Cosmos - Carl Sagan (1980)


In the orchards of the gods, he watches the canals …

—Enuma Elish, Sumer, c. 2500 B.C.

A man that is of Copernicus’ Opinion, that this Earth of ours is a Planet, carry’d round and enlightn’d by the Sun, like the rest of them, cannot but sometimes have a fancy … that the rest of the Planets have their Dress and Furniture, nay and their Inhabitants too as well as this Earth of ours.… But we were always apt to conclude, that ’twas in vain to enquire after what Nature had been pleased to do there, seeing there was no likelihood of ever coming to an end of the Enquiry … but a while ago, thinking somewhat seriously on this matter (not that I count my self quicker sighted than those great Men [of the past], but that I had the happiness to live after most of them) me thoughts the Enquiry was not so impracticable nor the way so stopt up with Difficulties, but that there was very good room left for probable Conjectures.

—Christiaan Huygens, New Conjectures Concerning the
Planetary Worlds, Their Inhabitants and Productions
c. 1690

Many years ago, so the story goes, a celebrated newspaper publisher sent a telegram to a noted astronomer: WIRE COLLECT IMMEDIATELY FIVE HUNDRED WORDS ON WHETHER THERE IS LIFE ON MARS. The astronomer dutifully replied: NOBODY KNOWS, NOBODY KNOWS, NOBODY KNOWS … 250 times. But despite this confession of ignorance, asserted with dogged persistence by an expert, no one paid any heed, and from that time to this, we hear authoritative pronouncements by those who think they have deduced life on Mars, and by those who think they have excluded it. Some people very much want there to be life on Mars; others very much want there to be no life on Mars. There have been excesses in both camps. These strong passions have somewhat frayed the tolerance for ambiguity that is essential to science. There seem to be many people who simply wish to be told an answer, any answer, and thereby avoid the burden of keeping two mutually exclusive possibilities in their heads at the same time. Some scientists have believed that Mars is inhabited on what has later proved to be the flimsiest evidence. Others have concluded the planet is lifeless because a preliminary search for a particular manifestation of life has been unsuccessful or ambiguous. The blues have been played more than once for the red planet.

Why Martians? Why so many eager speculations and ardent fantasies about Martians, rather than, say, Saturnians or Plutonians? Because Mars seems, at first glance, very Earthlike. It is the nearest planet whose surface we can see. There are polar ice caps, drifting white clouds, raging dust storms, seasonally changing patterns on its red surface, even a twenty-four-hour day. It is tempting to think of it as an inhabited world. Mars has become a kind of mythic arena onto which we have projected our earthly hopes and fears. But our psychological predispositions pro or con must not mislead us. All that matters is the evidence, and the evidence is not yet in. The real Mars is a world of wonders. Its future prospects are far more intriguing than our past apprehensions about it. In our time we have sifted the sands of Mars, we have established a presence there, we have fulfilled a century of dreams!

No one would have believed in the last years of the nineteenth century that this world was being watched keenly and closely by intelligences greater than man’s and yet as mortal as his own; that as men busied themselves about their various concerns, they were scrutinised and studied, perhaps almost as narrowly as a man with a microscope might scrutinise the transient creatures that swarm and multiply in a drop of water. With infinite complacency, men went to and fro over this globe about their little affairs, serene in their assurances of their empire over matter. It is possible that the infusoria under the microscope do the same. No one gave a thought to the older worlds of space as sources of human danger, or thought of them only to dismiss the idea of life upon them as impossible or improbable. It is curious to recall some of the mental habits of those departed days. At most, terrestrial men fancied there might be other men upon Mars, perhaps inferior to themselves and ready to welcome a missionary enterprise. Yet across the gulf of space, minds that are to our minds as ours are to those of the beasts that perish, intellects vast and cool and unsympathetic, regarded this Earth with envious eyes, and slowly and surely drew their plans against us.

These opening lines of H. G. Wells’ 1897 science fiction classic The War of the Worlds maintain their haunting power to this day.* For all of our history, there has been the fear, or hope, that there might be life beyond the Earth. In the last hundred years, that premonition has focused on a bright red point of light in the night sky. Three years before The War of the Worlds was published, a Bostonian named Percival Lowell founded a major observatory where the most elaborate claims in support of life on Mars were developed. Lowell dabbled in astronomy as a young man, went to Harvard, secured a semi-official diplomatic appointment to Korea, and otherwise engaged in the usual pursuits of the wealthy. Before he died in 1916, he had made major contributions to our knowledge of the nature and evolution of the planets, to the deduction of the expanding universe and, in a decisive way, to the discovery of the planet Pluto, which is named after him. The first two letters of the name Pluto are the initials of Percival Lowell. Its symbol is , a planetary monogram.

But Lowell’s lifelong love was the planet Mars. He was electrified by the announcement in 1877 by an Italian astronomer, Giovanni Schiaparelli, of canali on Mars. Schiaparelli had reported during a close approach of Mars to Earth an intricate network of single and double straight lines crisscrossing the bright areas of the planet. Canali in Italian means channels or grooves, but was promptly translated into English as canals, a word that implies intelligent design. A Mars mania coursed through Europe and America, and Lowell found himself swept up with it.

In 1892, his eyesight failing, Schiaparelli announced he was giving up observing Mars. Lowell resolved to continue the work. He wanted a first-rate observing site, undisturbed by clouds or city lights and marked by good “seeing,” the astronomer’s term for a steady atmosphere through which the shimmering of an astronomical image in the telescope is minimized. Bad seeing is produced by small-scale turbulence in the atmosphere above the telescope and is the reason the stars twinkle. Lowell built his observatory far away from home, on Mars Hill in Flagstaff, Arizona. He sketched the surface features of Mars, particularly the canals, which mesmerized him. Observations of this sort are not easy. You put in long hours at the telescope in the chill of the early morning. Often the seeing is poor and the image of Mars blurs and distorts. Then you must ignore what you have seen. Occasionally the image steadies and the features of the planet flash out momentarily, marvelously. You must then remember what has been vouchsafed to you and accurately commit it to paper. You must put your preconceptions aside and with an open mind set down the wonders of Mars.

Percival Lowell’s notebooks are full of what he thought he saw: bright and dark areas, a hint of polar cap, and canals, a planet festooned with canals. Lowell believed he was seeing a globe-girdling network of great irrigation ditches, carrying water from the melting polar caps to the thirsty inhabitants of the equatorial cities. He believed the planet to be inhabited by an older and wiser race, perhaps very different from us. He believed that the seasonal changes in the dark areas were due to the growth and decay of vegetation. He believed that Mars was, very closely, Earth-like. All in all, he believed too much.

Lowell conjured up a Mars that was ancient, arid, withered, a desert world. Still, it was an Earth-like desert. Lowell’s Mars had many features in common with the American Southwest, where the Lowell Observatory was located. He imagined the Martian temperatures a little on the chilly side but still as comfortable as “the South of England.” The air was thin, but there was enough oxygen to be breathable. Water was rare, but the elegant network of canals carried the life-giving fluid all over the planet.

What was in retrospect the most serious contemporary challenge to Lowell’s ideas came from an unlikely source. In 1907, Alfred Russel Wallace, co-discoverer of evolution by natural selection, was asked to review one of Lowell’s books. He had been an engineer in his youth and, while somewhat credulous on such issues as extrasensory perception, was admirably skeptical on the habitability of Mars. Wallace showed that Lowell had erred in his calculation of the average temperatures on Mars; instead of being as temperate as the South of England, they were, with few exceptions, everywhere below the freezing point of water. There should be permafrost, a perpetually frozen subsurface. The air was much thinner than Lowell had calculated. Craters should be as abundant as on the Moon. And as for the water in the canals:

Any attempt to make that scanty surplus [of water], by means of overflowing canals, travel across the equator into the opposite hemisphere, through such terrible desert regions and exposed to such a cloudless sky as Mr. Lowell describes, would be the work of a body of madmen rather than of intelligent beings. It may be safely asserted that not one drop of water would escape evaporation or insoak at even a hundred miles from its source.

This devastating and largely correct physical analysis was written in Wallace’s eighty-fourth year. His conclusion was that life on Mars—by this he meant civil engineers with an interest in hydraulics—was impossible. He offered no opinion on microorganisms.

Despite Wallace’s critique, despite the fact that other astronomers with telescopes and observing sites as good as Lowell’s could find no sign of the fabled canals, Lowell’s vision of Mars gained popular acceptance. It had a mythic quality as old as Genesis. Part of its appeal was the fact that the nineteenth century was an age of engineering marvels, including the construction of enormous canals: the Suez Canal, completed in 1869; the Corinth Canal, in 1893; the Panama Canal, in 1914; and, closer to home, the Great Lake locks, the barge canals of upper New York State, and the irrigation canals of the American Southwest. If Europeans and Americans could perform such feats, why not Martians? Might there not be an even more elaborate effort by an older and wiser species, courageously battling the advance of desiccation on the red planet?

We have now sent reconnaissance satellites into orbit around Mars. The entire planet has been mapped. We have landed two automated laboratories on its surface. The mysteries of Mars have, if anything, deepened since Lowell’s day. However, with pictures far more detailed than any view of Mars that Lowell could have glimpsed, we have found not a tributary of the vaunted canal network, not one lock. Lowell and Schiaparelli and others, doing visual observations under difficult seeing conditions, were misled—in part perhaps because of a predisposition to believe in life on Mars.

The observing notebooks of Percival Lowell reflect a sustained effort at the telescope over many years. They show Lowell to have been well aware of the skepticism expressed by other astronomers about the reality of the canals. They reveal a man convinced that he has made an important discovery and distressed that others have not yet understood its significance. In his notebook for 1905, for example, there is an entry on January 21: “Double canals came out by flashes, convincing of reality.” In reading Lowell’s notebooks I have the distinct but uncomfortable feeling that he was really seeing something. But what?

When Paul Fox of Cornell and I compared Lowell’s maps of Mars with the Mariner 9 orbital imagery—sometimes with a resolution a thousand times superior to that of Lowell’s Earthbound twenty-four-inch refracting telescope—we found virtually no correlation at all. It was not that Lowell’s eye had strung up disconnected fine detail on the Martian surface into illusory straight lines. There was no dark mottling or crater chains in the position of most of his canals. There were no features there at all. Then how could he have drawn the same canals year after year? How could other astronomers—some of whom said they had not examined Lowell’s maps closely until after their own observations—have drawn the same canals? One of the great findings of the Mariner 9 mission to Mars was that there are time-variable streaks and splotches on the Martian surface—many connected with the ramparts of impact craters—which change with the seasons. They are due to windblown dust, the patterns varying with the seasonal winds. But the streaks do not have the character of the canals, they are not in the position of the canals, and none of them is large enough individually to be seen from the Earth in the first place. It is unlikely that there were real features on Mars even slightly resembling Lowell’s canals in the first few decades of this century that have disappeared without a trace as soon as close-up spacecraft investigations became possible.

The canals of Mars seem to be some malfunction, under difficult seeing conditions, of the human hand/eye/brain combination (or at least for some humans; many other astronomers, observing with equally good instruments in Lowell’s time and after, claimed there were no canals whatever). But this is hardly a comprehensive explanation, and I have the nagging suspicion that some essential feature of the Martian canal problem still remains undiscovered. Lowell always said that the regularity of the canals was an unmistakable sign that they were of intelligent origin. This is certainly true. The only unresolved question was which side of the telescope the intelligence was on.

Lowell’s Martians were benign and hopeful, even a little godlike, very different from the malevolent menace posed by Wells and Welles in The War of the Worlds. Both sets of ideas passed into the public imagination through Sunday supplements and science fiction. I can remember as a child reading with breathless fascination the Mars novels of Edgar Rice Burroughs. I journeyed with John Carter, gentleman adventurer from Virginia, to “Barsoom,” as Mars was known to its inhabitants. I followed herds of eight-legged beasts of burden, the thoats. I won the hand of the lovely Dejah Thoris, Princess of Helium. I befriended a four-meter-high green fighting man named Tars Tarkas. I wandered within the spired cities and domed pumping stations of Barsoom, and along the verdant banks of the Nilosyrtis and Nepenthes canals.

Might it really be possible—in fact and not in fancy—to venture with John Carter to the Kingdom of Helium on the planet Mars? Could we venture out on a summer evening, our way illuminated by the two hurtling moons of Barsoom, for a journey of high scientific adventure? Even if all Lowell’s conclusions about Mars, including the existence of the fabled canals, turned out to be bankrupt, his depiction of the planet had at least this virtue: it aroused generations of eight-year-olds, myself among them, to consider the exploration of the planets as a real possibility, to wonder if we ourselves might one day voyage to Mars. John Carter got there by standing in an open field, spreading his hands and wishing. I can remember spending many an hour in my boyhood, arms resolutely outstretched in an empty field, imploring what I believed to be Mars to transport me there. It never worked. There had to be some other way.

Like organisms, machines also have their evolutions. The rocket began, like the gunpowder that first powered it, in China where it was used for ceremonial and aesthetic purposes. Imported to Europe around the fourteenth century, it was applied to warfare, discussed in the late nineteenth century as a means of transportation to the planets by the Russian schoolteacher Konstantin Tsiolkovsky, and first developed seriously for high altitude flight by the American scientist Robert Goddard. The German V-2 military rocket of World War II employed virtually all of Goddard’s innovations and culminated in 1948 in the two-stage launching of the V-2/WAC Corporal combination to the then-unprecedented altitude of 400 kilometers. In the 1950’s, engineering advances organized by Sergei Korolov in the Soviet Union and Wernher von Braun in the United States, funded as delivery systems for weapons of mass destruction, led to the first artificial satellites. The pace of progress has continued to be brisk: manned orbital flight; humans orbiting, then landing on the moon; and unmanned spacecraft outward bound throughout the solar system. Many other nations have now launched spacecraft, including Britain, France, Canada, Japan and China, the society that invented the rocket in the first place.

Among the early applications of the space rocket, as Tsiolkovsky and Goddard (who as a young man had read Wells and had been stimulated by the lectures of Percival Lowell) delighted in imagining, were an orbiting scientific station to monitor the Earth from a great height and a probe to search for life on Mars. Both these dreams have now been fulfilled.

Imagine yourself a visitor from some other and quite alien planet, approaching Earth with no preconceptions. Your view of the planet improves as you come closer and more and more fine detail stands out. Is the planet inhabited? At what point can you decide? If there are intelligent beings, perhaps they have created engineering structures that have high-contrast components on a scale of a few kilometers, structures detectable when our optical systems and distance from the Earth provide kilometer resolution. Yet at this level of detail, the Earth seems utterly barren. There is no sign of life, intelligent or otherwise, in places we call Washington, New York, Boston, Moscow, London, Paris, Berlin, Tokyo and Peking. If there are intelligent beings on Earth, they have not much modified the landscape into regular geometrical patterns at kilometer resolution.

But when we improve the resolution tenfold, when we begin to see detail as small as a hundred meters across, the situation changes. Many places on Earth seem suddenly to crystallize out, revealing an intricate pattern of squares and rectangles, straight lines and circles. These are, in fact, the engineering artifacts of intelligent beings: roads, highways, canals, farmland, city streets—a pattern disclosing the twin human passions for Euclidean geometry and territoriality. On this scale, intelligent life can be discerned in Boston and Washington and New York. And at ten-meter resolution, the degree to which the landscape has been reworked first really becomes evident. Humans have been very busy. These photos have been taken in daylight. But at twilight or during the night, other things are visible: oil-well fires in Libya and the Persian Gulf; deepwater illumination by the Japanese squid fishing fleet; the bright lights of large cities. And if, in daylight, we improve our resolution so we can make out things that are a meter across, then we begin to detect for the first time individual organisms—whales, cows, flamingos, people.

Intelligent life on Earth first reveals itself through the geometric regularity of its constructions. If Lowell’s canal network really existed, the conclusion that intelligent beings inhabit Mars might be similarly compelling. For life to be detected on Mars photographically, even from Mars orbit, it must likewise have accomplished a major reworking of the surface. Technical civilizations, canal builders, might be easy to detect. But except for one or two enigmatic features, nothing of the sort is apparent in the exquisite profusion of Martian surface detail uncovered by unmanned spacecraft. However, there are many other possibilities, ranging from large plants and animals to microorganisms, to extinct forms, to a planet that is now and was always lifeless. Because Mars is farther from the Sun than is the Earth, its temperatures are considerably lower. Its air is thin, containing mostly carbon dioxide but also some molecular nitrogen and argon and very small quantities of water vapor, oxygen and ozone. Open bodies of liquid water are impossible today because the atmospheric pressure on Mars is too low to keep even cold water from rapidly boiling. There may be minute quantities of liquid water in pores and capillaries in the soil. The amount of oxygen is far too little for a human being to breathe. The ozone abundance is so small that germicidal ultraviolet radiation from the Sun strikes the Martian surface unimpeded. Could any organism survive in such an environment?

To test this question, many years ago my colleagues and I prepared chambers that simulated the Martian environment as it was then known, inoculated them with terrestrial microorganisms and waited to see if anybody survived. Such chambers are called, of course, Mars Jars. The Mars Jars cycled the temperatures within a typical Martian range from a little above the freezing point around noon to about – 80°C just before dawn, in an anoxic atmosphere composed chiefly of CO2 and N2. Ultraviolet lamps reproduced the fierce solar flux. No liquid water was present except for very thin films wetting individual sand grains. Some microbes froze to death after the first night and were never heard from again. Others gasped and perished from lack of oxygen. Some died of thirst, and some were fried by the ultraviolet light. But there were always a fair number of varieties of terrestrial microbes that did not need oxygen; mat temporarily closed up shop when the temperatures dropped too low; that hid from the ultraviolet light under pebbles or thin layers of sand. In other experiments, when small quantities of liquid water were present, the microbes actually grew. If terrestrial microbes can survive the Martian environment, how much better Martian microbes, if they exist, must do on Mars. But first we must get there.

The Soviet Union maintains an active program of unmanned planetary exploration. Every year or two the relative positions of the planets and the physics of Kepler and Newton permit the launch of a spacecraft to Mars or Venus with a minimum expenditure of energy. Since the early 1960’s the U.S.S.R. has missed few such opportunities. Soviet persistence and engineering skills have eventually paid off handsomely. Five Soviet spacecraft—Veneras 8 through 12—have landed on Venus and successfully returned data from the surface, no insignificant feat in so hot, dense and corrosive a planetary atmosphere. Yet despite many attempts, the Soviet Union has never landed successfully on Mars—a place that, at least at first sight, seems more hospitable, with chilly temperatures, a much thinner atmosphere and more benign gases; with polar ice caps, clear pink skies, great sand dunes, ancient river beds, a vast rift valley, the largest volcanic construct, so far as we know, in the solar system, and balmy equatorial summer afternoons. It is a far more Earth-like world than Venus.

In 1971, the Soviet Mars 3 spacecraft entered the Martian atmosphere. According to the information automatically radioed back, it successfully deployed its landing systems during entry, correctly oriented its ablation shield downward, properly unfurled its great parachute and fired its retro-rockets near the end of its descent path. According to the data returned by Mars 3, it should have landed successfully on the red planet. But after landing, the spacecraft returned a twenty-second fragment of a featureless television picture to Earth and then mysteriously failed. In 1973, a quite similar sequence of events occurred with the Mars 6 lander, in that case the failure occurring within one second of touchdown. What went wrong?

The first illustration I ever saw of Mars 3 was on a Soviet postage stamp (denomination, 16 kopecks), which depicted the spacecraft descending through a kind of purple muck. The artist was trying, I think, to illustrate dust and high winds: Mars 3 had entered the Martian atmosphere during an enormous global dust storm. We have evidence from the U.S. Mariner 9 mission that near-surface winds of more than 140 meters per second—faster than half the speed of sound on Mars—arose in that storm. Both our Soviet colleagues and we think it likely that these high winds caught the Mars 3 spacecraft with parachute unfurled, so that it landed gently in the vertical direction but with breakneck speed in the horizontal direction. A spacecraft descending on the shrouds of a large parachute is particularly vulnerable to horizontal winds. After landing, Mars 3 may have made a few bounces, hit a boulder or other example of Martian relief, tipped over, lost the radio link with its carrier “bus” and failed.

But why did Mars 3 enter in the midst of a great dust storm? The Mars 3 mission was rigidly organized before launch. Every step it was to perform was loaded into the on-board computer before it left Earth. There was no opportunity to change the computer program, even as the extent of the great 1971 dust storm became clear. In the jargon of space exploration, the Mars 3 mission was preprogrammed, not adaptive. The failure of Mars 6 is more mysterious. There was no planet-wide storm when this spacecraft entered the Martian atmosphere, and no reason to suspect a local storm, as sometimes happens, at the landing site. Perhaps there was an engineering failure just at the moment of touchdown. Or perhaps there is something particularly dangerous about the Martian surface.

The combination of Soviet successes in landing on Venus and Soviet failures in landing on Mars naturally caused us some concern about the U.S. Viking mission, which had been informally scheduled to set one of its two descent craft gently down on the Martian surface on the Bicentennial of the United States, July 4, 1976. Like its Soviet predecessors, the Viking landing maneuver involved an ablation shield, a parachute and retro-rockets. Because the Martian atmosphere is only 1 percent as dense as the Earth’s, a very large parachute, eighteen meters in diameter, was deployed to slow the spacecraft as it entered the thin air of Mars. The atmosphere is so thin that if Viking had landed at a high elevation there would not have been enough atmosphere to brake the descent adequately: it would have crashed. One requirement, therefore, was for a landing site in a low-lying region. From Mariner 9 results and ground-based radar studies, we knew many such areas.

To avoid the probable fate of Mars 3, we wanted Viking to land in a place and time at which the winds were low. Winds that would make the lander crash were probably strong enough to lift dust off the surface. If we could check that the candidate landing site was not covered with sifting, drifting dust, we would have at least a fair chance of guaranteeing that the winds were not intolerably high. This was one reason that each Viking lander was carried into Mars orbit with its orbiter, and descent delayed until the orbiter surveyed the landing site. We had discovered with Mariner 9 that characteristic changes in the bright and dark patterns on the Martian surface occur during times of high winds. We certainly would not have certified a Viking landing site as safe if orbital photographs had shown such shifting patterns. But our guarantees could not be 100 percent reliable. For example, we could imagine a landing site at which the winds were so strong that all mobile dust had already been blown away. We would then have had no indication of the high winds that might have been there. Detailed weather predictions for Mars were, of course, much less reliable than for Earth. (Indeed one of the many objectives of the Viking mission was to improve our understanding of the weather on both planets.)

Because of communication and temperature constraints, Viking could not land at high Martian latitudes. Farther poleward than about 45 or 50 degrees in both hemispheres, either the time of useful communication of the spacecraft with the Earth or the period during which the spacecraft would avoid dangerously low temperatures would have been awkwardly short.

We did not wish to land in too rough a place. The spacecraft might have tipped over and crashed, or at the least its mechanical arm, intended to acquire Martian soil samples, might have become wedged or been left waving helplessly a meter too high above the surface. Likewise, we did not want to land in places that were too soft. If the spacecraft’s three landing pods had sunk deeply into a loosely packed soil, various undesirable consequences would have followed, including immobilization of the sample arm. But we did not want to land in a place that was too hard either—had we landed in a vitreous lava field, for example, with no powdery surface material, the mechanical arm would have been unable to acquire the samples vital to the projected chemistry and biology experiments.

The best photographs then available of Mars—from the Mariner 9 orbiter—showed features no smaller than 90 meters (100 yards) across. The Viking orbiter pictures improved this figure only slightly. Boulders one meter (three feet) in size were entirely invisible in such photographs, and could have had disastrous consequences for the Viking lander. Likewise, a deep, soft powder might have been indetectable photographically. Fortunately, there was a technique that enabled us to determine the roughness or softness of a candidate landing site: radar. A very rough place would scatter radar from Earth off to the sides of the beam and therefore appear poorly reflective, or radar-dark. A very soft place would also appear poorly reflective because of the many interstices between individual sand grains. While we were unable to distinguish between rough places and soft places, we did not need to make such distinctions for landing-site selection. Both, we knew, were dangerous. Preliminary radar surveys suggested that as much as a quarter to a third of the surface area of Mars might be radar-dark, and therefore dangerous for Viking. But not all of Mars can be viewed by Earth-based radar—only a swath between about 25° N and about 25° S. The Viking orbiter carried no radar system of its own to map the surface.

There were many constraints—perhaps, we feared, too many. Our landing sites had to be not too high, too windy, too hard, too soft, too rough or too close to the pole. It was remarkable that there were any places at all on Mars that simultaneously satisfied all our safety criteria. But it was also clear that our search for safe harbors had led us to landing sites that were, by and large, dull.

When each of the two Viking orbiter-lander combinations was inserted into Martian orbit, it was unalterably committed to landing at a certain latitude on Mars. If the low point in the orbit was at 21° Martian north latitude, the lander would touch down at 21° N, although, by waiting for the planet to turn beneath it, it could land at any longitude whatever. Thus the Viking science teams selected candidate latitudes for which there was more than one promising site. Viking 1 was targeted for 21° N. The prime site was in a region called Chryse (Greek for “the land of gold”), near the confluence of four sinuous channels thought to have been carved in previous epochs of Martian history by running water. The Chryse site seemed to satisfy all safety criteria. But the radar observations had been made nearby, not in the Chryse landing site itself. Radar observations of Chryse were made for the first time—because of the geometry of Earth and Mars—only a few weeks before the nominal landing date.

The candidate landing latitude for Viking 2 was 44° N; the prime site, a locale called Cydonia, chosen because, according to some theoretical arguments, there was a significant chance of small quantities of liquid water there, at least at some time during the Martian year. Since the Viking biology experiments were strongly oriented toward organisms that are comfortable in liquid water, some scientists held that the chance of Viking finding life would be substantially improved in Cydonia. On the other hand, it was argued that, on so windy a planet as Mars, microorganisms should be everywhere if they are anywhere. There seemed to be merit to both positions, and it was difficult to decide between them. What was quite clear, however, was that 44° N was completely inaccessible to radar site-certification; we had to accept a significant risk of failure with Viking 2 if it was committed to high northern latitudes. It was sometimes argued that if Viking 1 was down and working well we could afford to accept a greater risk with Viking 2. I found myself making very conservative recommendations on the fate of a billion-dollar mission. I could imagine, for example, a key instrument failure in Chryse just after an unfortunate crash landing in Cydonia. To improve the Viking options, additional landing sites, geologically very different from Chryse and Cydonia, were selected in the radar-certified region near 4° S latitude. A decision on whether Viking 2 would set down at high or at low latitude was not made until virtually the last minute, when a place with the hopeful name of Utopia, at the same latitude as Cydonia, was chosen.

For Viking 1, the original landing site seemed, after we examined orbiter photographs and late-breaking Earth-based radar data, unacceptably risky. For a while I worried that Viking 1 had been condemned, like the legendary Flying Dutchman, to wander the skies of Mars forever, never to find safe haven. Eventually we found a suitable spot, still in Chryse but far from the confluence of the four ancient channels. The delay prevented us from setting down on July 4, 1976, but it was generally agreed that a crash landing on that date would have been an unsatisfactory two hundredth birthday present for the United States. We deboosted from orbit and entered the Martian atmosphere sixteen days later.

After an interplanetary voyage of a year and a half, covering a hundred million kilometers the long way round the Sun, each orbiter/lander combination was inserted into its proper orbit about Mars; the orbiters surveyed candidate landing sites; the landers entered the Martian atmosphere on radio command and correctly oriented ablation shields, deployed parachutes, divested coverings, and fired retro-rockets. In Chryse and Utopia, for the first time in human history, spacecraft had touched down, gently and safely, on the red planet. These triumphant landings were due in considerable part to the great skill invested in their design, fabrication and testing, and to the abilities of the spacecraft controllers. But for so dangerous and mysterious a planet as Mars, there was also at least an element of luck.

Immediately after landing, the first pictures were to be returned. We knew we had chosen dull places. But we could hope. The first picture taken by the Viking 1 lander was of one of its own footpads—in case it were to sink into Martian quicksand, we wanted to know about it before the spacecraft disappeared. The picture built up, line by line, until with enormous relief we saw the footpad sitting high and dry above the Martian surface. Soon other pictures came into being, each picture element radioed individually back to Earth.

I remember being transfixed by the first lander image to show the horizon of Mars. This was not an alien world, I thought. I knew places like it in Colorado and Arizona and Nevada. There were rocks and sand drifts and a distant eminence, as natural and unselfconscious as any landscape on Earth. Mars was a place. I would, of course, have been surprised to see a grizzled prospector emerge from behind a dune leading his mule, but at the same time the idea seemed appropriate. Nothing remotely like it ever entered my mind in all the hours I spent examining the Venera 9 and 10 images of the Venus surface. One way or another, I knew, this was a world to which we would return.

The landscape is stark and red and lovely: boulders thrown out in the creation of a crater somewhere over the horizon, small sand dunes, rocks that have been repeatedly covered and uncovered by drifting dust, plumes of fine-grained material Mown about by the winds. Where did the rocks come from? How much sand had been blown by wind? What must the previous history of the planet have been to create sheared rocks, buried boulders, polygonal gouges in the ground? What are the rocks made of? The same materials as the sand? Is the sand merely pulverized rock or something else? Why is the sky pink? What is the air made of? How fast does the wind blow? Are there marsquakes? How do the atmospheric pressure and the appearance of the landscape change with the seasons?

For every one of these questions Viking has provided definitive or at least plausible answers. The Mars revealed by the Viking mission is of enormous interest—particularly when we remember that the landing sites were chosen for their dullness. But the cameras revealed no sign of canal builders, no Barsoomian aircars or short swords, no princesses or fighting men, no thoats, no footprints, not even a cactus or a kangaroo rat. For as far as we could see, there was not a sign of life.*

Perhaps there are large lifeforms on Mars, but not in our two landing sites. Perhaps there are smaller forms in every rock and sand grain. For most of its history, those regions of the Earth not covered by water looked rather like Mars today—with an atmosphere rich in carbon dioxide, with ultraviolet light shining fiercely down on the surface through an atmosphere devoid of ozone. Large plants and animals did not colonize the land until the last 10 percent of Earth history. And yet for three billion years there were microorganisms everywhere on Earth. To look for life on Mars, we must look for microbes.

The Viking lander extends human capabilities to other and alien landscapes. By some standards, it is about as smart as a grasshopper; by others, only as intelligent as a bacterium. There is nothing demeaning in these comparisons. It took nature hundreds of millions of years to evolve a bacterium, and billions to make a grasshopper. With only a little experience in this sort of business, we are becoming fairly skillful at it. Viking has two eyes as we do, but they also work in the infrared, as ours do not; a sample arm that can push rocks, dig and acquire soil samples; a kind of finger that it puts up to measure wind speed and direction; a nose and taste buds, of a sort, with which it senses, to a much higher precision than we can, the presence of trace molecules; an interior ear with which it can detect the rumbling of marsquakes and the gentler wind-driven jiggling of the spacecraft; and a means of detecting microbes. The spacecraft has its own self-contained radioactive power source. It radios all the scientific information it acquires back to Earth. It receives instructions from Earth, so human beings can ponder the significance of the Viking results and tell the spacecraft to do something new.

But what is the optimum way, given severe constraints on size, cost and power requirements, to search for microbes on Mars? We cannot—at least as yet—send microbiologists there. I once had a friend, an extraordinary microbiologist named Wolf Vishniac, of the University of Rochester, in New York. In the late 1950’s, when we were just beginning to think seriously about looking for life on Mars, he found himself at a scientific meeting where an astronomer expressed amazement that the biologists had no simple, reliable, automated instrument capable of looking for microorganisms. Vishniac decided he would do something about the matter.

He developed a small device to be sent to the planets. His friends called it the Wolf Trap. It would carry a little vial of nutrient organic matter to Mars, arrange for a sample of Martian soil to be mixed with it, and observe the changing turbidity or cloudiness of the liquid as the Martian bugs (if there were any) grew (if they would). The Wolf Trap was selected along with three other microbiology experiments to go aboard the Viking landers. Two of the other three experiments also chose to send food to the Martians. The success of the Wolf Trap required that Martian bugs like liquid water. There were those who thought that Vishniac would only drown the little Martians. But the advantage of the Wolf Trap was that it laid no requirements on what the Martian microbes must do with their food. They had only to grow. All the other experiments made specific assumptions about gases that would be given off or taken in by the microbes, assumptions that were little more than guesses.

The National Aeronautics and Space Administration, which runs the United States planetary space program, is subject to frequent and unpredictable budget cuts. Only rarely are there unanticipated budget increases. NASA scientific activities have very little effective support in the government, and so science is most often the target when money needs to be taken away from NASA. In 1971 it was decided that one of the four microbiology experiments must be removed, and the Wolf Trap was off-loaded. It was a crushing disappointment for Vishniac, who had invested twelve years in its development.

Many others in his place might have stalked off the Viking Biology Team. But Vishniac was a gentle and dedicated man. He decided instead that he could best serve the search for life on Mars by voyaging to the most Mars-like environment on Earth—the dry valleys of Antarctica. Some previous investigators had examined Antarctic soil and decided that the few microbes they were able to find were not really natives of the dry valleys, but had been blown there from other, more clement environments. Recalling the Mars Jars experiments, Vishniac believed that life was tenacious and that Antarctica was perfectly consistent with microbiology. If terrestrial bugs could live on Mars, he thought, why not in Antarctica—which was by and large warmer, wetter, and had more oxygen and much less ultraviolet light. Conversely, finding life in Antarctic dry valleys would correspondingly improve, he thought, the chances of life on Mars. Vishniac believed that the experimental techniques previously used to deduce no indigenous microbes in Antarctica were flawed. The nutrients, while suitable for the comfortable environment of a university microbiology laboratory, were not designed for the arid polar wasteland.

So on November 8, 1973, Vishniac, his new microbiology equipment and a geologist companion were transported by helicopter from McMurdo Station to an area near Mount Balder, a dry valley in the Asgard range. His practice was to implant the little microbiology stations in the Antarctic soil and return about a month later to retrieve them. On December 10, 1973, he left to gather samples on Mount Balder; his departure was photographed from about three kilometers away. It was the last time anyone saw him alive. Eighteen hours later, his body was discovered at the base of a cliff of ice. He had wandered into an area not previously explored, had apparently slipped on the ice and tumbled and bounced for a distance of 150 meters. Perhaps something had caught his eye, a likely habitat for microbes, say, or a patch of green where none should be. We will never know. In the small brown notebook he was carrying that day, the last entry reads, “Station 202 retrieved. 10 December, 1973. 2230 hours. Soil temperature, – 10°. Air temperature – 16°.” It had been a typical summer temperature for Mars.

Many of Vishniac’s microbiology stations are still sitting in Antarctica. But the samples that were returned were examined, using his methods, by his professional colleagues and friends. A wide variety of microbes, which would have been indetectable with conventional scoring techniques, was found in essentially every site examined. A new species of yeast, apparently unique to Antarctica, was discovered in his samples by his widow, Helen Simpson Vishniac. Large rocks returned from Antarctica in that expedition, examined by Imre Friedmann, turn out to have a fascinating microbiology—one or two millimeters inside the rock, algae have colonized a tiny world in which small quantities of water are trapped and made liquid. On Mars such a place would be even more interesting, because while the visible light necessary for photosynthesis would penetrate to that depth, the germicidal ultraviolet light would be at least partially attenuated.

Because the design of space missions is finalized many years before launch, and because of Vishniac’s death, the results of his Antarctic experiments did not influence the Viking design for seeking Martian life. In general, the microbiology experiments were not carried out at the low ambient Martian temperatures, and most did not provide long incubation times. They all made fairly strong assumptions about what Martian metabolism had to be like. There was no way to look for life inside the rocks.

Each Viking lander was equipped with a sample arm to acquire material from the surface and then slowly withdraw it into the innards of the spacecraft, transporting the particles on little hoppers like an electric train to five different experiments: one on the inorganic chemistry of the soil, another to look for organic molecules in the sand and dust, and three to look for microbial life. When we look for life on a planet, we are making certain assumptions. We try, as well as we can, not to assume that life elsewhere will be just like life here. But there are limits to what we can do. We know in detail only about life here. While the Viking biology experiments are a pioneering first effort, they hardly represent a definitive search for life on Mars. The results have been tantalizing, annoying, provocative, stimulating, and, at least until recently, substantially inconclusive.

Each of the three microbiology experiments asked a different kind of question, but in all cases a question about Martian metabolism. If there are microorganisms in the Martian soil, they must take in food and give off waste gases; or they must take in gases from the atmosphere and, perhaps with the aid of sunlight, convert them into useful materials. So we bring food to Mars and hope that the Martians, if there are any, will find it tasty. Then we see if any interesting new gases come out of the soil. Or we provide our own radioactively labeled gases and see if they are converted into organic matter, in which case small Martians are inferred.

By criteria established before launch, two of the three Viking microbiology experiments seem to have yielded positive results. First, when Martian soil was mixed with a sterile organic soup from Earth, something in the soil chemically broke down the soup—almost as if there were respiring microbes metabolizing a food package from Earth. Second, when gases from Earth were introduced into the Martian soil sample, the gases became chemically combined with the soil—almost as if there were photosynthesizing microbes, generating organic matter from atmospheric gases. Positive results in Martian microbiology were achieved in seven different samplings in two locales on Mars separated by 5,000 kilometers.

But the situation is complex, and the criteria of experimental success may have been inadequate. Enormous efforts were made to build the Viking microbiology experiments and test them with a variety of microbes. Very little effort was made to calibrate the experiments with plausible inorganic Martian surface materials. Mars is not the Earth. As the legacy of Percival Lowell reminds us, we can be fooled. Perhaps there is an exotic inorganic chemistry in the Martian soil that is able by itself, in the absence of Martian microbes, to oxidize foodstuffs. Perhaps there is some special inorganic, nonliving catalyst in the soil that is able to fix atmospheric gases and convert them into organic molecules.

Recent experiments suggest that this may indeed be the case. In the great Martian dust storm of 1971, spectral features of the dust were obtained by the Mariner 9 infrared spectrometer. In analyzing these spectra, O. B. Toon, J. B. Pollack and I found that certain features seem best accounted for by montmorillonite and other kinds of clay. Subsequent observations by the Viking lander support the identification of windblown clays on Mars. Now, A. Banin and J. Rishpon have found that they can reproduce some of the key features—those resembling photosynthesis as well as those resembling respiration—of the “successful” Viking microbiology experiments if in laboratory experiments they substitute such clays for the Martian soil. The clays have a complex active surface, given to adsorbing and releasing gases and to catalyzing chemical reactions. It is too soon to say that all the Viking microbiology results can be explained by inorganic chemistry, but such a result would no longer be surprising. The clay hypothesis hardly excludes life on Mars, but it certainly carries us far enough to say that there is no compelling evidence for microbiology on Mars.

Even so, the results of Banin and Rishpon are of great biological importance because they show that in the absence of life there can be a kind of soil chemistry that does some of the same things life does. On the Earth before life, there may already have been chemical processes resembling respiration and photosynthesis cycling in the soil, perhaps to be incorporated by life once it arose. In addition, we know that montmorillonite clays are a potent catalyst for combining amino acids into longer chain molecules resembling proteins. The clays of the primitive Earth may have been the forge of life, and the chemistry of contemporary Mars may provide essential clues to the origin and early history of life on our planet.

The Martian surface exhibits many impact craters, each named after a person, usually a scientist. Crater Vishniac lies appropriately in the Antarctic region of Mars. Vishniac did not claim that there had to be life on Mars, merely that it was possible, and that it was extraordinarily important to know if it was there. If life on Mars exists, we will have a unique opportunity to test the generality of our kind of life. And if there is no life on Mars, a planet rather like the Earth, we must understand why—because in that case, as Vishniac stressed, we have the classic scientific confrontation of the experiment and the control.

The finding that the Viking microbiology results can be explained by clays, that they need not imply life, helps to resolve another mystery: the Viking organic chemistry experiment showed not a hint of organic matter in the Martian soil. If there is life on Mars, where are the dead bodies? No organic molecules could be found—no building blocks of proteins and nucleic acids, no simple hydrocarbons, nothing of the stuff of life on Earth. This is not necessarily a contradiction, because the Viking microbiology experiments are a thousand times more sensitive (per equivalent carbon atom) than the Viking chemistry experiments, and seem to detect organic matter synthesized in the Martian soil. But this does not leave much margin. Terrestrial soil is loaded with the organic remains of once-living organisms; Martian soil has less organic matter than the surface of the Moon. If we held to the life hypothesis, we might suppose that the dead bodies have been destroyed by the chemically reactive, oxidizing surface of Mars—like a germ in a bottle of hydrogen peroxide; or that there is life, but of a kind in which organic chemistry plays a less central role than it does in life on Earth.

But this last alternative seems to me to be special pleading: I am, reluctantly, a self-confessed carbon chauvinist. Carbon is abundant in the Cosmos. It makes marvelously complex molecules, good for life. I am also a water chauvinist. Water makes an ideal solvent system for organic chemistry to work in and stays liquid over a wide range of temperatures. But sometimes I wonder. Could my fondness for materials have something to do with the fact that I am made chiefly of them? Are we carbon- and water-based because those materials were abundant on the Earth at the time of the origin of life? Could life elsewhere—on Mars, say—be built of different stuff?

I am a collection of water, calcium and organic molecules called Carl Sagan. You are a collection of almost identical molecules with a different collective label. But is that all? Is there nothing in here but molecules? Some people find this idea somehow demeaning to human dignity. For myself, I find it elevating that our universe permits the evolution of molecular machines as intricate and subtle as we.

But the essence of life is not so much the atoms and simple molecules that make us up as the way in which they are put together. Every now and then we read that the chemicals which constitute the human body cost ninety-seven cents or ten dollars or some such figure; it is a little depressing to find our bodies valued so little. However, these estimates are for human beings reduced to our simplest possible components. We are made mostly of water, which costs almost nothing; the carbon is costed in the form of coal; the calcium in our bones as chalk; the nitrogen in our proteins as air (cheap also); the iron in our blood as rusty nails. If we did not know better, we might be tempted to take all the atoms that make us up, mix them together in a big container and stir. We can do this as much as we want. But in the end all we have is a tedious mixture of atoms. How could we have expected anything else?

Harold Morowitz has calculated what it would cost to put together the correct molecular constitutents that make up a human being by buying the molecules from chemical supply houses. The answer turns out to be about ten million dollars, which should make us all feel a little better. But even then we could not mix those chemicals together and have a human being emerge from the jar. That is far beyond our capability and will probably be so for a very long period of time. Fortunately, there are other less expensive but still highly reliable methods of making human beings.

I think the lifeforms on many worlds will consist, by and large, of the same atoms we have here, perhaps even many of the same basic molecules, such as proteins and nucleic acids—but put together in unfamiliar ways. Perhaps organisms that float in dense planetary atmospheres will be very much like us in their atomic composition, except they might not have bones and therefore not need much calcium. Perhaps elsewhere some solvent other than water is used. Hydrofluoric acid might serve rather well, although there is not a great deal of fluorine in the Cosmos; hydrofluoric acid would do a great deal of damage to the kind of molecules that make us up, but other organic molecules, paraffin waxes, for example, are perfectly stable in its presence. Liquid ammonia would make an even better solvent system, because ammonia is very abundant in the Cosmos. But it is liquid only on worlds much colder than the Earth or Mars. Ammonia is ordinarily a gas on Earth, as water is on Venus. Or perhaps there are living things that do not have a solvent system at all—solid-state life, where there are electrical signals propagating rather than molecules floating about.

But these ideas do not rescue the notion that the Viking lander experiments indicate life on Mars. On that rather Earth-like world, with abundant carbon and water, life, if it exists, should be based on organic chemistry. The organic chemistry results, like the imaging and microbiology results, are all consistent with no life in the fine particles of Chryse and Utopia in the late 1970’s. Perhaps some millimeters beneath the rocks (as in the Antarctic dry valleys), or elsewhere on the planet, or in some earlier, more clement time. But not where and when we looked.

The Viking exploration of Mars is a mission of major historical importance, the first serious search for what other kinds of life may be, the first survival of a functioning spacecraft for more than an hour or so on any other planet (Viking 1 has survived for years), the source of a rich harvest of data on the geology, seismology, mineralogy, meteorology and half a dozen other sciences of another world. How should we follow up on these spectacular advances? Some scientists want to send an automatic device that would land, acquire soil samples, and return them to Earth, where they could be examined in great detail in the large sophisticated laboratories of Earth rather than in the limited microminiaturized laboratories that we are able to send to Mars. In this way most of the ambiguities of the Viking microbiology experiments could be resolved. The chemistry and mineralogy of the soil could be determined; rocks could be broken open to search for subsurface life; hundreds of tests for organic chemistry and life could be performed, including direct microscopic examination, under a wide range of conditions. We could even use Vishniac’s scoring techniques. Although it would be fairly expensive, such a mission is probably within our technological capability.

However, it carries with it a novel danger: back-contamination. If we wish on Earth to examine samples of Martian soil for microbes, we must, of course, not sterilize the samples beforehand. The point of the expedition is to bring them back alive. But what then? Might Martian microorganisms returned to Earth pose a public health hazard? The Martians of H. G. Wells and Orson Welles, preoccupied with the suppression of Bournemouth and Jersey City, never noticed until too late that their immunological defenses were unavailing against the microbes of Earth. Is the converse possible? This is a serious and difficult issue. There may be no micromartians. If they exist, perhaps we can eat a kilogram of them with no ill effects. But we are not sure, and the stakes are high. If we wish to return unsterilized Martian samples to Earth, we must have a containment procedure that is stupefyingly reliable. There are nations that develop and stockpile bacteriological weapons. They seem to have an occasional accident, but they have not yet, so far as I know, produced global pandemics. Perhaps Martian samples can be safely returned to Earth. But I would want to be very sure before considering a returned-sample mission.

There is another way to investigate Mars and the full range of delights and discoveries this heterogeneous planet holds for us. My most persistent emotion in working with the Viking lander pictures was frustration at our immobility. I found myself unconsciously urging the spacecraft at least to stand on its tiptoes, as if this laboratory, designed for immobility, were perversely refusing to manage even a little hop. How we longed to poke that dune with the sample arm, look for life beneath that rock, see if that distant ridge was a crater rampart. And not so very far to the southeast, I knew, were the four sinuous channels of Chryse. For all the tantalizing and provocative character of the Viking results, I know a hundred places on Mars which are far more interesting than our landing sites. The ideal tool is a roving vehicle carrying on advanced experiments, particularly in imaging, chemistry and biology. Prototypes of such rovers are under development by NASA. They know on their own how to go over rocks, how not to fall down ravines, how to get out of tight spots. It is within our capability to land a rover on Mars that could scan its surroundings, see the most interesting place in its field of view and, by the same time tomorrow, be there. Every day a new place, a complex, winding traverse over the varied topography of this appealing planet.

Such a mission would reap enormous scientific benefits, even if there is no life on Mars. We could wander down the ancient river valleys, up the slopes of one of the great volcanic mountains, along the strange stepped terrain of the icy polar terraces, or muster a close approach to the beckoning pyramids of Mars.* Public interest in such a mission would be sizable. Every day a new set of vistas would arrive on our home television screens. We could trace the route, ponder the findings, suggest new destinations. The journey would be long, the rover obedient to radio commands from Earth. There would be plenty of time for good new ideas to be incorporated into the mission plan. A billion people could participate in the exploration of another world.

The surface area of Mars is exactly as large as the land area of the Earth. A thorough reconnaissance will clearly occupy us for centuries. But there will be a time when Mars is all explored; a time after robot aircraft have mapped it from aloft, a time after rovers have combed the surface, a time after samples have been returned safely to Earth, a time after human beings have walked the sands of Mars. What then? What shall we do with Mars?

There are so many examples of human misuse of the Earth that even phrasing this question chills me. If mere is life on Mars, I believe we should do nothing with Mars. Mars then belongs to the Martians, even if the Martians are only microbes. The existence of an independent biology on a nearby planet is a treasure beyond assessing, and the preservation of that life must, I think, supersede any other possible use of Mars. However, suppose Mars is lifeless. It is not a plausible source of raw materials: the freightage from Mars to Earth would be too expensive for many centuries to come. But might we be able to live on Mars? Could we in some sense make Mars habitable?

A lovely world, surely, but there is—from our parochial point of view—much wrong with Mars, chiefly the low oxygen abundance, the absence of liquid water, and the high ultraviolet flux. (The low temperatures do not pose an insuperable obstacle, as the year-round scientific stations in Antarctica demonstrate.) All of these problems could be solved if we could make more air. With higher atmospheric pressures, liquid water would be possible. With more oxygen we might breathe the atmosphere, and ozone would form to shield the surface from solar ultraviolet radiation. The sinuous channels, stacked polar plates and other evidence suggest that Mars once had such a denser atmosphere. Those gases are unlikely to have escaped from Mars. They are, therefore, on the planet somewhere. Some are chemically combined with the surface rocks. Some are in subsurface ice. But most may be in the present polar ice caps.

To vaporize the caps, we must heat them; perhaps we could dust them with a dark powder, heating them by absorbing more sunlight, the opposite of what we do to the Earth when we destroy forests and grasslands. But the surface area of the caps is very large. The necessary dust would require 1,200 Saturn 5 rocket boosters to be transported from Earth to Mars; even then, the winds might blow the dust off the polar caps. A better way would be to devise some dark material able to make copies of itself, a little dusky machine which we deliver to Mars and which then goes about reproducing itself from indigenous materials all over the polar caps. There is a category of such machines. We call them plants. Some are very hardy and resilient. We know that at least some terrestrial microbes can survive on Mars. What is necessary is a program of artificial selection and genetic engineering of dark plants—perhaps lichens—that could survive the much more severe Martian environment. If such plants could be bred, we might imagine them being seeded on the vast expanse of the Martian polar ice caps, taking root, spreading, blackening the ice caps, absorbing sunlight, heating the ice, and releasing the ancient Martian atmosphere from its long captivity. We might even imagine a kind of Martian Johnny Appleseed, robot or human, roaming the frozen polar wastes in an endeavor that benefits only the generations of humans to come.

This general concept is called terraforming: the changing of an alien landscape into one more suitable for human beings. In thousands of years humans have managed to perturb the global temperature of the Earth by only about one degree through greenhouse and albedo changes, although at the present rate of burning fossil fuels and destroying forests and grasslands we can now change the global temperature by another degree in only a century or two. These and other considerations suggest that a time scale for a significant terraforming of Mars is probably hundreds to thousands of years. In a future time of greatly advanced technology we might wish not only to increase the total atmospheric pressure and make liquid water possible but also to carry liquid water from the melting polar caps to the warmer equatorial regions. There is, of course, a way to do it. We would build canals.

The melting surface and subsurface ice would be transported by a great canal network. But this is precisely what Percival Lowell, not a hundred years ago, mistakenly proposed was in fact happening on Mars. Lowell and Wallace both understood that the comparative inhospitability of Mars was due to the scarcity of water. If only a network of canals existed, the lack would be remedied, the habitability of Mars would become plausible. Lowell’s observations were made under extremely difficult seeing conditions. Others, like Schiaparelli, had already observed something like the canals; they were called canali before Lowell began his lifelong love affair with Mars. Human beings have a demonstrated talent for self-deception when their emotions are stirred, and there are few notions more stirring than the idea of a neighboring planet inhabited by intelligent beings.

The power of Lowell’s idea may, just possibly, make it a kind of premonition. His canal network was built by Martians. Even this may be an accurate prophecy: If the planet ever is terraformed, it will be done by human beings whose permanent residence and planetary affiliation is Mars. The Martians will be us.

*In 1938, a radio version, produced by Orson Welles, transposed the Martian invasion from England to the eastern United States, and frightened millions in war-jittery America into believing that the Martians were in fact attacking.

Isaac Newton had written “If the Theory of making Telescopes could at length be fully brought into practice, yet there would be certain Bounds beyond which Telescopes could not perform. For the Air through which we look upon the Stars, is in perpetual tremor.… The only remedy is the most serene and quiet Air, such as may perhaps be found on the tops of the highest mountains above the grosser Clouds.”

*There was a brief flurry when the uppercase letter B, a putative Martian graffito, seemed to be visible on a small boulder in Chryse. But later analysis showed it to be a trick of light and shadow and the human talent for pattern recognition. It also seems remarkable that the Martians should have tumbled independently to the Latin alphabet. But there was just a moment when resounding in my head was the distant echo of a word from my boyhood—Barsoom.

*The largest are 3 kilometers across at the base, and 1 kilometer high—much larger than the pyramids of Sumer, Egypt or Mexico on Earth. They seem eroded and ancient, and are, perhaps, only small mountains, sandblasted for ages. But they warrant, I think, a careful look.