#CarlSagan covers a wide range of scientific subjects, including the origin of life and a #perspective of our place in the universe...
A 13-part #documentary #series that covers a wide range of #scientific #subjects, including the #origin of #life and a #perspective of our place in the universe narrated by famous American #Scientist – #Carl #Sagan.
A 13-part #documentary #series that covers a wide range of #scientific #subjects, including the #origin of #life and a #perspective of our place in the universe narrated by famous American #Scientist – #Carl #Sagan.
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LearningTranscript
00:00:00Oh
00:00:30This is the age of planetary exploration
00:00:53when our ships have begun to sail the heavens.
00:00:56In those heavens, there are some worlds much like hell.
00:01:02Our planet is, in comparison, much like a heaven.
00:01:06But the gates of heaven and hell are adjacent and unmarked.
00:01:14The Earth is a lovely and more or less placid place.
00:01:19Things change, but slowly.
00:01:22You can lead a full life and never encounter anything in the way of a natural catastrophe more violent than a storm.
00:01:29And so we become complacent, relaxed, unconcerned.
00:01:34But in the history of the solar system, and even in human history, there are clear records of extraordinary and devastating catastrophes.
00:01:44We humans have now achieved the dubious distinction of being able to make our own major catastrophes, both intentional and inadvertent.
00:01:52On the landscapes of other planets, where the records of the past are better preserved, there is abundant evidence of major catastrophes.
00:02:01It'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.
00:02:09But even on the Earth, in this century, there have been bizarre natural events.
00:02:16In 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.
00:02:32They told of a blast of searing wind that knocked down people and whole forests.
00:02:39It happened, they said, on a summer's morning in the year 1908.
00:02:45In the late 1920s, L.A. Kulik, a Soviet scientist, organized expeditions to try and solve the mystery.
00:02:53He built boats to penetrate this trackless land, snowbound in winter, a swampy morass in summer.
00:03:07Eyewitnesses told of a ball of flame larger than the sun that had blazed across the sky twenty years before.
00:03:16Kulik assumed a giant meteorite had struck the Earth.
00:03:23He expected to find an enormous impact crater and rare meteorite fragments chipped off some distant asteroid.
00:03:35However, at ground zero, Kulik found upright trees stripped of their branches, but not a trace of the meteorite or its impact crater.
00:03:44He 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.
00:04:00Undaunted, 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.
00:04:17For more than twenty kilometers in every direction from ground zero, the trees were flattened radially outward, like broken matchsticks.
00:04:30There must have been a powerful explosion several kilometers above the ground.
00:04:35The pressure waves spreading out at the speed of sound has been reconstructed from barometric records at weather stations across Siberia, through Russia, and on into Western Europe.
00:04:46Dust from the explosion reflected so much sunlight back to Earth that people could read by it at night in London, ten thousand kilometers away.
00:04:55This really remarkable occurrence is called the Tunguska event.
00:05:08But what was it?
00:05:10Well, 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.
00:05:27But the radioactivity that you'd expect from matter-antimatter annihilation is to be found nowhere at the impact site.
00:05:35Or, 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.
00:05:50But the records of atmospheric shock waves give not a hint of something booming out of the North Atlantic later that day.
00:05:57Or 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.
00:06:14Well, if so, it's pretty startling that at the impact site there's not a piece, not the tiniest transistor of a crashed spacecraft.
00:06:24More prosaically, perhaps it was a large meteorite or small asteroid which hit the Earth.
00:06:30But even here, there are no observable telltale rocky or metallic fragments of the sort that you'd expect from such an impact.
00:06:40The 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.
00:06:56There seems to be only one explanation which is consistent with all these facts.
00:07:02And that explanation is this.
00:07:05In 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.
00:07:25It had been drifting for centuries through the inner solar system like an iceberg in the ocean of interplanetary space.
00:07:34But this time, by accident, there was a planet in the way.
00:07:46From 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.
00:08:05It 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.
00:08:17falling asleep at the end of the、 uh, water Bijan.
00:08:24The尽products of장und sorry I was saying goodbye happened.
00:08:27Now I was hoping so at all the light of your body of body to me, doesn't?
00:08:30If such an explosion
00:09:00happened today, it might be thought, especially in the panic of the moment, to be produced
00:09:06by a nuclear weapon. Such a cometary impact and fireball simulates all the effects of
00:09:13a 15 megaton nuclear burst, including the mushroom cloud. With one exception, there would be no
00:09:19radiation. So, could a rare but natural event, the impact of a comet with the Earth, trigger
00:09:26a nuclear war? It's a strange scenario. A small comet hits
00:09:32the Earth, as millions of them have during the history of our planet, and the response
00:09:36of our civilization is promptly to self-destruct. Maybe it's unlikely, but it might be a good
00:09:45idea to understand comets and collisions and catastrophes a little bit better than we do.
00:09:51Now, a comet, at least as far as we understand them today, is made mostly of ice. Water ice,
00:09:59maybe some ammonia ice, a little bit of methane ice. So, in striking the Earth's atmosphere,
00:10:06a modest cometary fragment will produce a great radiant fireball and a mighty blast wave. It'll
00:10:14burn trees and level forests and make a sound heard around the world, but it need not make
00:10:21a crater in the ground. Why? Because 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 left on the ground.
00:10:32We humans like to think of the heavens as stable, serene, unchanging. But comets suddenly appear
00:10:48and hang ominously in the sky night after night for weeks. So, the idea developed that the comet had
00:10:58to be there for a reason. And the reason was that comets were kind of predictions of disaster,
00:11:03that they foretold the deaths of princes and the fall of kingdoms. In 1066, for example, the Normans
00:11:11witnessed an apparition or appearance of Halley's Comet. Since a comet must, they thought, predict the
00:11:17fall of some kingdom, they promptly went ahead and invaded England. Here's King Harold of England
00:11:22looking a little glum. The events were noted in the Bayeux Tapestry, a kind of newspaper of the day.
00:11:29Or, in the early 13th century, Giotto, one of the founders of modern realistic painting,
00:11:35witnessed another apparition of comet Halley and inserted it into a nativity he was painting,
00:11:42a harbinger of a different sort of change of kingdoms. Around 1517, another great comet appeared,
00:11:50this time was seen in Mexico. And the Aztec emperor, Moctezuma, maybe this is he,
00:11:57promptly executed his astrologers. Why? Well, they hadn't predicted the comet and they sure
00:12:03hadn't explained it. Moctezuma was positive that the comet foretold some dreadful disaster. He became
00:12:10distant and gloomy and in that way helped to set the stage for the successful Spanish conquest
00:12:17west of Mexico under Cortez. In many cases, a superstitious belief in comets becomes a self-fulfilling
00:12:25prophecy. Here are two quite different representations of the Great Comet of 1577. This one pictured by the Turks.
00:12:35In 1705, Edmund Halley finally figured out that the same spectacular comet was booming by the Earth
00:12:55every 76 years, like clockwork. That comet is now called, appropriately, Comet Halley. And it's the same
00:13:02one that we talked about a moment ago, the Comet of 1066. At that point, the subject began to lose a little of its
00:13:09burden of superstition, but hardly all public fear of comets survived. And, well, for example, look at this
00:13:17terribly nasty comet of 1857 that, uh, some people figured would splinter the Earth. By 1910, Halley's comet
00:13:26returned once more. But this time, astronomers using a new tool, the spectroscope, had discovered
00:13:33cyanogen gas in the tail of a comet. Now, cyanogen is a poison. The Earth was to pass through this
00:13:41poisonous tail. The fact that the gas was astonishingly fabulously thin reassured almost nobody. So, for example,
00:13:50take a look at the headlines in the Los Angeles Examiner for May 9th, 1910. Say, has that comet cyanogened you yet?
00:14:01Entire human race due for free gaseous bath. Expect hijinks. Or, uh, take this from the San Francisco Chronicle,
00:14:10May 15th, 1910. Comet comes and husband reforms. Comet parties now fad in New York. Amazing stuff.
00:14:20In 1910, people were holding comet parties, not so much to celebrate the end of the world as to make merry
00:14:26before it happened. There were entrepreneurs who were hawking comet pills. I think I'm gonna take one for later.
00:14:34And there were those who were selling gas masks to, uh, protect against the cyanogen. And comet nuttiness didn't stop in 1910.
00:14:53Long before 1066, humans marveled at comets. Our generation is beginning to understand them.
00:15:03Mercury, Venus, Earth, and Mars are small planets, made mostly of rock and iron. Farther out, where it's colder,
00:15:18are the giant planets, made mostly of gas. But comets originate from a great cloud beyond the planets,
00:15:25almost halfway to the nearest star. Occasionally, one falls in, accelerated by the sun's gravity.
00:15:32Because it's made mostly of ice, a comet begins to evaporate as it approaches the sun. The vapor is blown back by the solar wind,
00:15:39forming the cometary tail. Then it's flung back into outer darkness, its orbit so large that it will not return for millions of years.
00:15:48These are the long period comets. And for everyone plunging close enough to the sun to be discovered,
00:15:55there may be a billion others slowly drifting beyond Pluto's orbit. Very rarely, a long period comet is captured in the inner solar system,
00:16:05becoming a short period comet. It passes near a major planet like Saturn. The planet provides a small gravitational tug,
00:16:14enough to deflect it into a much smaller orbit. Although few comets are captured this way, those that are become well known,
00:16:21because they all return in relatively short intervals. Once trapped in the inner solar system, among the planets,
00:16:28the chances of another near collision are increased. Here, a second encounter with Saturn further reduces the comet's orbital period to decades.
00:16:39A comet may take 10,000 years between close planetary encounters, but in this computer study, we've sped things up.
00:16:48A third encounter, this time with Jupiter, further reduces the comet's orbital period.
00:16:54Now, 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,
00:17:03the comet must slowly be eroding. Pieces of it break off. Sometimes, as we've seen, they even strike the Earth.
00:17:12In a few thousand years, if a short period comet hasn't hit a planet, it will have evaporated away almost entirely,
00:17:19leaving sand-sized fragments, which become meteors, and its core, which perhaps becomes an asteroid.
00:17:28Suppose I were a pretty typical comet, and what you would see would be a kind of tumbling snowball,
00:17:40spending most of my time out here in the outer solar system. I'd be a kilometer across.
00:17:47I'd be living most of my days in the gloom beyond Saturn orbiting the sun.
00:17:53But about once every century, I would find myself careening inward, faster and faster, towards the inner solar system.
00:18:02By 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.
00:18:12I'd be evaporating a little bit, and small pieces of dust and ice would be blown behind me by the solar wind, forming an incipient cometary tail.
00:18:23On the scale of such a solar system model, I, me, a cometary nucleus, would be smaller than a snowflake.
00:18:31Although, when fully developed, my tail would be longer than the spacing between the worlds.
00:18:40Now, sooner or later, comets on these long elliptical trajectories around the sun must collide with planets.
00:18:48The Earth and the moon must have been bombarded by comets and asteroids, the debris from the early history of the solar system.
00:18:58In interplanetary space, there are many more small objects than large objects.
00:19:03So, there must be, on a given planetary surface, many more impacts of small objects than of large objects.
00:19:10So, a thing like the Tunguska impact happens on the Earth maybe every thousand years.
00:19:16But the impact of a giant cometary nucleus, like Halley's Comet, let's say, happens only every billion years or so.
00:19:25Now, is there evidence of past collisions?
00:19:29When a large comet, or a large rocky asteroid, hits a planet, it makes a bowl-shaped crater.
00:19:36The few well-preserved impact craters on Earth were all formed fairly recently.
00:19:41The older ones have been softened, filled in, or rubbed out by running water and mountain building.
00:19:47Impacts make craters on other worlds, and about as often.
00:19:51But when the air is thin, when water rarely flows, when mountain building is feeble, the ancient craters are retained.
00:19:59This is the case on the Moon and Mercury and Mars, our neighboring terrestrial planets.
00:20:06They huddle around the Sun, their source of heat and light, a little bit like campers around a fire.
00:20:15All of them are about four and a half billion years old, and all bear witness to an age long gone of major collisions,
00:20:25which do not happen at that scale and frequency anymore.
00:20:30If we move out past the terrestrial planets, beyond Mars, we find ourselves in a different regime of the solar system,
00:20:40in the realm of Jupiter and the other giant or Jovian planets.
00:20:47These are great worlds composed largely of the gases hydrogen and helium, some other stuff, too.
00:20:54When 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.
00:21:07These are serious planets, not fragmentary little worldlets like the Earth.
00:21:13In fact, a thousand Earths would fit in the volume of Jupiter.
00:21:19If a comet or asteroid were to accidentally impact Jupiter, it would be very unlikely to leave a crater.
00:21:29It might make a momentary hole in the clouds, but that's it.
00:21:32Nevertheless, we know that the outer solar system has been subject to a many-billion-year history of impact cratering.
00:21:41Jupiter's moon, Callisto, is studded with thousands of craters, clear evidence of ancient collisions beyond Mars.
00:21:50And there are craters on other moons of Jupiter.
00:21:53Most of the thousands of large craters on our own moon were excavated billions of years ago.
00:21:59But were any recorded in historical times, the odds against it are about a thousand to one.
00:22:06Nevertheless, there's a possible eyewitness account of just such an event.
00:22:19It was the Sunday before the feast of St. John the Baptist in the summer of 1178.
00:22:27The monks of Canterbury Cathedral had just completed their evening prayers and were about to retire for the night.
00:22:34The scholarly brother, Gervaise, returned to his cell to read, while some of the others went outside to enjoy the gentle June air.
00:22:48In the midst of their recreation, they chanced to witness an astonishing sight, a violent explosion on the moon.
00:22:59This was a time when the heavens were thought to be changeless.
00:23:12The moon, the stars, and the planets were deemed pure because they followed an unvarying celestial routine.
00:23:20They were expected to behave without unseemly disruptions, like monks in a monastery.
00:23:26Was it wise to discuss such a vision?
00:23:29In every time and culture, there are pressures to conform to the prevailing prejudices.
00:23:44But there are also, in every place and epoch, those who value the truth, who record the evidence faithfully.
00:23:51Future generations are in their debt.
00:24:01A fire on the moon.
00:24:03Might it be some portent of ill fortune?
00:24:07Should the chronicler of the monastery be told?
00:24:11Was this event an apparition of the evil one?
00:24:16Gervaise of Canterbury was a historian, considered today a reliable reporter of the political and cultural events of his time.
00:24:25This is his account of the eyewitness testimony he was given.
00:24:30Now there was a bright new moon, and as usual in that phase, its horns were tilted toward the east.
00:24:37And suddenly, the upper horn split in two.
00:24:41From the midpoint of this division, a flaming torch sprang up, spewing out over a considerable distance, fire, hot coals, and sparks.
00:24:51After these transformations, Gervaise continued, the moon from horn to horn, that is, along its whole length, took on a blackish appearance.
00:25:02Gervaise took depositions from all the eyewitnesses.
00:25:11He later wrote,
00:25:12The present writer was given this report by men who saw it with their own eyes,
00:25:17and have prepared to stake their honor on an oath that they have made no addition or falsification.
00:25:23Gervaise committed the account to paper, enabling astronomers eight centuries later to try and reconstruct what really happened.
00:25:32It may be that 200 years before Chaucer, five monks saw an event more wonderful than many another celebrated Canterbury tale.
00:25:44If a small, drifting mountain were to hit the moon, it would set our satellites swinging like a bell.
00:25:51Eventually, the tremors would die down, but not in a mere 800 years.
00:25:55So is the moon still quivering from that impact?
00:25:58The Apollo astronauts emplaced arrays of special mirrors on the moon.
00:26:03Reflectors made by French scientists were also put on the moon by the Soviet Lunokhod vehicles.
00:26:08When a laser beam from Earth strikes one of these mirrors and bounces back, the round trip travel time can be measured.
00:26:15At the MacDonald Observatory of the University of Texas, a laser beam is prepared for firing at the reflectors on the moon,
00:26:23three hundred and eighty thousand kilometers away.
00:26:26By multiplying the travel time by the speed of light, the distance to that spot on the moon can be determined to a precision of seven to ten centimeters.
00:26:36The width of a hand.
00:26:42When such measurements are repeated over a period of years, even an extremely slight wobble in the moon's motion can be determined.
00:26:51The accuracy is phenomenal.
00:26:54The error is much less than one millionth of a percent.
00:27:00The moon, it turns out, is gently swinging like a bell, just as if it had been hit by an asteroid less than a thousand years ago.
00:27:15There may be physical evidence in the age of space flight for the account of the Canterbury monks in the 12th century.
00:27:22If 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.
00:27:37In billions of years, lunar rays are eroded, but not in hundreds.
00:27:41And there is a recent ray crater called Giordano Bruno, in exactly the region of the moon where an explosion was reported in 1178.
00:27:50The entire evolution of the moon is a story of catastrophes.
00:28:02Four and a half billion years ago, the moon was accreting from interplanetary boulders and craters were forming all over its surface.
00:28:09The energy so released helped melt the crust.
00:28:12After most of this debris was swept up by the moon, the surface cooled.
00:28:17But about 3.9 billion years ago, a great asteroid impacted.
00:28:24It generated an expanding shockwave and remelted some of the surface.
00:28:32The resulting basin was then flooded, probably by dark lava, producing one of the dry seas on the moon.
00:28:39More recent impacts excavated craters with bright rays named after Eratosthenes and Copernicus.
00:28:46The familiar features of the man in the moon are a chronicle of ancient impacts.
00:28:52Most of the original asteroids were swept up in the making of the moon and planets.
00:28:59Many still orbit the sun in the asteroid belt.
00:29:02Some, themselves almost fractured by gravity tides and by impacts with other asteroids,
00:29:08have been captured by planets, Phobos around Mars, for example.
00:29:12Or a close moon of Jupiter called Amalthea.
00:29:17Somewhat similar to the asteroid belt are the rings of Saturn,
00:29:21composed of millions of small, tumbling, icy moonlets.
00:29:26Maybe the rings of Saturn are a moon which was prevented from forming by the tides of Saturn.
00:29:33Or maybe it's the remains of a moon that wandered too close and was torn apart by the tides of Saturn.
00:29:40It's certainly a lovely place.
00:29:43Jupiter also has a newly discovered ring system which is invisible from the Earth.
00:29:49Now, there is a curious argument alleging major recent collisions in the solar system proposed by a psychiatrist named Emmanuel Velikovsky in 1950.
00:30:07He suggested that an object of planetary mass, which he called a comet, was somehow produced in the Jupiter system.
00:30:18He doesn't say exactly how it's produced, but maybe it's spat out of Jupiter.
00:30:34Anyway, however it was made some 3500 years ago, he imagines, it made repeated close encounters with Mars,
00:30:46with the Earth-Moon system, having as entertaining biblical consequences the parting of the Red Sea,
00:30:57so that Moses and the Israelites could safely avoid the host of Pharaoh,
00:31:02and the stopping of the Earth's rotation at the moment that Joshua commanded the sun to stand still in Gibeon.
00:31:10He also imagined that there was extensive flooding and volcanoes all over the Earth at that time.
00:31:16Well, then after a very complicated game of interplanetary billiards is completed,
00:31:25Velikovsky proposed that this comet entered into a stable, almost perfectly circular orbit,
00:31:34becoming the planet Venus, which he claimed never existed until then.
00:31:45Now, these ideas are almost certainly wrong.
00:31:50There is no objection in planetary astronomy to collisions.
00:31:53We've seen collision fragments and evidence throughout the solar system.
00:31:59The problem is with recent and major collisions.
00:32:03In any scale model of the solar system like this, it's impossible to have both the sizes of the planets
00:32:09and the sizes of their orbits to the same scale, because then the planets would be too small to see.
00:32:15If the planets were really to scale in such a model as grains of dust,
00:32:21it would then be entirely clear that a comet entering the inner solar system
00:32:26would have a negligible chance of colliding with a planet in only a few thousand years.
00:32:32Moreover, Venus is a rocky and metallic hydrogen-poor world,
00:32:39whereas Jupiter, the place that Velikovsky imagines it comes from,
00:32:43is made of almost nothing but hydrogen.
00:32:46There's no energy source in Jupiter to eject planets or comets.
00:32:51If one did enter the inner solar system, there is no way it could stop the Earth from rotating,
00:32:58and if it could, there's no way the Earth could start up-rotating again at anything like 24 hours a day.
00:33:04There's no geological evidence for flooding and volcanism 3,500 years ago.
00:33:10Babylonian astronomers observed Venus in its present stable orbit before Velikovsky said it existed.
00:33:19And so on.
00:33:27There are many hypotheses in science which are wrong. That's perfectly right.
00:33:31It's the aperture to finding out what's right. Science is a self-correcting process.
00:33:36To be accepted, new ideas must survive the most rigorous standards of evidence and scrutiny.
00:33:43The worst aspect of the Velikovsky affair is not that many of his ideas were wrong or silly or in gross contradiction to the facts.
00:33:52Rather, the worst aspect is that some scientists attempted to suppress Velikovsky's ideas.
00:33:59The suppression of uncomfortable ideas may be common in religion or in politics, but it is not the path to knowledge.
00:34:09And there's no place for it in the endeavor of science.
00:34:12We do not know beforehand where fundamental insights will arise from about our mysterious and lovely solar system.
00:34:22And the history of our study of the solar system shows clearly that accepted and conventional ideas are often wrong.
00:34:31And that fundamental insights can arise from the most unexpected sources.
00:34:37We've evolved on the planet Earth, and so we find it a congenial place.
00:34:43But just next door is Venus, until recently enveloped in mystery.
00:34:48It has almost the same size and mass as the Earth.
00:34:52Might our sister world be a balmy summer planet, a little warmer than the Earth because it's a little closer to the sun?
00:35:00Are there craters, volcanoes, mountains, oceans, life?
00:35:05The first person to look at Venus through a telescope was Galileo in 1609.
00:35:10But all he could see was a featureless disk.
00:35:13And as optical telescopes got bigger, that's all anybody could see.
00:35:17A disk with no details on it at all.
00:35:20Venus evidently was covered with an opaque layer, thick clouds concealing the surface.
00:35:27For centuries, even the composition of the clouds of Venus was unknown.
00:35:32I mean, you could go outside, look up, see Venus with the naked eye, observe sunlight reflected from the clouds of Venus.
00:35:40What were you looking at? What were the clouds made of? Nobody knew.
00:35:44As a result, imagination ran riot.
00:35:48The absence of anything you could see on Venus led some scientists and others to deduce that the surface was a swamp.
00:35:59The arguments, if we can dignify it with such a phrase, went something like this.
00:36:05I can't see a thing on the surface of Venus. Why not? Because it's covered with a dense layer of clouds.
00:36:11Well, what are clouds made of? Water, of course. Therefore, Venus must have an awful lot of water on it.
00:36:17Therefore, the surface must be wet. Well, if the surface is wet, it's probably a swamp.
00:36:21If there's a swamp, there's ferns. If there's ferns, maybe there's even dinosaurs.
00:36:27Observation. You couldn't see a thing.
00:36:29Conclusion. Dinosaurs.
00:36:32Well, if just looking at Venus was so unproductive, what else could you do?
00:36:37The next clue came from early work with that, a glass prism.
00:36:42An intense beam of ordinary white light is made to pass through a narrow slit and then through the prism.
00:36:48The result is to spread the white light out into its constituent rainbow of colors.
00:36:54This rainbow pattern is called a spectrum. Think about it. White light enters the prism.
00:37:02What comes out of the prism is colored light. Lots of colors. Where did they come from?
00:37:07They must have been hiding in the white light. White light must be a mixture of many colors.
00:37:12Here we see the spectrum running from violet, blue, green, yellow, orange to red.
00:37:19Since we see these colors, we call this the spectrum of visible light.
00:37:24The sun emits lots of visible light. The air is transparent to it.
00:37:29So our eyes evolve to work in visible light.
00:37:32But there are many other frequencies of light which our eyes can't detect.
00:37:35Beyond the violet is the ultraviolet. It's just as real, but you need instruments to detect it.
00:37:41Beyond the ultraviolet are the x-rays and then the gamma rays.
00:37:45On the other side of visible light, beyond the red, is the infrared. Again real, again invisible.
00:37:50Beyond the infrared are the radio waves.
00:37:54Now this entire range from the gamma rays way over there to the radio waves all the way over here
00:38:00are simply different kinds of light. They differ only in the frequency.
00:38:05They're all useful, by the way, in astronomy.
00:38:08But because of the limitations of our eyes, we have a prejudice, a bias, a chauvinism to this tiny rainbow band of visible light.
00:38:20Now, a spectrum can be used in a simple and elegant way to determine the chemical composition of the atmosphere of some distant planet or star.
00:38:29Different atoms and molecules absorb different frequencies or colors of light.
00:38:34And those absorbed or missing frequencies appear as black lines in the spectrum of the light we receive from the planet or star.
00:38:43Each and every substance has a characteristic fingerprint, a spectral signature, which permits it to be detected over a great distance.
00:38:53As a result, the gases in the atmosphere of Venus, at a distance of 60 million kilometers, have been determined, their composition's been determined from the Earth.
00:39:04It's amazing to me still. We can tell what a thing is made out of at an enormous distance away without ever touching.
00:39:14Our eyes can't see in the near-infrared part of the spectrum, but our instruments can.
00:39:20Here's the absorption pattern of lots and lots of carbon dioxide.
00:39:24Dark lines in characteristic patterns at specific frequencies.
00:39:28You'd detect a different set of infrared lines if, say, water vapor were present.
00:39:35If Venus were really soaking wet, then you should be able to determine that by finding the pattern of water vapor in its atmosphere.
00:39:44But around 1920, when this experiment was first performed, it was found that the Venus atmosphere seemed to have not a hint, not a smidgen, not a trace of water vapor above the clouds.
00:39:54And 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.
00:40:07But later, spectroscopic observations revealed the characteristic absorption lines of an enormous amount of carbon dioxide.
00:40:15So, some scientists thought there must be lots of carbon compounds on the surface, making this a planet covered with petroleum.
00:40:24Others agreed that the atmosphere was dry, but thought the surface was wet.
00:40:28With all that CO2, it had to be carbonated water.
00:40:31Venus, they thought, was covered with a vast ocean of seltzer.
00:40:35Now, the first hint of the true situation on Venus came not from the visible or the ultraviolet or the infrared part of the spectrum, but from over here in the radio region.
00:40:46We're used to the idea of radio signals from intelligent life, or at least semi-intelligent life, I mean, radio and television stations.
00:40:54But there are all kinds of reasons why natural objects should emit radio waves.
00:40:58One reason is that they're hot.
00:41:01And 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.
00:41:13But the real demonstration that the surface of Venus was astonishingly hot came when the first spacecraft penetrated the obscuring clouds of Venus and slowly settled on the surface of the nearest planet.
00:41:31These were the unmanned spacecraft of the Soviet Venera series.
00:41:38In our spaceship of the imagination, we retrace their course.
00:41:45From a distance, our sister planet seems serene and peaceful, its clouds motionless.
00:41:55These clouds are near the top of a great ocean of air, about a hundred kilometers thick, composed mainly of carbon dioxide.
00:42:07There's some nitrogen, a little water vapor and other gases, but only the merest trace of hydrocarbon.
00:42:13And the clouds turn out to be, not water, but a concentrated solution of sulfuric acid.
00:42:20Even in the high clouds, Venus is a thoroughly masky place.
00:42:34The clouds are stained yellow by sulfur.
00:42:46There are great lightning storms.
00:42:48As we descend, there are increasing amounts of the noxious gas sulfur dioxide.
00:42:53The pressure has become so high that early Venera spacecraft were crushed like old tin cans by the weight of the surrounding atmosphere.
00:43:05Beneath the clouds, in the dense, clear air, it's about as bright as on an overcast day on Earth.
00:43:10But the atmosphere is so thick that the ground seems to ripple and distort.
00:43:16The atmospheric pressure down here is 90 times that on Earth.
00:43:20The temperature is 380 degrees centigrade, 900 Fahrenheit.
00:43:25Hotter than the hottest household oven.
00:43:28This is a world marked by searing heat, crushing pressures, sulfurous gases, and a desolate, reddish landscape.
00:43:37Far from the balmy paradise imagined by some early scientists,
00:43:41Venus is the one place in the solar system most like hell.
00:43:47But today, as in ancient tradition, there are travelers who will dare a visit to the underworld.
00:43:59Venera 9 was the first spacecraft in human history to return a photograph from the surface of Venus.
00:44:05It found the rocks curiously eroded, perhaps by the corrosive gases,
00:44:10perhaps because the temperature is so high that the rocks are partly molten and sluggishly flow.
00:44:16The Soviet Venera spacecraft, their electronics long ago fried, are slowly corroding on the surface of Venus.
00:44:25They are the first spaceships from Earth ever to land on another planet.
00:44:35The reason Venus is like hell seems to be what's called the greenhouse effect.
00:44:40Ordinary visible sunlight penetrates the clouds and heats the surface.
00:44:44But the dense atmosphere blankets the surface and prevents it from cooling off to space.
00:44:49An atmosphere 90 times as dense as ours, made of carbon dioxide, water vapor, and other gases,
00:44:55lets in visible light from the sun, but will not let out the infrared light radiated by the surface.
00:45:01So the temperature rises until the infrared radiation trickling out to space just balances the sunlight reaching the surface.
00:45:09The greenhouse effect can make an Earth-like world into a planetary inferno.
00:45:21In this cauldron, there is not likely to be anything alive, even creatures very different from us.
00:45:26Organic and other conceivable biological molecules would simply fall to pieces.
00:45:33The hell of Venus is in stark contrast with the comparative heaven of its neighboring world, our little planetary home, the Earth.
00:45:54Here, the atmosphere is 90 times thinner.
00:46:01Here, the carbon dioxide and water vapor make a modest greenhouse effect,
00:46:06which heats the ground above the freezing point of water.
00:46:09Without it, our oceans would be frozen solid.
00:46:13A little greenhouse effect is a good thing.
00:46:30But Venus is an ominous reminder that on a world rather like the Earth, things can go wrong.
00:46:37There is no guarantee that our planet will always be so hospitable.
00:46:42To maintain this clement world, we must understand it and appreciate it.
00:46:51The Earth is a place to our eyes more beautiful than any other that we know.
00:46:57But this beauty has been sculpted by change.
00:47:01Gentle, almost indetectable change, and sudden, violent change.
00:47:06In the cosmos, there is no refuge from change.
00:47:11The Sphinx, human head, lion's body, constructed more than 5,500 years ago.
00:47:18That face was once crisp and cleanly rendered like this paw I'm standing on.
00:47:23The paw has been buried in the sand until recently and protected from erosion.
00:47:29The face is now muddled and softened because of thousands of years of sandblasting in the desert and a little rainfall.
00:47:39In New York City, there is an obelisk called Cleopatra's Needle.
00:47:43It comes from Egypt.
00:47:45In only a little more than a century, in New York's Central Park, the inscriptions on that obelisk have been almost totally obliterated.
00:47:54Not by sand and water, but by smog and industrial pollution, a little bit like the atmosphere of Venus.
00:48:02Slow erosion wipes out information.
00:48:06On the Earth, mountain ranges are destroyed by erosion in maybe tens of millions of years.
00:48:13Small impact craters in maybe hundreds of thousands of years.
00:48:17And the greatest artifacts of human beings in thousands or tens of thousands of years.
00:48:25In addition to such slow and uniform processes, there are rare but sudden catastrophes.
00:48:32The Sphinx is missing a nose.
00:48:35In an act of idle desecration, some soldiers once shot it off.
00:48:40If you wait long enough, everything changes.
00:48:44Slow, uniform processes, unheralded events, the sting of a sand grain, the fall of a drop of water,
00:48:59can over the ages totally rework the landscape.
00:49:15And rare violent processes, exceptional events that will not recur in a lifetime, also make major changes.
00:49:44Both the insignificant and the extraordinary are the architects of the natural world.
00:50:12This is all about the rocks.
00:50:15When the paint is on the right of the ground.
00:50:17It is nearly as low as the ambassadors.
00:50:19The paint is a large village.
00:50:21It is no longer possible of the stars, but the stars across the ocean.
00:50:24It is always possible of the stars.
00:50:29The seas of the earth, one of the flavors.
00:50:31With the trees, one of the forties, seven undeniable, a стро of a passer.
00:50:36The destruction of trees and grasslands makes the surface of the earth brighter.
00:51:03It reflects more sunlight back to space and cools our planet.
00:51:09After we discovered fire, we began to incinerate forests intentionally to clear the land by
00:51:15a process called slash-and-burn agriculture.
00:51:19And today, forests and grasslands are being destroyed frivolously, carelessly, by humans
00:51:28who are heedless of the beauty of our cousins, the trees, and ignorant of the possible climatic
00:51:35catastrophes which large-scale burning of forests may bring.
00:51:46The indiscriminate destruction of vegetation may alter the global climate in ways that
00:51:51no scientist can yet predict.
00:51:55It has already deadened large patches of the earth's life-supporting skin.
00:52:12And yet, we ravage the earth at an accelerated pace, as if it belonged to this one generation,
00:52:19as if it were ours to doeth as we please.
00:52:23The earth has mechanisms to cleanse itself, to neutralize the toxic substances in its system.
00:52:37But these mechanisms work only up to a point.
00:52:40Beyond some critical threshold, they break down.
00:52:44The damage becomes irreversible.
00:52:46The damage becomes irreversible.
00:53:14Our generation must choose.
00:53:17Which do we value more?
00:53:19Short-term profits or the long-term habitability of our planetary home?
00:53:28The world is divided politically.
00:53:30But 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.
00:53:42We have uncovered other worlds with choking atmospheres and deadly surfaces.
00:53:47Shall we then recreate these hells on earth?
00:53:53We have encountered desolate moons and barren asteroids.
00:53:57Shall we then scar and crater this blue-green world in their likeness?
00:54:12Natural catastrophes are rare.
00:54:23But they come often enough.
00:54:25We need not force the hand of nature.
00:54:29If we ruin the earth, there is no place else to go.
00:54:43This is not a disposable world.
00:54:46And we are not yet able to re-engineer other planets.
00:54:50The coolest 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, freezing out all its water, locking it in a perpetual ice age.
00:55:20Human activities brighten our landscape and our atmosphere.
00:55:25Might this ultimately make an ice age here?
00:55:28At 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.
00:55:57The 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.
00:56:07They 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.
00:56:21There are worlds that began with as much apparent promise as Earth.
00:56:26But 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?
00:56:54Most of the moons in the outer solar system have craters on them, made by cometary impacts.
00:57:00Some have such large craters, though, that if the impacting comets had been just a little bit bigger, the moons would have been shattered.
00:57:09What would the results of such a collision look like?
00:57:15Maybe 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.
00:57:34Some 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.
00:58:23Unless we change our ways.
00:58:25What do we have to do?
00:58:27Four things.
00:58:28One, 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.
00:58:44Three, reforestation on a grand scale.
00:58:47And 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.
00:59:01Now, no one has proposed that the trouble with Venus is that there once was Venusians who drove fuel inefficient cars.
00:59:08But our nearest neighbor, nevertheless, is a stark warning on the possible fate of an Earth-like world.
00:59:15This Typical
00:59:29If you need false reasons
00:59:30Comeстранic Finders
00:59:31Well, time today
00:59:34That's why.
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