Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay
Additional information on the bodies described here is available from links listed atthe end of this page.
Earth, as Sagan notes, is to us a lot like like Heaven, but nearby worlds fit ourconcepts 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 beinevitable in a million years.
For example, the Baraboo Range in Wisconsin consists of an ancient ridge of quartziteburied by marine sands during the Cambrian Period (about 550 million years ago). Onequarry has huge rounded boulders of quartzite up to 2-3 meters in size embedded inthe sandstone. To be rounded, the boulders had to be tossed around by water, but it wouldtake 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 occurredas often as they do in the Caribbean, there would have been about 250,000 duringthe time the rocks were being deposited. Even if hurricanes occurred only as often as theydo in New England, there would still have been 50,000. Hurricanes are infrequent ona 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'sto investigate what is now called the Tunguska Event of 1908. A huge blast burned andflattened trees for many kilometers, but left no crater. The lack of a crater has spawnedsuch exotic theories as antimatter, a mini-black hole, or a doomed spaceship (note thatSagan uses the term "other people" rather than "scientist" to describethese theorists.)
At the time Cosmos was produced, the idea that the event was due to the impact of alarge meteorite was in disfavor. Most scientists believed that the best explanation wasthe impact of a comet nucleus. The comet, made mostly of volatile materials, would releaseall its energy in the atmosphere and not form a crater.
Moving objects have energy, called kinetic energy. It takes energy to getsomething moving; that's why automobiles use a lot of fuel accelerating. When the objectstops, the kinetic energy is transformed into other forms: sound, mechanical effects, andheat. If you hammer vigorously on a large nail, you will hear sound, you will flatten thenail 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 kineticenergy would transform into heat instantly. Doing the math:
For one thing, a megaton, the unit of energy for large nuclear weapons, is defined as 5x 10^15 joules. That's the energy of a million tons of high explosives. The impact abovewould pack 90 megatons of energy. It would produce all the characteristics of alarge nuclear explosion except radiation. There would be a fireball, a blast wave, even amushroom cloud (the mushroom cloud occurs as the fireball rises and sucks smoke and dustup beneath it. All large explosions produce mushroom clouds, nuclear or not.) Thebarometric pulse from the shock wave was recorded across Europe and dust from the blastcircled the globe. But there was no crater.
An estimate of the pre-impact path of the object was a fairly good match to the orbitof a small comet called Encke's Comet. However, since Cosmos was produced, we have learneda lot more about impact processes. It now looks as if comets are too fragile to getthat close to the surface. For one thing, they are not nearly as dense or sturdy as solidice. For another, the stresses of a high-speed atmospheric entry are enormous; in humanterms, like falling off a tall building. The consensus now is the Tunguska object wasprobably a small fragile stony meteorite that fragmented and vaporized a few kilometersabove 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 satellitedata show that about once a month, Earth is hit by a meteor capable of releasing energyequivalent to a small nuclear weapon, say a kiloton or so. These bodies fragment in theatmosphere as mini-Tunguskas, showing up as fireballs that momentarily rival the Sun.There are enough differences between meteor impacts and nuclear blasts that normalmilitary intelligence can tell them apart. But in a tense situation, or if command andcommunications 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 theyare not even the same kind of idea. Catastrophism already assumes that certainkinds of events have occurred, and therefore the standard of proof for a catastrophe is not very high. Uniformitarianism uses cause and effect and the physicalevidence to try to determine what events occurred. The idea that most events on the Earthare gradual is a finding, based on the physical record, not an initial assumption. Nothingin uniformitarianism rules out catastrophes provided the physical evidence supports it(and real catastrophes leave absolutely unmistakable physical evidence.) A scientist whouses the physics of present day nuclear explosions to understand ancient meteor impacts isa 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 in1997, last visited the inner Solar System 4,200 years ago.
Because of their unpredictability, comets were long considered portents of disaster. Abrilliant 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 beyondhuman control, change almost never meant good news. If the best you can hope for is baresurvival, change is to be feared.
Comets probably formed in the vicinity of Uranus and Neptune and were flung intoextremely elongated orbits by encounters with the planets. On one of their rare visits tothe inner Solar System they may encounter a planet and be flung into an even more distantorbit, or possibly have their period shortened (Hale-Bopp's period was shortened byJupiter to 2,400 years.) Some comets have their periods shortened so much that they spendall their time in the Solar System.
Once captured, short-period comets are doomed. At each return gases boil off as thecomet 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 lightitself sweep the gases away to form the tail. The tail of a comet always points awayfrom the Sun. Halley's Comet has not always been in its present orbit (butcalculations have so far not revealed when it might have been captured) and eventually itwill either evaporate completely or cease to emit gases
One of the most famous of these regular visitors has returned every 75 or 76 yearssince at least 240 B.C. It has been recorded at every apparition since then. The firstknown drawing of it dates from 684. In 1066 it appeared just before the Norman invasion ofBritain and was recorded in the Bayeux Tapestry. In 1301 it was seen in Europe andincorporated into Giotto di Bondone's Nativity painting; the first fairly accurate pictureof a comet. It was noted in 1682. In 1705 Edmund Halley began calculating the orbits ofcomets, 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 andsuggested they were actually the same comet. If so, he predicted it would reappear in 1758or 1759. It did, though he didn't live to see it, and ever since then has borne the nameHalley's Comet.
Halley's Comet appeared in 1835, the year Mark Twain was born. He was fond of saying hecame in on Halley's Comet and would go out on it, and he did when it next appeared in1910. In that year, cyanogen (CN) was detected in the tail of Halley's Comet, which theEarth was due to pass through. Cyanogen is poisonous but the gases of the tail are theequivalent of evaporating a teaspoon of water and spreading the vapor through the GrandCanyon, and the Earth passed through the tail without harm. However, in addition to someholdovers of ancient comet fears, people were frightened by the cyanogen scare, andhucksters took advantage.
Modern comet hysteria has a parallel in the "Grand Alignments" of the planetsin 1962 (all five naked-eye planets were visible near the Sun during a total eclipse) and1982 (all nine planets within a 60-degree sector). Grand alignments seem to be connectedto events on earth: dull news days. How many times must such predictions fail beforepeople stop taking them seriously?
The 1985-86 apparition of Halley's Comet was disappointing to the general public. Thecomet was unusually far from Earth and dim, and in the light-polluted skies of most urbanareas it was invisible to the unaided eye. But for the first time the comet was visited byspacecraft: the Giotto probe of the European Space Agency, two Soviet Vega craft, and aJapanese probe. The apparition in 2061 should be much better, that in 2134 should passquite close to Earth and be very spectacular
Comet Kohoutek in 1973 was a good illustration of why astronomers cringe when the mediaannounces a bright comet. The press hype predicted a comet to rival Hale-Bopp, but thecomet 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'sComet 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 summerof 1178 by Gerves of Canterbury. Laser reflectors left on the Moon by Apollo astronautsare used for studies of earth-moon tidal interactions, but also can detect oscillationsdue 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 reportedhis strange events.
The laser reflection as seen in the video is simulated; the actual laser reflectionsare 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 cornerof 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 thatreason. Crude versions are used in bicycle reflectors because they relect headlights backto the source, wherever it is.
The spectacular collision of Comet Schumacher-Levy with Jupiter in 1993 was the firsttime a large planetary impact was ever observed. Had it happened a few decades earlier, wewould have lacked the technology to observe it properly and might not have even known ithappened.
The Solar System formed by accretion of small bodies into larger ones. Essentially, itformed by repeated impacts. Computer simulations show that when planets accrete, theydon't form a small number of large planets. Instead, hundreds of Moon- to Mars-sizedobjects form, and then those collide. Thus, the final stages of planetary accretion seemto 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 SolarSystem, 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 fartherout, frozen methane and ammonia.
Asteroids are leftover bits of inner Solar System material that never accreted into aplanet; comets are the outer Solar System equivalent. Meteors and meteorites are smallbits 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 theEarth's equatorial plane like most other satellites, but actually has its orbit prettymuch in the plane of the rest of the Solar System. Fission also suffers from the flaw thatyou simply can't fling something off the surface of the Earth into orbit. Studies of lunarsamples 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 asuccessful capture are very stringent, making such an event very unlikely.
A fourth theory has emerged in the last two decades: mega-impact. In thistheory, proto-Earth was hit a grazing blow by a Mars-sized object. An impact is a loteasier to arrange than a capture! This theory accounts for the plane of the Moon's orbitand its different composition from Earth.
The violence of such an event defies belief. It would take an hour or so for thecollision 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 andkilled by the radiant heat. For perhaps 10,000 years the Earth would have an atmosphere ofvaporized rock at a temperature close to the surface of the Sun.
Mega-impacts seem capable of explaining some other Solar System anomalies: the largecore of Mercury, the slow backward rotation of Venus, and perhaps the strange axial tiltof Uranus.
Most viewers of Cosmos are perplexed by the strange digression into this characterVelikovsky. Who was he, and what's the point?
Immanuel Velikovsky was born in Russia and trained as a psychologist under Freud. Hebecame convinced that myths of catastrophes in many cultures reflected great globalcatastrophes in early historic times and were responsible among other things for theaccounts of the plagues of Egypt, the parting of the Red Sea, and the Sun standing stillfor 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 provethe historical truth of the events in Exodus leading to the Israelite occupation ofPalestine in the Old Testament.
Velikovsky's book was published as Worlds in Collision in 1950. Here's thepart of the "suppression" account you don't hear. The book was originallyaccepted 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 hisbusiness elsewhere if Macmillan published the book. Macmillan dumped the book, (and theeditor who had accepted it) and it was picked up by Doubleday, which had an instantbest-seller.
What else was going on in America in 1950? The McCarthy Era. Shapley had come underfire from McCarthy's committee and he regarded Velikovsky as just another element in asociety-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 tobe behaving irresponsibly or unethically, especially if the author's reputation is goingto be used in the process? In effect, Shapley's reputation as an astronomer would havebeen used to sell Velikovsky's work. Imagine, for example, that Colin Powell's publisheraccepted a virulently racist book and hyped it in the same ads as Powell's book. Wouldn'tPowell have the right to change publishers? Actually, publishers have found crank scienceso 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 lifeas a cult figure. It probably would have done no harm to let him publish. Likewise, youhave to wonder exactly what the art community of turn of the century Vienna accomplishedby not admitting Adolf Hitler to art school. But sooner or later, some other cult leaderwill 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. Sooneror later, someone will have to say no.
From the late 1960's into the 1970's, Velikovsky's ideas enjoyed a resurgence when hissupporters claimed that many of Velikovsky's predictions had been borne out by planetaryexploration. In particular, Velikovsky had predicted Venus would be hot and Jupiter wouldemit radio waves. Venus is hot, but not because of a recent catastrophe, but because ofits atmosphere. Jupiter emits radio waves, but by a physical mechanism completelyunforeseen by Velikovsky (or anyone else at the time.) In 1976, the American Associationfor the Advancement of Science held a symposium featuring Velikovsky and many of hiscritics, 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. Theinvisibility of its surface gave rise to fanciful speculations. Spectroscopic measurementsfrom Earth had shown that Venus was very dry by the 1920's and that Venus was too hot forterrestrial life in 1956.
A casual reader of popular media would have known none of this at the time. Right up untilMariner 2 flew past Venus in 1962 and drove the final nail in the coffin of Venus asparadise, the popular media published speculations about life on Venus. Long after sciencehad conclusive evidence that Venus was hostile, the media simply ignored the data andcontinued to speculate, because the speculation was more exciting. Exactly the samething 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 raisingthe 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 OldTestament references traditionally used to support the idea of Lucifer being expelled fromHeaven could also be astronomical allusions to the planet Venus.
The Soviet Union, in a superb technical achievement, landed a series of spacecraft ofthe Venera series on Venus, returning pictures and other data for about an hour beforesuccumbing 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 ElectromagneticSpectrum. In order of decreasing wavelength, the electromagnetic spectrum includesradio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.Sound waves, which are mechanical disturbances transmitted by atoms in a material (likeair) are not electromagnetic. They are completely different kinds of waves.
Electromagnetic radiation is produced by many processes, but two of the most importantmechanisms are:
Objects that are very hot, like molten metal, a light-bulb filament, or the Sun, emitvisible light. Objects that are less hot emit infrared or radio waves. The pattern ofwavelengths 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 theboiling point of water.
Atoms and molecules can emit radiation. On the other hand, when radiation passesthrough a material, it can also absorb specific wavelengths of radiation. The infraredwavelengths 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 extensivecraters and lava plains, and earth-like on the inside with a large core. It has a hugeimpact 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 excerptsfrom 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 Renaissanceviolinist and composer whose works were so fiendishly difficult that the legend arose hehad sold his soul to the devil for perfection in his art (and a beautiful woman - whyshould 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 anddamnation, have obvious parallels to some of our environmental negligence.
Earth has a small greenhouse effect, due partially to carbon dioxide but mostly towater vapor. 90 percent of Earth's greenhouse effect is due to water vapor. Without agreenhouse effect, the Earth would be below freezing. As Sagan notes, "a littlegreenhouse 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 differentatmospheres 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 requireliquid water. If this process ever operated on Venus, it failed to prevent a runawaygreenhouse effect. Once the oceans evaporated, the runaway could continue unchecked.
On Mars, the small mass of the planet probably meant that atmospheric gases couldescape into space, so that Mars, even with an atmosphere of carbon dioxide, does not haveenough of a greenhouse effect to warm it appreciably.
Complicating matters is the fact that the Sun has brightened by about 30 percent overthe age of the Solar System, so that early Earth must have had a much thicker atmosphereto trap heat and keep its surface above freezing. The Sun is still brightening, and in afew billion years, even with no greenhouse effect, it will reach the boiling point ofwater. Carbonate rocks will break down, and combined with the water vapor in theatmosphere, will create a runaway greenhouse effect on Earth.
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Created 13 January 1998, Last Update 18 March 1998