chlorophyll, lichens,
and algae
chlorophyll
While Percival Lowell’s ideas about canals and life on Mars may have been losing favor among professional astronomers in the west, his influence nevertheless continued to extend eastward into Russia, where they guided the work of astronomer Gavriil Adrianovich Tikhov. Tikhov decided to prove that vegetation existed on Mars by looking for evidence of chlorophyll in the colors of Martian light. Born in Minsk in 1875, Tikhov trained first at Moscow University and then continued his education at the Sorbonne in Paris. He worked as an astronomer at Pulkovo Observatory for almost four decades. He served as a pilot watcher in the First World War and survived that war; he then endured and survived both the Russian Revolution and the civil wars that followed, always continuing his astronomical work.1
Tikhov began his searches for chlorophyll as early as 1909. He knew that light reflected from chlorophyll-containing vegetation on Earth appears green, so presumably he was looking for clues, using homemade colored filters, that Mars had green surface patches. Most plants on Earth do look green and, indeed, chlorophyll is the reason. The chlorophyll molecules (there are two different kinds of chlorophyll in plants, chlorophyll a and chlorophyll b) power photosynthesis; they do so by absorbing sunlight and transferring that solar energy to an electron that kick-starts the process of converting water and carbon dioxide into sugar and oxygen. Chlorophyll molecules are very good at their job of absorbing the energy in sunlight. Together, chlorophylls a and b absorb nearly 50–90 percent of the light in the violet and blue and 50–60 percent of the photons in the red. But they are poor absorbers—and therefore great reflectors—in the green and yellow.
Tikhov was unable to find any evidence for green reflected light in his earliest observations for chlorophyll on Mars. Yes, Mars has dark patches, but they simply don’t look green! Undaunted, and apparently unaffected by the evidence that seemed to have proven his initial hypothesis wrong, he planned to continue his search for chlorophyll on Mars during the next set of favorable Martian oppositions in 1918 and 1920. He designed and fabricated a primitive spectrograph for this purpose and didn’t let World War I or the Russian Civil War deter him. Despite the battles that took place around the Pulkovo Observatory during these years, he observed Mars, as planned; but he was unable to uncover evidence for chlorophyll in his Martian spectra. Two decades later, during the siege of Leningrad (formerly known as Saint Petersburg), which commenced in September 1941, Pulkovo Observatory was destroyed. After the war, Tikhov relocated to Kazakhstan and restarted his astronomical work at Alma-Ata Observatory.
As has often been stated, the absence of evidence is not evidence of absence, and in this case the absence of a definitive detection of chlorophyll on Mars did not, in his view, demonstrate that plant life did not exist, somewhere, on Mars. Imagination and belief trumped observational evidence; he allowed himself to believe that his measurements simply demonstrated that plant life on Mars must be different from plant life on Earth and that his telescopic evidence revealed that vegetation on Mars must grow without the benefit or need of chlorophyll. Undaunted by his apparent observational proofs that Martian life lacked chlorophyll, while at Alma-Ata he investigated the colors of terrestrial vegetation as a means toward gaining a better understanding of the colors of Mars. This line of reasoning led Tikhov to invent the field of research he called astrobotany, in which he would study the reflected light from plants, in particular plants that grew in extreme, Mars-like environments on Earth—at high elevation or in extremely low temperature environments—in order to look for spectra from plants that might lack the green-color signature of chlorophyll. Amazingly, he found that at very low temperatures the color of the reflected light from chlorophyll was not always green for some plants. In addition, some plants, when grown at low temperatures, will have colors other than green.
The robustness of conclusions with regard to the presence of vegetation on Mars depended strongly on the likelihood that the surface temperature of Mars was conducive to plant life, but astronomers in the nineteenth century had only been able to guess at that temperature, and the temperature calculations made by Lowell and Poynting in 1907 were neither accurate nor definitive.
In the early 1920s, William Weber Coblentz, a physicist who spent his entire career working for the National Bureau of Standards in Washington, DC, decided to measure the temperature of Mars. Working with Lampland at Lowell Observatory, he conducted a series of carefully planned observations to measure the intensity of light from Mars in the mid-infrared, at wavelengths from 8 to 15 microns. Using these measurements, and assumptions about the reflectivity of Mars and the effects of Earth’s atmosphere on these measurements, Coblentz measured the temperature of Mars in different Martian seasons and across separate surface regions that he characterized as either bright or dark. He found that the bright areas, with temperatures as low as freezing (0°C or 32°F) are cooler than the dark areas, which had temperatures of 10–16°C (50–60°F). Then, having associated bright areas on Earth with hot deserts, he concluded that this bright-and-cold versus dark-and-hot bimodality for landforms on Mars “is just the reverse of conditions here on Earth, where the surface of the bare desert areas becomes burning hot.”2 His logic defied reality. The opposite conclusion—that Mars is similar in this respect to Earth—is also easy to reach: the brightest areas on Earth are the cold polar caps and the warmest areas are continental regions far from the polar caps, just as they are on Mars.
Coblentz drew additional significant conclusions about Martian vegetation based on these temperature measurements, given other known conditions of Mars—most notably the relative dryness of Mars in comparison to Earth. “The observed high local temperatures of Mars,” he wrote, “can be explained best by the presence of vegetation which grows in the form of tussocks or thick tufts, such as pampas grasses, and the mosses and lichens that grow in the dry tundras of Siberia.” Coblentz obtained the answer he expected, the answer he perhaps wanted, and he was partly right: the highest temperatures on Mars would allow for the growth of vegetation. Most of the time on most of Mars, the temperature is close to –16°C (3°F) in the daytime, with nighttime temperatures plunging down to –90°C (–130°F); however, noontime temperatures at the equator in the summer can reach as high as 20°C (68°F). He, of course, had absolutely no evidence that pampas grasses, mosses, or lichens actually were growing on Mars, let alone that the Martian vegetation affects Martian temperatures.
In the 1920s and 1930s, American astronomers Vesto Slipher and Robert Trumpler and then Canadian astronomer Peter Millman independently pursued the same goal as Tikhov. Almost certainly, they observed Mars in complete ignorance of the work of Tikhov, who worked largely in isolation from western astronomers.
Slipher’s 1908 studies of Mars were of marginal value, at best, though Percival Lowell had trumpeted them as definitive evidence for the presence of water in the atmosphere of Mars.
What research was Slipher carrying out in the 1920s regarding Mars? Lowell Observatory, of course, by then had a long history of observing Mars, and Slipher and his brother Earl had continued that program of observations with regularity. Because Mars is well positioned for study from telescopes on Earth about every two years, not surprisingly the publication cycle for the Slipher brothers’ studies of Mars followed this same cycle. One or the other of them published at least four papers concerning observations of Mars in 1922, five more in 1924, and four more in 1927.
Vesto Slipher began chasing down evidence of chlorophyll at the urging of Percival Lowell during the years from 1905 through 1907, with no success. Two decades later, the technology for taking astronomical photographs had improved. Slipher now had available for his work new sensitizing dyes—pinaverdol for improving sensitivity in the green and yellow, pinacyanol for the red, and dicyanin A and kryptocyanine for the infrared—that he used in darkroom baths to modify the emulsions on commercial photographic plates before use. These new chemicals were supposed to “bring out strikingly the red color of Mars,” especially in comparison to the Moon. The reflection spectrum of chlorophyll, he noted, is bright in “the deep red, beyond the sensitivity of the eye.” He decided to look for this “deep red” signature of chlorophyll because proof of the existence of chlorophyll on Mars would prove that life exists on Mars and would therefore add to the legacy of Percival Lowell. Apparently, his experimental results were negative for the presence of chlorophyll on Mars, though he would describe those results rather cautiously. “The Martian spectra of the dark regions,” he reported in a paper published in 1924 in the journal Astronomical Society of the Pacific, “so far do not give any certain evidence of the typical reflection spectrum of chlorophyll.”3 In the years that followed, he was busy running Lowell Observatory and the search for Lowell’s Planet X, studying the atmospheres of Venus and the giant planets, and obtaining spectra of faint extended nebulae (some of these being clouds in the Milky Way galaxy, others in distant galaxies), but he rarely observed Mars again and never again published any additional details on this particular project. Whether he simply lost interest in Mars or chose not to pursue a line of research that might prove embarrassing to himself or to the legacy of the man whose name graced the observatory he directed, we don’t know.
By 1924, Trumpler’s reputation was secure, in large part because of his role in carrying out observational tests in Australia on September 21, 1922, of Einstein’s theory of relativity during a total eclipse of the Sun. While Sir Arthur Eddington had made Einstein world-famous when, during his eclipse expedition of May 29, 1919, he had photographed the bending of starlight by the gravitational pull of the Sun, Eddington’s measurements were on the edge of what could be measured. In contrast, Trumpler’s measurements of the deflection of starlight around the limb of the Sun represent the first secure confirmation of Einstein’s theory of relativity.
A few years later, Trumpler discovered that all of space within the Milky Way is filled with a haze caused by interstellar dust, a discovery that would lead authors to write his name into every textbook published in astronomy since the 1930s. This haze dims the light of distant stars and has the effect of blocking out more blue light than red light, making distant stars look redder than their true colors. For his important contributions to astronomy, Trumpler was elected to the National Academy of Sciences in 1932.
Trumpler described his studies of Mars in the Science News-Letter in 1927, and though he dismissed the artificiality of the network of lines seen on the surface of Mars, he did identify a “close relationship between the network and the extended dark areas of Mars which are of a bluish-green tinge.” This relationship, he wrote, “suggests the hypothesis that both are made visible by vegetation and that the network-lines represent lanes of greatest fertility.”4
Trumpler’s work influenced the opinion of none other than Princeton professor of astronomy Henry Norris Russell, whose work in creating the concept of what is now called the Hertzsprung-Russell diagram cannot be overstated in importance for all of astronomy. In 1926, Russell responded to an inquiry from a writer for the Science News-Letter about the recent observations made of Mars by noting that Mars has all the necessary conditions for life as we know it; in addition, he commented that the large green areas on Mars that change color with the Martian seasonal cycle make it probable that vegetable life exists on Mars.5
Fifteen years later, during the next observing period when Mars was well placed for study, Millman took up the challenge of looking for chlorophyll on Mars. Millman had cut his teeth, professionally, on Mars, as his first professional paper was a study of Mars, based on observations he had made while in Japan as a high school student at the Canadian Academy in Kobe. After concluding his studies at the University of Toronto and then spending four years at Harvard, Millman had attained the status of a specialist in the study of meteors.
Prior to 1939, nothing in Millman’s career led him down the path to a study of vegetation and a search for chlorophyll on Mars. He had studied binary stars and discovered a Cepheid star in the constellation Scorpio. Cepheids are arguably the most important kind of star known to astronomers. In the first decade of the twentieth century, Henrietta Leavitt, working at the Harvard College Observatory, had discovered that Cepheids, which were known to cycle in brightness from bright to faint to bright again, did so in a very dependable way. The brightest Cepheids required more than one hundred days to complete one cycle, while the faintest Cepheids completed one cycle of brightness changes in only a day. Once this understanding of the behavior of Cepheids was quantified, astronomers found that they could measure the period of variability of the light for any selected Cepheid and use that information to determine the distance to that Cepheid and the cluster of stars in which that particular Cepheid is embedded. Edwin Hubble had used Cepheids to prove, in 1925, that the Milky Way was not the only galaxy in the universe. He then used them again in 1929 and 1931 to discover that the universe is expanding. In the late 1930s, Cepheids were still among the most important objects of study for astronomers, and in fact remain similarly important well into the twenty-first century. The discovery of a Cepheid was an important discovery in the 1930s and indicated Millman had strong observing skills, because the discovery of such an object requires very careful measurements made over many months or even years. He also had published numerous articles on how to photograph and make spectroscopic observations of meteors, fireballs, and shooting stars. Added to this, he wrote papers in which he analyzed the spectra of meteors and studied the frequencies at which meteors struck Earth. Then, sticking out of his list of professional publications like a green patch on Mars is his paper “Is there vegetation on Mars,” which he published in 1939 in the journal The Sky, a magazine that, after only a few years of publication, was subsumed into Sky and Telescope.
Millman’s work on this project represents some of the most rational, sensible scientific effort ever expended on the subject of life on Mars, and merits great respect for the care and caution he brought to the project. In his paper, he wrote, “So much nonsense has been written about the planet . . . that it is easy to forget that Mars is still an object of serious scientific investigation.” He reminded his readers of the simple but compelling reasons many astronomers felt led to the hypothesis that Mars had vegetation.
Mars was well known to have polar caps that waxed and waned with the Martian seasons. When the southern polar cap shrank, presumably because the water ice it contained melted, “a wave of darkening proceeded from the polar regions towards the equator, and the seas, which in the winter were faded and inconspicuous, now became darker and greenish, starting with those nearest the pole. . . . It has been a natural and popular explanation that the seas are in reality vegetation which is nourished by the melting polar snow and goes through a seasonal cycle similar to the vegetation on earth. The greenish color of the seas has been considered an additional support to this hypothesis.”6 He then noted that this hypothesis should be tested, and he proceeded to design such a test. His approach was rational, logical, and unemotional.
In writing for his audience in The Sky, he explained that leaves are green because chlorophyll strongly reflects “yellow-green and yellow rays” but only weakly reflects light of shorter (violet, blue, blue-green) or longer (red) wavelengths. Chlorophyll, he continued, is also an extremely good reflector in the infrared. Thus, one could detect the signature of chlorophyll by looking for reflected light from Mars that was similar to reflected light from chlorophyll: strong green light, weak colors in the rest of the visible spectrum, and strong infrared light, just beyond the red. This approach, which was very similar to that used by Slipher a decade earlier, was the basis for Millman’s observing strategy.
Millman designed an observing plan using the 74-inch telescope (which saw first light in 1935) of the David Dunlap Observatory, located near Toronto. First, he obtained photographs of Mars in which he isolated two dark-colored neighboring regions of Mars known as Syrtis Major and Mare Tyrrhenum.a Next, he photographed an adjacent, lighter colored region. According to the hypothesis he was testing, Syrtis Major and Mare Tyrrhenum were dark because they were covered with vegetation, while the second region was lighter in color because it was free of vegetation. A comparison between the reflected colors of the two regions should show a distinct contrast, under the assumption that chlorophyll is present in Martian vegetation. Millman found, in his measurements, that although the dark colored seas “appear greenish in color to the eye,” the fact that they were “relatively strong in violet, blue, and blue-green light but weak in yellow, orange, and red . . . negates the existence of chlorophyll as an agent in producing that green color.” Were chlorophyll present, the yellow-green and yellow light would have been more intense than the other colors.
Figure 9.1. Plot of the intensity of reflected light from Mars (in units of stellar magnitudes) versus wavelength (in units of angstroms; 1 angstrom = one ten-billionth of a meter) or color of light. The upper part of the figure shows the intensity of reflected light from Martian seas (top line) and Martian deserts (bottom line). The seas are much brighter than the deserts in the colors indigo and blue, but are darker than the deserts in green and yellow. The bottom part of the figure shows the intensity of reflected light from chlorophyll, which is extremely reflective in the green and yellow and much less reflective at other colors. Clearly, neither the Martian seas nor deserts reflect light in a way that would be similar to the spectral reflectance signature of chlorophyll. Image from Millman, The Sky, 1939.
To check for consistency, Millman performed the same experiment with genuine, Earth-grown, Canadian green leaves and obtained exactly the result he expected for materials containing chlorophyll. The Canadian leaves reflected light very strongly in the yellow-green and yellow colors relative to the other colors.
Millman drew a rational conclusion from his measurements. His results, he wrote, “do not give any definitive evidence about the existence or non-existence of vegetation on the Martian surface. What they do seem to indicate is that the greenish color of the seas cannot be taken as a support for the vegetation hypothesis, since the green color does not seem to be like the green reflected from our terrestrial leaves.” He continued, again writing maybe all too reasonably, “Perhaps, after all, we are rather presumptuous to think that anything on Mars which might have some remote similarity to biologic life on the earth would develop under the same organic system as that found here.” Millman never returned to this project and never again studied Mars.
lichens
By the 1940s, the idea that we shared our solar system with walking, talking Martians had faded away; however, the possibility that Mars harbored some kind of life remained strong. Given the technological advances made by scientists and engineers during World War II in research areas that could impact astronomy, it was only a matter of time before a bright, young astronomer used those new tools to discover new ways to study Mars.
Gerard P. Kuiper, one of the giants of twentieth-century astronomy, was that bright young astronomer. During the late 1940s and 1950s, Kuiper worked at the University of Chicago’s Yerkes Observatory, in Williams Bay, Wisconsin, and at the University of Texas at Austin’s McDonald Observatory, near Fort Davis in western Texas. In 1947, Kuiper used a then-new device, a lead-sulfide photoconductive cell developed for military purposes earlier in the decade, in a new detector system for the 82-inch telescope at McDonald Observatory. This device was a very sensitive infrared spectrometer, fully a thousand times more sensitive in the infrared than the previous generation of instruments. With this new detector system, Kuiper was inventing modern infrared astronomy and using his new tool to study the planets.
In 1944, in a paper written while on a leave of absence from the University of Chicago and Yerkes Observatories to conduct “war research” at the Radiation Laboratory in Cambridge, Massachusetts, Kuiper discovered methane gas in the atmosphere of Saturn’s largest moon, Titan (Kuiper claimed that methane was the dominant gas in Titan’s atmosphere, though we now know that nitrogen is far more abundant), and in 1948 he discovered Miranda, the fifth discovered of the moons orbiting Uranus. In 1951, using known information for the orbits of comets, he predicted the existence of a disk of objects in the outer solar system located more distant from the Sun than even Neptune. That band of the solar system, which was not found observationally until the 1990s, is now known as the Kuiper Beltb; and NASA’s Kuiper Airborne Observatory, the KAO, was named to honor Kuiper, who died in 1973 after pioneering airborne infrared astronomy in the 1960s. The KAO was a C-141 military cargo plane with an infrared telescope built into the cargo hold. The KAO, which flew at altitudes as high as 45,000 feet in order to fly above 99 percent of the water vapor in Earth’s atmosphere, operated from 1975 until 1995. Astronomers flying on and working in the KAO received high-altitude survival training—if the tiny control room depressurized, they had 15 seconds to safely secure their oxygen masks before asphyxiating.
Using the 82-inch telescope at McDonald Observatory in 1948 (the second largest telescope in the world when it was completed in 1938), Kuiper obtained the first-ever color picture of Mars, which was such an exciting achievement that, in June, Life magazine published his spectacular photograph. The headline of the article that accompanied this picture was rather mundane but very much in tune with mainstream astronomy at the time: “Newest photographs and studies disclose that life on planet is limited to simple vegetation.” The text reported that the “green patches are vegetation, but of the lowest order, lichens, which live by drawing moisture from the air.” Kuiper informed readers of Life that “the lowly lichens represent life’s last stand on the Red Planet.”7
One might surmise that Kuiper’s photograph revealed something incredible about Mars: life. The remarkable news, however, was not that Kuiper had discovered life on Mars. The idea of life on Mars was old news; life on Mars was expected and assumed, known even, except for the details. No, the amazing news was that Kuiper thought he had proven that life on Mars was limited to lichens. Not little green men, not forests of trees, not even algae or moss. Just rootless, stemless, leafless lichens.
The lowly and primitive lichen is actually two different organisms, living together in a symbiotic relationship. Most of the lichen is composed of long cells of fungi, connected end-to-end to form long, tubular filaments. Unlike normal plant cells, many fungal cells contain multiple nuclei; also unlike normal plant cells, the cell walls of fungi are strong, as they receive structural support from the presence of chitin, a carbohydrate polymer molecule. The other living part of a lichen, which lives among the fungi cells, is photosynthetic, usually a type of algae known as green algae but sometimes an ancient form of bacteria called cyanobacteria (which are often called blue-green algae, although they are not plants).
During the Mars opposition campaign that begin in late 1947 and extended into early 1948, Kuiper succeeded in definitively demonstrating that the atmosphere of Mars contained a small amount of carbon dioxide and that CO2 was the dominant gas in the Martian atmosphere. Figuring out exactly how much CO2 was present was a great deal more difficult, however, than demonstrating that it was the most important gas in the Martian atmosphere. The answer he obtained from his calculations for the amount of CO2 in the Martian atmosphere was a bit too low, but he did correctly deduce that Mars’s atmosphere was very thin in comparison to Earth’s atmosphere.
As Time magazine noted in a report on Kuiper’s discoveries published in March 1948, Lowell had “believed that life on Mars is older and more highly developed than it is on Earth. . . . It is at least possible [according to Lowell] that Martians reached a stage of scientific civilization while man’s ancestors were still fish or reptiles. Perhaps the Martians have evolved by now into some superscientific stage.”8 Lowell’s views of Mars and Martians, however, were outdated by 1948, and Kuiper’s own discoveries about the presence and abundance of CO2 on Mars revealed that Mars wasn’t quite as hospitable as Percival Lowell had thought. The Time article on Kuiper’s new research noted that “Last autumn he found that the atmosphere contains a small amount of carbon dioxide, which is necessary to plants (the basic living organisms). Without any carbon dioxide, plants cannot live, but too much would indicate that there are no plants on Mars to consume it.”
What is missing from this explanation is that the atmosphere of Mars contains almost nothing except carbon dioxide. The Martian atmosphere does contain a small total amount of carbon dioxide, but the atmosphere contains almost nothing else. Kuiper suspected this was the case, though definitive proof of the thinness of the Martian atmosphere would not be found until two decades later. Kuiper did, nevertheless, assert that the climate of Mars resembles “earth at an elevation of 50,000 feet. . . . This probably would support lichen, since these plants act like sponges and suck up water vapor present in air. Rain is not necessary for their existence.”9 Kuiper’s Mars was a cold, dry, nearly airless place. For anyone expecting astronomers to find Martians, Kuiper’s news would have been disappointing.
Having figured this out mostly correctly, he moved on to study the polar caps of Mars. Solid CO2 (dry ice), he decided, was unlikely at the relatively warm (for dry ice) surface temperatures of Mars and at the atmospheric pressure he had measured for Mars. But the absence of solid CO2 was mere speculation. With his new infrared spectrometer, he could test his hypothesis, and so he set out to do so. “The observations of the Mars polar cap showed that the cap resembles a water-cell spectrum,” he concluded. After repeated observations and laboratory tests to obtain spectral comparisons of CO2 snow and terrestrial snow and frost, Kuiper had an answer: “The conclusion is that the Mars polar caps are not composed of CO2 and are almost certainly composed of H2O frost at low temperature.” Again, Time publicly praised the genius of Kuiper: “Last week Kuiper focused his spectrometer on the gleaming ice cap, dwindling fast in the Martian May. It turned out to be ‘water in the solid state.’”
In fact, Kuiper’s solution to the polar cap problem was partly right and partly wrong. We know now that the polar caps have permanent caps of water ice overlaid by seasonal (winter) caps of carbon dioxide ice.10 His observations that showed evidence of water ice were correct, though it took the planetary science community another half century and multiple space missions to Mars to correctly solve the riddle of the Martian polar caps.
Nevertheless, having solved the atmospheric problem correctly and the polar cap problem partially correctly, Kuiper was ready to move on to the next problem in need of a solution, “the nature of the green areas on Mars, often held to be vegetation because of the observed seasonal changes.” Kuiper designed an experiment using observations at wavelengths of 0.6, 0.8, 1.0, and 1.6 microns, in which he observed both the green areas of Mars and the surrounding, so-called desert regions.
Human eyes are sensitive to light in the visible spectrum, from violet at the short wavelength end (at about 0.4 microns) to red at the long wavelength end (at about 0.67 microns). And chlorophyll, Kuiper knew, reflects light very effectively in the green (wavelengths close to 0.51 microns) and yellow (wavelengths close to 0.57 microns), i.e., at wavelengths shortward of 0.6 microns, and absorbs light at other visible-light colors. Green plants also reflect light very strongly at infrared wavelengths in the vicinity of 0.8 to 1.0 microns, a region in which human eyes are not sensitive. Kuiper hypothesized that if he could measure the color contrast between the green regions of Mars and the Martian desert regions he might resolve the question of what was producing the green color on Mars. If Earthlike, chlorophyll-containing plants were present on Mars, the percentage of reflected light from Mars would be dramatically lower at 0.8 and 1.0 microns than at 0.6 and 1.6 microns. But if no chlorophyll were present, the percentage of reflected light would be indistinguishable at the four different wave bands.
Kuiper found that the contrast between the two regions did not change across the four wavelength bands, a result that ruled out chlorophyll as the source of the green color on Mars. With this very simple and elegant experiment, Kuiper had ruled out Earthlike “seed plants” as the dominant kind of vegetation that existed on Mars. As explained again in Time magazine, “They could not be vegetation like trees or grass.” He concluded that this was “not surprising, in view of the extreme rigors of the Martian climate, in particular the cold nights . . . Seed plants and ferns are both vascular plants containing a great deal of water. Such plants would undoubtedly freeze in the Martian climate.”
If not seed plants, what form of vegetative life could exist on Mars? Time explained Kuiper’s ideas to readers this way: “They might be lowly lichens like those that grow on the dry rocks near McDonald Observatory. Lichens need no water in liquid form. Martian lichen-like plants might get enough water out of vapor from the ice caps, which evaporate without melting.” Kuiper himself wrote, in his chapter “Planetary Atmospheres and Their Origin,” in his own 1948 book, The Atmospheres of the Earth and Planets, “The hardiest terrestrial plant life are the lichens, a symbiosis of fungi and algae.” Also, he made clear to his readers, the reflectance spectra of lichens are just like what he had found for Mars; that is, lichens show the same color contrast at 0.6, 0.8, 1.0, and 1.6 microns as does Mars. “The spectrum of these lichens and that of the Martian regions are similar between 0.5 and 1.7 µ [microns].”11 While not saying, directly, that he had found spectra of lichens on Mars, Kuiper certainly meant to imply something very close to that idea. In fact, he presented a calculation that demonstrates that “if all atmospheric water vapor were made available to the green areas [which cover, he estimated, one-third of Mars], they would receive a layer 0.02 mm thick. The height of the living parts of the ‘vegetation’ could hardly be more than ten times this amount, or 0.2 mm; presumably it is much less. This estimate is compatible with a lichen cover.”c Kuiper continued his arguments in favor of lichens by noting that the green areas on Mars should long ago have been covered with yellow dust from the Martian dust storms unless they had the power to regenerate their green colors. He also noted that the complete absence of oxygen cannot kill terrestrial lichens, so we can feel confident that lichens could survive on Mars without oxygen. In addition, lichens “produce very little oxygen, and even the traces set free would gradually escape the planet.” As a result, the fact that observers had not detected oxygen in the Martian atmosphere is consistent with lichen life on Mars. “No contradiction with the spectroscopic tests would thus result,” he writes. Ultimately, Kuiper summed up the arguments in favor of lichens on Mars favorably, but with a level of caution not evident in most of his arguments: “Final judgment should probably still be withheld.” Such caution, at least, was wise. If only he had taken his own advice.
Kuiper was careful in his later scientific papers in the Astrophysical Journal to merely imply that the spectra were consistent with the spectra of lichens and not to say that he had discovered lichens on Mars. He was perhaps a bit less cautious, however, when speaking with the reporter from Life, and later he found himself walking back his lichen claims just a bit.
In 1955, in the Publications of the Astronomical Society of the Pacific, Kuiper wrote that he did not think Mars had canals and that perhaps he had been misquoted about the whole lichen business, but not about the presence of some kind of plant life on Mars. “I did study Mars with the 82-inch telescope in 1948, 1950, and 1954, often under excellent conditions, with powers 660 and 900 times; and I have never seen a long, narrow canal nor a network of ‘fuzzy canals.’ I am personally convinced that the objective evidence which has led to this concept has been misinterpreted and erroneously represented on the drawings.”12 Having dismissed the existence of the canals, he then examined the evidence for the existence of vegetation. His conclusion, in 1955, was that though the canals did not exist, the evidence for vegetation probably did. That evidence was ambiguous but likely; in fact, his discoveries provided the weight of the evidence in favor of the biological hypothesis. In his own words, “Seasonal and secular variations in the extent of the dark areas have been described by several authors. . . . These variations have often been regarded as strong indication that the dark areas are covered with vegetation; but they are, of course, insufficient as proof. An inorganic explanation cannot be immediately ruled out, although such explanations as have been advanced appear improbable. The discovery of CO2 in the Martian atmosphere at the McDonald Observatory in 1947 and the infrared spectrum of the polar cap showing it to be frozen H2O not CO2, have greatly enhanced the a priori probability that some primitive vegetation exists on Mars.”
Given that he had embraced “primitive” vegetative life on Mars as a much more likely explanation for the color changes of the dark areas than weather effects or areology,d he addressed his earlier statements about lichens. “The hypothesis of plant life, for reasons developed elsewhere, appears still the most satisfactory explanation of the various shades of dark markings and their complex seasonal and secular changes. I should, however, correct the impression that I have supposed this hypothetical vegetation to be lichens. Actually, I said: ‘Particularly, the comparison with lichens must be regarded to have only heuristic value; it would be most surprising if similar species had developed on Mars as on the Earth.’” Indeed, these words did appear exactly as he quoted himself, near the end of his “Planetary Atmospheres and Their Origin” chapter in his 1948 book The Atmospheres of the Earth and Planets.
By the mid-1950s, with multiple observers on several continents working across three decades and none of them finding any evidence for chlorophyll on Mars, and with Lowell’s canals all but forgotten, Kuiper’s “primitive vegetation” was now the only remaining scientific leg on which the life-on-Mars argument still stood. Kuiper himself was becoming very careful in his professional writings, reminding his colleagues that his lichens-on-Mars idea was merely a hypothesis advanced to explain the seasonal and long-term changes in the dark markings on Mars, and he refrained further from any bolder statements to reporters from Time or Life magazines.
Writing in the March 1957 issue of the Astrophysical Journal, he backtracked a bit more when he noted that “The lack of vivid colors observed during the 1954 and 1956 oppositions (early and late spring, respectively, in the southern Martian hemisphere) suggests that an inorganic explanation for the dark markings be considered along with the vegetation hypothesis. . . . The most probable inorganic hypothesis would appear to be that the maria are lava fields, somewhat like the lunar maria and perhaps those of Mercury. On general grounds, such a unifying hypothesis would be attractive. . . . Apparently, while the sand fills the crevasses in the lava, it blows off the vitreous surface. Lava fields therefore have the ‘regenerative power’ after a dust storm that has been invoked as an argument favoring the vegetation hypothesis.”
Kuiper, though becoming more cautious, was unable to completely give up on the idea of Martian plant life. He stated in his very last sentence, “As a working hypothesis it is supposed that the maria are lava fields that may have a partial cover of some very hardy vegetation.”13 Kuiper, having backed away from asserting that he had discovered lichens on Mars, had covered all his bases.
The momentum had shifted away from the belief that our neighboring planet Mars, the pale red dot in the nighttime sky, was a planet brimming with life. Then, just when the idea of Martian life might have been extinguished for good, Bill Sinton showed up.
algae
One of the unfortunate legacies of the flawed work of Percival Lowell was that planetary astronomy was not a popular pursuit in the first half of the twentieth century. Instead, most astronomers studied stars and galaxies (and had great success in learning about the physics of stars and the universe). One had to be a bit foolish and quite a bit courageous to decide to pursue a career in this neglected backwater. Bill Sinton had fought with the Twenty-sixth Infantry Division in the Second World War. That difficult experience may have served him well when he decided to follow Kuiper’s lead and become one of the pioneers in developing the field of infrared astronomy and applying that approach to studying objects in the solar system. For his doctoral dissertation research at Johns Hopkins University, he measured the infrared spectrum and the temperature of Venus and other planets. He next directed his attention to the Moon and Mars.
One of the biggest obstacles he had to overcome was the enormous amount of infrared light (the “background signal”) that was emitted by the detector he was using to measure the infrared light from distant, astronomical objects. All objects emit light, and the kind of light they emit most effectively depends on their temperatures (this is known as “blackbody radiation”). Objects with temperatures of millions of degrees emit light most effectively in the form of X-rays. Objects like the Sun, with temperatures of thousands of degrees, emit visible light best. Objects at room temperature (hundreds of degrees Kelvin), like buildings, telescopes, astronomers, and astronomical detectors, are prolific emitters in the infrared. At temperatures below about 100 degrees (Kelvin), objects emit microwaves or radio waves prolifically but give off almost no infrared, visible, ultraviolet, or X-ray photons.
Sinton knew that he could decrease the intensity of the problematic infrared background signal by reducing the temperature of the detector. In principle, when the detector was cool enough the infrared light emitted by the detector would approach a negligible level; consequently, detecting the infrared signal from the Moon or Mars would become possible. Working with the 61-inch telescope at the Harvard College Observatory (installed in 1934), he used an instrument he built by himself, one that was cooled by liquid nitrogen down to almost 300 degrees below zero (–287°F, or 96 K), a remarkably cold temperature for an astronomical detector system at that time. At such a low temperature, infrared studies of Mars became possible.
Sinton decided to study Mars at the infrared wavelength of about 3.4 microns in order to search for signs of vegetation. Thanks to Kuiper, as was well known, he wrote, “there is already important evidence pointing toward the presence of vegetation on Mars.”14 Using his pioneering skills in infrared astronomy, he was going to gather substantive, spectroscopic proof.
At wavelengths from 3 to 4 microns, Mars emits much less light of its own than the amount of infrared sunlight it reflects. Material on the Martian surface, however, could have an effect on the spectrum of the reflected sunlight. In particular, laboratory work published in 1948 by a team of chemists had shown that when two carbon atoms each share an electron with a single hydrogen atom, as would be expected in organice molecules, that combination of atoms is especially good at both absorbing and emitting light at a wavelength of 3.46 microns. If the organic molecule is very lightweight, for example a methane molecule, which is made up of only one carbon atom and four hydrogen atoms (CH4), the band at which the molecule is an effective absorber and emitter of light can shift to a wavelength as short as 3.3 microns. In general, larger and heavier organic molecules, like those in biological materials on Earth, shift this spectral feature into the 3.4–3.5-micron range. Sinton designed an experiment to study Mars and look for this spectral feature, under the assumption that plant life on Mars would show this same signature as plant life on Earth.
If we imagine that an incoming source of light (e.g., sunlight) includes approximately the same amount of light at 3.1, 3.2, 3.3, 3.4, 3.5, and 3.6 microns, then when light waves at all of these distinct wavelengths reflect off a surface that includes leaves or moss or grass or lichens, the organic material, because it is full of those C-H bonds, absorbs light effectively in the 3.4–3.5-micron range and reflects light effectively at the other wavelengths. If we were to plot the intensity of reflected light (the y-axis value) as a function of the wavelength of light (the x-axis value), we would see a constant amount of light at 3.1, 3.2, 3.3, and 3.4 microns, a drop in the light intensity from 3.4 to 3.5 microns, and a rise in the light intensity from 3.5 to 3.6 microns. This drop in the light intensity across a short range of wavelengths is an absorption band, similar to those Fraunhofer bands first discovered a century and a half earlier. Sinton obtained infrared spectra of terrestrial biological materials in order to demonstrate the presence of this absorption band in light reflected from living things, including a lily of the valley leaf, a maple leaf, two types of lichen, and a moss, all of which did show some sort of absorption feature in the 3.4–3.5-micron range. Then he collected infrared spectra of Mars and compared them to his test spectra.
The result? The Martian spectra he obtained in late 1956 showed “a depression at the wave length of the organic band” at 3.46 microns. In a paper he published in the Astrophysical Journal in 1957, he wrote that this dip did not prove that lichens are present but did indicate “that organic molecules are present.”15
Figure 9.2. A plot that compares the amount of reflected light from the Sun (top), a bright region on Mars (middle), and a dark region on Mars (bottom), as a function of wavelength. Long wavelength (about 4 microns) is to the left; short wavelength (about 3 microns) is to the right. The apparent drops in the intensity of reflected light at 3.67, 3.56, and 3.43 microns in the dark regions of Mars, in contrast to the absence of these apparent drops in both the bright regions of Mars and in direct sunlight, were interpreted by Sinton as evidence for vegetation on Mars. Image from Sinton, Lowell Observatory Bulletin, 1959.
Sinton was correct in asserting that the presence of a bona fide absorption band at this wavelength would demonstrate that some material that is good at absorbing light at 3.46 microns exists somewhere along the light path between the Harvard College telescope in Massachusetts and the surface of Mars. He was on weaker ground in arguing that that material was organic. Whatever was absorbing the light in the 3.46-micron band might be in Mars’s atmosphere, but it also could be in Earth’s atmosphere. Sinton quickly concluded, however, that he had more knowledge about that absorbing material than he, in fact, actually had. He claimed that “the dip at the significant wave length is therefore additional evidence for vegetation. This evidence, together with the strong evidence given by the seasonal changes, makes it extremely likely that plant life exists on Mars.” Unlike Kuiper, Sinton did not take a cautious approach in his professional writing. He jumped in with both feet, concluding without equivocation that he had uncovered clear evidence for Martian vegetation.
Two years later, when Mars returned to a position in the sky where it was again well positioned for viewing from Earth, Sinton returned to his studies of Mars. In 1958, the steady advance of technology offered Sinton the benefit of improved equipment. In addition, his first success in supposedly detecting life on Mars was leverage for moving his observing program from the 61-inch Harvard Observatory telescope to the biggest telescope on the planet at that time, the 200-inch Hale telescope on Palomar Mountain in southern California, which had opened in 1948. For this project, he was awarded two weeks of use of this telescope during the time of the month astronomers call “bright time,” this being the nights just before, during, and after the night of the month when the Moon is full and when moonlight illuminates much of the night sky.
This approach is quite typical in astronomy. Use your initial discovery on your “small” local telescope as leverage to obtain access to observing time on a much bigger telescope. With a bigger telescope, one collects more light from the astronomical target in the same amount of observing time than one would with the smaller telescope, thereby gaining efficiency and a more reliable scientific result. In addition, bigger, newer telescopes tend to be placed on mountains with better weather conditions and usually have more modern equipment than smaller, older telescopes. As a result, with the bigger telescope, in this case with the biggest telescope in the world, Bill Sinton expected to carry out the definitive observing program in the study of Mars and prove the existence of plant life on Mars.
Sinton’s goal in 1958 was to remove any doubt about the reality of the absorption band he had claimed to have identified in his 1956 observations. In his own mind, without any reservations, he achieved that goal: “the reality and distribution of the band were established.” His new discoveries were published in 1959 in the prestigious scientific journal, Science, which maximized the impact of his work and generated the most fame for himself and publicity for his work.16
According to Sinton, the Martian spectra, in particular those taken when observing the dark patches on Mars, revealed absorption bands that were strongest at 3.43 and 3.56 microns and showed a third absorption feature at 3.67 microns (later revised to 3.45, 3.58, and 3.69 microns17). The “bands near 3.5 microns” are “most probably produced by organic molecules,” he wrote, and these are “produced in localized regions in relatively short spans of time. Growth of vegetation certainly seems to be the most logical explanation for the appearance of organic molecules.”
The longest wavelength band had Sinton puzzled, because “it had not been seen in any terrestrial plants.” However, perhaps through serendipity, he conducted laboratory spectral tests of the lichen physica and the alga cladophora. Both physica and cladophora showed a shallow absorption feature at about 3.7 microns, in addition to deeper absorption bands at about 3.43 and 3.56 microns.
Sinton concluded that the similarity of the Martian spectrum to that of cladophora demonstrated that what was observed on Mars was “produced by carbohydrate molecules present in the plant. The attachment of an oxygen atom to one of the carbon atoms shifts the resonance of a hydrogen atom attached to the same carbon to a longer wavelength. Thus, the evidence points not only to organic molecules but to carbohydrates as well.” Sinton suggested that this spectral signature was not only proof for the presence of life on Mars but was also a clue about the need for Martian plants to have large food storage capacities.
Sinton had hit an apparent grand slam. He had not just uncovered evidence for life on Mars. He had discovered spectral features that revealed what kind of life exists on Mars. Not Kuiper’s lichens. Instead, Martian algae very closely resemble terrestrial algae that produce and store carbohydrates. Soon, other scientists would refer to the 3.4–3.7-micron spectral features on Mars as the Sinton bands, and Science magazine would become the town square for arguments for and against the Sinton bands as proof of evidence for life on Mars.
Notably, in 1958 Sinton had obtained solar spectra in the afternoon by observing light reflected off a piece of aluminum (which was thought to be a near-perfect reflector). Presumably, Sinton assumed he could use the reflected-light spectrum of aluminum to correct the Martian spectra for any spectral effects due to the Sun or to Earth’s atmosphere, but these spectra perhaps were insufficient for this task. He did not obtain (nighttime) lunar spectra, with the Moon at a similar height in the sky as Mars, as a comparison, perhaps because, based on his 1956 observations, he had concluded that a (reflected) solar spectrum was a better calibration source than a lunar spectrum. In actuality, a lunar spectrum probably would have done a better job in correcting for terrestrial atmospheric effects, and if he had used this method, he might have discovered his error sooner rather than later.
In a letter to Science published in 1961, Norman Colthup, of Stamford Research Laboratories, in Connecticut, and a giant in the field of infrared spectroscopy, wrote to agree that the band seen by Sinton at 3.43 microns was almost certainly due to a carbon-hydrogen bond in “carbohydrates and protein organic matter in plants which resembled terrestrial plants.”18 Colthup goes on to say that the only likely source of the two spectral features at 3.56 and 3.67 microns must be molecules known as “organic aldehydes (but not formaldehyde),” because organic aldehydes “are among the few materials with a strong band near 3.67 microns” in addition to the band at 3.43 microns.
Aldehydes are chemical compounds that contain the elemental group CHO, in which a carbon atom shares two electrons with an oxygen atom and shares a third electron with a hydrogen atom. Because a carbon atom has four electrons available for sharing, the carbon atom in the CHO group still has one more electron bond it can share with yet another atom. How that fourth electron is shared determines the actual chemical species for this compound (e.g., is it methanal, ethanal, or propanal?). Aldehyde is in the ethanal family. As Colthup noted, acetaldehyde is a very effective absorber at wavelengths very close to 3.58 and 3.68 microns, which, he argued, makes it a great match to the Martian spectral signatures.
Colthup identified the specific aldehyde, acetaldehyde (or ethanal, C2H4O), as the most likely source of the Sinton bands and attributes the presence of this material on Mars to the near absence of oxygen on Mars, since this particular molecule preferentially forms in an oxygen-poor environment. Just in case he had not gone too far already, Colthup continued by going much too far: “If I may be permitted to speculate a bit, acetaldehyde may be an end product of certain anaerobic metabolic processes.” He pointed out that one such process is fermentation, in which carbohydrates are converted to acetaldehyde and finally to alcohol. “This process yields much less energy for the organism than conventional oxidation . . . but certain organisms on Earth use fermentation as their source of energy when oxygen is not available, and perhaps this happens on Mars.”19
As often happens in the competitive, scientific marketplace of ideas, not everyone agreed with Colthup and Sinton. In 1962, Donald G. Rea, of the Space Sciences Laboratory at the University of California at Berkeley, began the attack. In a major review of the studies of planetary atmospheres carried out by astronomers up until that time, he pointed out that his own calculations showed that at the temperatures and pressures of the Martian atmosphere at the Martian surface, the vapor pressure should force acetaldehyde molecules into the gas phase and thereby into the atmosphere.20 In fact, “the high volatility of this chemical would ensure a high concentration in the atmosphere . . . It should then be observed over the entire disk [of Mars] and not be restricted to certain areas.” Sinton, however, saw these supposed absorption bands in reflected light only from the dark areas on Mars. Since the absorption bands were not evident from the light areas of Mars, Rea argued that the conclusion drawn by Colthup was wrong. Rea knew that Sinton’s flawed conclusions were vulnerable to his laboratory work and, like a dog with a bone, he was not about to let go.
A year later Rea and two of his colleagues in the Department of Chemistry at Berkeley, T. Belsky and Melvin Calvin, presented a large body of laboratory spectra that revealed additional problems with Sinton’s work.21 Some of these problems were instrumental. For example, when light from Mars was focused by a telescope onto Sinton’s detector, the light had to first pass through a window made of Plexiglas in order to enter the sealed, supercooled device that holds the infrared detector. Rea and his colleagues asserted that the measured wavelength of the light wave changed as a result of being transmitted through the Plexiglas. The result of using the Plexiglas window, they claimed, is that the experimenters might not be able to know the correct wavelength of the spectral feature on Mars before it was transformed.
Other problems had to do with which absorption bands are properly identified with the alga cladophora or with other biological materials Sinton tested, including lichens and the lily agapanthus. The Berkeley chemists showed that Sinton’s assignment of a single spectral feature to cladophora was a marginal conclusion, at best, and that the Mars spectra were missing other spectral signatures of cladophora. If they have one of the cladophora spectral features, shouldn’t they also have the others, they asked? They knew the answer was “yes.” Perhaps, then, the evidence for cladophora was not as definitive as Sinton claimed. In 1963, Rea, Belsky, and Calvin mostly raised questions but provided no firm answers. The questions, however, were a sign that trouble was brewing for Sinton and his supporters.
Another team of Berkeley chemists, James Shirk, William Haseltine, and George Pimentel, were similarly unconvinced that the Sinton bands were evidence of the presence of algae or of active fermentation processes on Mars. They went looking for other possible explanations. In January 1965, they found one: water. In fact, the explanation they found centered on the presence of both heavy water and semi-heavy water in the Martian atmosphere.
Just as the three isotopologues of water (H2O, HDO, and D2O) respond differently to the force of gravity, they also absorb and emit light at slightly different wavelengths. Shirk and his colleagues found that HDO and D2O absorb light at wavelengths that match the wavelengths of the Sinton bands more closely than does acetaldehyde. In other words, “the reported absorptions could be attributed to HDO or D2O in the Martian atmosphere.”22 Their explanation raised other interesting questions in need of answers. Why, for example did the dark areas on Mars show a higher water vapor content than the light areas?
In March 1965, the algae and the fermenters on Mars finally met their Waterloo in Denver, Colorado. Using observations of the Sun obtained at the University of Denver and measurements of the total amount of water vapor that would have been in the air above Palomar Mountain in Earth’s atmosphere when Sinton used the Hale Telescope to make his original observations of Mars in the late 1950s, the team of Rea and Brian T. O’Leary, together with Sinton himself drew the conclusion that “there seems to be a correlation between the intensities of the 3.58 and 3.69-micron features and the amount of telluric water vapor in the optical path. An important corollary is that there is no evidence for attributing these spectral features to Mars.”23 That rather quiet, low-key sentence bears repeating: there is no evidence for attributing these spectral features to Mars. The Sinton bands at 3.58 and 3.69 microns are telluric! They have everything to do with the water content of Earth’s atmosphere and nothing to do with anything in Mars’s atmosphere. Sinton had detected water, specifically heavy water, on Earth, not life on Mars.
What about the third Sinton band at 3.45 microns? In hindsight, an impartial referee who might examine the original spectrum published by Sinton in his 1959 paper and who looks for the unexplained spectral feature at 3.45 microns is unlikely to find it. No such feature ever existed. No such absorption band ever needed to be understood. All Sinton had measured was noise.
To his great credit, Sinton was one of the co-authors of the paper that demolished a decade of his own work. Such a high-profile, public acknowledgment of such a major scientific error is rare and brave. Thereafter, Sinton had a long and moderately distinguished scientific career. He helped to establish the observatories on the summit of Mauna Kea in Hawai’i for the University of Hawai’i, studied the volcanoes on Jupiter’s moon Io, and conducted extensive infrared studies of the Moon and the atmospheres of the planets Uranus and Neptune.
Science magazine had been the purveyor of the yin and yang of the debate about the Sinton bands for a decade. Now that debate was over and a new era in planetary science had begun when NASA’s Mariner 4 spacecraft, launched in November 1964, returned the first-ever close-up image of Mars on July 15, 1965. During the Mariner 4 flyby, the spacecraft took twenty-one pictures of Mars. To commemorate the one-year anniversary of that historic event, in the July 15, 1966, issue of Science, the magazine turned to Ernst Julius Öpik to offer some perspective on Mars. Estonian by birth, Russian by education, Öpik finished his career at Armagh Observatory in Northern Ireland, where he edited the Irish Astronomical Journal. He was inducted into the United States’ National Academy of Sciences in 1960 and received the Gold Medal from the Royal Astronomical Society in 1975. Öpik was one of the giants of astrophysics and planetary astronomy in the twentieth century, and he had significant weight to toss around when he explained what astronomers knew about life on Mars for Science.
Öpik summed up what we Earthlings thought we knew at the time about Mars in his twelve-page article, “The Martian Surface.”24 “The story of the infrared bands near 3.6 microns is very instructive,” he wrote, pointing out that the scarcity of our observations is problematic. “These bands were at first attributed to the CH bond characteristic of complex hydrocarbons and indicative of some kind of organic matter. Then it was shown that they fitted absorption bands of heavy water, HDO, much better. This led to speculations on the enrichment of deuterium on Mars, through preferential escape to space of the lighter hydrogen, or through some other mechanisms. Finally, it turned out that the bands belong to heavy water in the terrestrial atmosphere and have no bearing on Mars.” Clear, succinct, and accurate.
The surface of Mars, he continued, has regions of different colors. About 70 percent (the continents) of the surface can be characterized as orange, yellow, or red. The rest of the regions (the maria) are darker, and are “sometimes described as greenish or bluish, but they are only less reddish” than the other areas. Here, finally, Öpik had pulled back the curtain and acknowledged that Mars had never looked green. It only sometimes looks a bit less red. And, as has been known for centuries, the shades and colorations change with the seasons.
How are we to understand all of this, he asked? Despite all the evidence to the contrary, despite the barrenness of Mars revealed by the admittedly grainy Mariner 4 images, despite the implosion of the Sinton bands as evidence of Martian life, despite his acknowledgment that Mars has never shown any green colors, but is only sometimes less red, Öpik, like so many astronomers before, found himself trapped by the weight of astronomers’ long love affair with the idea of Martian life. Öpik’s answer: “Against this background, the survival and permanence of the Martian surface markings seem to suggest a certain regenerative property. . . . Vegetation growing in favorable places on the drifting dust could serve as an explanation. . . . That vegetation could survive the extreme cold and dryness of the Martian climate may seem incredible. Yet the very rigor of the environment may help in this respect: the nocturnal temperatures are so low that hoarfrost may be deposited (as is apparently observed on the soil and hard-frozen plants; when this melts, drops of liquid water may be utilized by the plants while they are warming up in the morning sun. . . . However doubtful the vegetation hypothesis of the maria may appear, it is difficult to find an alternative that accounts for all the facts.”
No evidence for chlorophyll. No proof of lichens or primitive vegetation. No CH bands due to algae. No “green” areas. But still the idea of life persisted. By 1965, Mars was also known to be incredibly cold, oxygen-deprived, and nearly water-free. Because the Martian atmosphere is so thin and has no ozone layer, the surface is exposed to lethal levels of ultraviolet light from the Sun and is bombarded by deadly cosmic rays from the Sun and beyond. Yet, despite all of those clear reasons to dismiss vegetation as a possibility on the Martian surface, Öpik couldn’t do it. The desire to find life on Mars was too deeply ingrained in our mid-twentieth-century human psyche for Öpik to let go of that possibility.
Two giants of late twentieth-century planetary astronomy, James Pollack and Carl Sagan, finally put this issue to rest with their work a year later, in 1967. Sagan was already well known for studies of the atmospheres of Mars and Venus and for his speculative book Intelligent Life in the Universe, published in 1966, though not yet world-famous for his work on the Viking missions and his narration of the Cosmos television series on National Public Television. Pollack would go on to play important roles in virtually every NASA mission to every planet for decades, including Mariner 9 (to Mars), Viking 1 and 2 (both to Mars), Voyager 1 and 2 (to Jupiter, Saturn, Uranus, and Neptune), Pioneer Venus, Mars Observer, and Galileo (to Jupiter). He would become an expert on the physics of Saturn’s rings, on the formation of giant planets, and on the evolution of Earth’s early atmosphere.
Pollack and Sagan used radar maps of Mars, collected with one of NASA’s deep space network antennas, the Goldstone tracking station in California, managed by the Jet Propulsion Laboratory, to study and understand the surface of Mars. With one of the Goldstone dishes, they sent out radio waves that bounced off the surface of Mars; they used the same giant dishes to measure the reflected signals. The dark areas of Mars, they discovered, were at higher elevations than the bright areas. In addition, the dark regions that exhibit long-term changes in coloration have smaller slopes and elevations than the dark regions that do not undergo secular changes in color. As a result, they “hypothesize that the secular changes are due to the movement of sand and dust from the bright areas onto and off from adjacent dark areas of shallow slopes.” The so-called regenerative properties of the dark areas, that is, the ability of the dark areas to darken after lightening, “are due to winds scouring small deposited particles off the sloping highlands.”25
In other words, Martian weather systems drive windblown sand across the surface. The sand grains are preferentially deposited in the lowland “deserts.” The sand grains are highly reflective, so the deserts look fairly bright. When the small sand particles are blown onto and cover the shallow, sloped surfaces, these regions brighten and look like the deserts. When the winds scour the small particles of sand off the shallow slopes, these sloped surfaces rapidly darken. The darkening of these surfaces had been interpreted (and misunderstood) as the rapid regeneration of plant life in Martian springtime.
Sagan and Pollack later presented a quantitative model for windblown dust particles on Mars to support their initially, purely descriptive hypothesis.26 While they concluded that “the success of windblown dust models does not, of course, argue against life on Mars,” the success of their windblown dust models did finally put an end to the idea that the changing colors on Mars are caused by waves of greenery that roll across Mars as melting water from the polar caps ushers Martian springtime into bloom. No more trees; no more moss; no more lichens; no more algae. Just windblown sand.
a In modern usage, all names for craters, mountains, valleys, and all other surface features on Mars must be approved by the International Astronomical Union’s Working Group for Planetary System Nomenclature. The IAU first approved names for 126 surface features on Mars in 1958 (including Syrtis Major and Mare Tyrrhenum), approved three more names in 1967, 273 names in 1973, 528 names between 1976 and 1979, and has been kept busy approving names for Mars surface features ever since. See: https://planetarynames.wr.usgs.gov/Page/MARS/target
b In 1949, Irishman Kenneth Essex Edgeworth had made a similar prediction, suggesting the possibility of a stable reservoir of comets and small planets in the trans-Neptune zone of the solar system.
c Italics in original text.
d Geology on Mars.
e An organic molecule must contain one or more carbon atoms and must have carbon-hydrogen (C-H) bonds.