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00:00:24This is the age of planetary exploration.
00:00:52When our ships have begun to sail the heavens.
00:00:56In those heavens there are some worlds much like hell.
00:01:00Our planet is, in comparison, much like a heaven.
00:01:04But the gates of heaven and hell are adjacent and unmarked.
00:01:10The Earth is a lovely and more or less placid place.
00:01:16Things change, but slowly.
00:01:20You can lead a full life and never encounter anything in the way of a natural catastrophe more violent than a storm.
00:01:26And so we become complacent, relaxed, unconcerned.
00:01:32But in the history of the solar system, and even in human history, there are clear records of extraordinary and devastating catastrophes.
00:01:46We humans have now achieved the dubious distinction of being able to make our own major catastrophes, both intentional and inadvertent.
00:01:54On the landscapes of other planets, where the records of the past are better preserved, there is abundant evidence of major catastrophes.
00:02:02It's all a matter of time scale.
00:02:04An event which is improbable in a hundred years may be inevitable in a hundred million.
00:02:10But even on the Earth, in this century, there have been bizarre natural events.
00:02:17In 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.
00:02:33They told of a blast of searing wind that knocked down people and whole forests.
00:02:40It happened, they said, on a summer's morning in the year 1908.
00:02:46In the late 1920s, L.A. Kulik, a Soviet scientist, organized expeditions to try and solve the mystery.
00:02:55He built boats to penetrate this trackless land, snowbound in winter, a swampy morass in summer.
00:03:05Eyewitnesses told of a ball of flame larger than the sun that had blazed across the sky 20 years before.
00:03:17Kulik assumed a giant meteorite had struck the Earth.
00:03:22He expected to find an enormous impact crater and rare meteorite fragments chipped off some distant asteroid.
00:03:33However, at ground zero, Kulik found upright trees stripped of their branches, but not a trace of the meteorite or its impact crater.
00:03:45He was deeply puzzled, but he thought perhaps there were meteorite fragments buried in the swampy ground.
00:03:51So he set about digging trenches and pumping out the water, but the expected meteoritic rock and iron was nowhere to be found.
00:04:01Undaunted, Kulik went on to make a thorough survey, despite the swarms of insects and other hardships,
00:04:07because he had discovered something that, in his own words, exceeded all the tales of the eyewitnesses and my wildest expectations.
00:04:15For more than 20 kilometers in every direction from ground zero, the trees were flattened radially outward like broken matchsticks.
00:04:27There must have been a powerful explosion several kilometers above the ground.
00:04:36The pressure waves spreading out of the speed of sound has been reconstructed from barometric records at weather stations across Siberia, through Russia, and on into Western Europe.
00:04:47Dust from the explosion reflected so much sunlight back to Earth that people could read by it at night, in London, 10,000 kilometers away.
00:05:02This really remarkable occurrence is called the Tunguska event.
00:05:09But what was it?
00:05:12Well, 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.
00:05:28But the radioactivity that you'd expect from matter-antimatter annihilation is to be found nowhere at the impact site.
00:05:36Or, perhaps, other scientists have suggested, it was a mini black hole from space which impacted the Earth in Siberia, tunneled its way through the solid body of our planet, and plunged out the other side.
00:05:51But the records of atmospheric shock waves give not a hint of something booming out of the North Atlantic later that day.
00:05:58Or 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.
00:06:15Well, if so, it's pretty startling that at the impact site there's not a piece, not the tiniest transistor of a crashed spacecraft.
00:06:25More prosaically, perhaps it was a large meteorite or a small asteroid which hit the Earth.
00:06:31But even here, there are no observable telltale rocky or metallic fragments of the sort that you'd expect from such an impact.
00:06:41The 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.
00:06:57There seems to be only one explanation which is consistent with all these facts.
00:07:02And that explanation is this.
00:07:06In 1908, a piece of a comet hit the Earth.
00:07:19No one saw it approach, a small point of light lost in the glare of the morning sun.
00:07:26It had been drifting for centuries through the inner solar system, like an iceberg in the ocean of interplanetary space.
00:07:35But this time, by accident, there was a planet in the way.
00:07:47From the time and direction of its approach, the object that hit the Earth seems to have been a fragment of a comet named NK, hurtling at more than 100,000 kilometers an hour.
00:08:06It was a mountain of ice about the size of a football field and weighing almost a million tons.
00:08:13There was no warning until it plunged into the atmosphere.
00:08:18There was no warning.
00:08:21There was no warning.
00:08:22THE END
00:08:52If such an explosion happened today,
00:09:02it might be thought, especially in the panic of the moment,
00:09:06to be produced by a nuclear weapon.
00:09:08Such a cometary impact and fireball
00:09:11simulates all the effects of a 15-megaton nuclear burst,
00:09:15including the mushroom cloud.
00:09:16With one exception, there would be no radiation.
00:09:20So, could a rare but natural event,
00:09:23the impact of a comet with the Earth,
00:09:25trigger a nuclear war?
00:09:29It's a strange scenario.
00:09:31A small comet hits the Earth,
00:09:33as millions of them have during the history of our planet,
00:09:35and the response of our civilization is promptly to self-destruct.
00:09:39Maybe it's unlikely,
00:09:43but it might be a good idea
00:09:46to understand comets and collisions and catastrophes
00:09:49a little bit better than we do.
00:09:52Now, a comet, at least as far as we understand them today,
00:09:56is made mostly of ice.
00:09:58Water ice, maybe some ammonia ice,
00:10:01a little bit of methane ice.
00:10:02So, in striking the Earth's atmosphere,
00:10:06a modest cometary fragment
00:10:08will produce a great radiant fireball
00:10:12and a mighty blast wave.
00:10:14It'll burn trees and level forests
00:10:16and make a sound heard around the world,
00:10:19but it need not make a crater in the ground.
00:10:22Why?
00:10:23Because the ices in the comet are all melted in the impact,
00:10:27and there's going to be very few recognizable pieces of comet
00:10:30left on the ground.
00:10:38We humans like to think of the heavens
00:10:41as stable, serene, unchanging.
00:10:46But comets suddenly appear
00:10:48and hang ominously in the sky night after night for weeks.
00:10:55So, the idea developed
00:10:57that the comet had to be there for a reason.
00:10:59And the reason was
00:11:00that comets were kind of predictions of disaster,
00:11:03that they foretold the deaths of princes
00:11:05and the fall of kingdoms.
00:11:07In 1066, for example,
00:11:09the Normans witnessed an apparition
00:11:12or appearance of Halley's Comet.
00:11:15Since a comet must, they thought,
00:11:16predict the fall of some kingdom,
00:11:18they promptly went ahead and invaded England.
00:11:21Here's King Harold of England
00:11:22looking a little glum.
00:11:24The events were noted in the Bayeux Tapestry,
00:11:26a kind of newspaper of the day.
00:11:29Or, in the early 13th century,
00:11:32Giotto, one of the founders of modern realistic painting,
00:11:35witnessed another apparition of Comet Halley
00:11:38and inserted it into a nativity he was painting,
00:11:41a harbinger of a different sort of change of kingdoms.
00:11:45Around 1517, another great comet appeared,
00:11:50this time was seen in Mexico,
00:11:52and the Aztec emperor, Moctezuma,
00:11:55maybe this is he,
00:11:57promptly executed his astrologers.
00:11:59Why?
00:12:00Well, they hadn't predicted the comet
00:12:02and they sure hadn't explained it.
00:12:04Moctezuma was positive that the comet
00:12:06foretold some dreadful disaster.
00:12:09He became distant and gloomy,
00:12:12and in that way helped to set the stage
00:12:14for the successful Spanish conquest of Mexico under Cortez.
00:12:19In many cases,
00:12:20a superstitious belief in comets
00:12:23becomes a self-fulfilling prophecy.
00:12:25Here are two quite different representations
00:12:29of the Great Comet of 1577,
00:12:32this one pictured by the Turks,
00:12:35and this one by the Germans.
00:12:39In 1705, Edmund Halley finally figured out
00:12:51that the same spectacular comet
00:12:53was booming by the Earth every 76 years,
00:12:56like clockwork.
00:12:57That comet is now called, appropriately,
00:13:00Comet Halley,
00:13:01and it's the same one that we talked about a moment ago,
00:13:04the Comet of 1066.
00:13:05At that point, the subject began to lose
00:13:07a little of its burden of superstition,
00:13:11but hardly all public fear of comets survived.
00:13:15Well, for example,
00:13:16look at this terribly nasty comet of 1857
00:13:19that some people figured would splinter the Earth.
00:13:24By 1910, Halley's comet returned once more,
00:13:28but this time astronomers using a new tool,
00:13:31the spectroscope,
00:13:32had discovered cyanogen gas
00:13:35in the tail of a comet.
00:13:37Now, cyanogen is a poison.
00:13:40The Earth was to pass through this poisonous tail.
00:13:44The fact that the gas was astonishingly,
00:13:46fabulously thin,
00:13:48reassured almost nobody.
00:13:49So, for example,
00:13:51take a look at the headlines
00:13:52in the Los Angeles Examiner
00:13:54for May 9, 1910.
00:13:57Say, has that comet cyanogened you yet?
00:14:02Entire human race due for free gaseous bath.
00:14:05Expect hijinks.
00:14:08Or, uh,
00:14:09take this from the San Francisco Chronicle,
00:14:11May 15, 1910.
00:14:13Comet comes and
00:14:14husband reforms.
00:14:16Comet parties now fad in New York.
00:14:19Amazing stuff.
00:14:21In 1910, people were holding comet parties
00:14:23not so much to celebrate the end of the world
00:14:26as to make merry before it happened.
00:14:28There were entrepreneurs
00:14:29who were hawking comet pills.
00:14:33I think I'm going to take one for later.
00:14:35And there were those who were
00:14:37selling gas masks
00:14:39to protect against the cyanogen.
00:14:44And comet nuttiness
00:14:45didn't stop in 1910.
00:14:53Long before 1066,
00:14:56humans marveled at comets.
00:14:57Our generation
00:14:59is beginning to understand them.
00:15:09Mercury, Venus, Earth, and Mars
00:15:12are small planets
00:15:13made mostly of rock and iron.
00:15:16Farther out,
00:15:17where it's colder,
00:15:19are the giant planets
00:15:20made mostly of gas.
00:15:22But comets originate
00:15:23from a great cloud
00:15:24beyond the planets
00:15:25almost halfway
00:15:26to the nearest star.
00:15:28Occasionally,
00:15:29one falls in,
00:15:30accelerated by the sun's gravity.
00:15:32Because it's made mostly of ice,
00:15:34a comet begins to evaporate
00:15:35as it approaches the sun.
00:15:37The vapor is blown back
00:15:39by the solar wind
00:15:40forming the cometary tail.
00:15:42Then it's flung back
00:15:43into outer darkness,
00:15:45its orbit so large
00:15:46that it will not return
00:15:47for millions of years.
00:15:49These are the
00:15:50long-period comets.
00:15:52And for everyone
00:15:53plunging close enough
00:15:54to the sun to be discovered,
00:15:56there may be a billion others
00:15:57slowly drifting
00:15:59beyond Pluto's orbit.
00:16:01Very rarely,
00:16:02a long-period comet
00:16:03is captured
00:16:04in the inner solar system,
00:16:06becoming a short-period comet.
00:16:08It passes near
00:16:09a major planet
00:16:10like Saturn.
00:16:12The planet provides
00:16:13a small gravitational tug,
00:16:15enough to deflect it
00:16:16into a much smaller orbit.
00:16:18Although few comets
00:16:18are captured this way,
00:16:20those that are
00:16:21become well-known
00:16:21because they all return
00:16:22in relatively short intervals.
00:16:25Once trapped
00:16:26in the inner solar system
00:16:27among the planets,
00:16:28the chances of another
00:16:29near collision
00:16:30are increased.
00:16:31Here, a second encounter
00:16:35with Saturn
00:16:36further reduces
00:16:37the comet's orbital period
00:16:38to decades.
00:16:40A comet may take
00:16:4110,000 years
00:16:42between close
00:16:43planetary encounters,
00:16:44but in this computer study,
00:16:46we've sped things up.
00:16:49A third encounter,
00:16:50this time with Jupiter,
00:16:52further reduces
00:16:53the comet's orbital period.
00:16:54Now, the comet
00:16:57must approach the sun
00:16:58and grow a tail
00:16:59every few years.
00:17:01Since the dust
00:17:02and gas in the tail
00:17:03are lost forever to space,
00:17:05the comet must slowly
00:17:06be eroding.
00:17:07Pieces of it
00:17:08break off.
00:17:09Sometimes, as we've seen,
00:17:11they even strike the Earth.
00:17:13In a few thousand years,
00:17:14if a short-period comet
00:17:15hasn't hit a planet,
00:17:17it will have evaporated away
00:17:19almost entirely,
00:17:20leaving sand-sized fragments
00:17:23which become meteors
00:17:24and its core,
00:17:26which perhaps
00:17:27becomes an asteroid.
00:17:31Suppose I were
00:17:32a pretty typical comet,
00:17:35and what you would see
00:17:36would be a kind of
00:17:37a tumbling snowball,
00:17:41spending most of my time
00:17:43out here
00:17:44in the outer solar system.
00:17:45I'd be a kilometer across.
00:17:47I'd be living most of my days
00:17:49in the gloom beyond Saturn
00:17:51orbiting the sun.
00:17:53But about once every century,
00:17:55I would find myself
00:17:56careening inward,
00:17:58faster and faster,
00:17:59towards the inner solar system.
00:18:03By the time I would
00:18:04cross the orbit of Jupiter
00:18:07on my way to the orbit of Mars,
00:18:09I'd be heating up
00:18:10because I'd be getting
00:18:11closer to the sun.
00:18:12I'd be evaporating a little bit,
00:18:14and small pieces of dust and ice
00:18:17would be blown behind me
00:18:18by the solar wind
00:18:20forming an incipient cometary tail.
00:18:24On the scale of such
00:18:25a solar system model,
00:18:27I, me, a cometary nucleus,
00:18:29would be smaller
00:18:30than a snowflake.
00:18:32Although, when fully developed,
00:18:34my tail would be longer
00:18:36than the spacing
00:18:38between the worlds.
00:18:39Now, sooner or later,
00:18:43comets on these long
00:18:45elliptical trajectories
00:18:46around the sun
00:18:47must collide with planets.
00:18:49The Earth and the moon
00:18:51must have been bombarded
00:18:53by comets and asteroids,
00:18:56the debris from the early history
00:18:58of the solar system.
00:18:59In interplanetary space,
00:19:01there are many more small objects
00:19:02than large objects,
00:19:03so there must be,
00:19:05on a given planetary surface,
00:19:07many more impacts
00:19:08of small objects
00:19:09than of large objects.
00:19:11So, a thing like
00:19:13the Tunguska impact
00:19:14happens on the Earth
00:19:15maybe every thousand years,
00:19:17but the impact
00:19:18of a giant cometary nucleus
00:19:20like Halley's Comet,
00:19:22let's say,
00:19:22happens only every
00:19:23billion years or so.
00:19:26Now, is there evidence
00:19:27of past collisions?
00:19:30When a large comet
00:19:31or a large rocky asteroid
00:19:34hits a planet,
00:19:35it makes a bowl-shaped crater.
00:19:37The few well-preserved
00:19:38impact craters on Earth
00:19:40were all formed
00:19:41fairly recently.
00:19:42The older ones
00:19:42have been softened,
00:19:44filled in,
00:19:44or rubbed out
00:19:45by running water
00:19:46and mountain building.
00:19:48Impacts make craters
00:19:49on other worlds
00:19:50and about as often.
00:19:52But when the air is thin,
00:19:53when water rarely flows,
00:19:56when mountain building
00:19:56is feeble,
00:19:57the ancient craters
00:19:58are retained.
00:20:00This is the case
00:20:00on the Moon
00:20:01and Mercury
00:20:02and Mars
00:20:02are neighboring
00:20:03terrestrial planets.
00:20:07They huddle
00:20:08around the Sun,
00:20:09their source of heat
00:20:11and light,
00:20:12a little bit like
00:20:13campers around a fire.
00:20:15All of them
00:20:16are about four and a half
00:20:17billion years old
00:20:18and all bear witness
00:20:20to an age long gone
00:20:22of major collisions
00:20:25which do not happen
00:20:27at that scale
00:20:27and frequency anymore.
00:20:30If we move out past
00:20:33the terrestrial planets,
00:20:35beyond Mars,
00:20:36we find ourselves
00:20:38in a different regime
00:20:39of the solar system,
00:20:40in the realm of Jupiter
00:20:42and the other giant
00:20:45or Jovian planets.
00:20:47These are great worlds
00:20:49composed largely
00:20:51of the gases,
00:20:52hydrogen and helium,
00:20:53some other stuff too.
00:20:54when we look
00:20:55at the surface,
00:20:56we do not see
00:20:57a solid surface,
00:21:00but only
00:21:00an occasional
00:21:02patch of atmosphere
00:21:03and a complex array
00:21:05of multicolored clouds.
00:21:07These are serious planets,
00:21:09not fragmentary
00:21:11little worldlets
00:21:12like the Earth.
00:21:13In fact,
00:21:15a thousand Earths
00:21:17would fit
00:21:17in the volume
00:21:19of Jupiter.
00:21:19If a comet
00:21:22or asteroid
00:21:23were to accidentally
00:21:25impact Jupiter,
00:21:27it would be very unlikely
00:21:29to leave a crater.
00:21:30It might make
00:21:30a momentary hole
00:21:31in the clouds,
00:21:32but that's it.
00:21:33Nevertheless,
00:21:34we know that
00:21:35the outer solar system
00:21:37has been subject
00:21:38to a many billion year
00:21:39history of impact cratering.
00:21:43Jupiter's moon,
00:21:44Callisto,
00:21:45is studded
00:21:45with thousands
00:21:46of craters,
00:21:47clear evidence
00:21:48of ancient collisions
00:21:49beyond Mars,
00:21:50and there are craters
00:21:52on other moons
00:21:53of Jupiter.
00:21:54Most of the thousands
00:21:55of large craters
00:21:56on our own moon
00:21:57were excavated
00:21:58billions of years ago,
00:22:00but were any recorded
00:22:01in historical times,
00:22:03the odds against it
00:22:04are about
00:22:05a thousand to one.
00:22:06Nevertheless,
00:22:16there's a possible
00:22:17eyewitness account
00:22:18of just such an event.
00:22:20It was the Sunday
00:22:21before the feast
00:22:23of St. John the Baptist
00:22:24in the summer
00:22:26of 1178.
00:22:28The monks
00:22:29of Canterbury Cathedral
00:22:30had just completed
00:22:31their evening prayers
00:22:32and were about
00:22:33to retire
00:22:34for the night.
00:22:34The scholarly brother
00:22:36Gervaise
00:22:37returned to his cell
00:22:38to read,
00:22:39while some of the others
00:22:40went outside
00:22:41to enjoy
00:22:42the gentle June air.
00:22:50In the midst
00:22:51of their recreation,
00:22:52they chanced
00:22:53to witness
00:22:54an astonishing sight,
00:22:56a violent explosion
00:22:57on the moon.
00:23:04This was a time
00:23:09when the heavens
00:23:10were thought
00:23:11to be changeless.
00:23:13The moon,
00:23:13the stars,
00:23:14and the planets
00:23:14were deemed pure
00:23:16because they followed
00:23:18an unvarying celestial routine.
00:23:21They were expected
00:23:22to behave
00:23:22without unseemly disruptions,
00:23:24like monks
00:23:26in a monastery.
00:23:28Was it wise
00:23:29to discuss
00:23:29such a vision?
00:23:37In every time
00:23:38and culture,
00:23:40there are pressures
00:23:40to conform
00:23:42to the prevailing prejudices.
00:23:44But there are also,
00:23:45in every place
00:23:46and epoch,
00:23:47those who value
00:23:48the truth,
00:23:49who record
00:23:50the evidence faithfully.
00:23:52Future generations
00:23:53are in their debt.
00:23:54A fire
00:24:02on the moon.
00:24:03Might it be
00:24:04some portent
00:24:05of ill fortune?
00:24:08Should the chronicler
00:24:09of the monastery
00:24:09be told?
00:24:12Was this event
00:24:12an apparition
00:24:14of the evil one?
00:24:18Gervais of Canterbury
00:24:19was a historian,
00:24:21considered today
00:24:22a reliable reporter
00:24:23of the political
00:24:24and cultural events
00:24:25of his time.
00:24:27This is his account
00:24:28of the eyewitness testimony
00:24:30he was given.
00:24:31Now there was
00:24:32a bright new moon,
00:24:34and as usual
00:24:35in that phase,
00:24:36its horns
00:24:36were tilted
00:24:37toward the east.
00:24:38And suddenly,
00:24:40the upper horn
00:24:40split in two.
00:24:42From the midpoint
00:24:43of this division,
00:24:44a flaming torch
00:24:45sprang up,
00:24:46spewing out
00:24:47over a considerable distance,
00:24:49fire,
00:24:50hot coals,
00:24:50and sparks.
00:24:52After these transformations,
00:24:54Gervais continued,
00:24:56the moon from horn
00:24:57to horn,
00:24:58that is,
00:24:58along its whole length,
00:25:00took on a blackish appearance.
00:25:07Gervais took depositions
00:25:09from all the eyewitnesses.
00:25:11He later wrote,
00:25:12the present writer
00:25:14was given this report
00:25:15by men who saw it
00:25:16with their own eyes
00:25:17and have prepared
00:25:18to stake their honor
00:25:19on an oath
00:25:20that they have made
00:25:21no addition
00:25:21or falsification.
00:25:24Gervais committed
00:25:25the account to paper,
00:25:26enabling astronomers
00:25:27eight centuries later
00:25:29to try and reconstruct
00:25:30what really happened.
00:25:31It may be that
00:25:34200 years before Chaucer,
00:25:36five monks
00:25:37saw an event
00:25:38more wonderful
00:25:39than many another
00:25:40celebrated Canterbury tale.
00:25:45If a small,
00:25:46drifting mountain
00:25:47were to hit the moon,
00:25:49it would set our satellites
00:25:50swinging like a bell.
00:25:52Eventually,
00:25:52the tremors
00:25:53would die down,
00:25:54but not in a mere
00:25:55800 years.
00:25:56So is the moon
00:25:57still quivering
00:25:57from that impact?
00:25:59The Apollo astronauts
00:26:00emplaced arrays
00:26:01of special mirrors
00:26:03on the moon.
00:26:04Reflectors made
00:26:05by French scientists
00:26:06were also put on the moon
00:26:07by the Soviet
00:26:07Lunachad vehicles.
00:26:09When a laser beam
00:26:10from Earth
00:26:11strikes one of these mirrors
00:26:12and bounces back,
00:26:14the round-trip travel time
00:26:15can be measured.
00:26:16At the MacDonald Observatory
00:26:18of the University of Texas,
00:26:20a laser beam
00:26:21is prepared for firing
00:26:22at the reflectors
00:26:23on the moon
00:26:24380,000 kilometers away.
00:26:27By multiplying
00:26:29the travel time
00:26:30by the speed of light,
00:26:31the distance to that spot
00:26:32on the moon
00:26:33can be determined
00:26:34to a precision
00:26:35of 7 to 10 centimeters,
00:26:37the width of a hand.
00:26:43When such measurements
00:26:44are repeated
00:26:45over a period of years,
00:26:46even an extremely
00:26:48slight wobble
00:26:49in the moon's motion
00:26:50can be determined.
00:26:52The accuracy
00:26:53is phenomenal.
00:26:55The error
00:26:56is much less
00:26:57than one millionth
00:26:59of a percent.
00:27:02The moon,
00:27:03it turns out,
00:27:04is gently swinging
00:27:06like a bell,
00:27:07just as if it had been hit
00:27:08by an asteroid
00:27:09less than 1,000 years ago.
00:27:15So,
00:27:15there may be
00:27:16physical evidence
00:27:18in the age
00:27:18of space flight
00:27:19for the account
00:27:20of the Canterbury monks
00:27:21in the 12th century.
00:27:26If 800 years ago,
00:27:28a big asteroid
00:27:29hit the moon,
00:27:30the crater
00:27:30should be prominent today,
00:27:32still surrounded
00:27:33by bright rays,
00:27:35thin streamers
00:27:36of dust
00:27:36spewed out
00:27:37by the impact.
00:27:38In billions of years,
00:27:39lunar rays are eroded,
00:27:41but not in hundreds.
00:27:42And there is
00:27:43a recent ray crater
00:27:44called Giordano Bruno,
00:27:46in exactly
00:27:47the region
00:27:48of the moon
00:27:48where an explosion
00:27:49was reported
00:27:50in 1178.
00:27:57The entire evolution
00:27:59of the moon
00:27:59is a story
00:28:01of catastrophes.
00:28:02Four and a half
00:28:03billion years ago,
00:28:04the moon was accreting
00:28:05from interplanetary boulders
00:28:07and craters
00:28:08were forming
00:28:08all over its surface.
00:28:10The energy
00:28:10so released
00:28:11helped melt the crust.
00:28:13After most of this debris
00:28:14was swept up
00:28:15by the moon,
00:28:16the surface cooled.
00:28:18But about 3.9 billion years ago,
00:28:21a great asteroid impacted.
00:28:28It generated
00:28:29an expanding shockwave
00:28:31and remelted
00:28:31some of the surface.
00:28:33The resulting basin
00:28:33was then flooded,
00:28:35probably by dark lava,
00:28:37producing one of the dry
00:28:38seas on the moon.
00:28:40More recent impacts
00:28:41excavated craters
00:28:42with bright rays
00:28:44named after
00:28:45Eratosthenes
00:28:46and Copernicus.
00:28:47The familiar features
00:28:48of the man in the moon
00:28:49are a chronicle
00:28:51of ancient impacts.
00:28:54Most of the original asteroids
00:28:56were swept up
00:28:57in the making
00:28:58of the moon and planets.
00:28:59Many still orbit the sun
00:29:01in the asteroid belt.
00:29:03Some,
00:29:04themselves almost fractured
00:29:05by gravity tides
00:29:07and by impacts
00:29:08with other asteroids,
00:29:09have been captured
00:29:10by planets,
00:29:10Phobos around Mars,
00:29:11for example,
00:29:12or a close moon of Jupiter
00:29:15called Amalthea.
00:29:19Somewhat similar
00:29:19to the asteroid belt
00:29:20are the rings of Saturn
00:29:21composed of millions
00:29:23of small,
00:29:24tumbling, icy moonlets.
00:29:26Maybe the rings
00:29:27of Saturn
00:29:28are a moon
00:29:30which was prevented
00:29:31from forming
00:29:32by the tides of Saturn,
00:29:34or maybe it's
00:29:35the remains of a moon
00:29:36that wandered too close
00:29:37and was torn apart
00:29:39by the tides of Saturn.
00:29:41It's certainly
00:29:41a lovely place.
00:29:44Jupiter also
00:29:45has a newly discovered
00:29:46ring system
00:29:48which is invisible
00:29:49from the Earth.
00:29:53Now,
00:29:54there is a
00:29:56curious argument
00:29:58alleging major
00:30:00recent collisions
00:30:00in the solar system
00:30:02proposed by
00:30:04a psychiatrist
00:30:05named Emmanuel Velikovsky.
00:30:07in 1950.
00:30:09He suggested
00:30:11that an object
00:30:12of planetary mass,
00:30:14which he called
00:30:14a comet,
00:30:16was somehow
00:30:16produced
00:30:17in the Jupiter system.
00:30:19He doesn't say
00:30:20exactly how
00:30:21it's produced,
00:30:22but maybe
00:30:24it's spat out
00:30:30of Jupiter.
00:30:34anyway,
00:30:36however,
00:30:37it was made
00:30:37some 3,500 years ago,
00:30:40he imagines.
00:30:41It made
00:30:42repeated
00:30:43close encounters
00:30:44with Mars,
00:30:46with the Earth-Moon system,
00:30:49having as
00:30:50entertaining
00:30:52biblical consequences
00:30:54the parting
00:30:56of the Red Sea
00:30:57so that
00:30:58Moses and the Israelites
00:30:59could safely
00:31:01avoid
00:31:01the host
00:31:02of Pharaoh
00:31:03and the stopping
00:31:04of the Earth's
00:31:04rotation
00:31:05at the moment
00:31:05that Joshua
00:31:06commanded the sun
00:31:08to stand still
00:31:09in Gibeon.
00:31:11He also imagined
00:31:12that there was
00:31:12extensive flooding
00:31:13and volcanoes
00:31:14all over the Earth
00:31:15at that time.
00:31:16Well,
00:31:18then,
00:31:18after a
00:31:19very complicated
00:31:21game of
00:31:22interplanetary
00:31:23billiards
00:31:24is completed,
00:31:26Velikovsky
00:31:27proposed
00:31:28that this
00:31:29comet
00:31:30entered into
00:31:31a stable,
00:31:33almost perfectly
00:31:33circular orbit,
00:31:35becoming
00:31:35the planet
00:31:39Venus,
00:31:40which he claimed
00:31:42never existed
00:31:42until then.
00:31:44now,
00:31:46these ideas
00:31:48are almost
00:31:49certainly wrong.
00:31:51There is no
00:31:52objection in
00:31:52planetary astronomy
00:31:53to collisions.
00:31:54We've seen
00:31:54collision fragments
00:31:56and evidence
00:31:57throughout the
00:31:58solar system.
00:31:59The problem
00:32:00is with recent
00:32:01and major
00:32:03collisions.
00:32:04In any scale
00:32:05model of the
00:32:05solar system
00:32:06like this,
00:32:06it's impossible
00:32:07to have both
00:32:08the sizes of
00:32:09the planets
00:32:09and the sizes
00:32:10of their orbits
00:32:11to the same scale
00:32:12because then
00:32:13the planets
00:32:14would be
00:32:14too small
00:32:14to see.
00:32:16If the planets
00:32:17were really
00:32:17to scale
00:32:18in such a model
00:32:18as grains
00:32:20of dust,
00:32:22it would then
00:32:22be entirely
00:32:23clear that a
00:32:25comet entering
00:32:25the inner
00:32:26solar system
00:32:27would have
00:32:28a negligible
00:32:28chance of
00:32:29colliding
00:32:29with a planet
00:32:30in only a few
00:32:31thousand years.
00:32:33Moreover,
00:32:34Venus is a
00:32:36rocky and
00:32:38metallic hydrogen
00:32:39poor world,
00:32:40whereas Jupiter,
00:32:41the place
00:32:42that Volkovsky
00:32:42imagines it
00:32:43comes from,
00:32:44is made of
00:32:44almost nothing
00:32:45but hydrogen.
00:32:46There's no
00:32:47energy source
00:32:48in Jupiter
00:32:48to eject
00:32:50planets or
00:32:51comets.
00:32:51If one did
00:32:52enter the
00:32:54inner solar
00:32:54system,
00:32:55there is no
00:32:56way it could
00:32:57stop the Earth
00:32:58from rotating,
00:32:59and if it could,
00:33:00there's no way
00:33:00the Earth could
00:33:01start up rotating
00:33:02again at anything
00:33:03like 24 hours
00:33:04a day.
00:33:05There's no
00:33:06geological evidence
00:33:07for flooding
00:33:08and volcanism
00:33:093,500 years
00:33:10ago.
00:33:11Babylonian
00:33:12astronomers
00:33:13observed Venus
00:33:14in its present
00:33:15stable orbit
00:33:16before Volkovsky
00:33:18said it
00:33:19existed.
00:33:20And so
00:33:22on.
00:33:27There are
00:33:28many hypotheses
00:33:29in science
00:33:30which are wrong.
00:33:31That's perfectly
00:33:31right.
00:33:32It's the aperture
00:33:33to finding out
00:33:33what's right.
00:33:34Science is a
00:33:35self-correcting
00:33:36process.
00:33:37To be accepted,
00:33:38new ideas
00:33:39must survive
00:33:40the most rigorous
00:33:41standards of
00:33:42evidence and
00:33:43scrutiny.
00:33:44The worst aspect
00:33:46of the Volkovsky
00:33:46affair is not
00:33:47that many of
00:33:48his ideas
00:33:49were wrong
00:33:50or silly
00:33:51or in gross
00:33:51contradiction
00:33:52to the facts.
00:33:54Rather,
00:33:55the worst aspect
00:33:56is that some
00:33:56scientists
00:33:58attempted to
00:33:58suppress
00:33:59Volkovsky's
00:34:00ideas.
00:34:01The suppression
00:34:02of uncomfortable
00:34:03ideas may be
00:34:04common in
00:34:05religion or
00:34:06in politics,
00:34:07but it is not
00:34:08the path to
00:34:09knowledge,
00:34:09and there's no
00:34:10place for it
00:34:11in the endeavor
00:34:12of science.
00:34:13We do not know
00:34:14beforehand where
00:34:16fundamental insights
00:34:17will arise from
00:34:19about our
00:34:20mysterious and
00:34:21lovely solar
00:34:22system.
00:34:24And the history
00:34:24of our study
00:34:25of the solar
00:34:26system shows
00:34:26clearly that
00:34:28accepted and
00:34:29conventional ideas
00:34:30are often
00:34:31wrong, and
00:34:32that fundamental
00:34:33insights can
00:34:34arise from
00:34:35the most
00:34:36unexpected
00:34:36sources.
00:34:39We've evolved
00:34:40on the planet
00:34:41Earth, and so
00:34:42we find it a
00:34:42congenial place.
00:34:44But just next
00:34:45door is Venus,
00:34:47until recently
00:34:48enveloped in
00:34:49mystery.
00:34:50It has almost
00:34:50the same size
00:34:51and mass as
00:34:52the Earth.
00:34:53Might our
00:34:54sister world
00:34:54be a balmy
00:34:56summer planet,
00:34:57a little warmer
00:34:58than the Earth
00:34:58because it's a
00:34:59little closer
00:34:59to the sun?
00:35:01Are there
00:35:01craters, volcanoes,
00:35:03mountains, oceans,
00:35:04life?
00:35:06The first person
00:35:07to look at Venus
00:35:08through a telescope
00:35:08was Galileo in
00:35:091609, but all
00:35:11he could see
00:35:12was a featureless
00:35:13disk.
00:35:15And as optical
00:35:15telescopes got
00:35:16bigger, that's
00:35:16all anybody could
00:35:17see, a disk
00:35:19with no details
00:35:20on it at all.
00:35:21Venus evidently
00:35:22was covered
00:35:22with an opaque
00:35:24layer, thick
00:35:25clouds concealing
00:35:27the surface.
00:35:28For centuries,
00:35:29even the
00:35:30composition of
00:35:31the clouds
00:35:31of Venus
00:35:32was unknown.
00:35:33I mean, you
00:35:33could go outside,
00:35:35look up,
00:35:36see Venus
00:35:37with the naked
00:35:37eye, observe
00:35:38sunlight reflected
00:35:40from the clouds
00:35:40of Venus.
00:35:41What were you
00:35:42looking at?
00:35:42What were the
00:35:43clouds made of?
00:35:44Nobody knew.
00:35:46As a result,
00:35:47imagination ran
00:35:48riot.
00:35:49The absence
00:35:50of anything
00:35:51you could see
00:35:51on Venus
00:35:52led some
00:35:54scientists and
00:35:54others to
00:35:55deduce that
00:35:56the surface
00:35:57was a swamp.
00:35:59The argument,
00:36:01if we can
00:36:01dignify it
00:36:02with such a
00:36:03phrase,
00:36:04went something
00:36:05like this.
00:36:05I can't see a
00:36:06thing on the
00:36:07surface of Venus.
00:36:08Why not?
00:36:09Because it's
00:36:09covered with a
00:36:10dense layer of
00:36:11clouds.
00:36:12Well, what are
00:36:13clouds made of?
00:36:14Water, of course.
00:36:15Therefore, Venus
00:36:16must have an awful
00:36:16lot of water on it.
00:36:18Therefore, the
00:36:18surface must be
00:36:19wet.
00:36:19Well, if the
00:36:20surface is wet,
00:36:21it's probably a
00:36:21swamp.
00:36:22If there's a
00:36:23swamp, there's
00:36:23ferns.
00:36:24If there's
00:36:24ferns, maybe
00:36:26there's even
00:36:26dinosaurs.
00:36:28Observation, you
00:36:29couldn't see a
00:36:29thing.
00:36:30Conclusion, dinosaurs.
00:36:33Well, if just
00:36:34looking at Venus
00:36:35was so unproductive,
00:36:36what else could
00:36:37you do?
00:36:38The next clue came
00:36:39from early work
00:36:40with that, a
00:36:41glass prism.
00:36:43An intense beam
00:36:44of ordinary white
00:36:45light is made to
00:36:46pass through a narrow
00:36:47slit and then
00:36:48through the prism.
00:36:49The result is to
00:36:50spread the white
00:36:51light out into its
00:36:52constituent rainbow
00:36:54of colors.
00:36:56This rainbow
00:36:57pattern is called
00:36:58a spectrum.
00:36:59Think about it.
00:37:01White light
00:37:01enters the prism.
00:37:03What comes out of
00:37:03the prism is
00:37:04colored light.
00:37:06Lots of colors.
00:37:07Where did they
00:37:07come from?
00:37:08They must have
00:37:08been hiding in
00:37:09the white light.
00:37:10White light must
00:37:11be a mixture of
00:37:12many colors.
00:37:13Here we see the
00:37:14spectrum running
00:37:15from violet, blue,
00:37:18green, yellow,
00:37:18orange to red.
00:37:20Since we see
00:37:21these colors, we
00:37:22call this the
00:37:23spectrum of visible
00:37:24light.
00:37:25The sun emits
00:37:26lots of visible
00:37:27light.
00:37:28The air is
00:37:28transparent to it,
00:37:29so our eyes evolve
00:37:31to work in visible
00:37:32light.
00:37:32But there are many
00:37:33other frequencies of
00:37:34light which our eyes
00:37:35can't detect.
00:37:36Beyond the violet
00:37:37is the ultraviolet.
00:37:38It's just as real,
00:37:40but you need
00:37:40instruments to detect
00:37:41it.
00:37:42Beyond the ultraviolet
00:37:43are the x-rays
00:37:44and then the gamma
00:37:44rays.
00:37:46On the other side
00:37:46of visible light,
00:37:47beyond the red,
00:37:48is the infrared,
00:37:49again real,
00:37:50again invisible.
00:37:51Beyond the infrared
00:37:52are the radio waves.
00:37:54Now this entire range
00:37:56from the gamma rays
00:37:57way over there
00:37:58to the radio waves
00:37:59all the way over here
00:38:00are simply different
00:38:02kinds of light.
00:38:04They differ only in
00:38:05the frequency.
00:38:06They're all useful,
00:38:07by the way,
00:38:08in astronomy.
00:38:09But because of
00:38:10the limitations of our
00:38:11eyes, we have a
00:38:13prejudice, a bias,
00:38:15a chauvinism to this
00:38:17tiny rainbow band
00:38:19of visible light.
00:38:20Now, a spectrum
00:38:22can be used in a
00:38:23simple and elegant way
00:38:25to determine the
00:38:25chemical composition
00:38:26of the atmosphere
00:38:27of some distant
00:38:28planet or star.
00:38:30Different atoms and
00:38:31molecules absorb
00:38:32different frequencies
00:38:33or colors of light.
00:38:35And those absorbed
00:38:36or missing frequencies
00:38:38appear as black lines
00:38:40in the spectrum of the
00:38:41light we receive
00:38:42from the planet or star.
00:38:44Each and every substance
00:38:45has a characteristic
00:38:47fingerprint,
00:38:49a spectral signature,
00:38:51which permits it to be
00:38:52detected over a great
00:38:53distance.
00:38:54As a result,
00:38:56the gases in the
00:38:57atmosphere of Venus
00:38:57at a distance of
00:38:5960 million kilometers
00:39:01have been determined,
00:39:03their composition's been
00:39:04determined from the
00:39:05Earth.
00:39:05It's amazing to me
00:39:07still.
00:39:07We can tell what a
00:39:08thing is made out of
00:39:09at an enormous distance
00:39:11away, without ever
00:39:12touching.
00:39:14Our eyes can't see
00:39:17in the near-infrared
00:39:18part of the spectrum,
00:39:19but our instruments can.
00:39:21Here's the absorption
00:39:22pattern of lots and lots
00:39:23of carbon dioxide,
00:39:25dark lines in
00:39:26characteristic patterns
00:39:27at specific frequencies.
00:39:29You'd detect a
00:39:31different set of
00:39:31infrared lines if, say,
00:39:33water vapor were
00:39:34present.
00:39:36If Venus were really
00:39:38soaking wet, then you
00:39:40should be able to
00:39:40determine that by
00:39:41finding the pattern of
00:39:43water vapor in its
00:39:44atmosphere.
00:39:45But around 1920, when
00:39:46this experiment was
00:39:47first performed, it
00:39:49was found that the
00:39:49Venus atmosphere seemed
00:39:50to have not a hint,
00:39:51not a smidgen, not a
00:39:53trace of water vapor
00:39:54above the clouds.
00:39:56And so, instead of a
00:39:57swampy, soaking wet
00:39:59surface, it was
00:40:00suggested that Venus was
00:40:01bone dry, a desert
00:40:03planet with clouds
00:40:05composed of fine
00:40:06silicate dust.
00:40:08But later,
00:40:09spectroscopic observations
00:40:10revealed the
00:40:11characteristic absorption
00:40:12lines of an enormous
00:40:14amount of carbon dioxide.
00:40:16So, some scientists
00:40:18thought there must be
00:40:19lots of carbon compounds
00:40:20on the surface,
00:40:21making this a planet
00:40:22covered with petroleum.
00:40:25Others agreed that the
00:40:26atmosphere was dry,
00:40:27but thought the surface
00:40:28was wet.
00:40:29With all that CO2,
00:40:30it had to be carbonated
00:40:32water.
00:40:33Venus, they thought,
00:40:33it was covered with a
00:40:34vast ocean of seltzer.
00:40:37Now, the first hint of
00:40:38the true situation on
00:40:39Venus came not from the
00:40:41visible or the ultraviolet
00:40:42or the infrared part of
00:40:43the spectrum, but from
00:40:44over here in the radio
00:40:46region.
00:40:47We're used to the idea of
00:40:49radio signals from
00:40:50intelligent life, or at
00:40:52least semi-intelligent
00:40:52life, I mean radio and
00:40:54television stations, but
00:40:55there are all kinds of
00:40:56reasons why natural objects
00:40:57should emit radio waves.
00:40:59One reason is that
00:41:01they're hot, and when in
00:41:031956, Venus was for the
00:41:05first time observed by a
00:41:06radio telescope, the planet
00:41:08was discovered to be
00:41:09emitting radio waves as if
00:41:11it were at an extremely
00:41:13high temperature.
00:41:14But the real demonstration
00:41:16that the surface of Venus
00:41:17was astonishingly hot came
00:41:19when the first spacecraft
00:41:21penetrated the obscuring
00:41:23clouds of Venus and slowly
00:41:26settled on the surface of
00:41:28the nearest planet.
00:41:31These were the unmanned
00:41:33spacecraft of the Soviet
00:41:35Venera series.
00:41:39In our spaceship of the
00:41:41imagination, we retrace
00:41:43their course.
00:41:45From a distance, our sister
00:41:47planet seems serene and
00:41:49peaceful, its clouds
00:41:51motionless.
00:41:55These clouds are near the
00:41:57top of a great ocean of
00:41:58air, about 100 kilometers
00:42:00thick, composed mainly of
00:42:02carbon dioxide.
00:42:07There's some nitrogen, a
00:42:09little water vapor and
00:42:10other gases, but only the
00:42:11nearest trace of hydrocarbon.
00:42:14And the clouds turn out to
00:42:15be not water, but a
00:42:17concentrated solution of
00:42:19sulfuric acid.
00:42:20Even in the high clouds,
00:42:32Venus is a thoroughly nasty
00:42:34place.
00:42:34The clouds are stained yellow
00:42:45by sulfur.
00:42:46There are great lightning
00:42:47storms.
00:42:48As we descend, there are
00:42:50increasing amounts of the
00:42:51noxious gas sulfur dioxide.
00:42:54The pressures become so high
00:42:55that early Venera spacecraft
00:42:57were crushed like old tin cans
00:42:59by the weight of the
00:43:00surrounding atmosphere.
00:43:01Beneath the clouds, in the
00:43:07dense, clear air, it's about
00:43:09as bright as on an overcast
00:43:10day on Earth.
00:43:11But the atmosphere is so
00:43:12thick that the ground seems
00:43:13to ripple and distort.
00:43:17The atmospheric pressure
00:43:18down here is 90 times that
00:43:20on Earth.
00:43:21The temperature is 380 degrees
00:43:23centigrade, 900 Fahrenheit,
00:43:26hotter than the hottest
00:43:28household oven.
00:43:28This is a world marked by
00:43:31searing heat, crushing
00:43:32pressures, sulfurous gases,
00:43:35and a desolate, reddish
00:43:36landscape.
00:43:37Far from the balmy paradise
00:43:39imagined by some early
00:43:40scientists, Venus is the one
00:43:43place in the solar system
00:43:44most like hell.
00:43:45But today, as in ancient
00:43:54tradition, there are travelers
00:43:55who will dare a visit
00:43:57to the underworld.
00:44:00Venera 9 was the first
00:44:01spacecraft in human history
00:44:02to return a photograph
00:44:03from the surface of Venus.
00:44:05It found the rocks
00:44:06curiously eroded, perhaps
00:44:09by the corrosive gases,
00:44:11perhaps because the temperature
00:44:12is so high that the rocks
00:44:13are partly molten and sluggishly
00:44:15flow.
00:44:17The Soviet Venera spacecraft,
00:44:19their electronics long ago
00:44:21fried, are slowly corroding
00:44:23on the surface of Venus.
00:44:25They are the first
00:44:26spaceships from Earth
00:44:27ever to land on another planet.
00:44:35The reason Venus is like hell
00:44:37seems to be what's called
00:44:38the greenhouse effect.
00:44:41Ordinary visible sunlight
00:44:42penetrates the clouds
00:44:43and heats the surface.
00:44:45But the dense atmosphere
00:44:46blankets the surface
00:44:47and prevents it from
00:44:48cooling off to space.
00:44:50An atmosphere 90 times
00:44:52as dense as ours,
00:44:53made of carbon dioxide,
00:44:55water vapor, and other gases,
00:44:56lets in visible light
00:44:57from the sun,
00:44:58but will not let out
00:44:59the infrared light
00:45:01radiated by the surface.
00:45:02So the temperature rises
00:45:04until the infrared radiation
00:45:05trickling out to space
00:45:07just balances the sunlight
00:45:09reaching the surface.
00:45:13The greenhouse effect
00:45:15can make an Earth-like world
00:45:17into a planetary inferno.
00:45:21In this cauldron,
00:45:23there is not likely
00:45:24to be anything alive,
00:45:25even creatures
00:45:26very different from us.
00:45:27Organic and other
00:45:28conceivable biological molecules
00:45:30would simply fall to pieces.
00:45:32The hell of Venus
00:45:47is in stark contrast
00:45:49with the comparative heaven
00:45:50of its neighboring world,
00:45:53our little planetary home,
00:45:55the Earth.
00:45:57Here, the atmosphere
00:46:00is 90 times thinner.
00:46:01Here, the carbon dioxide
00:46:04and water vapor
00:46:05make a modest greenhouse effect
00:46:07which heats the ground
00:46:08above the freezing point
00:46:09of water.
00:46:10Without it,
00:46:12our oceans would be
00:46:13frozen solid.
00:46:15A little greenhouse effect
00:46:16is a good thing.
00:46:18But Venus is an ominous reminder
00:46:32that on a world
00:46:33rather like the Earth,
00:46:35things can go wrong.
00:46:38There is no guarantee
00:46:39that our planet
00:46:40will always be so hospitable.
00:46:43To maintain this clement world,
00:46:45we must understand it
00:46:46and appreciate it.
00:46:51The Earth
00:46:52is a place to our eyes
00:46:54more beautiful
00:46:55than any other
00:46:57that we know.
00:46:58But this beauty
00:46:59has been sculpted
00:47:00by change.
00:47:02Gentle,
00:47:03almost indetectable change
00:47:04and sudden,
00:47:05violent change.
00:47:07In the cosmos,
00:47:08there is no refuge
00:47:09from change.
00:47:10The Sphinx,
00:47:13human head,
00:47:14lion's body,
00:47:15constructed more than
00:47:165,500 years ago.
00:47:19That face
00:47:20was once crisp
00:47:21and cleanly rendered
00:47:22like this paw
00:47:23I'm standing on.
00:47:24The paw has been buried
00:47:25in the sand
00:47:27until recently
00:47:27and protected
00:47:28from erosion.
00:47:30The face
00:47:31is now muddled
00:47:32and softened
00:47:34because of thousands
00:47:35of years
00:47:36of sandblasting
00:47:37in the desert
00:47:37and a little rainfall.
00:47:39In New York City,
00:47:42there is an obelisk
00:47:43called Cleopatra's Needle.
00:47:44It comes from Egypt.
00:47:46In only a little more
00:47:47than a century
00:47:48in New York's
00:47:49Central Park,
00:47:50the inscriptions
00:47:51on that obelisk
00:47:52have been almost
00:47:53totally obliterated,
00:47:55not by sand
00:47:57and water,
00:47:57but by smog
00:47:59and industrial pollution,
00:48:00a little bit like
00:48:01the atmosphere of Venus.
00:48:03Slow erosion
00:48:04wipes out information.
00:48:07On the Earth,
00:48:08mountain ranges
00:48:09are destroyed
00:48:10by erosion
00:48:11in maybe
00:48:12tens of millions
00:48:12of years,
00:48:13small impact craters
00:48:14in maybe hundreds
00:48:16of thousands
00:48:16of years,
00:48:17and the greatest
00:48:18artifacts of human beings
00:48:20in thousands
00:48:21or tens of thousands
00:48:23of years.
00:48:25In addition
00:48:26to such slow
00:48:27and uniform processes,
00:48:29there are rare
00:48:30but sudden
00:48:31catastrophes.
00:48:33The Sphinx
00:48:34is missing a nose.
00:48:35in an act
00:48:37of idle desecration,
00:48:38some soldiers
00:48:39once shot it off.
00:48:41If you wait long enough,
00:48:42everything changes.
00:48:44slow, uniform processes,
00:49:06unheralded events,
00:49:07the sting of a sand grain,
00:49:10the fall of a drop of water,
00:49:11can over the ages
00:49:13totally rework
00:49:14the landscape.
00:49:15and rare violent processes,
00:49:39exceptional events
00:49:40that will not recur
00:49:41in a lifetime,
00:49:42also make major changes.
00:49:44both the insignificant
00:50:07and the extraordinary
00:50:09are the architects
00:50:10of the natural world.
00:50:12meaning
00:50:14of
00:50:16Stop shooting
00:50:16leave your hands
00:50:17in a nunca
00:50:17érica
00:50:20waiting Familien
00:50:21for all angles
00:50:24to your help
00:50:25in a new coming
00:50:26on MARCIA
00:50:26and thedrum complexities
00:50:26and even
00:50:27wat
00:50:29to your
00:50:31at
00:50:31which
00:50:33is
00:50:34come in
00:50:34and
00:50:35save some
00:50:36money
00:50:37for more
00:50:38and
00:50:39to your
00:50:40become
00:50:41The destruction of trees and grasslands makes the surface of the Earth brighter.
00:51:04It reflects more sunlight back to space and cools our planet.
00:51:07After we discovered fire, we began to incinerate forests intentionally
00:51:14to clear the land by a process called slash-and-burn agriculture.
00:51:19And today, forests and grasslands are being destroyed frivolously, carelessly,
00:51:27by humans who are heedless of the beauty of our cousins the trees
00:51:32and ignorant of the possible climatic catastrophes.
00:51:37Which large-scale burning of forests may bring.
00:51:46The indiscriminate destruction of vegetation may alter the global climate
00:51:50in ways that no scientist can yet predict.
00:51:55It has already deadened large patches of the Earth's life-supporting skin.
00:51:59And yet, we ravage the Earth at an accelerated pace,
00:52:16as if it belonged to this one generation,
00:52:19as if it were ours to doeth as we please.
00:52:22The Earth has mechanisms to cleanse itself,
00:52:34to neutralize the toxic substances in its system.
00:52:38But these mechanisms work only up to a point.
00:52:40Beyond some critical threshold, they break down.
00:52:43The damage becomes irreversible.
00:52:47The damage becomes irreversible.
00:52:49The damage becomes irreversible.
00:53:20The profits or the long-term habitability of our planetary home.
00:53:27The world is divided politically, but ecologically it is tightly interwoven.
00:53:33There are no useless threads in the fabric of the ecosystem.
00:53:37If you cut any one of them, you will unravel many others.
00:53:41We have uncovered other worlds with choking atmospheres and deadly surfaces.
00:53:47Shall we then recreate these hells on Earth?
00:53:50We have encountered desolate moons and barren asteroids.
00:53:57Shall we then scar and crater this blue-green world in their likeness?
00:54:02Natural catastrophes are rare, but they come often enough.
00:54:25We need not force the hand of nature.
00:54:38If we ruin the Earth, there is no place else to go.
00:54:43This is not a disposable world, and we are not yet able to re-engineer other planets.
00:54:49The cruelest desert on Earth is far more hospitable than any place on Mars.
00:55:05The bright sandy surface and dusty atmosphere of Mars reflect enough sunlight back to space to cool the planet,
00:55:14freezing out all its water, locking it in a perpetual ice age.
00:55:19At the same time, we are releasing vast quantities of carbon dioxide, increasing the greenhouse effect.
00:55:36The Earth need not resemble Venus very closely, for it to become barren and lifeless.
00:55:42It 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.
00:55:54The study of the global climate, the sun's influence, the comparison of the Earth with other worlds,
00:56:04these are subjects in their earliest stages of development.
00:56:08They are funded poorly and grudgingly.
00:56:11And meanwhile, we continue to load the Earth's atmosphere with materials about whose long-term influence we are almost entirely ignorant.
00:56:19There are worlds that began with as much apparent promise as Earth, but something went wrong.
00:56:29Knowing that worlds can die alerts us to our danger.
00:56:34If a visitor arrived from another world, what account would we give of our stewardship of the planet Earth?
00:56:49In the history of the solar system, have worlds ever been destroyed?
00:56:55Most of the moons in the outer solar system have craters on them, made by cometary impacts.
00:57:02Some have such large craters, though, that if the impacting comets had been just a little bit bigger, the moons would have been shattered.
00:57:12What would the results of such a collision look like?
00:57:15Maybe, maybe, a planetary ring.
00:57:22The idea has been growing that little worlds are every now and then demolished by a cometary impact.
00:57:28The fragments then slowly coalesce and a moon arises again from its own ashes.
00:57:35Some moons may have been destroyed and reconstituted many times.
00:57:40For our own world, the peril is more subtle.
00:57:44Since this series was first broadcast, the dangers of the increasing greenhouse effect have become much more clear.
00:57:51We burn fossil fuels like coal and gas and petroleum, putting more carbon dioxide into the atmosphere and thereby heating the Earth.
00:58:01The hellish conditions on Venus are a reminder that this is serious business.
00:58:06Computer 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.
00:58:25What do we have to do four things?
00:58:29One, much more efficient use of fossil fuels.
00:58:33Why not cars that get 70 miles a gallon instead of 25?
00:58:37Two, research and development on safe alternative energy sources, especially solar power.
00:58:43Three, 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.
00:58:56Every one of these steps makes sense apart from greenhouse warming.
00:59:00Now, 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.
00:59:30One, much more efficient use of Venusians who drove fuel inefficient cars, which is the key step in curbing world, which is the key step in curbing world.
01:00:00One, much more efficient use of Venusians who drove fuel inefficient cars, which is the key step in curbing world.
01:00:30One, much more efficient use of Venusians who drove fuel inefficient cars, which is the key step in curbing world-oriented cars, which is the key step in curbing world.
01:00:33One, much more efficient use of Venusians who drove fuel Entwicklung to fuel fuel innovative cars, which is the key step in curbing world.
01:00:59AVAILABLE NOW
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