To be sure, astronomers of the 1950s knew enough about Mars to suspect a hash, topographically dull planet, but they also thought it might be hospitable to some kind of primitive life: lichens perhaps, or some other symbiotic algae/fungi organism cling to the rocks along the edge of the polar ice caps. Even as late as the early 1960s some astronomers believed in the essential correctness of the Lowellian view. After all, their theories were based on 350 years of telescopic observations and an impressive body of data.

The Mariner missions to Mars revolutionised our ideas about the Red Planet. When Mariner 4 swept past Mars in July 1965, its cameras revealed a craggy, crater-scarred landscape. For astronomers and the public at large, it was a startling discovery. The next two spacecraft in the series Mariners 6 and 7 did little to change the initial picture: Mars was an arid, dusty, desolate world. But Mariner 9 (the first orbiter) and Vikings 1 and 2, with orbiters and landers, started changing scientists’ views. Mars was (and still is) lifeless, yet it’s clear that rivers have flowed and perhaps even oceans once existed there.

Forty years ago, however, all that still lay in the future. A look through the pages of any pre-Mariner book about the Red Planet reveals just how misleading the early telescopic data was. Typical of such books is The Exploration of Mars by the German-American rocket engineers Willy Ley and Wernher von Braun. Published in 1956, this handsome volume traces the history of Martian studies, recaps the latest theories on surface conditions, and outlines the mechanics of a manned Mars mission. Included among its many illustrations are twenty-one paintings by the renowned space artist Chesley Bonestell, sixteen of them in colour. 

In the 1950s much of what astronomers believed about Mars was based on visual, radiometric, and spectroscopic analyses. Because atmospheric turbulence blurs fine planetary detail, photography was of limited use in recording what the eye detected, and technologies such as charge-coupled devices and digital enhancement were still decades away. So while the debate over intelligent life on Mars was long over, the question of the canals - a gridwork of fine, dark lines that had been reported from time to time crossing the planet’s ochre surface - remained unresolved right up until the Mariner missions. 

On the subject of the canals, The Exploration of Mars is a model of mid-century optimism. Ley and von Braun acknowledge that skilled observers like Asaph Hall and K. Graff never saw a canal, but they are firmly on the side of those who had Giovanni Schiaparelli, William H. Pickering, and Percival Lowell. Why had the canals eluded Hall and Graff? According to the authors, "seeing" was the likely culprit. Graff, working near Hamburg, Germany, and Hall, observing from Washington, D.C., simply lacked the steady skies of Schiaparelli in Milan, Pickering in Jamaica, and Lowell in Arizona. 

Another argument in support of the existence of canals was that observers always drew the most prominent ones in the same places. The authors have so little enthusiasm for Vincenzo Cerulli’s explanation of these features as "induced optical illusions" (that is, very fine and roughly linear detail perceived as continuous lines) that they reject his theory on the basis of a flawed experiment conducted by the English astronomer Edward Waiter Maunder back at the turn of the century. Their conclusion: 

To judge by The Exploration of Mars, the question of the canals was about the only issue dividing planetary scientists. To be sure, there was still much to learn about the planet next door, but in general astronomers were confident they had pieced together a reasonably accurate picture. 

What’s more, that picture was an encouraging one. For example, daytime surface temperatures, if not exactly balmy, were at least tolerable. Noontime desert temperatures were thought to range from about 14 to 41 degrees Fahrenheit; the noontime range in the dark areas—presumed to be low ground—was 50 to 68 degrees. Of course, the night side of the planet was bitterly cold, averaging perhaps 94 degrees below zero. These measurements, made at Lowell Observatory during the opposition of 1924, were still regarded as reliable in 1956. 

Atmospheric pressure was also believed to be significantly higher than present-day values. "The best determinations," the authors report, "permit the conclusion that the pressure at the surface is somewhere between 62 and 70 millibars of mercury, which corresponds to that of a height of between 16.5 and 17.5 kilometres (10.3 and 11 miles) in Earth’s atmosphere." Thin, but not quite as rarefied as the approximately 8 millibars of atmospheric pressure measured by the Viking landers.

Determining the composition of the Martian atmosphere proved particularly troublesome. The best estimate was that it contained 96% nitrogen and 4% argon, with traces of water vapour and carbon dioxide. ("The presence of carbon dioxide has been shown by the spectroscope; in fact, the percentage of carbon dioxide in the Martian atmosphere is higher than in the terrestrial atmosphere.") Today weknow that the Martian atmosphere is 95% carbon dioxide with only about 2% to 3% nitrogen and even less argon. 

Martian clouds fell into one of two categories -- yellow clouds of airborne dust and white clouds composed of ice crystals. However, large grey clouds had occasionally been spotted, confirming the existence of active volcanoes on the planet. 

In their discussion of Martian surface features, Ley and von Braun describe a world that, in its topography at least, is not fundamentally different from Percival Lowell's Mars. The polar caps, most astronomers agreed, were indeed frozen water. The only major disagreement was over just how thick the caps were – estimates ranged from I/10 of an inch to IO inches -- and whether or not they were composed of snow or ice. On the last point the authors, citing a Russian study conducted during the opposition of 1939, opt for ice covered by the feathery, frozen dew known as hoarfrost. 

Astronomers had also determined the nature of the dark fringe that was sometimes observed along the edges of the polar caps. This feature was thought to be either moist ground or, as seemed more likely, a contrast effect caused by the brilliance of the ice cap. Later, the French-American astronomer Gerard de Vaucouleurs, after repeated observations, concluded that it was indeed moistened soil. The polar ice really did melt -- but only after a great deal of evaporation had taken place. 

Interestingly, the Red Planet's most conspicuous surface feature is glossed over in one short paragraph: 

"Physically the most extended feature of Mars is, of course, the desert which covers three-quarters of the surface. Except for Baumann and a few others theorisers who follow more or less in his footsteps, there has never been any serious disagreement about its nature, namely that it is a desert coloured by a fairly high percentage of iron compounds, a large part of which might even have come from space, as Arrhenius thought." 

Such a brief account indicates the limitations of Earth-based observations. Canals aside, surface details were just too faint and too fleeting for Mars observers to imagine these reddish areas as anything other than flat, rather bland, planet-wide deserts. 

By far, the Red Planet's most intriguing surface features were the so-called "dark areas." While long regarded as vegetation, there had never been a satisfactory response to Swedish chemist Svante August Arrhenius' contention that they represented hygroscopic colour changes caused by moisture. That explanation, according to Ley and von Braun, had recently been given by Russian astronomer E. J. Opik. If Arrhenius' theory was correct, Opik argued, the dark areas should have been covered over by dust storms long ago. Yet these markings always reappeared after such storms, which meant they could only be something with the power to break through the dust - something alive. 

But what kind of vegetation? The authors, weighing the relative merits of mosses and lichens, make the case for lichen-type vegetation. Unlike mosses, they point out, lichens can flourish on bare rock and in any climate or altitude. Moreover, they have no root system and extract moisture directly from the air. Still, Martian vegetation need not resemble lichens either in shape or structure. There was no way to tell, across 35 million miles of space, "just what biochemical tricks have evolved under Martian conditions."

 Other possibilities existed too.

 Admittedly, one new theory seriously challenged this tantalising picture. At the 1955 meeting of the American Astronomical Society, it was suggested that differences between old and new Martian maps were the result of volcanism. Two features thought to have been formed by volcanic fallout were the funnel-shaped bays near Sinus Sabaeus and the Thoth-Nepenthes canal. Ley and von Braun, preferring a lusher Mars, reject this theory on the grounds that it does not account for the majority of dark areas or for their obvious seasonal changes in colour.

 But time was running out. Less than a decade later the view of Mars as a harsh but not inhospitable world of rolling deserts and exotic vegetation had become, like the Mars of Percival Lowell, part of the Red Planet's historical mystique. In the decades since Mariner 4, a new Mars has taken shape. Today's Mars is a far more hostile place – a bleak world of towering shield volcanoes, vast interconnected canyons, huge impact basins, and global dust storms that rage for months at a time.

 This year Russia will launch the first spacecraft since Viking designed to place sophisticated instruments on the Martian surface. If all goes well, the mission promises to answer many of the questions astronomers still have about our neighbour in space. So while the fabled Mars of the 1950s no longer exists, it's likely the planet will again be reshaped in new and unexpected ways by the Russian mission and by follow-up missions already on the drawing board.

A frequent contributor to Astronomy, Edmund A. Fortier is a high school teacher living in North Scituate, Massachusetts.