The Search for Life on Mars — A Scientific Romance, by Don Dixon

IT IS LATE AFTERNOON. THE WESTERING sun shines brightly in a salmon sky, casting soft shadows on the rusty desert. The warm colors are deceptive, for the air temperature is a brisk 16° below zero, Celsius, but this is normal for late spring. A 20-miles-per-hour southwesterly breeze sighs over the rocky plain, adding to the chill. The wind has little force in the thin air, however, and its only effect is to disturb an occasional precariously perched grain of sand, or to nudge a wisp of cirrus cloud over the horizon. All is quiet and still. In every respect this is a typical, uneventful day on the Plains of Gold.

Suddenly there is a flash at the zenith, a metallic glint of reflected sunlight. A sparkling, starlike object is falling from space. It grows at an alarming rate, and soon reveals itself to be a strange insectoid entity - squat, gray, its voice an angry, crackling hiss. It gropes toward the ground with three extended legs and kicks up billowing clouds of dust with its fiery breath. A padlike foot touches ground. There is sudden silence. The object bounces slightly and then nestles comfortably into the soft red soil. After a moment of stillness, a drift of sand collapses and buries one of the pads, as if to hide this ungainly alien. Unmindful, the invader shakes itself and extends several feelers, testing the air.

Twenty light-minutes away, at the Jet Propulsion Laboratory in Pasadena, California, several hundred technicians, scientists, and journalists wait tensely. It is shortly after 5:00 A.M., July 20, 1976, and although most of the people assembled in Von Karman Auditorium have been keeping vigil throughout the long night, no one seems tired. There is an electric air of expectation among the members of the international press corps, and a sense of fraternity. Not only is there an ample story for everyone here tonight, but also a universal hope for success. Even those critical of the space program find themselves caught up by the sheer magnitude of what is about to happen, and, for the small coterie of science-fiction writers and astronomical artists assembled, it is like Christmas morning.

Minutes pass, palms begin to sweat. Whatever is happening out there in space has already happened twenty minutes ago; the radio signal, crawling along at 186,000 miles per second, takes that long to reach us. There is a curious alteration in our perception of time, a sense of being in two places at once. If our distant emissary was doomed, disaster has already occurred, and no amount of urging can rescue it. Still, we clench our fists and hope.

Finally, at 5:12:07 A.M., the word comes: "Touchdown. We have touchdown." There is cheering and applause, followed by a collective sigh of relief. Everyone is grinning. Strangers slap one another on the back in passing. We've done it!

After a brief break to attend to long-neglected biological functions, the journalists engage in a sort of Brownian motion, picking their way over what seem to be miles of coaxial cable, in search of the monitor, the one with the clearest picture. At last it is located, on the right side of the room, and a pecking order is established in front of it. The screen remains dark until 5:54, and then a sliver of light appears on the left-hand edge. There are murmured exclamations. As the picture builds, we see small pebbles embedded in soft-looking soil, and then ... the edge of a saucer-shaped footpad. It's really true. We've landed on Mars!

The landing of the Viking I spacecraft that morning — not coincidentally, the seventh anniversary of the first manned lunar landing — was the culmination of a century-old dream. The very name Mars conjures up all the romance of space travel and symbolizes our hope of someday finding extraterrestrial life. No other world is so like Earth and yet, in our imaginations at least, so weirdly different, so full of delicious mystery. A modern-day equivalent of the medieval quest for the Grail, Viking's search for life on the Red Planet was fueled by a similar blend of spiritual longing and rich tradition, lightly seasoned with fact. Just as the legend of the Grail provided a motive for early European explorations, so the romance of Martian lore has been a significant driving force in our exploration of the planets.

HISTORICAL OBSERVATIONS

Because its reddish tint suggested the colors of fire and blood to the ancients, Mars has traditionally been associated with the god of war. Indeed, when Mars shone with particular brilliance every two years, a war was usually in progress somewhere, so the presumption of a correlation is understandable. It is easy to imagine a Chaldean astronomer-priest noting the rising of Nergal, the warrior chief, opposite the setting Sun, and hastening to warn the authorities of impending hostilities. Mars at its closest is outshone in the night sky only by the Moon and Venus, and it must have figured strongly in early religions.

This association of Mars with war has persisted throughout history. To the Egyptians, Mars was Horus the Red, the flaming hawk (later Hellenized to Ares). The Romans adopted the god, the "star," and the symbol - a shield and spear - and gave the planet its modern name, from whence derive words such as martial and the French mardi ("Tuesday"). Even after Mars's nature as a small, rather Earthlike world was established in the mid-nineteenth century, the stigma persisted. In H. G. Wells's 1898 novel, The War of the Worlds, the inhabitants of the planet Mars were naturally the ones who "slowly and surely drew their plans against us."

Bad press notwithstanding, Mars has played a significant role in the history of astronomy. It may be assumed that Mars's sinister reputation lent zeal to the observational and record-keeping activities of the first astronomers. Such records were of value to the Alexandrian astronomer Ptolemy when he compiled his Almagest in the second century A.D. In an effort to predict, if not explain, the seemingly erratic motions of the planets, Ptolemy resurrected and refined a theory that the Greek scholar Hipparchus had propounded more than two centuries earlier namely, that Earth was located at the center of the universe and that all other bodies orbited around it. Mystical tradition. required that all bodies follow "ideal" circular paths, so, in order to explain the retrograde loop that certain planets, including Mars, had been observed to perform in their journeys among the "fixed" stars, Ptolemy had to invent a complex system of small circles (epicycles) within larger circles (deferents). This "Ptolemaic" system worked fairly well. It could be used to predict the approximate positions of the planets years in advance. It was not perfect, however; corrections were constantly necessary. One planet in particular refused to conform to predictions: Mars. This deviation from theory was galling to astronomers. Imperfection could be tolerated on Earth, but the heavens werethe eternal domain of the gods, and therefore perfect. The minor deviations that the other planets exhibited could reasonably be attributed to observational error. Such was not the case with Mars. Something was wrong with Ptolemy's theory.

One astronomer who was intrigued by this problem was the Danish nobleman Tycho Brahe, whose "mural quadrant" at Uraniborg allowed him to measure a star's altitude above the horizon to a small fraction of a degree, thus greatly reducing the possibility of error over earlier efforts. During his short but productive career, from about 1569 to his death at 54 in 1601, Tycho made many thousands of positional observations. The data he obtained allowed him to devise a theory of planetary motion that was just an intuitive leap away from the correct one. While retaining Ptolemy's concept of the Earth as a pivotal center, Tycho adapted part of the Copernican theory, which had been published in 1543. According to Tycho's theory, the Moon and Sun orbited the Earth, while the rest of the planets orbited the Sun. Orbits were still assumed to be circular, although Tycho's observations of the comet of 1577 led him to suspect that its path might be somewhat oval. This speculation marked a deviation from tradition and was a tribute to his scientific honesty.

Tycho's assistant, Johann Kepler (1571-1631), apparently accepted the basic Sun-centered system of Copernicus quite early in his career. He assumed that all the planets, including Earth, revolved about the Sun, and that the driving force of their motion came from the Sun. Thus the closer planets, such as Mercury and Venus, would move faster in their orbits than did Jupiter and Saturn. This was a departure from earlier theory, which attributed the difference in orbital periods merely to the size of the planets' orbits. Tycho t'oo felt that Copernicus's circular epicycles and deferent~ were a contrivance, and that the planets moved along a simple, continuous curve. Using the wealth of observations left by Tycho, Kepler set about finding that curve. In 1609, the year before Galileo first turned a telescope toward the heavens, Kepler published De Motibus SteUae Martis [On the Motions of Mars}, in which he announced that Mars, and, by extension, every planet, moved in an elliptical path, with the Sun at one of the foci.

Kepler's choice of Mars as a subject was logical as well as fortuitous. Mercury, being so close to the Sun, was too difficult to observe (indeed, Copernicus is said never to have seen it). The motions of Venus and Jupiter differed too little from circular parhs, so they could fit either the Ptolemaic or Copernican models. Saturn moved too slowly in its orbit for Tycho to have collected data through a full revolution. The only planet left was Mars, which had steadfastly refused to be predictable. This was because Mars's orbit is highly elliptical; its perihelion is only 83 percent of its aphelion.

This variation in distance has interesting consequences. Earth, on its inner orbit, overtakes the slower-moving Mars every 780 days. At that time,. known as opposition, the two planets are closest, and observations are most fruitful. The rest of the time, Mars is too far away to be seen very clearly. Since Earth's orbit is nearly circular, while Mars's is highly eccentric, the amount of separation at opposition varies greatly. At the most favorable oppositions, which occur at intervals of about 15 to 17 years, Mars is only 56 million kilometers away - 140 times farther than the Moon. At unfavorable oppositions, the gap widens to 10 1 million kilometers, almost twice as far. By the time serious telescopic studies of Mars commenced late in the seventeenth century, it was understood that Mars was a planet that had to be observed on the fly, so to speak. Useful observations could be made only during the couple of months of opposition, and an astronomer could expect to see only three or four good oppositions in his lifetime. This tantalizing behavior has contributed much to the Martian mystique.

Even at its closest Mars is difficult to observe, for it subtends an arc of only 25 seconds. * This is the apparent size of a quarter seen from a distance of 0.15 kilometer. Its saving grace is that it is the only planet whose surface can be charted effectively from Earth. All the others are either covered with clouds or, in the case of Mercury, unfavorably located. The first recognizable drawing of Mars was made by the Dutch mathematician and physicist Christian Huygens in 1659. It depicts the dark, wedge-shaped area that later became known as Syrtis Major. By timing the appearance of such markings, Huygens was able to determine that Mars rotated on its axis approximately once every 24 hours. In 1666, the Italian Gian Cassini made a more accurate estimate of 24 hours, 40 minutes, which is about 2.5 minutes longer than the actual rotation rate. Cassini also seems to have been the first to glimpse the white polar caps of Mars. His nephew, Giacomo F. Maraldi, noticed in 1719 that the ice caps were not quite centered on the poles (which is also the case on Earth), and observed that Mars's appearance seemed to change from one night to the next. It is possible that he was seeing dust storms or other cloud formations.

Sir William Herschel, personal astronomer of King George III of England, determined in the early 1780s that Mars's axis of rotation was tilted about 30 degrees toward the plane of its orbit. Since Earth's similar tilt of 23.5 degrees is responsible for the seasons, Herschel concluded that the Martian environment might be rather like our own. He also noted that the polar caps almost entirely disappeared in summer, and hence might not be very thick.

It is of interest to note that once the planets were recognized as material bodies, much like the Earth, it was automatically assumed that they were inhabited. An influential book by Bernard de Fontenelle, published in 1688, argued that there was a "plurality of worlds." (He used the phrase not in its traditional sense - that there are, philosophically speaking, a multitude of world-models - but literally; he suggested that there was life on other planets.) The Church was still reeling from the Reformation, and such unorthodoxy was in disfavor. Nevertheless, these ideas persisted. There was already rich literary ground to nourish their growth. Nearly a century earlier, Kepler himself had written a speculative book entitled Somnium [The Dream}, in which he described living conditions on the Moon. Mindful of the fact that life must adapt to its environment (in itself a noteworthy insight, for the time), Kepler attempted to create a plausible scenario. Because of the low gravity, "everything ... is monstrously large in size. Growth is very rapid." To avoid the two-week-long lunar night, "[The Moon's inhabitants} have no safe and secure established dwelling, but instead wander about their world in troops . . ." Point by point, Kepler suggested how the unique properties of the Moon's environment would determine the form and behavior of its inhabitants.

The Moon was known to be very different from Earth, so it was expected that its inhabitants would tend to be rather bizarre, having little in common with humankind. Mars, on the other hand, seemed rather Earthlike, with its four seasons and 24.5-hour day. It seemed plausible that not only exotic animals, but people, rational beings, could be found there, and that they might even reveal their existence across the interplanetary gulf. When Mars drew unusually near during the opposition of 1877, astronomers turned their telescopes toward the Red Planet with this possibility in mind.

During a century of Martian cartography, a reasonable if misleading interpretation of the planet's features had been established: the white areas were snow, the orange areas were land, and the dark, gray patches were seas. All designations of Martian features followed this convention. Hence Richard Proctor's 1867 map bears names such as Herschel Continent, Copernicus Land, and Beer Sea (after the German banker, not the beverage). When Father Angelo Secchi charted 'some broad, gray streaks connecting two of the "seas" in 1869, it was reasonable for him to refer to them as canali, or channels. He saw them as purely natural waterways. A compatriot, Giovanni Schiaparelli, director of the Brera Observatory in Milan, was at this time preparing his own map of Mars. He was determined not only to place features far more accurately on the geodesic grid but also to give them prettier names. Schiaparelli drew on mythology and geography to find names such as Elysium and Utopia for Martian "lands," and assigned such euphonic appellations as Meridiani Sinus [Meridian Bay} and Solis Lacus [Lake of the Sun} to the "water." When, in 1877, he observed a number of dark linear features crossing the light areas, he naturally adopted Father Secchi's term canali to identifY them, since the word fit his own nomenclature scheme so well. He, too, assumed that they were natural channels.

Western culture was then in the midst of a technological boom, and people were eager to discover new marvels. An inexact translation of the word canali by the press sparked the following line of reasoning: If there are canals, there must be (or at least must have been) canal builders; therefore Mars is inhabited. The idea of a planetwide irrigation system dovetailed neatly with what was then known about Mars. For a century, it had been suspected that there was a dearth of water on the planet. And since the polar ice caps shrank to almost nothing during summer, they must be quite thin. As the ice caps receded, there seemed to be a corresponding expansion and darkening of the "seas," suggesting that they were being fed by melting snow and that coastal vegetation was benefiting from the increased moisture. The planet's smaller size and lower gravity implied a thin, weakly held atmosphere, such as that found at high elevations on Earth, and the general clarity of the Martian atmosphere, together with the tenuous nature of those clouds that did occasionally appear, supported the idea that Mars was a planet in the process of drying up, and was possibly older than Earth. What could be more natural than for intelligent Martians to build a network of canals to transport the precious potar runoff to equatorial farms'

Percival Lowell, wealthy scion of the influential Boston Lowells, was captivated by the poignant scenario of an advanced people uniting to conserve and equitably distribute their world's dwindling water reserves. In 1894, he established an observatory under the clear skies of Flagstaff, Arizona, for the express purpose of studying the canals of Mars. He was quite successful. While other astronomers complained that they couldn't even see the canals, Lowell charted hundreds of them. His canals, furthermore, were not the fuzzy streaks drawn by Schiaparelli, but geometrically thin lines that arced hundreds of kilometers across the Martian desert, intersecting at "oases." He observed the curious process of "gemination," or twinning, that Schiaparelli had described, whereby a canal seemed to manifest a companion running parallel to it, 50 to 600 kilometers away, as if Martian engineers had opened valves on an alternate branch of the network. Lowell drew canals across the "seas," concluding that these were actually arable lowlands, rather than open bodies of water. These Lowellian farms were as neatly bounded as any on Earth, although Martian surveyors apparently had a penchant for triangles and trapezoids, disdaining the less avantgarde rectangle. One could only conclude that the canals, as drawn by Lowell, were the product of intelligence. The only question is, as Carl Sagan has observed, which end of the telescope the intelligence was on.

To be sure, not everyone who accepted the existence of the canals believed them to be engineering works. Schiaparelli himself maintained that they were probably geological features. Alternative explanations ranged from cracks in a shrinking crust to ridges raised by the gravitational pull of atmosphere-grazing asteroids. Many astronomers, moreover, believed that the sharp, linear features drawn by Lowell simply did not exist, that they were optical illusions created by the distortions of Earth's atmosphere and the mind's tendency to abstract patterns from randomly distributed features on the Martian surface, in much the same way that we learn to see constellations.

In fairness to Lowell, observing Mars through a telescope is a tedious business calculated to set spots, if not canals, dancing before one's eyes. Even at opposition, a magnification factor of 300 or more is required to resolve detail on the surface, and such a degree of enlargement also magnifies the tremulous distortions caused by small variations in the density of Earth's atmosphere as it moves through the light path. On even the best nights, the planetary disk blurs and shimmers, and an observer must sit with eyes glued to the eyepiece, waiting for those rare moments of perfect "seeing" when the image steadies and features assume crystalline sharpness; then he must frantically sketch what he sees. Under such conditions, there is a natural tendency to sketch what one expects, or hopes, to see, and Lowell had strong motivation to see canals. Because of the limitations of Earthbound observatories, the existence or nonexistence of the canals was an issue that could not be resolved until the space age, and even then more than seven years would elapse between the first close-up photograph of Mars and the final word on the canals.

Following Lowell's death in 1916, study of Mars entered a hiatus, primarily because the thrust of astronomical research had turned away from the planets to the distant realm of stars, nebulae, and galaxies. A few astronomers, notably V. M. Slipher of Lowell Observatory, carried on exhaustive photographic studies of Mars, confirming the existence of a seasonal "wave of darkening" that was thought to be evidence of vegetation if not intelligence, but attempts to spectroscopically observe water or freeoxygen in the Martian atmosphere indicated that Mars was even drier and more inhospitable than anyone had thought. An effort to detect chlorophyll turned up negative, suggesting that the dark patches on Mars were probably not covered with plants. The dream of Martian life began to fade. When the spacecraft Mariner IV was launched toward Mars on November 5, 1964, few space scientists expected to find evidence of Lowell's planetary engineers, but most still believed that Mars harbored simple, hardy plants, such as lichen.

The exploration of Mars by instrumented spacecraft provides an object lesson on the dangers inherent in generalizing from insufficient data. Like blind men examining an elephant, space scientists were forced, with each new mission, to reevaluate their conclusions regarding the nature of the beast they were studying. A true picture did not emerge until 1971, when the entire planet was surveyed by an orbiting spacecraft.

On July 14, 1965, the spacecraft Mariner IV flew within 10,000 kilometers of Mars , transmitting 15 useful pictures of the planet. Although the probe's trajectory carried it over several charted canals, none were apparent in the photographs. Instead, Mariner's camera revealed a heavily cratered surface reminiscent of the Moon, indicating to most scientists that Mars, like the Moon, was geologically dead, and had changed very little in billions of years. To the general public, the following syllogism seemed reasonable: The Moon has craters and is lifeless; Mars has craters, therefore it too must be lifeless. Results of the radio occultation experiment, which measured the density of Mars's atmosphere, seemed to support this bad logic: surface air pressure was less than 10 millibars, the equivalent of a terrestrial altitude of 30 kilometers. The Martian atmosphere was a near-vacuum. In a single day, Mariner IV toppled the Lowellian dream. Gone was the mysterious world of master engineers, and in its place was a desiccated, barren wasteland.

This was not the last word on Martian biology, however. Questions still abounded. The seasonal darkening was a real effect; it had been photographed. Experiments designed to simulate the Martian environment - thin carbon dioxide atmosphere, low temperatures, ultraviolet radiation-in so-called "Mars jars" indicated that a plethora of terrestrial organisms, ranging in complexity from anaerobic bacteria to turtles, could survive exposure on Mars. It seemed reasonable to suppose that iflife had arisen during an earlier, more clement epoch, it would have adapted to the present harsh conditions. In addition and this is a fact that was largely overlooked at the time - Mariner IVs photographs covered only 1 percent of the planet. Had a similar mission been launched from Mars to Earth, what generalizations might Martian investigators have drawn from a photographic pass over the Pacific Ocean or the Sahara Desert? Clearly, more missions were necessary.

In February and March of 1969, twin spacecraft, Mariner VI and Mariner VII, were launched on fly-by missions to Mars. Improved communication and television systems permitted the spacecraft to take more pictures at greater resolution than had Mariner IV. On July 30, 1969, Mariner VI recorded 25 near-encounter pictures of Mars. These were followed on August 4 by Mariner VII's 33 close-up photos, which included coverage of the South Polar Cap. The smallest surface details that could be photographed were 300 meters across, but less than 20 percent of Mars was imaged at this resolution.

The 1969 missions revealed a Mars that was as geologically fascinating as it was biologically hostile. There were more craters than expected, some fresh and Moonlike, others eroded and filled with windblown dust, and there was terrain that geologists characterized as "chaotic" possibly the result of the surface slumping into basins left by melting permafrost. The ultraviolet spectrometer detected none of the ozone that shields Earth's surface from most of the Sun's ultraviolet radiation, and observed that the South Polar Cap was reflecting nearly the full solar ultraviolet flux, indicating that the Martian surface was blasted by this sterilizing radiation. Most discouraging of all, the infrared radiometer indicated that the South Polar Cap had the same temperature as dry ice - frozen carbon dioxide - and might have very little water ice after all. Mars seemed drier, colder, and more barren than ever before, and there was definitely no evidence of canals.

On November 18, 1971, Mariner IX fired its braking rocket and became the first spacecraft to orbit Mars (or,indeed, any other planet). During the weeks preceding its arrival, long-range photographs had revealed a fascinating if frustrating phenomenon on the planet: a dust storm. Such storms had been observed previously, and generally occurred around the time of perihelion, which this was. Usually, some details could be seen through the dust clouds. But this time the storm was planetwide and completely opaque. It was as if Mars, confronted with a spacecraft capable of unveiling its secrets, was making a lastditch effort to avoid scrutiny.

Gradually the dust settled, and a planet as intriguing as any Lowellian fantasy was revealed. The first features to emerge were huge volcanoes, far larger than any on Earth, bespeaking internal fires in Mars's past. Photos and instrument data showed the residual North Polar Cap - the tiny remnant that persists through summer - to be a fascinating structure that was probably composed of water ice and exhibited many of the features characteristic of glaciers, suggesting that it was probably quite thick. The case for extensive water on Mars was strengthened by the discovery of dozens of long, sinuous valleys meandering through the desert. These ancient riverbeds were certainly "canali" if not canals, and were identical in appearance to terrestrial arroyos caused by flash flooding in arid land. This suggested to some that it might have rained recently on Mars, at least by geologic standards. * Evidence for plate tectonics - continental drift - also came to light: There was a huge gash near the equator, a "grand canyon" 5000 kilometers long, 150 kilometers wide, and 10 kilometers deep. It had fluted edges characteristic of rain-and-winderoded valleys on Earth, and a central ridge similar to that found in the Great Rift Valley in eastern Equatorial Africa. Clearly, this was not the dead, moonlike planet earlier missions had described.

During the course of 1972, Mariner's wide, looping orbit allowed it to completely map Mars and observe the planet for more than half a Martian year. The mystery of the wave of darkening was explained by the discovery of seasonally shifting winds that altered the distribution of fine sand or dust on the dark, rocky surface. A full global survey revealed that Mars has two geologically disparate I:remispheres, apparently a quality common to all terrestrial planets. One side is characterized by extensive volcanism and faulting, while the other is ancient and cratered. Earlier missions had photographed only the cratered side, giving rise to the Moonlike model of the planet. The Mariner IX data gave rise to the two new models of Mars that were much more exciting from a biological perspective.

The "precessional spring" model supposed that Mars is currently in the grip of a global ice age, much as Earth was more than 10,000 years ago. Because of the present orientation of its rotation axis, Mars experiences winter in its Northern Hemisphere near the time of aphelion. On Earth, a similar effect serves only to slightly moderate Northern Hemisphere seasons and to mildly exacerbate those in the Southern Hemisphere. As a factor in heating efficiency, the very slight deviation from circularity in the Earth's orbit is far less important than the angle at which sunlight impinges on a given area of the surface. Because of the highly elliptical nature of Mars's orbit, however, its distance from the Sun strongly affects its temperature. Currently, the North Pole of Mars is tilted away from the Sun when the Sun is most distant, making for extremely cold northern winters and summers that are mild at their warmest. If most of Mars's water is bound up in the vestigial Northern Ice Cap, as appears to be the case, this means that the water never thaws, and that, unlike the carbon dioxide on Mars, it does not circulate seasonally between the poles.

Proponents of this model suggested that since Mars's axis precesses, like that of a spinning top, every 178,000 years, the situation was different approximately 50,000 years ago. The seasons were milder then, and the water circulated between the hemispheres. It was at this time (and at similar periods throughout Mars's history) that the Martian rains fell. If this theory is correct, then we are now observing Mars during the Great Winter, when it is a frigid, dry, nearly airless world. Fifty thousand years ago, our late Pleistocene ancestors may have seen Mars as a blue star in their sky. During Mars's Great Spring, advocates of this theory suggested, rain clouds scudded over the Martian desert, filling the canyons and arroyos with life-giving water. Since airborne seeds frozen for millennia in Antarctic ice can be thawed and germinated, why shouldn't Martian organisms lay dormant during the Great Winter, and spring into life when the rains finally come?

This is an appealing theory, but it has faults. Planetary scientists generally assume that the last wa~e of impactcrater formation ended approximately 4 billion years ago, and that from then on, erosional processes began to erase the craters. The proportion of young, fresh craters to old ones is therefore considered to be an indication of the relative age of a given area. When geologists studied those features that apparently bespoke an active, still-evolving Mars, such as the volcanic mountains and the river valleys, they found craters; and this was enough to convince them that most such features were billions, rather than thousands, of years old. The post-Mariner IX consensus was that Mars was planetary fossil that died in its geologic infancy and had remained in more or less its present state throughout the history of the solar system. Requiescat in pace, Marti, was the prevailing sentiment.

Hope springs eternal in the exobiologist's breast, however. Mars was once Earthlike, after all; there was liquid water in the planet's history. If Mars was wet for even a few million years, life may have developed; and if the planet dried up gradually, Martian organisms might have adapted to the present conditions. J'o test this supposition, two Viking spacecraft, equipped with marvelously ingenious automated biology labs, were dispatched to Mars in 1976. Both spacecraft were sterilized, to avoid contaminating Mars (and the life-detection instruments) with terrestrial organisms, and landing sites that were relatively "low, warm, wet and safe" were chosen after a month's orbital surveillance. Viking I landed at 22.5° N, 48.0° W, in the region known as Chryse Planitia, on July 20, 1976; Viking II touched down on the temperate plain of Utopia, at 48°N, 226° W, an area believed to be an ancient river delta. On September 3, 1976. .. .

The Viking spacecraft were equipped to conduct four lifedetection experiments. In the gas-exchange experiment, nutrient solution was added to a sample of Martian soil, and the changing composition of gases in the test chamber was monitored over several Martian days. A surprising amount of oxygen was evolved, but since the sample was kept in total darkness, it is unlikely that any biological mechanism, such as photosynthesis, was responsible. The water in the nutrient was probably reacting with peroxide compounds in the soil. In the pyrolytic-release experiment, Martian soil was exposed to radioactive carbon dioxide, the supposition being that any microorganisms present would incorporate the radioactively labeled atoms into their bodies during metabolic processes. After an incubation period, the gas was drained off and the soil was heated to 175°C. A small amount of radioactive gas was indeed driven out of the soil by heating, suggesting that biological processes could be taking place. A second sample, heat-sterilized prior to incubation, incorporated less radioactive gas - a reaction that was to be expected, if microorganisms were responsible. In the labeled-release experiment, radioactively tagged nutrients were added to a soil sample, and radioactive gases indeed evolved from it.

The pyrolytic and labeled-release results seemed to suggest that Martians did exist, albeit tiny ones; but a test of soil chemistry indicated that there were no complex organic compounds. On Earth, all life is made up of long-chain, carbon-based molecules, and the fact that none were detected on Mars - with the exception of those found in a weak residue of cleaning fluid in the equipment - bodes ill for the prospect of Martian biology. Either the population of microorganisms is extremely small, or "fancy chemistry" took place when liquid water was introduced into the bone-dry, highly oxidized soil. Laboratory simulations using synthetic, biologically sterile Martian soil suggest that the latter is the case.

The fourth life-detection experiment was the facsimilecamera system. While no trails of footprints appeared in any of the Viking photographs, recent studies of "stretched," highly exaggerated color images suggest that the surfaces of Martian rocks have patches on them that change shape and color' during the year. Whether this is due to exotic chemistry, biology ,or some other process is unknown.

In many respects, the quest for life on Mars was a longshot. There are places on Earth, such as the dry valleys of the Antarctic, that barely sustain even microbial life, although the environment there is infinitely more hospitable than any found on Mars. Still, the Antarctic environment is a geologically recent one, and it might be argued that there has not been sufficient time for many organisms to have adapted so as to fill that particular niche. It is often difficult to prove a negative, and the final answer to the question of life on Mars may come only after the entire planet has been systematically explored by automated roving vehicles or, eventually, by astronauts. Indeed, the answer may never be found. Who can say that there is not, deep within some hidden, steaming vent on the slopes of Olympus Mons, a small patch of organized matter, survivor of a bygone age, desperately clinging to existence? We can only wish our Martian cousins well; real or imaginary, they have led us out of our planetary cradle.

ADDENDUM, May 23, 2007.

It is curious to read this again after more than a quarter century. Our cleverly designed rovers have rolled across the Martian landscape, sampling, scraping, and photographing this most delightfully alien planet. Orbiting probes have mapped most of it to the resolution of a low-flying plane. The ruins of the Old Ones have not turned up, but we've found things almost as tantalyzing:

— Runoff valleys cut recently through the slopes of canyons and craters, as if subsurface ice has thawed and gushed forth briefly before boiling into the near-vacuum that Mars calls an atmosphere.

— Clear evidence that Mars once had extensive oceans.

— intriguing pockets of methane gas, a chemical that could persist only if it were being continually renewed either by volcanism or biological activity. (And there are NO signs of active volcanism).

— And, of course, the Mystery Meteorite ALH 84001, a rock recovered from Antactica that almost certainly came from Mars, and which contains minerals that may be the product of biology. Tests have shown that most of the possibly biological signatures coud have been produced by abiologic processes...but the rock has been found to contain small specks of magnetite with virtually the same structure as that produced by earthly bacteria.

Mars continues to beckon.