Cosmos
Page 12
If Venus were soaking wet, it should be easy to see the water vapor lines in its spectrum. But the first spectroscopic searches, attempted at Mount Wilson Observatory around 1920, found not a hint, not a trace, of water vapor above the clouds of Venus, suggesting an arid, desert-like surface, surmounted by clouds of fine drifting silicate dust. Further study revealed enormous quantities of carbon dioxide in the atmosphere, implying to some scientists that all the water on the planet had combined with hydrocarbons to form carbon dioxide, and that therefore the surface of Venus was a global oil field, a planet-wide sea of petroleum. Others concluded that there was no water vapor above the clouds because the clouds were very cold, that all the water had condensed out into water droplets, which do not have the same pattern of spectral lines as water vapor. They suggested that the planet was totally covered with water—except perhaps for an occasional limestone-encrusted island, like the cliffs of Dover. But because of the vast quantities of carbon dioxide in the atmosphere, the sea could not be ordinary water; physical chemistry required carbonated water. Venus, they proposed, had a vast ocean of seltzer.
Schematic diagram of the electromagnetic spectrum. The wavelength of light is measured in Ångstroms (Å), micrometers (μm), centimeters (cm) and meters (m).
The first hint of the true situation came not from spectroscopic studies in the visible or near-infrared parts of the spectrum, but rather from the radio region. A radio telescope works more like a light meter than a camera. You point it toward some fairly broad region of the sky, and it records how much energy, in a particular radio frequency, is coming down to Earth. We are used to radio signals transmitted by some varieties of intelligent life—namely, those who run radio and television stations. But there are many other reasons for natural objects to give off radio waves. One is that they are hot. And when, in 1956, an early radio telescope was turned toward Venus, it was discovered to be emitting radio waves as if it were at an extremely high temperature. But the real demonstration that the surface of Venus is astonishingly hot came when the Soviet spacecraft of the Venera series first penetrated the obscuring clouds and landed on the mysterious and inaccessible surface of the nearest planet. Venus, it turns out, is broiling hot. There are no swamps, no oil fields, no seltzer oceans. With insufficient data, it is easy to go wrong.
When I greet a friend, I am seeing her in reflected visible light, generated by the Sun, say, or by an incandescent lamp. The light rays bounce off my friend and into my eye. But the ancients, including no less a figure than Euclid, believed that we see by virtue of rays somehow emitted by the eye and tangibly, actively contacting the object observed. This is a natural notion and can still be encountered, although it does not account for the invisibility of objects in a darkened room. Today we combine a laser and a photocell, or a radar transmitter and a radio telescope, and in this way make active contact by light with distant objects. In radar astronomy, radio waves are transmitted by a telescope on Earth, strike, say, that hemisphere of Venus that happens to be facing the Earth, and bounce back. At many wavelengths the clouds and atmosphere of Venus are entirely transparent to radio waves. Some places on the surface will absorb them or, if they are very rough, will scatter them sideways and so will appear dark to radio waves. By following the surface features moving with Venus as it rotates, it was possible for the first time to determine reliably the length of its day—how long it takes Venus to spin once on its axis. It turns out that, with respect to the stars, Venus turns once every 243 Earth days, but backwards, in the opposite direction from all other planets in the inner solar system. As a result, the Sun rises in the west and sets in the east, taking 118 Earth days from sunrise to sunrise. What is more, it presents almost exactly the same face to the Earth each time it is closest to our planet. However the Earth’s gravity has managed to nudge Venus into this Earth-locked rotation rate, it cannot have happened rapidly. Venus could not be a mere few thousand years old but, rather, it must be as old as all the other objects in the inner solar system.
Radar pictures of Venus have been obtained, some from ground-based radar telescopes, some from the Pioneer Venus vehicle in orbit around the planet. They show provocative evidence of impact craters. There are just as many craters that are not too big or too small on Venus as there are in the lunar highlands, so many that Venus is again telling us that it is very old. But the craters of Venus are remarkably shallow, almost as if the high surface temperatures have produced a kind of rock that flows over long periods of time, like taffy or putty, gradually softening the relief. There are great mesas here, twice as high as the Tibetan plateau, an immense rift valley, possibly giant volcanoes and a mountain as high as Everest. We now see before us a world previously hidden entirely by clouds—its features first explored by radar and by space vehicles.
The surface temperatures on Venus, as deduced from radio astronomy and confirmed by direct spacecraft measurements, are around 480°C or 900°F, hotter than the hottest household oven. The corresponding surface pressure is 90 atmospheres, 90 times the pressure we feel from the Earth’s atmosphere, the equivalent of the weight of water 1 kilometer below the surface of the oceans. To survive for long on Venus, a space vehicle would have to be refrigerated as well as built like a deep submersible.
Something like a dozen space vehicles from the Soviet Union and United States have entered the dense Venus atmosphere, and penetrated the clouds; a few of them have actually survived for an hour or so on the surface.* Two spacecraft in the Soviet Venera series have taken pictures down there. Let us follow in the footsteps of these pioneering missions, and visit another world.
In ordinary visible light, the faintly yellowish clouds of Venus can be made out, but they show, as Galileo first noted, virtually no features at all. If the cameras look in the ultraviolet, however, we see a graceful, complex swirling weather system in the high atmosphere, where the winds are around 100 meters per second, some 220 miles per hour. The atmosphere of Venus is composed of 96 percent carbon dioxide. There are small traces of nitrogen, water vapor, argon, carbon monoxide and other gases, but the only hydrocarbons or carbohydrates present are there in less than 0.1 parts per million. The clouds of Venus turn out to be chiefly a concentrated solution of sulfuric acid. Small quantities of hydrochloric acid and hydrofluoric acid are also present. Even at its high, cool clouds, Venus turns out to be a thoroughly nasty place.
High above the visible cloud deck, at about 70 kilometers altitude, there is a continuous haze of small particles. At 60 kilometers, we plunge into the clouds, and find ourselves surrounded by droplets of concentrated sulfuric acid. As we go deeper, the cloud particles tend to get bigger. The pungent gas, sulfur dioxide, SO2, is present in trace amounts in the lower atmosphere. It is circulated up above the clouds, broken down by ultraviolet light from the Sun and recombined with water there to form sulfuric acid—which condenses into droplets, settles, and at lower altitudes is broken down by heat into SO2 and water again, completing the cycle. It is always raining sulfuric acid on Venus, all over the planet, and not a drop ever reaches the surface.
The sulfur-colored mist extends downwards to some 45 kilometers above the surface of Venus, where we emerge into a dense but crystal-clear atmosphere. The atmospheric pressure is so high, however, that we cannot see the surface. Sunlight is bounced about by atmospheric molecules until we lose all images from the surface. There is no dust here, no clouds, just an atmosphere getting palpably denser. Plenty of sunlight is transmitted by the overlying clouds, about as much as on an overcast day on the 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. As nearly as we can make out, at least some places on the surface are strewn fields of jumbled, softened irregular rocks, a hostile, barren landscape relieved only here and there by the eroded remnants of a derelict spacecraft from a distant planet, utterly invisible through the thick, cloudy, poisonous atmosphere.*
&nb
sp; Venus is a kind of planet-wide catastrophe. It now seems reasonably clear that the high surface temperature comes about through a massive greenhouse effect. Sunlight passes through the atmosphere and clouds of Venus, which are semi-transparent to visible light, and reaches the surface. The surface being heated endeavors to radiate back into space. But because Venus is much cooler than the Sun, it emits radiation chiefly in the infrared rather than the visible region of the spectrum. However, the carbon dioxide and water vapor† in the Venus atmosphere are almost perfectly opaque to infrared radiation, the heat of the Sun is efficiently trapped, and the surface temperature rises—until the little amount of infrared radiation that trickles out of this massive atmosphere just balances the sunlight absorbed in the lower atmopshere and surface.
Our neighboring world turns out to be a dismally unpleasant place. But we will go back to Venus. It is fascinating in its own right. Many mythic heroes in Greek and Norse mythology, after all, made celebrated efforts to visit Hell. There is also much to be learned about our planet, a comparative Heaven, by comparing it with Hell.
The Sphinx, half human, half lion, was constructed more than 5,500 years ago. Its face was once crisp and cleanly rendered. It is now softened and blurred by thousands of years of Egyptian desert sandblasting and by occasional rains. In New York City there is an obelisk called Cleopatra’s Needle, which came from Egypt. In only about a hundred years in that city’s Central Park, its inscriptions have been almost totally obliterated, because of smog and industrial pollution—chemical erosion like that in the atmosphere of Venus. Erosion on Earth slowly wipes out information, but because they are gradual—the patter of a raindrop, the sting of a sand grain—those processes can be missed. Big structures, such as mountain ranges, survive tens of millions of years; smaller impact craters, perhaps a hundred thousand*; and large-scale human artifacts only some thousands. In addition to such slow and uniform erosion, destruction also occurs through catastrophes large and small. The Sphinx is missing a nose. Someone shot it off in a moment of idle desecration—some say it was Mameluke Turks, others, Napoleonic soldiers.
On Venus, on Earth and elsewhere in the solar system, there is evidence for catastrophic destruction, tempered or overwhelmed by slower, more uniform processes: on the Earth, for example, rainfall, coursing into rivulets, streams and rivers of running water, creating huge alluvial basins; on Mars, the remnants of ancient rivers, perhaps arising from beneath the ground; on Io, a moon of Jupiter, what seem to be broad channels made by flowing liquid sulfur. There are mighty weather systems on the Earth—and in the high atmosphere of Venus and on Jupiter. There are sandstorms on the Earth and on Mars; lightning on Jupiter and Venus and Earth. Volcanoes inject debris into the atmospheres of the Earth and Io. Internal geological processes slowly deform the surfaces of Venus, Mars, Ganymede and Europa, as well as Earth. Glaciers, proverbial for their slowness, produce major reworkings of landscapes on the Earth and probably also on Mars. These processes need not be constant in time. Most of Europe was once covered with ice. A few million years ago, the present site of the city of Chicago was buried under three kilometers of frost. On Mars, and elsewhere in the solar system, we see features that could not be produced today, landscapes carved hundreds of millions or billions of years ago when the planetary climate was probably very different.
There is an additional factor that can alter the landscape and the climate of Earth: intelligent life, able to make major environmental changes. Like Venus, the Earth also has a greenhouse effect due to its carbon dioxide and water vapor. The global temperature of the Earth would be below the freezing point of water if not for the greenhouse effect. It keeps the oceans liquid and life possible. A little greenhouse is a good thing. Like Venus, the Earth also has about 90 atmospheres of carbon dioxide; but it resides in the crust as limestone and other carbonates, not in the atmosphere. If the Earth were moved only a little closer to the Sun, the temperature would increase slightly. This would drive some of the CO2 out of the surface rocks, generating a stronger greenhouse effect, which would in turn incrementally heat the surface further. A hotter surface would vaporize still more carbonates into CO2, and there would be the possibility of a runaway greenhouse effect to very high temperatures. This is just what we think happened in the early history of Venus, because of Venus’ proximity to the Sun. The surface environment of Venus is a warning: something disastrous can happen to a planet rather like our own.
The principal energy sources of our present industrial civilization are the so-called fossil fuels. We burn wood and oil, coal and natural gas, and, in the process, release waste gases, principally CO2, into the air. Consequently, the carbon dioxide content of the Earth’s atmosphere is increasing 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. In the burning of coal and oil and gasoline, we are also putting sulfuric acid into the atmosphere. Like Venus, our stratosphere even now has a substantial mist of tiny sulfuric acid droplets. Our major cities are polluted with noxious molecules. We do not understand the long-term effects of our course of action.
But we have also been perturbing the climate in the opposite sense. For hundreds of thousands of years human beings have been burning and cutting down forests and encouraging domestic animals to graze on and destroy grasslands. Slash-and-burn agriculture, industrial tropical deforestation and overgrazing are rampant today. But forests are darker than grasslands, and grasslands are darker than deserts. As a consequence, the amount of sunlight that is absorbed by the ground has been declining, and by changes in the land use we are lowering the surface temperature of our planet. Might this cooling increase the size of the polar ice cap, which, because it is bright, will reflect still more sunlight from the Earth, further cooling the planet, driving a runaway albedo* effect?
Our lovely blue planet, the Earth, is the only home we know. Venus is too hot. Mars is too cold. But the Earth is just right, a heaven for humans. After all, we evolved here. But our congenial climate may be unstable. We are perturbing our poor planet in serious and contradictory ways. Is there any danger of driving the environment of the Earth toward the planetary Hell of Venus or the global ice age of Mars? The simple answer is that nobody knows. The study of the global climate, the comparison of the Earth with other worlds, are subjects in their earliest stages of development. They are fields that are poorly and grudgingly funded. In our ignorance, we continue to push and pull, to pollute the atmosphere and brighten the land, oblivious of the fact that the long-term consequences are largely unknown.
A few million years ago, when human beings first evolved on Earth, it was already a middle-aged world, 4.6 billion years along from the catastrophes and impetuosities of its youth. But we humans now represent a new and perhaps decisive factor. 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? The Earth is a tiny and fragile world. It needs to be cherished.
*That meteors and meteorites are connected with the comets was first proposed by Alexander von Humboldt in his broad-gauge popularization of all of science, published in the years 1845 to 1862, a work called Kosmos. It was reading Humboldt’s earlier work that fired the young Charles Darwin to embark on a career combining geographical exploration and natural history. Shortly thereafter he accepted a position as naturalist aboard the ship H.M.S. Beagle, the event that led to The Origin of Species.
*The Earth is r = 1 astronomical unit = 150,000,000 kilometers from the Sun. Its roughly circular orbit then has a circumference of 2пr ≈ 109 km. Our planet circulates once along this path every year. One year = 3 × 107 seconds. So the
Earth’s orbital speed is 109 km/3 × 107 sec ≈ 30 km/sec. Now consider the spherical shell of orbiting comets that many astronomers believe surrounds the solar system at a distance ≈ 100,000 astronomical units, almost halfway to the nearest star. From Kepler’s third law (p. 50) it immediately follows that the orbital period about the Sun of any one of them is about (105) = 107.5 ≈ 3 × 107 or 30 million years. Once around the Sun is a long time if you live in the outer reaches of the solar system. The cometary orbit is 2пa = 2п × 105 × 1.5 × 108 km ≈ 1014 km around, and its speed is therefore only 1014 km/1015 sec = 0.1 km/sec ≈ 220 miles per hour.
*On Mars, where erosion is much more efficient, although there are many craters there are virtually no ray craters, as we would expect.
*As far as I know, the first essentially nonmystical attempt to explain a historical event by cometary intervention was Edmund Halley’s proposal that the Noachic flood was “the casual Choc [shock] of a Comet.”
†The Adda cylinder seal, dating from the middle of the third millennium B.C., prominently displays Inanna, the goddess of Venus, the morning star, and precursor of the Babylonian Ishtar.
*It is, incidentally, some 30 million times more massive than the most massive comet known.
*Light is a wave motion; its frequency is the number of wave crests, say, entering a detection instrument, such as a retina, in a given unit of time, such as a second. The higher the frequency, the more energetic the radiation.
*Pioneer Venus was a successful U.S. mission in 1978–79, combining an orbiter and four atmospheric entry probes, two of which briefly survived the inclemencies of the Venus surface. There are many unexpected developments in mustering spacecraft to explore the planets. This is one of them: Among the instruments aboard one of the Pioneer Venus entry probes was a net flux radiometer, designed to measure simultaneously the amount of infrared energy flowing upwards and downwards at each position in the Venus atmosphere. The instrument required a sturdy window that was also transparent to infrared radiation. A 13.5-karat diamond was imported and milled into the desired window. However, the contractor was required to pay a $12,000 import duty. Eventually, the U.S. Customs service decided that after the diamond was launched to Venus it was unavailable for trade on Earth and refunded the money to the manufacturer.