
The sun emits lots of visible light.
The air is transparent to it.
So our eyes evolved
to work in visible light.
But there are many other frequencies
of light which our eyes can't detect.
Beyond the violet
is the ultraviolet.
It's just as real, but you need
instruments to detect it.
Beyond the ultraviolet are
the x-rays and then the gamma rays.
On the other side of visible light,
beyond the red...
...is the infrared,
again real, again invisible.
Beyond the infrared
are the radio waves.
Now, this entire range from
the gamma rays way over there...
...to the radio waves
all the way over here...
...are simply different
kinds of light.
They differ only in the frequency.
They're all useful, by the way,
in astronomy.
But because of the limitations
of our eyes...
...we have a prejudice,
a bias, a chauvinism...
...to this tiny rainbow band
of visible light.
Now, a spectrum can be used
in a simple and elegant way...
...to determine the chemical
composition of the atmosphere...
...of a planet or star.
Different atoms and molecules
absorb...
...different frequencies
or colors of light.
And those absorbed or missing
frequencies appear as black lines...
...in the spectrum of the light
we receive from the planet or star.
Each and every substance
has a characteristic fingerprint...
...a spectral signature...
...which permits it to be detected
over a great distance.
As a result, the gases
in the atmosphere of Venus...
...at a distance of
60 million kilometers...
...their composition's been determined
from the Earth.
It's amazing to me still, we can tell
what a thing is made out of...
...at an enormous distance away,
without ever touching it.
Our eyes can't see in the near
infrared part of the spectrum.
But our instruments can.
Here's the absorption pattern
of lots and lots of carbon dioxide:
Dark lines in characteristic patterns
at specific frequencies.
You'd detect a different set
of infrared lines...
...if, say, water vapor were present.
If Venus were really soaking wet,
then you could determine that...
...by finding the pattern
of water vapor in its atmosphere.
But around 1920, when this experiment
was first performed...
...the Venus atmosphere seemed to
have not a hint...
...not a smidgen, not a trace
of water vapor above the clouds.
And so instead of a swampy,
soaking wet surface...
...it was suggested that Venus
was bone-dry, a desert planet...
...with clouds composed
of fine silicate dust.
But later, spectroscopic
observations revealed...
...the characteristic
absorption lines...
...of an enormous amount
of carbon dioxide.
Scientists thought there must be lots
of carbon compounds on the surface...
...making this a planet
covered with petroleum.
Others agreed that the atmosphere was
dry but thought the surface was wet.
With all that CO 2,
it had to be carbonated water.
Venus, they thought, was covered
with a vast ocean of seltzer.
The first hint of the true situation
on Venus came...
...not from the visible, ultraviolet
or infrared part of the spectrum...
...but from over here
in the radio region.
We're used to the idea of
radio signals from intelligent life...
...or at least semi-intelligent life,
radio and television stations.
But there are all kinds of reasons
why natural objects emit radio waves.
One reason is that they're hot.
And when, in 1956...
...Venus was, for the first time,
observed by a radio telescope...
...the planet was discovered
to be emitting radio waves...
...as if it were at
an extremely high temperature.
But the real demonstration that Venus'
surface was astonishingly hot...
...came when the first spacecraft
penetrated the clouds of Venus...
...and slowly settled on the surface
of the nearest planet.
These were the unmanned spacecraft
of the Soviet Venera series.
In our spaceship of the imagination,
we retrace their course.
From a distance, our sister planet
seems serene and peaceful...
...its clouds motionless.
These clouds are near the top
of a great ocean of air...
...about 100 kilometers thick,
composed mainly of carbon dioxide.
There's some nitrogen, a little
water vapor and other gases...
...but only the merest trace
of hydrocarbons.
The clouds turn out to be,
not water...
...but a concentrated solution
of sulfuric acid.
Even in the high clouds...
...Venus is a thoroughly nasty place.
The clouds are stained yellow
by sulfur.
There are great lightning storms.
As we descend, there are
increasing amounts...
...of the noxious gas sulfur dioxide.
The pressures become so high
that early Venera spacecraft...
...were crushed like old tin cans...
...by the weight
of the surrounding atmosphere.
Beneath the clouds
in the dense, clear air...
...it's as bright as
on an overcast day on Earth.
But the atmosphere is so thick
that the ground seems to ripple...
...and distort.
The atmospheric pressure down here is
90 times that on Earth.
The temperature is 380 degrees
centigrade, 900 degrees Fahrenheit.
Hotter than the hottest
household oven.
This is a world
marked by searing heat...
...crushing pressures,
sulfurous gases...
...and a desolate, reddish landscape.
Far from the balmy paradise
imagined by some early scientists...
...Venus is the one place
in the solar system most like hell.
But today, as in ancient tradition...
...there are travelers who will dare
a visit to the underworld.
Venera 9 was the first spacecraft
in human history...
...to return a photograph
from the surface of Venus.
It found the rocks curiously eroded...
...perhaps by the corrosive gases...
...perhaps because
the temperature is so high...
...that the rocks are partly molten
and sluggishly flow.
The Soviet Venera spacecraft,
their electronics long ago fried...
...are slowly corroding
on the surface of Venus.
They are the first spaceships
from Earth...
...ever to land on another planet.
The reason Venus is like hell...
...seems to be what's called
the greenhouse effect.
Ordinary visible sunlight penetrates
the clouds and heats the surface.
But the dense atmosphere
blankets the surface...
...and prevents it from
cooling off to space.
An atmosphere 90 times
as dense as ours...
...made of carbon dioxide,
water vapor and other gases...
...lets in visible light
from the sun...
...but will not let out the infrared
light radiated by the surface.
The temperature rises...
...until the infrared radiation
trickling out to space...
...just balances the sunlight
reaching the surface.
The greenhouse effect can make
an Earth-like world...
...into a planetary inferno.
In this caldron, there's not likely
to be anything alive...
...even creatures
very different from us.
Organic and other conceivable
biological molecules...
...would simply fall to pieces.
The hell of Venus is
in stark contrast...
...with the comparative heaven
of its neighboring world...
...our little planetary home,
the Earth.
Here, the atmosphere is
90 times thinner.
Here, the carbon dioxide
and water vapor...
...make a modest greenhouse effect...
...which heats the ground
above the freezing point of water.
Without it, our oceans
would be frozen solid.
A little greenhouse effect
is a good thing.
But Venus is an ominous reminder...
...that on a world
rather like the Earth...
...things can go wrong.
There is no guarantee that our planet
will always be so hospitable.
To maintain this clement world...
...we must understand it
and appreciate it.
The Earth is a place to our eyes...
...more beautiful than
any other that we know.
But this beauty has been
sculpted by change:
Gentle, almost undetectable change...
...and sudden, violent change.
In the cosmos,
there is no refuge from change.
The Sphinx:
human head, lion's body...
...constructed more than
5500 years ago.
That face was once crisp
and cleanly rendered...
...like this paw I am standing on.
The paw has been buried
in the sand until recently...
...and protected from erosion.
The face is now muddled
and softened...
...because of thousands of years
of sandblasting in the desert...
...and a little rainfall.
In New York City, there is an obelisk
called Cleopatra's Needle...
...which comes from Egypt.
In only a little more than a century
in New York's Central Park...
...the inscriptions on that obelisk
have been almost totally obliterated.
Not by sand and water...
...but by smog
and industrial pollution.
A bit like the atmosphere of Venus.
Slow erosion wipes out information.
On the Earth...
...mountain ranges are destroyed
by erosion...
...in maybe tens of millions
of years...
...small impact craters in maybe
hundreds of thousands of years.
And the greatest artifacts
of human beings...
...in thousands
or tens of thousands of years.
In addition to such slow
and uniform processes...
...there are rare but sudden
catastrophes.
The Sphinx is missing a nose.
In an act of idle desecration,
some soldiers once shot it off.
If you wait long enough,
everything changes.
Slow, uniform processes,
unheralded events:
The sting of a sand grain...
...the fall of a drop of water...
...can, over the ages,
totally rework the landscape.
And rare, violent processes...
...exceptional events
that will not recur in a lifetime...
...also make major changes.
Both the insignificant
and the extraordinary...
...are the architects
of the natural world.
The destruction of trees
and grasslands...
...makes the surface
of the Earth brighter.
It reflects more sunlight
back to space and cools our planet.
After we discovered fire...
...we began to incinerate forests
intentionally...
...to clear the land
by a process called...
..."slash and burn" agriculture.
And today, forests and grasslands
are being destroyed...
...frivolously, carelessly
by humans who are...
...heedless of the beauty
of our cousins the trees...
...and ignorant of the possible
climatic catastrophes...
...which large-scale burning
of forests may bring.
(TREES BREAKING)
The indiscriminate destruction
of vegetation...
...may alter the global climate...
...in ways that no scientist
can yet predict.
It has already deadened
large patches...
...of the Earth's
life-supporting skin.
And yet, we ravage the Earth
at an accelerated pace...
...as if it belonged
to this one generation...
...as if it were ours
to do with as we please.
The Earth has mechanisms
to cleanse itself...
...to neutralize the toxic substances
in its system.
But these mechanisms work
only up to a point.
Beyond some critical threshold,
they break down.
The damage becomes irreversible.
Our generation must choose.
Which do we value more:
short-term profits...
...or the long-term habitability
of our planetary home?
The world is divided politically.
But ecologically
it is tightly interwoven.
There are no useless threads
in the fabric of the ecosystem.
If you cut any one of them,
you will unravel many others.
We have uncovered other worlds...
...with choking atmospheres
and deadly surfaces.
Shall we then re-create
these hells on Earth?
We have encountered desolate moons
and barren asteroids.
Shall we then scar and crater this
blue-green world in their likeness?
Natural catastrophes are rare.
But they come often enough.
We need not force the hand of nature.
If we ruin the Earth,
there is no place else to go.
This is not a disposable world.
And we are not yet able
to re-engineer other planets.
The cruelest desert on Earth...
...is far more hospitable
than any place on Mars.
The bright, sandy surface
and dusty atmosphere of Mars...
...reflect enough sunlight
back to space to cool the planet...
...freezing out all its water,
locking it in a perpetual ice age.
Human activities brighten
our landscape and our atmosphere.
Might this ultimately
make an ice age here?
At the same time, we are releasing
vast quantities of carbon dioxide...
...increasing the greenhouse effect.
The Earth need not
resemble Venus very closely...
...for it to become
barren and lifeless.
It may not take much
to destabilize the Earth's climate...
...to convert this heaven,
our only home in the cosmos...
...into a kind of hell.
The study of the global climate,
the sun's influence...
...the comparison of the Earth
with other worlds...
These are subjects in their
earliest stages of development.
They are funded poorly
and grudgingly.
Meanwhile, we continue to load the
Earth's atmosphere with materials...
...about whose long-term influence
we are almost entirely ignorant.
There are worlds that began with
as much apparent promise as Earth.
But something went wrong.
Knowing that worlds can die
alerts us to our danger.
If a visitor arrived from another
world, what account would we give...
...of our stewardship
of the planet Earth?
In the history of the solar system,
have worlds ever been destroyed?
Most of the moons in the outer
solar system have craters on them...
...made by cometary impacts.
Some have such
large craters though...
...that if the impacting comets
had been just a little bit bigger...
...the moons would have
been shattered.
What would the results of
such a collision look like?
(EXPLOSION)
Maybe a planetary ring.
The idea has been growing
that little worlds are...
...every now and then,
demolished by a cometary impact.
The fragments then slowly coalesce,
and a moon arises again...
...from its own ashes.
Some moons may have been destroyed
and reconstituted many times.
For our own world,
the peril is more subtle.
Since this series was
first broadcast...
...the dangers of the increasing
greenhouse effect...
...have become much more clear.
We burn fossil fuels, like coal
and gas and petroleum...
...putting more carbon dioxide
into the atmosphere...
...and thereby heating the Earth.
The hellish conditions on Venus
are a reminder that...
...this is serious business.
Computer models that
successfully explain...
...the climates of other planets...
...predict the deaths of forests...
...parched croplands,
the flooding of coastal cities...
...environmental refugees...
...widespread disasters
in the next century...
...unless we change our ways.
What do we have to do?
Four things.
One: much more efficient use
of fossil fuels.
Why not cars that get 70 miles
a gallon instead of 25?
Two: research and development
on safe alternative energy sources...
...especially solar power.
Three: reforestation
on a grand scale.
And four: helping to bring
the billion poorest people...
...on the planet
to self-sufficiency...
...which is the key step
in curbing world population growth.
Every one of these steps makes sense
apart from greenhouse warming.
No one has proposed
that the trouble with Venus is...
...that there once was Venusians
who drove fuel-inefficient cars.
But our nearest neighbor,
nevertheless, is a stark warning...
...on the possible fate
of an Earth-like world.