Pale Blue Dot: A Vision of the Human Future in Space - Carl Sagan, Ann Druyan (1997)


This is another world

Which is not of men.



You can see it shining brilliantly in the twilight, chasing the Sun down below the western horizon. Upon first glimpsing it each night, people were accustomed to make a wish (“upon a star”). Sometimes the wish came true.

Or you can spy it in the east before dawn, fleeing the rising Sun. In these two incarnations, brighter than anything else in the sky except only the Sun and the Moon, it was known as the evening and the morning star. Our ancestors did not recognize it was a world, the same world, never too far from the Sun because it is in an orbit about it interior to the Earth’s. Just before sunset or just after sunrise, we can sometimes see it near some fluffy white cloud, and then discover by the comparison that Venus has a color, a pale lemon-yellow.

You peer through the eyepiece of a telescope—even a big telescope, even the largest optical telescope on Earth—and you can make out no detail at all. Over the months, you see a featureless disk methodically going through phases, like the Moon: crescent Venus, full Venus, gibbous Venus, new Venus. There is not a hint of continents or oceans.

Some of the first astronomers to see Venus through the telescope immediately recognized that they were examining a world enshrouded by clouds. The clouds, we now know, are droplets of concentrated sulfuric acid, stained yellow by a little elemental sulfur. They lie high above the ground. In ordinary visible light there’s no hint of what this planet’s surface, some 50 kilometers below the cloud tops, is like, and for centuries the best we had were wild guesses.

You might conjecture that if we could take a much finer look there might be breaks in the clouds, revealing day by day, in bits and pieces, the mysterious surface ordinarily hidden from our view. Then the time of guesses would be over. The Earth is on average half cloud-covered. In the early days of Venus exploration, we saw no reason that Venus should be 100 percent overcast. If instead it was only 90 percent, or even 99 percent, cloud-covered, the transient patches of clearing might tell us much.

In 1960 and 1961, Mariner1 and 2, the first American spacecraft designed to visit Venus, were being prepared. There were those, like me, who thought the ships should carry video cameras so they could radio pictures back to Earth. The same technology would be used a few years later when Rangers 7, 8, and 9 would photograph the Moon on the way to their crash landings—the last making a bull’s-eye in the crater Alphonsus. But time was short for the Venus mission, and cameras were heavy. There were those who maintained that cameras weren’t really scientific instruments, but rather catch-as-catch-can, razzle-dazzle, pandering to the public, and unable to answer a single straightforward, well-posed scientific question. I thought myself that whether there are breaks in the clouds was one such question. I argued that cameras could also answer questions that we were too dumb even to pose. I argued that pictures were the only way to show the public—who were, after all, footing the bill—the excitement of robotic missions. At any rate, no camera was flown, and subsequent missions have, for this particular world, at least partly vindicated that judgment: Even at high resolution from close flybys, in visible light it turns out there are no breaks in the clouds of Venus, any more than in the clouds of Titan.* These worlds are permanently overcast.

In the ultraviolet there is detail, but due to transient patches of high-altitude overcast, far above the main cloud deck. The high clouds race around the planet much faster than the planet itself turns: super-rotation. We have an even smaller chance of seeing the surface in the ultraviolet.

When it became clear that the atmosphere of Venus was much thicker than the air on Earth—as we now know, the pressure at the surface is ninety times what it is here—it immediately followed that in ordinary visible light we could not possibly see the surface, even if there were breaks in the clouds. What little sunlight is able to make its tortuous way through the dense atmosphere to the surface would be reflected back, all right; but the photons would be so jumbled by repeated scattering off molecules in the lower air that no image of surface features could be retained. It would be like a “whiteout” in a polar snowstorm. However, this effect, intense Rayleigh scattering, declines rapidly with increasing wavelength; in the near-infrared, it was easy to calculate, you could see the surface if there were breaks in the clouds—or if the clouds were transparent there.

So in 1970 Jim Pollack, Dave Morrison, and I went to the McDonald Observatory of the University of Texas to try to observe Venus in the near-infrared. We “hypersensitized” our emulsions; the good old-fashioned* glass photographic plates were treated with ammonia, and sometimes heated or briefly illuminated, before being exposed at the telescope to light from Venus. For a time the cellars of McDonald Observatory reeked of ammonia. We took many pictures. None showed any detail. We concluded that either we hadn’t gone far enough into the infrared, or the clouds of Venus were opaque and unbroken in the near infrared.

More than 20 years later, the Galileo spacecraft, making a close flyby of Venus, examined it with higher resolution and sensitivity, and at wavelengths a little further into the infrared than we were able to reach with our crude glass emulsions. Galileo photographed great mountain ranges. We already knew of their existence, though; a much more powerful technique had earlier been employed: radar. Radio waves effortlessly penetrate the clouds and thick atmosphere of Venus, bounce off the surface, and return to Earth, where they are gathered in and used to make a picture. The first work had been done, chiefly, by American ground-based radar at JPL’s Goldstone tracking station in the Mojave Desert and at the Arecibo Observatory in Puerto Rico, operated by Cornell University.

Then the U.S. Pioneer 12, the Soviet Venera 15 and 16 and the U.S. Magellan missions inserted radar telescopes into orbit around Venus and mapped the place pole to pole. Each spacecraft would transmit a radar signal to the surface and then catch it as it bounced back. From how reflective each patch of surface was and how long it took the signal to return (shorter from mountains, longer from valleys), a detailed map of the entire surface was slowly and painstakingly constructed.

The world so revealed turns out to be uniquely sculpted by lava flows (and, to a much lesser degree, by wind), as described in the next chapter. The clouds and atmosphere of Venus have now become transparent to us, and another world has been visited by the doughty robot explorers from Earth. Our experience with Venus is now being applied elsewhere—especially to Titan, where once again impenetrable clouds hide an enigmatic surface, and radar is beginning to give us hints of what might lie below.

VENUS HAD LONG BEEN THOUGHT of as our sister world. It is the nearest planet to the Earth. It has almost the same mass, size, density, and gravitational pull as the Earth does. It’s a little closer to the Sun than the Earth, but its bright clouds reflect more sunlight back to space than our clouds do. As a first guess you might very well imagine that, under those unbroken clouds, Venus was rather like Earth. Early scientific speculation included fetid swamps crawling with monster amphibians, like the Earth in the Carboniferous Period; a world desert; a global petroleum sea; and a seltzer ocean dotted here and there with limestone-encrusted islands. While based on some scientific data, these “models” of Venus—the first dating from the beginnings of the century, the second from the 1930s, and the last two from the mid-1950s—were little more than scientific romances, hardly constrained by the sparse data available.

Then, in 1956, a report was published in The Astrophysical Journal by Cornell H. Mayer and his colleagues. They had pointed a newly completed radio telescope, built in part for classified research, on the roof of the Naval Research Laboratory in Washington, D.C., at Venus and measured the flux of radio waves arriving at Earth. This was not radar: No radio waves were bounced off Venus. This was listening to radio waves that Venus on its own emits to space. Venus turned out to be much brighter than the background of distant stars and galaxies. This in itself was not very surprising. Every object warmer than absolute zero (–273°C) gives off radiation throughout the electromagnetic spectrum, including the radio region. You, for example, emit radio waves at an effective or “brightness” temperature of about 35°C, and if you were in surroundings colder than you are, a sensitive radio telescope could detect the faint radio waves you are transmitting in all directions. Each of us is a source of cold static.

What was surprising about Mayer’s discovery was that the brightness temperature of Venus is more than 300°C, far higher than the surface temperature of the Earth or the measured infrared temperature of the clouds of Venus. Some places on Venus seemed at least 200° hotter than the normal boiling point of water. What could this mean?

Soon there was a deluge of explanations. I argued that the high radio brightness temperature was a direct indication of a hot surface, and that the high temperatures were due to a massive carbon dioxide/water vapor greenhouse effect—in which some sunlight is transmitted through the clouds and heats the surface, but the surface experiences enormous difficulty in radiating back to space because of the high infrared opacity of carbon dioxide and water vapor. Carbon dioxide absorbs at a range of wavelengths through the infrared, but there seemed to be “windows” between the CO2 absorption bands through which the surface could readily cool off to space. Water vapor, though, absorbs at infrared frequencies that correspond in part to the windows in the carbon dioxide opacity. The two gases together, it seemed to me, could pretty well absorb almost all the infrared emission, even if there was very little water vapor—something like two picket fences, the slats of one being fortuitously positioned to cover the gaps of the other.

There was another very different category of explanation, in which the high brightness temperature of Venus had nothing to do with the ground. The surface could still be temperate, clement, congenial. It was proposed that some region in the atmosphere of Venus or in its surrounding magnetosphere emitted these radio waves to space. Electrical discharges between water droplets in the Venus clouds were suggested. A glow discharge in which ions and electrons recombined at twilight and dawn in the upper atmosphere was offered. A very dense ionosphere had its advocates, in which the mutual acceleration of unbound electrons (“free-free emission”) gave off radio waves. (One proponent of this idea even suggested that the high ionization required was due to an average of 10,000 times greater radioactivity on Venus than on Earth—perhaps from a recent nuclear war there.) And, in the light of the discovery of radiation from Jupiter’s magnetosphere, it was natural to suggest that the radio emission came from an immense cloud of charged particles trapped by some hypothetical very intense Venusian magnetic field.

In a series of papers I published in the middle 1960s, many in collaboration with Jim Pollack,* these conflicting models of a high hot emitting region and a cold surface were subjected to a critical analysis. By then we had two important new clues: the radio spectrum of Venus, and the Mariner 2 evidence that the radio emission was more intense at the center of the disk of Venus than toward its edge. By 1967 we were able to exclude the alternative models with some confidence, and conclude that the surface of Venus was at a scorching and un-Earthlike temperature, in excess of 400°C. But the argument was inferential, and there were many intermediate steps. We longed for a more direct measurement.

In October 1967—commemorating the tenth anniversary of Sputnik 1—the Soviet Venera 4 spacecraft dropped an entry capsule into the clouds of Venus. It returned data from the hot lower atmosphere, but did not survive to the surface. One day later, the United States spacecraft Mariner 5 flew by Venus, its radio transmission to Earth skimming the atmosphere at progressively greater depths. The rate of fading of the signal gave information about atmospheric temperatures. Although there seemed to be some discrepancies (later resolved) between the two sets of spacecraft data, both clearly indicated that the surface of Venus is very hot.

Since then a progression of Soviet Venera spacecraft and one cluster of American spacecraft from the Pioneer 12 mission have entered the deep atmosphere or landed on the surface and measured directly—essentially by sticking out a thermometer—the surface and near-surface temperatures. They turn out to be about 470°C, almost 900°F. When such factors as calibration errors of terrestrial radio telescopes and surface emissivity are taken into account, the old radio observations and the new direct spacecraft measurements turn out to be in good accord.

Early Soviet landers were designed for an atmosphere somewhat like our own. They were crushed by the high pressures like a tin can in the grasp of a champion arm wrestler, or a World War II submarine in the Tonga Trench. Thereafter, Soviet Venus entry vehicles were heavily reinforced, like modern submarines, and successfully landed on the searing surface. When it became clear how deep the atmosphere is and how thick the clouds, Soviet designers became concerned that the surface might be pitch-black. Veneras 9 and 10 were equipped with floodlights. They proved unnecessary. A few percent of the sunlight that falls on the top of the clouds makes it through to the surface, and Venus is about as bright as on a cloudy day on Earth.

The resistance to the idea of a hot surface on Venus can, I suppose, be attributed to our reluctance to abandon the notion that the nearest planet is hospitable for life, for future exploration, and perhaps even, in the longer term, for human settlement. As it turns out there are no Carboniferous swamps, no global oil or seltzer oceans. Instead, Venus is a stifling, brooding inferno. There are some deserts, but it’s mainly a world of frozen lava seas. Our hopes are unfulfilled. The call of this world is now more muted than in the early days of spacecraft exploration, when almost anything was possible and our most romantic notions about Venus might, for all we then knew, be realized.

MANY SPACECRAFT CONTRIBUTED to our present understanding of Venus. But the pioneering mission was Mariner 2. Mariner 1 failed at launch and—as they say of a racehorse with a broken leg—had to be destroyed. Mariner 2 worked beautifully and provided the key early radio data on the climate of Venus. It made infrared observations of the properties of the clouds. On its way from Earth to Venus, it discovered and measured the solar wind—the stream of charged particles that flows outward from the Sun, filling the magnetospheres of any planets in its way, blowing back the tails of comets, and establishing the distant heliopause. Mariner 2 was the first successful planetary probe, the ship that ushered in the age of planetary exploration.

It’s still in orbit around the Sun, every few hundred days still approaching, more or less tangentially, the orbit of Venus. Each time that happens, Venus isn’t there. But if we wait long enough, Venus will one day be nearby and Mariner 2 will be accelerated by the planet’s gravity into some quite different orbit. Ultimately, Mariner 2, like some planetesimal from ages past, will be swept up by another planet, fall into the Sun, or be ejected from the Solar System.

Until then, this harbinger of the age of planetary exploration, this minuscule artificial planet, will continue silently orbiting the Sun. It’s a little as if Columbus’s flagship, the Santa María, were still making regular runs with a ghostly crew across the Atlantic between Cádiz and Hispaniola. In the vacuum of interplanetary space, Mariner 2 should be in mint condition for many generations.

My wish on the evening and morning star is this: that late in the twenty-first century some great ship, on its regular gravity-assisted transit to the outer Solar System, intercepts this ancient derelict and heaves it aboard, so it can be displayed in a museum of early space technology—on Mars, perhaps, or Europa, or Iapetus.

*For Titan, imaging revealed a succession of detached hazes above the main layer of aerosols. So Venus works out to be the only world in the Solar System for which spacecraft cameras working in ordinary visible light haven’t discovered something important. Happily, we’ve now returned pictures from almost every world we’ve visited. (NASA’s International Cometary Explorer, which raced through the tail of Comet Giacobini-Zimmer in 1985, flew blind, being devoted to charged particles and magnetic fields.)

*Today many telescopic images are obtained with such electronic contrivances as charge-coupled devices and diode arrays, and processed by computer—all technologies unavailable to astronomers in 1970.

*James B. Pollack made important contributions to every area of planetary science. He was my first graduate student and a colleague ever since. He converted NASA’s Ames Research Center into a world leader in planetary research and the post-doctoral training of planetary scientists. His gentleness was as extraordinary as his scientific abilities. He died in 1994 at the height of his powers.