Additional information on the bodies described here is available from links listed at the end of this page.
Earth, as Sagan notes, is to us a lot like like Heaven, but nearby worlds fit our concepts of hell. Processes on the planets are both small-scale and gradual, or vast, sudden, and infrequent. Time scale is important; something rare in a human lifetime may be inevitable in a million years.
For example, the Baraboo Range in Wisconsin consists of an ancient ridge of quartzite buried by marine sands during the Cambrian Period (about 550 million years ago). One quarry has huge rounded boulders of quartzite up to 2-3 meters in size embedded in the sandstone. To be rounded, the boulders had to be tossed around by water, but it would take waves over 10 meters high to do that, waves comparable to those in a hurricane.
During the Cambrian Period, Wisconsin was at tropical latitudes. If hurricanes occurred as often as they do in the Caribbean, there would have been about 250,000 during the time the rocks were being deposited. Even if hurricanes occurred only as often as they do in New England, there would still have been 50,000. Hurricanes are infrequent on a human time scale, common on the geologic time scale.
Cosmos shows rare footage of the Siberian expeditions by L.A. Kulik in the late 1920's to investigate what is now called the Tunguska Event of 1908. A huge blast burned and flattened trees for many kilometers, but left no crater. The lack of a crater has spawned such exotic theories as antimatter, a mini-black hole, or a doomed spaceship (note that Sagan uses the term "other people" rather than "scientist" to describe these theorists.)
At the time Cosmos was produced, the idea that the event was due to the impact of a large meteorite was in disfavor. Most scientists believed that the best explanation was the impact of a comet nucleus. The comet, made mostly of volatile materials, would release all its energy in the atmosphere and not form a crater.
Moving objects have energy, called kinetic energy. It takes energy to get something moving; that's why automobiles use a lot of fuel accelerating. When the object stops, the kinetic energy is transformed into other forms: sound, mechanical effects, and heat. If you hammer vigorously on a large nail, you will hear sound, you will flatten the nail and feel the impact in your arm, and the nail will get warm, perhaps even hot.
A comet head entering the atmosphere would vaporize above the surface, and its kinetic energy would transform into heat instantly. Doing the math:
For one thing, a megaton, the unit of energy for large nuclear weapons, is defined as 5 x 10^15 joules. That's the energy of a million tons of high explosives. The impact above would pack 90 megatons of energy. It would produce all the characteristics of a large nuclear explosion except radiation. There would be a fireball, a blast wave, even a mushroom cloud (the mushroom cloud occurs as the fireball rises and sucks smoke and dust up beneath it. All large explosions produce mushroom clouds, nuclear or not.) The barometric pulse from the shock wave was recorded across Europe and dust from the blast circled the globe. But there was no crater.
An estimate of the pre-impact path of the object was a fairly good match to the orbit of a small comet called Encke's Comet. However, since Cosmos was produced, we have learned a lot more about impact processes. It now looks as if comets are too fragile to get that close to the surface. For one thing, they are not nearly as dense or sturdy as solid ice. For another, the stresses of a high-speed atmospheric entry are enormous; in human terms, like falling off a tall building. The consensus now is the Tunguska object was probably a small fragile stony meteorite that fragmented and vaporized a few kilometers above the surface. A comet would have fragmented tens of kilometers above the surface; dramatic, but not the Tunguska event.
Could an impact trigger nuclear war? Recently declassified Defense Department satellite data show that about once a month, Earth is hit by a meteor capable of releasing energy equivalent to a small nuclear weapon, say a kiloton or so. These bodies fragment in the atmosphere as mini-Tunguskas, showing up as fireballs that momentarily rival the Sun. There are enough differences between meteor impacts and nuclear blasts that normal military intelligence can tell them apart. But in a tense situation, or if command and communications break down, it could get dicey.
A central theme of this episode is the contrast between two world views:
Catastrophism and Uniformitarianism are often treated as opposites, but in reality they are not even the same kind of idea. Catastrophism already assumes that certain kinds of events have occurred, and therefore the standard of proof for a catastrophe is not very high. Uniformitarianism uses cause and effect and the physical evidence to try to determine what events occurred. The idea that most events on the Earth are gradual is a finding, based on the physical record, not an initial assumption. Nothing in uniformitarianism rules out catastrophes provided the physical evidence supports it (and real catastrophes leave absolutely unmistakable physical evidence.) A scientist who uses the physics of present day nuclear explosions to understand ancient meteor impacts is a uniformitarian, not a catastrophist.
Comets are small solid masses mostly made of water ice and frozen ammonia and methane. They travel extremely elongated elliptical orbits. Comet Hale-Bopp, so spectacular in 1997, last visited the inner Solar System 4,200 years ago.
Because of their unpredictability, comets were long considered portents of disaster. A brilliant comet in 1517 persuaded the Aztec ruler Montezuma that disaster was imminent, helping to lay the stage for Cortez' conquest. In a world where most things were beyond human control, change almost never meant good news. If the best you can hope for is bare survival, change is to be feared.
Comets probably formed in the vicinity of Uranus and Neptune and were flung into extremely elongated orbits by encounters with the planets. On one of their rare visits to the inner Solar System they may encounter a planet and be flung into an even more distant orbit, or possibly have their period shortened (Hale-Bopp's period was shortened by Jupiter to 2,400 years.) Some comets have their periods shortened so much that they spend all their time in the Solar System.
Once captured, short-period comets are doomed. At each return gases boil off as the comet is warmed by the Sun; Halley's comet lost several meters of material in 1986. Particles from the Sun (the "solar wind") plus the pressure exerted by light itself sweep the gases away to form the tail. The tail of a comet always points away from the Sun. Halley's Comet has not always been in its present orbit (but calculations have so far not revealed when it might have been captured) and eventually it will either evaporate completely or cease to emit gases
One of the most famous of these regular visitors has returned every 75 or 76 years since at least 240 B.C. It has been recorded at every apparition since then. The first known drawing of it dates from 684. In 1066 it appeared just before the Norman invasion of Britain and was recorded in the Bayeux Tapestry. In 1301 it was seen in Europe and incorporated into Giotto di Bondone's Nativity painting; the first fairly accurate picture of a comet. It was noted in 1682. In 1705 Edmund Halley began calculating the orbits of comets, on the assumption that the orbits were parabolas. He found one set of orbits, including the comet of 1682, all seemed to have the same orientation in space and suggested they were actually the same comet. If so, he predicted it would reappear in 1758 or 1759. It did, though he didn't live to see it, and ever since then has borne the name Halley's Comet.
Halley's Comet appeared in 1835, the year Mark Twain was born. He was fond of saying he came in on Halley's Comet and would go out on it, and he did when it next appeared in 1910. In that year, cyanogen (CN) was detected in the tail of Halley's Comet, which the Earth was due to pass through. Cyanogen is poisonous but the gases of the tail are the equivalent of evaporating a teaspoon of water and spreading the vapor through the Grand Canyon, and the Earth passed through the tail without harm. However, in addition to some holdovers of ancient comet fears, people were frightened by the cyanogen scare, and hucksters took advantage.
Modern comet hysteria has a parallel in the "Grand Alignments" of the planets in 1962 (all five naked-eye planets were visible near the Sun during a total eclipse) and 1982 (all nine planets within a 60-degree sector). Grand alignments seem to be connected to events on earth: dull news days. How many times must such predictions fail before people stop taking them seriously?
The 1985-86 apparition of Halley's Comet was disappointing to the general public. The comet was unusually far from Earth and dim, and in the light-polluted skies of most urban areas it was invisible to the unaided eye. But for the first time the comet was visited by spacecraft: the Giotto probe of the European Space Agency, two Soviet Vega craft, and a Japanese probe. The apparition in 2061 should be much better, that in 2134 should pass quite close to Earth and be very spectacular
Comet Kohoutek in 1973 was a good illustration of why astronomers cringe when the media announces a bright comet. The press hype predicted a comet to rival Hale-Bopp, but the comet failed to brighten as expected and was only faintly visible to the eye. To this day, "Comet Kohoutek" is a synonym for something that fizzles.
For information on Halley's Comet see Mankind's Comet, by Guy Ottewell and Fred Schaaf, Astronomical Workshop, Greenville, SC, 1985. The raft of books that came out on Halley's Comet in 1985 and 1986 fall into two classes: this one and all the others.
The most likely account of a historic impacts on the Moon is that related in the summer of 1178 by Gerves of Canterbury. Laser reflectors left on the Moon by Apollo astronauts are used for studies of earth-moon tidal interactions, but also can detect oscillations due to an impact. The Moon does appear to be oscillating, and there is a fresh crater, Giordano Bruno, just over the visible edge of the Moon in the area where Gerves reported his strange events.
The laser reflection as seen in the video is simulated; the actual laser reflections are too faint to see. The reflector need not be very precisely aligned, by the way. A "corner reflector", consisting of three perpendicular mirrors forming one corner of a box, will reflect any light ray back out along exactly the same path it came in on. Thus, corner reflectors need not be aligned and are used in laser ranging for just that reason. Crude versions are used in bicycle reflectors because they relect headlights back to the source, wherever it is.
The spectacular collision of Comet Schumacher-Levy with Jupiter in 1993 was the first time a large planetary impact was ever observed. Had it happened a few decades earlier, we would have lacked the technology to observe it properly and might not have even known it happened.
The Solar System formed by accretion of small bodies into larger ones. Essentially, it formed by repeated impacts. Computer simulations show that when planets accrete, they don't form a small number of large planets. Instead, hundreds of Moon- to Mars-sized objects form, and then those collide. Thus, the final stages of planetary accretion seem to be violent on a scale undreamed of a few decades ago.
The Solar System probably formed from the same material as the Sun. In the inner Solar System, it was hot enough that most of the lightest elements would have remained as gases, and the inner Solar System is dominated by worlds of silicon, magnesium, iron and oxygen. In the outer Solar System it was cold enough for frozen water to form, and still farther out, frozen methane and ammonia.
Asteroids are leftover bits of inner Solar System material that never accreted into a planet; comets are the outer Solar System equivalent. Meteors and meteorites are small bits of leftovers, or small pieces of asteroids knocked loose by impacts.
Until recently, there were three theories for the formation of the Moon:
Fission and co-creation suffer from the flaw that the Moon should be orbiting in the Earth's equatorial plane like most other satellites, but actually has its orbit pretty much in the plane of the rest of the Solar System. Fission also suffers from the flaw that you simply can't fling something off the surface of the Earth into orbit. Studies of lunar samples suggest that the Moon formed in a hotter part of the Solar System than the Earth. That plus the plane of the Moon's orbit argues for capture, but the requirements for a successful capture are very stringent, making such an event very unlikely.
A fourth theory has emerged in the last two decades: mega-impact. In this theory, proto-Earth was hit a grazing blow by a Mars-sized object. An impact is a lot easier to arrange than a capture! This theory accounts for the plane of the Moon's orbit and its different composition from Earth.
The violence of such an event defies belief. It would take an hour or so for the collision to occur, so things would look like they were happening in slow motion. However, if you were watching from a few Earth radii away you would be blinded by the light and killed by the radiant heat. For perhaps 10,000 years the Earth would have an atmosphere of vaporized rock at a temperature close to the surface of the Sun.
Mega-impacts seem capable of explaining some other Solar System anomalies: the large core of Mercury, the slow backward rotation of Venus, and perhaps the strange axial tilt of Uranus.
Most viewers of Cosmos are perplexed by the strange digression into this character Velikovsky. Who was he, and what's the point?
Immanuel Velikovsky was born in Russia and trained as a psychologist under Freud. He became convinced that myths of catastrophes in many cultures reflected great global catastrophes in early historic times and were responsible among other things for the accounts of the plagues of Egypt, the parting of the Red Sea, and the Sun standing still for Joshua at the battle of Gibeon.
Velikovsky was in some ways a Jewish fundamentalist. Exodus, not Genesis, was his obsession. He was an early adherent of the Zionist movement and he wanted to prove the historical truth of the events in Exodus leading to the Israelite occupation of Palestine in the Old Testament.
Velikovsky's book was published as Worlds in Collision in 1950. Here's the part of the "suppression" account you don't hear. The book was originally accepted by MacMillan, who wanted to publish it not as a trade book but as a textbook. Harlow Shapley, their best-selling astronomy author, was livid, and vowed to take his business elsewhere if Macmillan published the book. Macmillan dumped the book, (and the editor who had accepted it) and it was picked up by Doubleday, which had an instant best-seller.
What else was going on in America in 1950? The McCarthy Era. Shapley had come under fire from McCarthy's committee and he regarded Velikovsky as just another element in a society-wide attack on science and reason. He was determined to go down fighting.
Also, a lot has been made of Velikovsky's free-speech rights, but what about Shapley's? Doesn't an author have the right to withdraw his business from a publisher he considers to be behaving irresponsibly or unethically, especially if the author's reputation is going to be used in the process? In effect, Shapley's reputation as an astronomer would have been used to sell Velikovsky's work. Imagine, for example, that Colin Powell's publisher accepted a virulently racist book and hyped it in the same ads as Powell's book. Wouldn't Powell have the right to change publishers? Actually, publishers have found crank science so profitable that they wouldn't hesitate to dump an academically respected author today. If the Velikovsky affair happened today, Shapley, not Velikovsky, would have been dumped.
In retrospect, Velikovsky was helped by the controversy and spent the rest of his life as a cult figure. It probably would have done no harm to let him publish. Likewise, you have to wonder exactly what the art community of turn of the century Vienna accomplished by not admitting Adolf Hitler to art school. But sooner or later, some other cult leader will come along and demand not just publication and recognition, but research funds, inclusion of his theories in textbooks, control of scientific journals, and so on. Sooner or later, someone will have to say no.
From the late 1960's into the 1970's, Velikovsky's ideas enjoyed a resurgence when his supporters claimed that many of Velikovsky's predictions had been borne out by planetary exploration. In particular, Velikovsky had predicted Venus would be hot and Jupiter would emit radio waves. Venus is hot, but not because of a recent catastrophe, but because of its atmosphere. Jupiter emits radio waves, but by a physical mechanism completely unforeseen by Velikovsky (or anyone else at the time.) In 1976, the American Association for the Advancement of Science held a symposium featuring Velikovsky and many of his critics, including Sagan. That's how Velikovsky ended up in Cosmos. It is easily the worst editorial decision in Cosmos since the video doesn't give viewers enough context to understand what the issues are, and the discussion quickly became dated.
Venus is nearly as big as Earth in size and mass but totally covered by clouds. The invisibility of its surface gave rise to fanciful speculations. Spectroscopic measurements from Earth had shown that Venus was very dry by the 1920's and that Venus was too hot for terrestrial life in 1956.
A casual reader of popular media would have known none of this at the time. Right up until Mariner 2 flew past Venus in 1962 and drove the final nail in the coffin of Venus as paradise, the popular media published speculations about life on Venus. Long after science had conclusive evidence that Venus was hostile, the media simply ignored the data and continued to speculate, because the speculation was more exciting. Exactly the same thing happened with the controversy over "canals" on Mars.
Venus has an atmosphere 90 times denser than Earth's, made mostly of carbon dioxide. The carbon dioxide creates a runaway greenhouse effect, trapping solar heat and raising the surface temperature on Venus to 900 degrees F. The clouds are droplets of H2SO4 - sulfuric acid and form a layer about 100 kilometers thick. Far from a tropical paradise, Venus is uncannily close to our traditional picture of Hell. Intriguingly, the Old Testament references traditionally used to support the idea of Lucifer being expelled from Heaven could also be astronomical allusions to the planet Venus.
The Soviet Union, in a superb technical achievement, landed a series of spacecraft of the Venera series on Venus, returning pictures and other data for about an hour before succumbing to the heat. Electronics simply cannot function at the temperatures on Venus, so long-duration missions on the surface will require a substantial cooling system.
The varieties of electromagnetic radiation collectively make up the Electromagnetic Spectrum. In order of decreasing wavelength, the electromagnetic spectrum includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. Sound waves, which are mechanical disturbances transmitted by atoms in a material (like air) are not electromagnetic. They are completely different kinds of waves.
Electromagnetic radiation is produced by many processes, but two of the most important mechanisms are:
Objects that are very hot, like molten metal, a light-bulb filament, or the Sun, emit visible light. Objects that are less hot emit infrared or radio waves. The pattern of wavelengths emitted and the wavelength of strongest radiation are dictated by temperature. The discovery that Venus emits radio waves proved that it was quite hot - above the boiling point of water.
Atoms and molecules can emit radiation. On the other hand, when radiation passes through a material, it can also absorb specific wavelengths of radiation. The infrared wavelengths absorbed by carbon dioxide and water vapor are diagnostic, so we can tell, without going there, that Venus had abundant carbon dioxide but little if any water.
Not discussed in Cosmos, Mercury is Moon-like on the outside with extensive craters and lava plains, and earth-like on the inside with a large core. It has a huge impact basin, the Caloris Basin. Refer to the link below for additional information.
Cosmos received high praise for its use of classical music, both modern and older, and nowhere is the musical sophistication of the series better shown. The somber piano music during the scenes of environmental havoc is from Liszt's Totentanz (thanks to reader Lee Hodges for that information). This piece works in excerpts from an old liturgical chant, the Dies Irae or Day of Wrath, dealing with the final judgment and damnation of sinners. One English translation begins:
Day of wrath, that dreadful day
Shall all the world in ashes lay
The music is also reminiscent of Rachmaninoff's Rhapsody on a Theme by Paganini, which also uses the Dies Irae theme. Paganini was a Renaissance violinist and composer whose works were so fiendishly difficult that the legend arose he had sold his soul to the devil for perfection in his art (and a beautiful woman - why should Faust have all the fun?). Mindful of the legend, Rachmaninoff worked in the Dies Irae.
The themes of selling one's soul to the devil, resulting in final destruction and damnation, have obvious parallels to some of our environmental negligence.
Earth has a small greenhouse effect, due partially to carbon dioxide but mostly to water vapor. 90 percent of Earth's greenhouse effect is due to water vapor. Without a greenhouse effect, the Earth would be below freezing. As Sagan notes, "a little greenhouse effect is a good thing".
Some of the other issues noted:
Venus is too hot, Mars is too cold, and Earth is just right, yet with different atmospheres any of the three planets could be habitable, or uninhabitable.
Earth has almost as much carbon dioxide as Venus, but locked into carbonate rocks. Nowadays these are mostly created by biological activity, but early in Earth history, inorganic processes probably formed these rocks. However, the necessary reactions require liquid water. If this process ever operated on Venus, it failed to prevent a runaway greenhouse effect. Once the oceans evaporated, the runaway could continue unchecked.
On Mars, the small mass of the planet probably meant that atmospheric gases could escape into space, so that Mars, even with an atmosphere of carbon dioxide, does not have enough of a greenhouse effect to warm it appreciably.
Complicating matters is the fact that the Sun has brightened by about 30 percent over the age of the Solar System, so that early Earth must have had a much thicker atmosphere to trap heat and keep its surface above freezing. The Sun is still brightening, and in a few billion years, even with no greenhouse effect, it will reach the boiling point of water. Carbonate rocks will break down, and combined with the water vapor in the atmosphere, will create a runaway greenhouse effect on Earth.
For further information on topics covered above:
Return to Cosmos Index
Created 13 January 1998, Last Update 18 March 1998
Not an official UW Green Bay site