Cosmos: Chapter 4 - Heaven and Hell
This chapter has two main subjects. The first is comets -- what they are, where they come from, and how they have been interpreted by humans throughout history. The second subject is the planet Venus -- what its atmosphere is like, the techniques we have used to study it, and what Venus can teach us about potential changes to Earth's atmosphere.
Sagan begins the discussion on comets by describing the Tunguska Event, in which it is believed that a piece of a comet struck the Earth's atmosphere in central Siberia in 1908. The impact caused a huge forest fire and sent atmospheric shock waves around the world, but investigators were puzzled to find no foreign debris at the center of the blast site. This, they ultimately concluded, was because the comet disintegrated in the air before hitting the ground, thus leaving no impact crater.
Sagan explains that this is quite possible since comets are huge, dirty snowballs made up mostly of ice and dust. He explains further that similar, but much smaller, events happen all the time in the form of meteors, which are simply the dust particles left behind by a comet. When the Earth's orbit takes it through the trail of dust left behind by a comet, some of the dust streaks through the upper atmosphere and burns up from friction. Sagan calls this "a natural fireworks display, an entertainment in the heavens."
Sagan describes how comets were interpreted before we knew what we were observing:
"Comets have always evoked fear and awe and superstition. Their occasional apparitions disturbingly challenged the notion of an unalterable and divinely ordered Cosmos. It seemed inconceivable that a spectacular streak of milk-white flame, rising and setting with the stars night after night, was not there for a reason, did not hold some portent for human affairs. So the idea arose that comets were harbingers of disaster, auguries of divine wrath -- that they foretold the deaths of princes, the fall of kingdoms."
Sagan describes many of the interesting interpretations that were ascribed to individual comets. He also explains how Issac Newton demystified comets by claiming that they were like planets and orbited around the Sun. This assertion, Sagan says, "led his friend Edmund Halley in 1707 to calculate that the comets of 1531, 1607 and 1682 were apparitions at 76-year intervals of the same comet, and [he] predicted its return in 1758. The comet duly arrived and was named for him posthumously."
Sagan explains that comets originate in "a vast spherical cloud of a trillion cometary nuclei" orbiting the Sun almost halfway to the nearest star. Gravitational forces from other stars occasionally pull these comets into the inner solar system. [Note: A piece of Sagan trivia. Sagan claimed that he never used the infamous phrase "billions and billions", in part, he said, because it was too vague. "How many billion are in 'billions and billions'?", he asked. However, in this segment of the television series, he does use the phrase "trillions and trillions" to describe the number of nuclei in the cometary cloud.]
Sagan explains that comets are responsible for the craters that we see on the moons and planets of the inner solar system. We don't see many craters on the Earth, however, because erosion wipes them away. He talks about how likely it would be for a comet to hit the Earth within the course of a human lifetime. Not very likely, he concludes. Using rough approximations based on the number of craters visible on the moon, he estimates collisions with small cometary fragments such as the Tunguska Event every thousand years, small comets yielding craters one kilometer across every ten thousand years, and large comets such as Halley's comet once every billion years.
Perhaps beating these odds, Sagan recounts a description of an event witnessed by the Canterbury monks in 1178:
"There was a bright New Moon, and as usual in that phase its horns were tilted towards the east. Suddenly, the upper horn split in two. From the midpoint of the division, a flaming torch sprang up, spewing out fire, hot coals and sparks."
From the evidence of a fairly recent crater in the region of the moon described by the monks, and a slight "wobble" in the moon's motion detected through laser reflection off special mirrors placed on the moon by Apollo astronauts, it appears that the monks may have witnessed a cometary collision on the moon.
Sagan then discusses his opinions of a book entitled Worlds in Collision, "published in 1950 by a psychiatrist named Immanuel Velikovsky." [Sagan had been engaged in an extended debate over the theories expressed in this book.] Velikovsky imagined that the planet Venus arose as a "comet" from the planet Jupiter. Sagan explains why he thinks "these ideas are almost certainly wrong." His main point is something else, however:
"The worst aspect of the Velikovsky affair is not that his hypotheses were wrong or in contradiction to firmly established facts, but that some who called themselves scientists attempted to suppress Velikovsky's work. Science is generated by and devoted to free inquiry: the idea that any hypothesis, no matter how strange, deserves to be considered on its merits. The suppression of uncomfortable ideas may be common in religion and politics, but it is not the path to knowledge; it has no place in the endeavor of science. We do not know in advance who will discover fundamental new insights."
The Velikovsky discussion segues us into a discussion of the planet Venus, which Sagan studied closely during his career. He describes how Venus was an enigma for ages because the surface is obscured by thick clouds. Sagan and others determined that "the atmosphere of Venus is composed of 96 percent carbon dioxide. ... The surface temperatures are around 480 degrees Celsius or 900 degrees Fahrenheit, hotter than the hottest household oven. ... The clouds of Venus turn out to be chiefly a concentrated solution of sulfuric acid. ... Venus turns out to be a thoroughly nasty place."
But how did they do it? How could the composition of Venus be determined without a sample to be tested? In describing how the answers were determined, Sagan teaches us about the nature of light. He explains how light is a wave which changes properties based on its wavelength. He takes us through the spectrum of short wavelength (gamma rays, x-rays, ultraviolet light), medium wavelength (visible light), and long wavelength (infrared light and radio waves.) He then explains the technique known as spectroscopy:
"Different molecules and chemical elements absorb different frequencies or colors of light, sometimes in the visible and sometimes elsewhere in the spectrum. In the spectrum of a planetary atmosphere a single dark line represents ... the absorption of sunlight during its brief passage through the air of another world. Each such line is made by a particular kind of molecule or atom. Every substance has its characteristic spectral signature. The gases on Venus can be identified from the Earth, 60 million kilometers away. We can divine the composition of the Sun; ... of distant galaxies analyzed through the collective light of a hundred billion constituent stars. Astronomical spectroscopy is an almost magical technique. It amazes me still."
Sagan concludes the chapter with a comparison of Venus and Earth. "With searing heat, crushing pressures, noxious gases and everything suffused in an eerie, reddish glow, Venus seems less the goddess of love than the incarnation of hell. ... There is much to be learned about our planet, a comparative Heaven, by comparing it with Hell."
Sagan discusses the possibilities for the environmental future of the Earth, a topic on which he was sometimes criticized. He says that the runaway "greenhouse effect" which causes the high temperatures on Venus could also occur on Earth if the amount of carbon dioxide in the atmosphere reaches a critical level. [Sagan was instrumental in popularizing the notion of a greenhouse effect.]
"The carbon dioxide content of the Earth's atmosphere is rising dramatically. The possibility of a runaway greenhouse effect suggests that we have to be careful: Even a one or two-degree rise in the global temperature can have catastrophic consequences.
"Our intelligence and our technology have given us the power to affect the climate. How will we use this power? Are we willing to tolerate ignorance and complacency in matters that affect the entire human family? Do we value short-term advantages above the welfare of the Earth? Or will we think on longer time scales, with concern for our children and our grandchildren, to understand and protect the complex life-support systems of our planet?"
Tom |
