|The Planet Mars: A History of Observation and Discovery|
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Partly because of the heated, seemingly endless, and finally inconclusive controversies about the Martian canals, and partly because of the rise of astrophysics in the years after the First World War, planetary astronomy was long relatively neglected by professional astronomers, at least in the United States. There were a few exceptions. William H. Pickering published his "Mars reports" in Popular Astronomy from 1914 to 1930, based largely on his observations at Mandeville, Jamaica, with an 11-inch (28-cm) refractor and, after this instrument was returned to Harvard in 1925, a 12.5-inch (32-cm) reflector. Pickering was never short of theories to explain the canals. In Arequipa in 1892, he had proposed that they were strips of vegetation lining the banks of actual canals too small to be seen, an idea Lowell enthusiastically adopted, though Pickering himself later changed his mind in favor of the theory that the lines were vegetation growing along the border of cracks similar to the steam cracks he had observed in Hawaii. (He also, incidentally, strongly advocated the existence of clouds, vegetation, and even insects on the Moon!) Most professional astronomers paid little attention to Pickering, and he died, a bitter man, in 1938.1
Lowell's observatory at Flagstaff continued to be an active center for Martian research after its founder's death. (Antoniadi wrote to Gabrielle Flammarion, whom Camille married after his first wife, Sylvie, died in 1919, of the fortune Lowell had left: "We can therefore talk for a long time about canals on Mars"!)2 Earl C. Slipher was Flagstaff's leading Mars observer; a photographic specialist, he eventually obtained more than 100,000 images covering twenty-seven oppositions between 1905 and his death in 1964. He was a great admirer of Percival Lowell and remained a firm believer in the Martian canals, whose existence, he was convinced, was proved by his photographs.3 His map from the late 1950s was adopted by the U.S. Air Force as its official chart for use with its Mars spacecraft missions---it is covered with a Lowellian canal network!
In Europe, there were a number of active Mars observers. Undoubtedly the greatest observer during the interwar years was E. M. Antoniadi, who used the Grand Lunette at Meudon to study the planet during its oppositions between 1924 and 1941. Antoniadi never belonged to the observatory staff, and referred to himself simply as the "astronome volontaire à l'Observatoire de Meudon." His classic book La Planète Mars, based largely on his studies at Meudon, was published in 1930; it contains very complete descriptions of the changing surface features and will always be of great value to Mars observers (fig. 19).4 Antoniadi died in Occupied France in 1944.5
Other leading European observers included the Italian Mentore Maggini, who in 1926 became director of Vincenzo Cerulli's Collurania Observatory; Georges Fournier, like Antoniadi a onetime assistant of Flammarion at Juvisy, who from 1907 worked for René Jarry-Desloges at his various observing stations in France and at Sétif, Algeria; Gerard de Vaucouleurs, who carried out detailed studies in 1939 and 1941 with the 8-inch (20-cm) refractor at the observatory founded by Julien Péridier at Le Houga; and Bernard Lyot, Henri Camichel, Audouin Dollfus, and Jean-Henri Focas, who made extensive observations with the 24-inch (61-cm) refractor at Pic du Midi Observatory in the French Pyrenees, a site renowned for the quality of its seeing. Focas, like Antoniadi of Greek descent, produced the most highly detailed map of Mars ever made by a visual observer of the planet. His map expands on Antoniadi's description of the "leopard-skin" structure of the dark areas by showing them broken up into a fine mottling of small, granular patches. Apart from the visual and photographic studies of Mars, great work was done by observers armed with spectroscopes, polarimeters, and thermocouples.
It is their results that I summarize in the present chapter. I shall have very little more to say about the canals; instead I will describe the views of the planet's atmosphere, clouds, polar caps, and dark and light areas that prevailed in the years leading up to the beginning of the spacecraft era. Suffice it to say that although the canal theory had largely fallen into disrepute after Antoniadi's observations in 1909, in other respects the Lowellian synthesis remained influential. Indeed, Lowell's ideas about the water, atmosphere, and vegetation "survived and prospered. . . . [T]hese parts of Lowell's dream were repeatedly confirmed, and even embellished, by observers of Mars after 1916."6
According to the kinetic theory of gases, Mars should have a thin atmosphere. The planet, after all, is only a tenth as massive as the Earth; this means that it has a weak gravitational pull, and any gas molecules that reach a velocity of 5 km/sec (3.1 miles/sec) will escape into space. Mars thus was expected to have lost all of its hydrogen but to have retained heavier gases such as nitrogen, oxygen, and water vapor; moreover, their relative proportions were expected to be similar to those found on Earth (78 percent nitrogen, 21 percent oxygen, and 1 percent other gases, including argon and water vapor).
Various attempts were made to determine the surface pressure of the Martian atmosphere, but, unfortunately, all involved indirect measurements. An early calculation, published by Lowell in 1908, was based on the planet's albedo---its ability to reflect light. Lowell estimated what the total reflectivity of the Martian surface would be if there were no atmosphere at all. Since almost five-eights of the planet was desert (albedo 0.16), and three-eights consisted of blue-green areas (albedo 0.07), this came out to be around 0.13, whereas the actual albedo of Mars was 0.27. Correcting for the fact that some of the sunlight would be absorbed by the atmosphere before it reached the ground, Lowell gave a figure of 0.10 for the albedo of the surface. This left 0.17 for the albedo of the atmosphere, which he compared with an estimated albedo of the Earth's atmosphere of 0.75. Because albedo is a measure of the scattering of light, Lowell estimated that the mass of the atmosphere per unit area of Mars would be 0.17/0.75 = 0.23 of the mass per unit area of the terrestrial atmosphere. Given the fact that the force of gravity on Mars is 0.38 times that of Earth, the atmospheric pressure came out to 0.23 x 0.38 = 0.087 of the Earth's barometric pressure, or 87 millibars (1 bar is approximately equivalent to the atmospheric pressure of Earth).7
In general, later investigators confirmed this result. De Vaucouleurs, for instance, using a method that depended on how the brightness of various patches varied with distance from the central meridian of the planet (since a feature near the limb would be seen through a greater thickness of atmosphere), arrived at a figure right around 85 millibars.8
Attempts were also made to determine the composition of the Martian atmosphere. Though nitrogen is notoriously difficult to detect with the spectroscope, oxygen and water vapor should be detectable if they are present. W. W. Campbell's negative results in 1894 at Lick Observatory and in 1909 from the summit of Mount Whitney have already been discussed. The next careful study was made by Walter S. Adams and Theodore Dunham with a spectroscope attached to the 100-inch (2.54-m) reflector at Mount Wilson Observatory in the 1930s. Instead of comparing the Martian spectrum with that of the Moon, as earlier spectroscopists had done, Adams and Dunham used a more reliable method, first proposed by Lowell, which depends on the Doppler effect. The upshot is this: if a body is approaching the Earth, the lines in its spectrum will be shifted slightly toward the violet end of the spectrum; if it is receding, the lines will be shifted slightly toward the red end. In the case of Mars, its orbital motion relative to the Earth produces displacements in the positions of lines in its spectrum compared with those due to the atmosphere of Earth. These displacements are greatest when Mars is near quadrature---that is, at right angles relative to the Earth and Sun. But despite using sensitive instruments, Adams and Dunham failed to identify any lines in the spectrum of Mars that indicated the presence of either oxygen or water vapor.9
The first substance positively detected in the Martian atmosphere was neither oxygen nor water vapor but carbon dioxide, and it was found by Gerard Peter Kuiper at the McDonald Observatory in Texas in 1947. By subtracting out the Earth's contribution to the carbon dioxide bands in the spectrum of the Moon, Kuiper found that there was about twice as much carbon dioxide on Mars as on Earth. This discovery was surprising at the time, but it did little to change the overall view of the planet. After all, since carbon dioxide accounts for only 0.03 percent of the Earth's atmosphere, most astronomers naturally assumed that it must be a minor constituent on Mars as well. In 1950, the best guess as to composition of the Martian atmosphere was made by de Vaucouleurs: nitrogen, 98.5 percent; argon, 1.2 percent; carbon dioxide, 0.25 percent; and oxygen, less than 0.1 percent.10
Thin as it was, however, the Martian atmosphere was obviously substantial enough to support clouds. First, there were the yellow clouds, so-called because they appeared bright when observed through a yellow filter. Actually the name is somewhat misleading because almost everything on Mars appears bright in yellow light; it is better to refer to them simply as dust clouds. Some of these clouds may have been seen by Beer and Mädler as long ago as the 1830s, and still others were seen by Lockyer in 1862, but the first really satisfactory observations were made by Schiaparelli in 1877.
The clouds were particularly well observed in 1909. Fournier noted clouds as early as June, and Antoniadi recorded on August 12 that Mars appeared "citron yellow, and it was nearly impossible to see patches normally as dark as the Mare Tyrrhenum, Syrtis Major and the Sinus Sabaeus! It was truly a unique spectacle. On the following days Mare Cimmerium began to be lightly obscured, and the yellow cloud again covered almost the whole of the planet. . . . Only the Mare Sirenum retained its normal intensity."11 At the following opposition, in 1911, Antoniadi found large parts of Mare Erythraeum covered for several weeks with yellowish clouds, and again in August 1924 the planet was almost completely covered "and presented a cream color similar to that of Jupiter."12 In Antoniadi's drawing made on December 24, the dark patch Nodus Gordii---now known to be one of the Martian shield volcanoes---was virtually the only visible feature on the disk.
Antoniadi thought that the yellow clouds consisted of fine dust carried aloft by the winds. They were, he noted, most often seen when Mars was near perihelion. This was only to be expected; after all, at perihelion the solar heat was half again as great as at aphelion, and this ought to produce stronger winds, so that the lighter sand and dust particles would be more easily picked up from the Martian deserts.13
There were major dust storms in 1941 and 1956 as well. The latter began on August 20 with a bright cloud over the Hellas-Noachis region that was discovered at both the Kwasan Observatory in Kyoto and the National Science Museum in Tokyo, Japan. This cloud quickly expanded through the southern hemisphere, and by mid-September, when Mars came to opposition, had developed into a planet-encircling storm---observers at the time noted, as Antoniadi had in 1924, that the disk had become virtually blank.
Apart from the dust clouds and planet-encircling storms, observers have also noted whitish clouds (also referred to as "bluish" clouds, because they appear most brilliant when the planet is observed through a blue filter). Every autumn and winter a polar hood forms over the north polar cap, often extending as far south as latitude 50° N, at which time it obscures much of Mare Acidalium. In addition, limb hazes are commonly seen in spring and summer at the planet's morning limb. Still other whitish clouds form predominantly in the late afternoon, disperse during the night, and then re-form the following afternoon; since they frequently appear over the same region for days at a time, these are referred to as "recurrent" clouds. Often they resemble a light mist; at other times they become so bright that they rival the polar caps. Areas commonly affected by recurrent clouds include Syrtis Major, Elysium, Arcadia, Nix Olympica, and the Tharsis region. The latter is the site of the famous "W" clouds (as seen in an inverted telescopic image; right-side up they would be "M" clouds), first photographed by E. C. Slipher at the 1907 opposition. This group of clouds has been observed many times over the years, although the exact W (or M) pattern has seldom been repeated. By analogy to the Earth, observers assumed that these recurring clouds formed preferentially over elevated areas---that is, that they were orographic clouds made of water-ice crystals which formed as moist air rose up windward slopes.
Curiously, the Martian atmosphere did not seem to be equally transparent in all wavelengths. When photographed through filters, the familiar surface details of Mars always showed up well in red and yellow, but (with the exception of the polar caps) they usually disappeared in blue, violet, and ultraviolet. This was noted as long ago as 1909 by C. O. Lampland at the Lowell Observatory, and it was long thought to imply that there was something in the Martian atmosphere that acted as a screen to block out short-wavelength radiation, in the same way that stratospheric ozone does on Earth---though, given the scarcity of atmospheric oxygen on Mars, no one ever seriously suggested that ozone itself was responsible. The supposed screen was referred to as the "Blue Haze" or the "Violet Layer," though actually the name was something of a misnomer because there was no visible haze or layer. The importance of such a screen to the question of life on the planet was obvious: if the haze existed, it would provide some protection against damaging ultraviolet radiation from the Sun.
Beginning in 1926, E. C. Slipher began to suspect that the Violet Layer was not permanent; at that opposition and again in 1928, the dark areas were revealed at times in blue photographs. However, "extreme caution suggested waiting for more tell-tale examples . . . to thoroughly clinch the evidence."14 The telltale example came in May 1937, when, for several days around opposition, the dark markings, including Syrtis Major and Sinus Sabaeus, were as conspicuous in photographs taken with a blue filter as in those taken with yellow or red filters. Later studies showed other blue clearings; sometimes they seemed to affect only one hemisphere of Mars, at other times the whole planet. Oddly enough, many of the more remarkable episodes seemed to take place at or close to the date of the opposition. After reviewing several thousand photographs taken over a number of years, Slipher concluded that "the clearings were so striking in character and were so closely associated with the oppositions that such occurrences were accepted to be the rule."15
At that time the dark areas were generally believed to be tracts of vegetation. In 1941, Seymour L. Hess pointed out that if the Violet Layer actually screened out shortwave radiation, then the growth of vegetation ought to be arrested during a blue clearing. In October of that year, Hess found that this actually seemed to be the case; during a blue clearing, the wave of darkening seemed to halt for several days, and resumed only after conditions had again returned to normal---a development that seemed to furnish very strong arguments both for the existence of a Violet Layer and for the presence of vegetation on Mars.
What was the Violet Layer? There were various theories, including one favored by Kuiper and Hess that it consisted of tiny ice crystals. The Estonian-born astronomer E. J. Öpik suggested that it might be made up of a mixture of surface dust and small particles of carbon black formed by decomposition of carbon dioxide by solar ultraviolet radiation. The point is that no one really knew, and the whole question remained among the unsolved mysteries of Mars until after the beginning of the spacecraft era.
We now know, however, that the Violet Layer does not exist. The blue clearings have been rather mundanely explained as due to phase-angle effects of light-scattering by airborne dust, which causes occasional enhancement of the low-contrast differences between the light and dark areas in blue light.16
Though water vapor in the Martian atmosphere eluded detection by spectroscope, there was never much doubt that water did exist on Mars. The Ranyard-Stoney theory, that the polar caps were frozen carbon dioxide, had never really caught on; most investigators still supported William Herschel's idea from 1784 that they were water ice. Antoniadi went so far as to state, "This theory of Herschel's is correct."
Lowell had made much of a dark band which he observed accompanying the cap in its retreat. "At pressures of anything like one [Earth] atmosphere or less," he wrote,
|carbon dioxide passes at once from the solid to the gaseous state. Water, on the other hand, lingers on in the intermediate stage of a liquid. Now, as the Martian cap melts it shows surrounded by a deep blue band which accompanies it in its retreat, shrinking to keep pace with the shrinking of the cap. . . . This badge of blue ribbon about the melting cap, therefore, shows conclusively that carbon dioxide is not what we see, and leaves us with the only alternative that we know of: water.17|
Antoniadi was skeptical of the reality of Lowell's band, which he regarded as a mere contrast effect. But de Vaucouleurs, from his observations in 1939, found that the band was not generally visible in winter, when the cap appeared largest; it presented its full development only during the spring, which could hardly have been the case were it illusory.18 In April 1950 Kuiper came to the same conclusion after observations with the 82-inch (2.08-m) refractor at the McDonald Observatory.
That a band of some sort existed was, however, hardly definite proof that it consisted of water. The blue color alleged by Lowell was a particularly weak argument in support of this conclusion, as Alfred Russel Wallace had pointed out in Is Mars Habitable? in 1907: "It is perfectly well known that although water, in large masses and by transmitted light, is of a blue colour, yet shallow water by reflected light is not so; and in the case of the liquid produced by the snow-caps of Mars, which the whole conditions of the planet show must be shallow, and also be more or less turbid, it cannot possibly be the cause of the `deep blue' tint said to result from the melting of the snow."19
But though inferences based on the color of the Lowell band did not hold water, so to speak, most investigators continued to hold firmly to the notion that the caps were water ice. Kuiper in 1948 showed that their spectrum resembled that of ordinary snow, and not carbon dioxide snow, which was quite different. Thus he concluded: "The Martian polar caps are not composed of carbon dioxide and are certainly composed of water frost at low temperature [much below 0°C]."20 Moreover, the alternative---caps of frozen carbon dioxide---seemed even more unlikely because it did not seem possible that the temperature of Mars, even at the poles, ever got cold enough for carbon dioxide to freeze. Although various attempts had been made to calculate the temperature of Mars on theoretical grounds---including Lowell's well-known "warm as the south of England" estimate of 1907---the first direct measures of the temperature of the planet date only from the 1920s. For this purpose D. H. Menzel, W. W. Coblentz, and C. O. Lampland at the Lowell Observatory and E. Petit and S. B. Nicholson at Mount Wilson used a thermocouple, an instrument that consists of a circuit formed with wires of different metals soldered end to end. If a very minute amount of heat---say, from part of a planet's surface---warms one of the two joints, a tiny electric current is set up that can be measured with a galvanometer, and this leads to a determination of the planet's surface temperature. The thermocouple results showed that the temperature in the bright areas at the equator of Mars sometimes reached 10--20°C (50--68°F), and reached 20--30°C (68--86°F) in the dark areas, while the temperature in the south polar region in 1924 was -50--70°C (-58--94°F). At the low atmospheric pressures on Mars, a temperature of at least -100°C (-148°F) would be required for solid carbon dioxide to form a deposit on the surface. Thus the conclusion once again seemed to follow: the caps must be water ice.
Even if they were formed of water ice, however, the caps undoubtedly contained only minute quantities of water. From the rapidity of their melting, de Vaucouleurs estimated that they were only a few centimeters thick. Thus, the planet was obviously extremely arid.
Lowell had compared the salmon-colored areas of the planet to the Painted Desert of Arizona. There had never been any doubt that this is what they were---dusty deserts, desolate almost beyond the ability of anyone to imagine and vast by terrestrial standards, since altogether they occupied an area ten times larger than the combined areas of the Saharan, Libyan, and Nubian Deserts on Earth. As for their composition, the pioneering polariscopic observations of Bernard Lyot, who studied the specific way light was reflected from Mars and compared it with that of various minerals, indicated that the deserts might be covered with a layer of volcanic ash. Later, Audouin Dollfus demonstrated a close match with pulverized limonite, a compound rich in iron oxides.
The seasonal wave of darkening affecting the dark areas also seemed to be well established. Again, Lowell's observations were largely confirmed, and indeed embellished, by a number of other skillful observers of the planet. Thus Fournier wrote: "As the spring advances, the dark shading progressively encroaches from the pole towards the equator across the seas. This advance, across wide expanses and along channels, takes place at a variable speed, but nearly always very rapidly; a few weeks only suffice to change the landscape completely."21
Some of the markings seemed to change in a regular way with the seasons---for example, Antoniadi claimed that Syrtis Major appeared broad and dark in winter, narrow and weak in summer; Pandorae Fretum underwent a pronounced darkening each summer, and so on. Still other changes did not appear to be seasonal. Thus Nepenthes-Thoth showed a dramatic broadening in 1888 and again between 1911 and 1929; and Solis Lacus, whose classical form had been an ellipse with its long axis lying east-west, underwent a marked transformation in 1926 so that its long axis ran north and south. These changes were carefully recorded by Antoniadi, who was certain that they were due to "an invasion by the dark or greenish vegetation on to the lightly-shaded or rosy areas adjacent to the great dark areas."22
The changes of color that affected the dark markings also seemed to be seasonal. Lowell, to whom the disk presented "washes of color, the one robin's-egg blue, the other roseate ochre," had remarked that some of the dark areas changed from bluish green to brown and then to yellow with the arrival of midsummer in the southern hemisphere, reminding him of the changes observed in terrestrial vegetation during dry summers and autumn. Antoniadi paid no less minute attention to the coloring of the planet. In 1924, he and his associate Fernand Baldet described complex color changes spreading from the polar regions. "Not only the green areas," Antoniadi wrote, "but also the greyish or blue surfaces, turned under my eyes to brown, lilac-brown or even carmine, while other green or bluish regions remained unaffected. . . . It was almost exactly the color of leaves which fall from trees in summer and autumn in our latitudes."23
These observations were somewhat suspect because they were made with large refractors, in which chromatic aberration can never be entirely eliminated. Observations with reflectors were less objectionable. Thus E. E. Barnard, using the 60-inch (1.52-m) reflector at Mount Wilson in 1911, described the disk of Mars as "feeble salmon," while dark markings such as Syrtis Major and Mare Tyrrhenum were "light-gray."24 G. P. Kuiper, using the 82-inch reflector at McDonald Observatory, similarly found that in the spring of 1956 the dark areas were predominantly neutral gray, although he also noted "a touch of moss green" in some of the equatorial regions.25
The greenish colors naturally suggested chlorophyll. Chlorophyll looks green because it reflects light in the green part of the spectrum; however, it absorbs strongly in the infrared. Therefore, if the dark areas on Mars contained chlorophyll-bearing plants, they ought to look dark in infrared photographs. In fact, however, just the opposite was true. Thus, any plants on Mars could not contain chlorophyll; instead, they were widely believed to be primitive but hardy organisms, perhaps similar to terrestrial lichens.
Various attempts were made to detect the presence of organic materials spectroscopically, and there was a brief flurry of excitement in 1959 when W. M. Sinton announced that he had found strong absorption bands in the spectrum of the dark areas that seemed to coincide with those produced by carbon-hydrogen bonds in organic molecules. Alas, the Sinton absorption bands were later found to be caused by molecules in the terrestrial atmosphere; they had nothing at all to do with Mars.26
The vivid colors, too, finally proved to be illusory. The true tones of the Martian dark areas are drab, dull reds and gray-browns; the greens and blues are mere results of a visual physiology effect known as simultaneous contrast, which causes a relatively neutral-toned area surrounded by a yellow-orange field to appear bluish green to the eye. The effect has been well known (except to astronomers, evidently!) since the nineteenth century---it was first pointed out by a French chemist named M. E. Chevreul in 1839.27 Chevreul, the director of dyeing at France's national tapestry workshop (Manufactures Royales des Gobelins), had been charged with improving the intensity and fastness of dyes, and realized that the apparent brightness of a color depends more on the colors surrounding it than on the intensity of the color itself. Thus, he wrote, "where the eye sees at the same time two contiguous colors, they will appear as dissimilar as possible, both in their optical composition and in the height of their tone. We have, then, at the same time, simultaneous contrast of color properly so called and contrast of tone."28 Simultaneous contrast was briefly considered by nineteenth-century astronomers John Herschel and François Arago as an explanation of the greenish blue colors of Mars, but then promptly forgotten. These subjective colors, together with the linelike visions of the color-blind Italian astronomer G. V. Schiaparelli, created the compelling dream of a living world that so long dominated our views of Mars.
Alternative (that is, nonvegetative) explanations of the intensity changes in Martian surface markings began with the view of Svante Arrhenius, the Nobel Prize--winning chemist, who suggested in 1912 that certain minerals known as hygroscopic salts might be responsible; these salts absorb water and show dramatic darkening on contact with it. But Arrhenius's idea never received much support. Another theory was worked out by University of Michigan astronomer Dean B. McLaughlin. In a series of papers written between 1954 and 1956, McLaughlin proposed that Mars was actively volcanic. According to this theory, dark ash that spewed from volcanic vents flared downwind, in the process giving rise to the characteristic caret-shaped dark areas on Mars such as Sinus Meridiani (Dawes' forked bay). Seasonal variations in wind direction produced a redistribution in the primary ash deposits, which changed the shape and darkness of the Martian markings. Thus, according to McLaughlin, the well-documented changes were due to windblown dust rather than to the annexation of deserts by vegetation.29
After McLaughlin put forward his theory, Tsuneo Saheki, a Japanese astronomer, went so far as to suggest that a very bright "flare" that he had recorded at the Martian terminator on December 8, 1951, had been nothing less than a Martian volcano caught during an actual eruption. Though the volcanic explanation is doubtful at best, Saheki's description is nevertheless interesting: "When I first looked at Mars, . . . I saw Tithonius Lacus just inside the limb. Very soon afterwards, a very small and extremely brilliant spot became visible at the east end of this marking. At first I could not believe my eyes, because the appearance was so completely unexpected."30 The spot brightened further until, briefly, it surpassed the north polar cap itself; however, in less than an hour it had faded completely from view.31
G. P. Kuiper roundly criticized McLaughlin's theory, in large part because the existence of active Martian volcanoes seemed to be incompatible with the extreme scarcity of water vapor in the planet's atmosphere.32 It still seems so today, but at least McLaughlin had the kernel of an important idea. Kuiper himself, who had hitherto embraced the vegetation hypothesis, changed his mind within the year, in large part because of the impression made on him by the great dust storm of 1956. The obvious ability of winds to move dust on Mars led him to propose that the dark areas might be dust-covered lava fields. As McLaughlin had in his original theory, Kuiper invoked seasonal removal of the dust by wind currents to explain the "wave of darkening."33 In 1958, a Russian astronomer named V. V. Sharanov arrived independently at the same explanation. The air currents on Mars, he wrote, "vary from season to season, depositing dust at some times of the year and blowing it away at other times. Thus, for instance, the inherently dark surface . . . may brighten at a definite time of the year as a result of settling of light-colored dust blown over from the desert areas."34
Just as the idea that only the vegetation hypothesis could account for the observed phenomena of Mars was beginning to be called into question, other basic tenets of the Lowellian Mars were also falling by the wayside. Lowell had argued that the dark areas were dry sea bottoms, but radar studies in the early 1960s indicated that at least some of them were elevated rather than low-lying areas. This, incidentally, destroyed yet another argument once regarded as compelling support of the vegetation theory. In 1950, E. J. Öpik had pointed out that if the dark areas were low, they ought to be quickly covered by yellowish dust unless they had some means of regenerating themselves. At the time this was thought to prove that they were tracts of vegetation. If, however, the dark expanses were actually elevated areas, there was no need to believe this---the dust might just as well be removed by wind scouring.
The long-standing belief in a flat Mars, based on the smoothness of its terminator as observed from Earth, was also called into question. Clyde Tombaugh, a former assistant astronomer at Lowell Observatory and later a professor at New Mexico State University, argued in 1961 that mountains on Mars would not be detectable from the shadows they cast at the terminator unless they were at least 8,500 meters (27,500 ft) tall, and more rounded features would have to be even taller. Incidentally, Tombaugh, Öpik, and Ralph B. Baldwin all independently suggested around 1950 that there might be numerous impact craters on Mars.35
The century-long quest for Martian water vapor finally came to fruition at the mid-winter opposition of 1963. The detection of very minute amounts of water vapor in the Martian atmosphere was announced by Audouin Dollfus, who set up a special telescope at the Jungfraujoch high in the Swiss Alps, and by H. Spinrad, G. Münch, and L. D. Kaplan, who used a photographic emulsion especially sensitive to infrared radiation to record the spectrum of Mars near quadrature with the 100-inch reflector at Mount Wilson.36 The latter team found that the average amount of precipitable water on Mars (that is, the equivalent thickness of liquid water if all the atmospheric water were to be condensed onto the surface) was only about 14 micrometers, compared with 1,000 micrometers of precipitable water in even the driest desert areas of Earth. In addition, they estimated that the partial pressure of carbon dioxide on Mars was 4.2 millibars and that the total atmospheric pressure at the surface could not be more than about 25 millibars.37 This was a drastic downward revision from the previously accepted figure of 85 millibars. Thus, Mars was drier and had an even thinner atmosphere than anyone had realized.
Parenthetically, the reason so many of the earlier investigators were so far wrong about the thickness of the Martian atmosphere was that most based their estimates on albedo studies. They had assumed the Martian atmosphere to be usually clear and transparent, but this is not the case; often there is at least some dust present, which makes the atmosphere more reflective than they had supposed.
By the time Spinrad, Münch, and Kaplan published the results of their monumental study, a very different view of Mars was beginning to come into focus. In many ways this new Mars resembled the modern view of the planet: a bone-dry world with an extremely rarefied atmosphere, a surface with perhaps considerable relief, and changes in its markings that were the result of windblown dust rather than vegetation. However, the paradigm shift was not yet complete by 1965 when the first spacecraft reconnaissance of Mars took place, and most astronomers and the lay public were shocked, even depressed, by what it revealed.