by Carl Sagan
While we may disagree with some of the steps in his reasoning, we probably all agree that the gentleman did the right thing.
My informant decided that to search for all the ancient and legendary gods of old would be too tiring a task. Instead, he set his sights on only a few: Jupiter, Mercury, and the goddess on the obverse face of the old British penny–not everyone’s first choice of the most interesting gods, but surely a representative trio. To his (and my) astonishment, he found–incarcerated in the very asylum in which he had committed himself–Jupiter, Mercury, and the goddess on the obverse face of the old English penny. These gods readily admitted their identities and regaled him with stories of the days of yore when nectar and ambrosia flowed freely.
And then my correspondent succeeded beyond his hopes. One day, over a bowl of Bing cherries, he encountered “God Almighty,” or at least a facsimile thereof. At least the Personage who offered him the Bing cherries modestly acknowledged being God Almighty. God Almighty luckily had a small spaceship on the grounds of the asylum and offered to take my informant on a short tool around the Solar System–which was no sooner said than done.
“And this, Dr. Sagan, is how I can assure you that the planets are inhabited.”
The letter then concluded something as follows: “But all this business about life elsewhere is so much speculation and not worth the really serious interest of a scientist such as yourself. Why don’t you address yourself to a really important problem, such as the construction of a trans-Canadian railroad at high northern latitudes?” There followed a detailed sketch of the proposed railway route and a standard expression of the sincerity of his good wishes.
Other than stating my serious intent to work on a trans-Canadian railroad at high northern latitudes, I have never been able to think of an appropriate response to this letter.
12. The Venus Detective Story
One of the reasons that planetary astronomy is such a delight these days is that it is possible to find out what’s really right. In the old days, you could make any guess you liked, however improbable, about a planetary environment, and there was little chance that anyone could ever prove you wrong. Today, spacecraft hang like swords of Damocles over each hypothesis spun by planetary theoreticians, and the theoreticians can be observed in a curious amalgam of hope and fear as each new burst of spacecraft planetary information comes winging in.
Back when astronomers had telescopes, eyeballs, and very little else to assist their observations, Venus beckoned as a sister world. By the late nineteenth century, it was known that Venus had about the same mass and radius as Earth. Venus is the closest planet to Earth, and it was natural to assume that it was, in other respects, Earth-like.
Immanuel Kant imagined a race of amorous quasi-humans on Venus. Emanuel Swedenborg and Annie Besant, a founder of theosophy, found–by methods described as spirit travel and astral projection–creatures very like humans on Venus. In more recent years, some of the more spectacularly audacious flyingsaucer accounts–for example, those of George Adamski–populated Venus with a race of benign and powerful beings, many of whom seem to have been garbed in long hair and long white robes–clear symbolism, in pre-1963 America, of deep spiritual intent. There is a long history of wishful thinking, bemused speculation, and conscious and unconscious fraud, which produced a popular expectation that our nearest planetary neighbor is habitable by humans, and is possibly even already inhabited by creatures rather like us.
It was, therefore, with a sense of considerable surprise, and even annoyance, that the results of the first radio observations of Venus were greeted. These measurements, performed in 1956 by C. H. Mayer and colleagues at the U. S. Naval Research Laboratory, found Venus to be a much more intense source of radio emission than had been expected. From Venus’ distance to the Sun and the amount of sunlight it reflects back to space, the planet should be cool. Because Venus reflects so much sunlight back to space, its temperature ought to be even less than the Earth’s, despite its closeness to the Sun. Mayer’s group found that Venus, at a radio wavelength of 3 centimeters, was giving off as much radiation as it would if it were a hot body at a temperature of about 600 degrees Fahrenheit. Later observations with many different radio telescopes at many different radio frequencies confirmed the general conclusion that Venus had a “brightness temperature” of about 600 degrees to 800 degrees.
Nevertheless, there was great reluctance in the scientific community to believe that the radio emission came from the Venus surface. A hot object emits radiation at many wavelengths. Why did Venus seem hot only at radio wavelengths? How could the surface of Venus be kept so hot? And finally–since psychological factors may be unconsciously compelling, even in science–a Venus hotter than the hottest household oven is simply less pleasant a prospect than the Venus populated, in the long tradition from Kant to Adamski, by gracious humans of amorous or spiritual inclinations.
This problem of the origin of the Venus radio emission was a major part of my doctoral dissertation. I wrote some twenty scientific papers concerning it between 1961 and 1968, when the problem was finally considered settled. I look back on this period with pleasure. The Venus radio story is very much like a detective story where there are clues littering the pages. Some are vital to the solution; others are false clues, leading in the wrong direction. Sometimes the right answer can be deduced by bearing in mind all the relevant facts and requiring reasonable logical consistency and plausibility.
There were several things we knew about Venus. We knew how the brightness temperature varied with radio frequency. We knew how Venus reflected back to Earth radio waves sent out by large radar telescopes. Man’s first successful planetary probe–the United States’ Mariner 2–found in 1962 that Venus was brighter at radio wavelengths at its center than at its edge.
To be matched against such observations were a variety of theories. They fell into two general categories: The hot-surface model, in which the radio emission came from the solid surface of the planet; and the cold-surface model, in which the radio emission came from somewhere else–from an ionized layer in the atmosphere of Venus, or from electrical discharges between droplets in the clouds of Venus, or from a hypothesized great belt of rapidly moving electrically charged particles surrounding Venus (like those that, in fact, surround the Earth and Jupiter). These latter models permitted the surface to be cold by placing the intense radio emission above the surface. If you wanted sailing ships on Venus, you were a cold-surface model advocate.
We systematically compared the cold-surface models with the observations and found that they all ran into serious troubles. The model in which the radio emission came from the ionosphere, for example, predicted that Venus should not reflect radio waves at all. But radar telescopes had found radio waves reflected from Venus with an efficiency of 10 or 20 percent. To circumvent such difficulties, advocates of the ionospheric model constructed very elaborate hypotheses in which there were many ionized layers with especially constructed holes in them to let radar through the ionosphere, hit the surface of Venus, and return to Earth. At the same time there could not be too many holes; otherwise, the radio emission would not be as intense as observed. These models seemed to me to be far too detailed and arbitrary in their requirements.
Just before the remarkable spacecraft observations of Venus of 1968, I submitted a paper to Nature, the British scientific journal, in which I summarized these conclusions and deduced that only the hot-surface model was consistent with all the evidence. I had earlier proposed a specific theory, in terms of the greenhouse effect, to explain how the surface of Venus could be at such high temperatures. But my conclusions against cold-surface models in 1968 did not depend upon the validity of the greenhouse explanation: It was just that a hot surface explained the data and a cold surface did not. Because of my interest in exobiology, I would have preferred a habitable Venus, but the facts led elsewhere. In a paper published in 1962, I had concluded from indirect evidence that the average surface temperature on Venu
s was about 800 degrees F and the average surface atmospheric pressure about fifty times larger than at the surface of Earth.
In 1968, an American spacecraft, Mariner 5, flew by Venus, and a Soviet spacecraft, Venera 4, entered its atmosphere. By the year 1974 there had been five Soviet instrumented capsules that entered the Venus atmosphere. The last three touched down and returned data from the planetary surface. They were the first craft of mankind to land on the surface of another planet. The average temperature on Venus turns out to be about 900 degrees F; the average pressure at the surface appears to be about ninety atmospheres. My early conclusions were approximately correct, just slightly too conservative.
It is interesting, now that we know by direct measurements the actual conditions on Venus, to read some of the criticism of the hot-surface model published in the 1960s. The year after receiving my Ph.D., I was offered, by a wellknown planetary astronomer, ten-to-one odds that the surface pressure on Venus was no more than ten times that on Earth. I gladly offered my ten dollars against his hundred; to his credit, he paid off–after the Soviet landing observations were in hand.
Theory and spacecraft interact in other ways. For example, Venera 4 radioed its last temperature/pressure point at 450 degrees F and twenty atmospheres. The Soviet scientists concluded that these were the surface conditions on Venus. But ground-based radio data had already shown that the surface temperature must be much higher. Combining radar with Mariner 5 data, we knew that the surface of Venus was far below where the Soviet scientists concluded Venera 4 had landed. It now appears that the designers of the first Venera spacecraft, believing the models of cold-surface theoreticians, built a relatively fragile spacecraft, which was crushed by the weight of the Venus atmosphere far above the surface–much as a submarine, not designed for great depths, will be crushed at the ocean bottoms.
At the 1968 Tokyo meeting of COSPAR, the Committee on Space Research of the International Council of Scientific Unions, I proposed that the Venera 4 spacecraft had ceased operating some fifteen miles above the surface. My colleague, Professor A. D. Kuzmin, of the Lebedev Physical Institute, in Moscow, argued that it had landed on the surface. When I noted that the radio and radar data did not put the surface at the altitude deduced for the Venera 4 touchdown, Dr. Kuzmin proposed that Venera 4 had landed atop a high mountain. I argued that ground-based radar studies of Venus had shown mountains a mile high, at most, and that it was exceptionally unlikely Venera 4 would land on the only fifteen-mile-high mountain on Venus, even if such a mountain were possible. Professor Kuzmin replied by asking me what I thought was the probability that the first German bomb to fall on Leningrad in World War II would kill the only elephant in the Leningrad zoo. I admitted that the chance was very small, indeed. He responded, triumphantly, with the information that such was indeed the fate of the Leningrad elephant.
The designers of subsequent Soviet entry probes were, despite the Leningrad zoo, cautious enough to increase the structural strength of the spacecraft in each successive mission. Venera 7 was able to withstand pressures of 180 times that at the surface of the Earth, a quite adequate margin for the actual Venus surface conditions. It transmitted twenty minutes’ worth of data from the Venus surface before being fried. Venera 8, in 1972, transmitted more than twice as long. The surface pressure is not at twenty atmospheres, and the spectacular Mount Kuzmin does not exist.
The principal conclusion about the scientific method that I draw from this history is this: While theory is useful in the design of experiments, only direct experiments will convince everyone. Based only on my indirect conclusions, there would today still be many people who did not believe in a hot Venus. As a result of the Venera observations, everyone acknowledges a Venus of crushing pressures, stifling heat, dim illumination, and strange optical effects.
That our sister planet should be so different from Earth is a major scientific problem, and studies of Venus are of the greatest interest in understanding the earliest history of Earth. In addition, it helps to calibrate the reliability of astral projection and spirit travel of the sorts popularized by Emanuel Swedenborg, Annie Besant, and innumerable present-day imitators, none of whom caught a glimmering of the true nature of Venus.
13. Venus Is Hell
The planet Venus floats, serene and lovely, in the sky of Earth, a bright pinpoint of yellowish-white light. Seen or photographed through a telescope, a featureless disc is discerned; a vast unbroken and enigmatic cloud layer shields the surface from our view. No human eye has seen the ground of our nearest planetary neighbor.
But we now know a great deal about Venus. From radio telescope and spacevehicle observations, we know that the surface temperature is about 900 degrees Fahrenheit. The atmospheric pressure at the surface of Venus is about ninety times that which we experience at the surface of the Earth. Since the planet’s gravity is about as strong as the Earth’s, there are about ninety times more molecules in the atmosphere of Venus as in the atmosphere of Earth. This dense atmosphere acts as a kind of insulating blanket, keeping the surface hot through the greenhouse effect and smoothing out temperature differences from place to place. The pole of Venus is probably not significantly colder than its equator, and on Venus it is as hot at midnight as at noon.
Forty miles above the surface is the thick cloud layer that we see from Earth. At least until recently, no one knew the composition of these clouds. I had proposed that they were constituted in part of water, a cosmically very abundant material, which could account for many but by no means all of the observed properties of the Venus clouds. But there were many other candidate materials proposed, among them, ammonium chloride, carbon suboxide, various silicates and oxides, solutions of hydrochloric acid, a hydrated ferric chloride, carbohydrates, and hydrocarbons. These last two materials were proposed by Immanuel Velikovsky in his speculative romance Worlds in Collision to provide manna for the Israelites during their forty years of wandering in the desert. The other candidate materials were proposed on somewhat firmer grounds. Yet each of them ran afoul of one or more of the observations.
But recently a material has been proposed that is in excellent quantitative agreement with all of the measurements. The American astronomer Andrew T. Young has shown that the clouds of Venus are very likely a concentrated solution of sulfuric acid. A 75 percent solution of H2SO4 precisely matches the index of refraction of the Venus clouds determined by polarimetric observations from the Earth. None of the other materials comes close. Such a solution is liquid at the temperatures and pressures at which the Venus clouds reside. Sulfuric acid has an absorption feature, determined by infrared spectroscopy, at a wavelength of 11.2 microns. Of all the materials proposed, only H2SO4 has such an absorption feature. The Soviet entry spacecraft of the Venera series have found large quantities of water vapor below the visible clouds of Venus. Ground-based observers looking for water vapor spectroscopically have found only a tiny amount of water vapor above the clouds of Venus. The two observations are in accord only if a very effective drying agent is present between these two regions. Sulfuric acid is such an agent.
In the Earth’s atmosphere there are water droplets at altitude, and water vapor in the atmosphere below. Likewise on Venus: If there are sulfuric acid droplets in the high clouds, there must be gaseous sulfuric acid below, with a relatively high concentration near the surface. Astronomers in Earth-bound observatories have also found unmistakable evidence of hydrochloric acid and hydrofluoric acid as gases in the upper atmosphere of Venus. They also must exist in a fair concentration–for example, the relative proportions of Los Angeles smog in Los Angeles air–in the lower atmosphere of Venus. These three acids are an extremely corrosive mixture. Any spacecraft that is to survive on the Venus surface must not only be bulwarked against the high pressures but protected against the corrosive atmosphere.
The Soviet Union is engaged in a very active program of unmanned exploration of Venus. We now know there is enough light for photography at midday on the Venus surface. The time will
come, in not too many years, I think, when we will have our first photographs of the surface of Venus. What does the surface of Venus look like? To some extent we can already make predictions.
Because of the very dense atmosphere of Venus, there are some interesting optical effects. The most important such effect is due to Rayleigh scattering, named after the British Lord Rayleigh. When sunlight strikes the clear, dust-free atmosphere of the Earth, it is scattered. Photons strike the molecules of the Earth’s atmosphere and are bounced off. Many such bounces may occur. But because the molecules of air are very much smaller than the wavelength of light, it turns out that short wavelengths are scattered or bounced away by the air molecules more efficiently than long wavelengths. Blue light is scattered much better than red light. This was a fact known to Leonardo da Vinci, who painted distant landscapes in an exquisite cerulean blue. It is why we talk of purple mountains; it is why the sky is blue. The light from the sun is scattered about in our atmosphere–some of it being scattered up and out again, but other fractions of sunlight being scattered about by the molecules of our atmosphere and then, from quite a different direction than that of the Sun, scattered back down to our eyeballs. In the absence of an atmosphere, as on the Moon, the sky is black. When we look at a sunset we are seeing the Sun through a longer path in the Earth’s atmosphere than when we view it at noon. Blue light has been preferentially scattered out of this path, leaving only the red light to strike our eyes. The beauty of the sunset, the sky, and distant landscapes are all due to Rayleigh scattering.