After the rather dismal landscape revealed by the flyby Mariners, the Mariner 9 images, "the most exciting ever obtained in planetary exploration,"1 gave a much-needed impetus to Mars studies. But as successful as it was, Mariner 9's reconnaissance had taken place entirely from orbit, and the one thing that it had not been able to do was search for life on the surface. We pass quickly over several Russian spacecraft, including two landers, which sent back depressingly meager results in 1974,2 and turn to the events of 1975--76, a year that will forever be remembered in the annals of Martian exploration.
Ninety-eight years after Schiaparelli made his memorable telescopic study of the planet and gave the surface features their enchanting names, including Chryse, the Land of Gold, the great attempt began. On August 20 and September 8, 1975, the two Viking spacecraft were launched toward Mars from Cape Canaveral, Florida. Though it was far from being their only purpose in going to Mars, the search for life certainly became the dominant theme of the Viking missions (fig. 20).
Each spacecraft consisted of an orbiter and a lander component. The orbiters were equipped with a more sophisticated television camera system than the one that had flown aboard Mariner 9, and each carried instruments for measuring the composition, pressure, temperature, and water vapor content of the Martian atmosphere. Their main purposes, however, at least initially, were to scout suitable landing sites and then to provide a radio link between the landers down on the surface and the Earth. Only after these tasks were completed could the orbiters be spared to carry out other investigations.
Unfueled, each lander weighed about 600 kilograms. In addition to a miniature laboratory needed to carry out the all-important biological experiments, the landers carried television cameras and an array of sensitive meteorological and seismological devices.
Prospective landing sites had been chosen well in advance. Lander 1 was to touch down at the point at which one of the largest Martian outflow channels (Maja Valles) debouched into Chryse Planitia, since this location was thought to offer the best hope of finding water and near-surface ice---and hence organic molecules, if any happened to be present. Lander 2 was to land at Cydonia, near the point of maximum extent of the north polar cap, again a favorable site for water.
The journeys across interplanetary space were uneventful. Viking 1, actually the second Viking launched, was inserted into Martian orbit on June 19, 1976. Its orbit was an ellipse with a periapsis of 1,513 kilometers and an apoapsis of 33,000 kilometers; each revolution was completed in 24.66 hours---the period was synchronized with the planet's rotation so that the orbiter could assume a stationary position over the intended landing site. Originally the mission planners had hoped that the first landing could be attempted on July 4, 1976, the two hundredth anniversary of the signing of the American Declaration of Independence, but the orbiter photographs showed that the Chryse site was more rugged than expected. There was no choice but to look elsewhere, and by the time an acceptably smooth location had been found---it lay farther to the west, though still within Chryse Planitia---the landing had to be delayed until July 20 (coincidentally, the seventh anniversary of the first Apollo lunar landing).
Early that morning---1:50 A.M. at mission control at the Jet Propulsion Laboratory in Pasadena, California---the Viking 1 lander, enclosed within a cone-shaped aluminum aeroshell, separated smoothly from the orbiter and began its descent toward the surface. For part of its descent the spacecraft would be traveling at 16,000 kilometers per hour, and since Mars, unlike the Moon, has an atmosphere, air friction could not be ignored---thus the capsule was equipped with a heat shield. In the later stages of descent, when the lander had reached an altitude of 6 kilometers above Mars, a parachute was deployed. At this point the aeroshell was jettisoned and the legs of the lander were extended. Less than a minute later, when the craft had reached an altitude of 1,200 meters above the ground, the parachute was released and retro-rockets were fired to slow the craft for the rest of its descent. Just before it came to rest on the surface, the lander was traveling at a speed of only 10 kilometers per hour.
Scientists on Earth did not know its fate for another nineteen minutes, the time required for the radio signals to travel the 342 million kilometers between Mars and the Earth. Fortunately, the lander narrowly missed a boulder large enough to cause a crash and settled safely on the surface---the precise location was at 22.5° N, 48° W, in Chryse Planitia. At the resolution of the orbiter images, the surface had appeared featureless apart from impact craters and wrinkle ridges---there was little, indeed, to distinguish it from one of the wide gray plains of the Moon.
Within twenty-five seconds of the lander's arrival, one of the television cameras began to scan the surface. The first picture took five minutes to scan---and then, of course, another nineteen minutes to transmit from the orbiter relay link to Earth. The field of view, which lay only 1.5 to 2 meters away from the camera, was strewn with rocks up to 10 centimeters across. The surface was obviously fairly firm; part of one of the footpads was visible, and it had penetrated less than 4 centimeters into the Martian soil.
Immediately after sending back this image, the same camera began scanning a 300° panorama to show the landscape around the spacecraft. Again the image showed rocks of various sizes, most of them angular in shape and coarsely pitted---the whole area was evidently a lava field, and the rocks appeared to be basaltic. It was an exhilarating moment; viewers back on Earth felt as if they were actually standing on the Martian surface. Another panorama, taken by the second television camera, included the 60° sector that had been hidden from the first camera by the body of the spacecraft; this showed an almost Saharan expanse to the northeast with large dune drifts of fine-grained materials. Obviously, aeolian processes were important.
Telescopic observers have long been fascinated by the compelling, if partly illusory, colors of Mars, and the Viking imaging team also had a hard time getting the Martian colors right. They made colored pictures by mixing images obtained through a tricolor wheel (a wheel containing blue, green, and red filters), but the calibration was uncertain, and the earliest published images showed the Martian surface as a very piquant orange-red.3 Meanwhile, the Martian sky---surprisingly bright and rather unromantically described as "similar to a smoggy day in Los Angeles"---was given a bluish cast that was eerily Earthlike.4 Later, someone suggested that Mars's atmosphere was probably too thin for molecular (Rayleigh) scattering to produce a blue sky color, and that instead the sky brightness must be due to a suspension of fine reddish dust, which would make the sky salmon pink---and so it became in later images (although I must admit that I never found the effect quite believable).5 In this case, the Viking imaging team seems to have bent too far in the other direction to avoid creating the illusion of an Earthlike Mars. Because the dust load of the Martian atmosphere is extremely variable, the color of the sky ranges, at different times, from salmon pink to yellow to light blue to dark blue to purple. Since the images returned by the Viking lander just after it landed show shadows cast by the rocks as razor sharp, the atmosphere was then fairly clear. Ironically, after all the trouble taken with it, the sky seems to have been blue after all!
After the excitement of viewing the first images of the Martian surface subsided, attention turned to the all-important biological experiments. The question of how to detect organisms on Mars, should they exist, had been carefully considered by the biologists on the Viking team (alas, Lowell's notion of large-brained Martians had long ago been discarded; no one expected to find anything more sophisticated than lowly microbes). The biologists designed three experiments, described below, to test for the presence of life.
1. Pyrolytic release experiment. A tiny sample (0.1 g) of Martian soil is first scooped up with the lever arm of the lander, then placed inside the test chamber. Carbon dioxide and carbon monoxide labeled with the radioactive isotope carbon-14 are then admitted, the mixture is incubated under a sunlamp for several days, and everything is heated in order to break down (pyrolyze) any organic compounds present. Finally, hydrogen gas is admitted into the chamber to sweep the pyrolysis products into a gas chromatograph and mass spectrograph capable of detecting carbon-14. Since any organisms present should carry out metabolic processes during which they will assimilate carbon-14 from the gas in the chamber, detection of carbon-14 would be a positive result, though in and of itself not entirely conclusive, since a first peak of radioactivity might equally well be due to chemical processes not involving living organisms. In order to rule out this latter possibility, other samples (serving as controls) are sterilized by heating before the carbon source is admitted.
2. Labeled release experiment. Again, a sample of Martian material is placed into the chamber, and a moist nutrient material containing carbon-14 is added. Any Martian organisms present will metabolize the nutrient material and release carbon-14-labeled gas, which is then registered by the detector.
3. Gas exchange experiment (popularly known as the "chicken soup" experiment). At the beginning of the experiment, the atmosphere within the chamber consists of carbon dioxide and the inert gases helium and krypton; a nutrient material and water vapor are added to the soil sample. On suddenly finding themselves in a water- and nutrient-rich environment, the Martian organisms will respond with a vigorous spurt of metabolism, resulting in the sudden buildup of gases in the chamber.
Apart from these three experiments, the mass spectrograph was designed to make sensitive measurements in a direct attempt to detect organic materials on Mars.
Though straightforward enough in theory, the experiments produced confusing results. The pyrolytic release experiment showed two peaks, and at first was felt to be weakly positive; however, later attempts to duplicate the effect were unsuccessful. The labeled release experiment showed an immediate---and startling---rise in the level of carbon-14 radioactivity immediately after the nutrient was introduced into the chamber. This data strip seemed to attest to a positive reaction, so much so that the experiment team immediately rushed out and ordered a bottle of champagne. The "chicken soup" experiment also produced dramatic and unexpected results; when the samples were humidified, there was a sudden burst of oxygen---something that had never occurred in earlier tests with terrestrial samples. However, there was a very weak response when the nutrient material was added.
On the whole, the consensus was that, instead of being produced by organisms of some kind, the chemical reactions observed were entirely due to a highly oxidizing substance in the rust red Martian soil, perhaps iron peroxide or superoxide. Some of the organic compounds in the nutrient would have been sensitive to oxidizing materials. The same exotic chemistry can explain the evolution of the large amounts of oxygen when water vapor was added to the gas-exchange experiment. The fact that the mass spectrograph failed to identify any organic compounds (except for some known contaminants such as methylchloride and freon-E) would appear to argue rather strongly against biological explanations for the results.6 On the other hand, the data from the labeled release experiment remain in dispute, and a small minority of scientists still maintain that the Viking results are positive evidence for the existence of microorganisms on Mars. The last word remains to be said.7
So much for the results at the Viking 1 site. Meanwhile, of course, Viking 2 was on its way; it arrived in Martian orbit on August 7, but because the Viking 1 experiments were then in full swing, the landing was delayed until September 3. Again there were last-minute changes. The original site in Cydonia was too rough, and the lander was directed to an alternate site at Utopia Planitia, on the vast northern plains. Despite a momentary power shortage in the orbiter, which caused a temporary loss of the main communications link, the Viking 2 lander touched down safely at 48° N, 225.7° W, some 7,400 kilometers northeast of where the Viking 1 lander was already resting on the surface.
As the first panoramic view began to come back from Utopia, the horizon appeared strongly sloped to the right; one of the lander's footpads had come to rest on a boulder, canting the spacecraft at an 8° angle. Though the orbiter photographs had suggested that this would be a region of dunes, Utopia was no less a forest of rocks than the Viking 1 site had been. There were differences, of course. Unlike the rather varied rock forms of Chryse, Utopia's rocks proved to be larger on average and more evenly distributed across the surface, with no bedrock outcrops or large drifts, and this created a singularly monotonous appearance. The most striking feature about the rocks, apart from their uniform size and distribution, was their extensive pitting. They looked rather like terrestrial vesicular volcanic rocks, in which pits form when cavities are created around small gas bubbles in volcanic lava.
Whereas the Chryse site consisted of gently rolling plains, Utopia proved to be remarkably flat, presumably because of its proximity to the 90-kilometer impact crater, Mie, whose rim lay 170 kilometers east of the landing site. The orbiter photographs showed that a broad lobe of ejecta material from Mie ran just southeast of the site. Aeolian processes were again evident, but the drifts were smaller than in Chryse---they occurred only in patches between clusters of rocks and as small windtails. Finally, the same suite of biological experiments was carried out at the Viking 2 site, and they too failed to find conclusive evidence for or---for a minority of scientists---against the existence of organic life.
The landers had originally been designed to function for only ninety days. In fact, all four spacecraft---the two landers and the two orbiters---continued to perform much longer than expected. Orbiter 2 transmitted data until July 25, 1978; Lander 2 (VL2) until April 11, 1980; and Orbiter 1 transmitted until August 7, 1980. The longest record was provided by Lander 1---later renamed the Mutch Memorial Station (MMS) in honor of geologist and landing team leader Timothy Mutch, who fell to his death in 1980 while climbing in the high Himalayas; it remained in contact until November 13, 1982, a total of 6.4 Earth, or 3.4 Martian, years.
Apart from the data obtained in the biology experiments, the landers gathered a tremendous amount of useful information about Martian surface conditions. Among other things, they deployed meteorological booms with temperature, pressure, and wind sensors, which effectively served as in situ meteorological stations for three Martian years.
When the landers first arrived, it was early summer in the northern hemisphere, and the atmosphere was nearly dust-free. The diurnal cycle of temperature variation repeated very nearly from one Sol to the next; at MMS, the diurnal temperature range was 50°C, with a minimum at dawn of -83°C (-118°F) and a maximum in the early afternoon of -33°C (-28°F). (Remember, this was the Martian tropics!) The temperatures at the more northerly VL2 site were five to ten degrees colder. Incidentally, because the thin Martian air has virtually no capacity to hold heat, the air temperatures were some twenty degrees colder than the surface temperatures.
Obviously, then, Mars is a very cold place---Martian temperatures are a far cry from the south of England temperatures that Percival Lowell predicted. The landers' temperature measures were supplemented by those of the orbiters. The warmest temperatures, found in some of the southern hemisphere "oases," reached as high as 22°C (60°F) in early afternoon in midsummer, but even so, the temperature still plummeted to -53°C (-63°F) at night.
The atmospheric pressure at MMS in the summer was 6.7 millibars; at VL2, because of its lower elevation, the pressure was slightly higher, 7.4 millibars. Winds at both sites were very light, with speeds of 2 meters per second at night and up to 7 meters per second during the day.
Later, the two stations recorded significant seasonal changes. By mid-autumn the polar hood had developed over the north polar cap and storms began passing regularly north of the landing sites, causing fluctuations in pressures and winds, especially at the more northerly VL2 station. In early 1977, a major dust storm occurred; its onset was heralded in orbiter images taken in February that revealed an extensive cloud swirling about the high ground of Claritas Fossae, west of Solis Planum. Within a few days this storm had grown to global proportions. The landers recorded that during the dust storm the Martian atmosphere became much more opaque; the diurnal temperature range narrowed sharply from fifty degrees to only about ten degrees, and the wind speeds picked up considerably---indeed, within only an hour of the storm's arrival at MMS they had increased to 17 meters per second, with gusts up to 26 meters per second. Nevertheless, no actual transport of material was observed at either site, only a gradual brightening and loss of contrast of the surface material.
The dust began to clear slowly over the next several weeks, and as it did so, a thin coating of water ice formed on the rocks at the VL2 site.8 In May 1977 a second global dust storm developed; the regular dust-free seasonal cycle of pressure and temperature variations was not reestablished until late 1977.
The atmospheric pressure at the two sites also showed seasonal variations. The values climbed from 6.7 millibars at MMS and 7.4 millibars at VL2 in summer to 8.8 millibars and 10 millibars in winter. This behavior had been expected on the basis of the behavior of the seasonal polar caps. Recall that the Martian atmosphere is 95 percent carbon dioxide, and that seasonal caps form at the poles when carbon dioxide freezes out as a frost onto the surface. Because of the longer southern hemisphere winters, the southern seasonal cap becomes much more extensive than its northern counterpart and incorporates more carbon dioxide from the atmosphere; conversely, during the short but hot southern hemisphere summer, much of this carbon dioxide sublimes away again to produce a net re-release of large amounts of carbon dioxide into the atmosphere. Some 30 percent of the Martian atmosphere is cycled through the seasonal polar caps in this way, and this is bound to have a dramatic effect on the seasonal wind patterns.
It is worth reviewing the different behavior of the two caps in some detail. The south seasonal cap, at its maximum extent, is roughly circular around the pole and reaches to about latitude 60° S throughout most of its circumference (and even somewhat farther, to about 50° S, within the Argyre basin; among other notable outliers is the great Hellas basin, which during the winter is filled with carbon dioxide frost). As the cap retreats, it develops rifts and becomes irregular in outline. Near midsummer, a large peninsular section, located at about 70° S and 330° W, breaks off, forming the detached island of Novissima Thyle (as Schiaparelli called it), or the Mountains of Mitchel. The spacecraft photographs show no mountains, however, only a southward-facing slope on which the frost continues to linger; it consists partly of the scarp of a large impact basin centered some 6° from the pole. The further retreat of the cap reveals an underlying residual cap of carbon dioxide ice, which persists throughout most summers. Its highly characteristic "swirl" pattern is produced by the uneven removal of frost by wind along the slopes of valleys. As long as the residual cap remains, the temperature is buffered to the frost point of carbon dioxide (-125°C [-193°F] at a surface pressure of 6.1 millibars), well below the sublimation point of water.
The northern cap behaves quite differently. When it emerges from the polar hood in early spring, it covers its greatest extent, to approximately 65° N latitude. It too shows rifts and outliers as it begins to retreat, though because the terrain is smoother than in the south there is nothing comparable to the Mountains of Mitchel. Dunes belt the region between 75° and 85° N, forming a collar around the cap. The north seasonal cap is smaller and darker than the southern one---mainly because it is laid down during the part of the Martian year when there is generally more dust in the atmosphere, which precipitates with carbon dioxide frost onto the surface. It also sublimates more efficiently, and vanishes completely by summer solstice, leaving behind an underlying residual cap which, unlike its counterpart, consists not of frozen carbon dioxide but of water ice---one of the surprises of the Viking mission.9 In all, the amount of water it contains would probably cover Mars to a depth of 10 to 40 meters if it were distributed evenly across the surface. After the carbon dioxide frost disappears, the residual northern cap is no longer buffered to the frost point of carbon dioxide as the southern cap generally is, and in summer may attain a temperature as high as -68°C (-90°F), close to the frost point for water on Mars. These temperatures are consistent with observations of large amounts of water vapor in the northern hemisphere during these times.
The atmosphere of Mars is thin, cold, and bone dry. In some ways it resembles the stratosphere of Earth, though with at least one important difference---in the stratosphere, the relative humidity is very low, whereas the atmosphere of Mars is nearly saturated. The saturated water vapor pressures for some Martian temperatures are as follows:
Water vapor is a minor component of the Martian atmosphere. The main component, of course, is carbon dioxide, which tends to freeze out at higher altitudes into a haze of fine crystals. As already noted, the amount of carbon dioxide varies seasonally by about 30 percent. Based on a total atmospheric pressure of 7.5 millibars, the following values for the composition of the Martian atmosphere have been derived from the Viking lander measurements: carbon dioxide, 95.32 percent; nitrogen, 2.7 percent; argon, 1.6 percent; oxygen, 0.13 percent; carbon monoxide, 0.7 percent; water vapor, 0.03 percent; inert gases, trace.
The atmosphere, thin as it is, can nonetheless transport significant amounts of sand and dust. The Viking orbiters noted many features formed by wind, including vast dune fields, especially near the south polar cap; wind-eroded hills, or yardings; and windblown streaks, both light and dark. Indeed, as noted by V. A. Firsoff, "the dusky areas seen in the distant views of Mars dissolve in the close-ups into swarms of dark `streaks' and `splotches.' The streaks are elongated markings 10 or more km long, frequently in the form of `crater-tails,' which may be comet- or fan-shaped or resemble the flame of a candle, including its central and peripheral shading. Splotches are irregular or rounded, often found within craters, centrally or on the side, and may `wash over' the walls."10 The dark streaks ("shredded streaks") that are found in great abundance in some areas, such as Syrtis Major, develop when rocks are scoured by very high winds; the light streaks consist of fine sand blown by the prevailing winds. Clearly, the broad features visible from Earth represent an integrated view of these small details. Changes observed in them, including the seasonal "wave of darkening" (if it exists), may be related, as suggested by Carl Sagan and others, to "the alternate deposition and deflation of windblown dust having detectable contrast with respect to basement material."11
This discussion of wind features brings us at last to the famous dust storms of Mars. Dust clouds, stirred up by rising cells of warm air which carry dust from the surface high into the atmosphere, tend to arise preferentially in certain areas; in the southern hemisphere, the active areas include the circumference of the south polar cap, Hellas, Hellespontus-Noachis, and Claritas Fossae--Solis Planum; in the northern hemisphere, some active areas are Chryse-Acidalium, Isidis Planitia--Syrtis Major, and Cerberus. Usually the dust arises at several points on the planet at once; indeed, a cloud's apparent movement may be partly illusory, because small storms tend to form simultaneously and then coalesce with one another. The storms may remain fleeting and localized, or they may spread through a latitudinal corridor around the entire planet (planet-encircling storms). The largest storms are global events like those of 1956 and 1971. A planet-encircling storm occurred in 1973, and two occurred during 1977 while the Viking landers were on the planet; yet another seems to have been developing in 1982 just as the Viking mission was coming to an end. These massive storms (planet-encircling or global) always occur during southern spring and summer, when Mars is near perihelion (Ls = 251°), though the actual interval is rather broad---they have been observed to begin anywhere from Ls = 204° to 310°.
Among the factors critical to triggering the development of major storms, the most important is the fact that there is increased heating near perihelion, which in turn produces stronger winds. Again, the atmospheric pressure is 30 percent greater during the southern hemisphere spring and summer than it is in winter, owing to the release of large amounts of carbon dioxide from the seasonal polar cap---which itself enhances the atmosphere's capacity to carry dust. Finally, since the areas where dust clouds generally develop are areas of slopes---or, in the case of the polar cap, of high temperature gradients---topographic factors further enhance the near-surface winds, which is why local clouds tend generally to arise in the same areas. Once aloft, the suspended dust becomes a major absorber both of solar radiation and of heat being reradiated from the surface; it is certainly a much more effective absorber than the thin, cold Martian air.
Dust storms on Mars are an example of a so-called chaotic phenomenon. Positive feedback mechanisms amplify the initial disturbance, but the interplay of these factors is unpredictable---it may lead to large-scale storms, but this is not the inevitable result. (There must, obviously, be negative feedback mechanisms as well, which dampen out the dust storm activity eventually, but at present these are even less satisfactorily understood than the positive feedback mechanisms.)
In general, dust is transported from the dark highland areas of the southern hemisphere, which contain numerous rocks and outcrops where active erosion is taking place, to the bright "deserts" of the northern hemisphere, such as Tharsis, Arabia, and Elysium, which contain significant deposits of fine dust. The north polar cap is also a major depository of dust.
Even a thin coating of dust is enough to account for many of the long-observed changes in Martian albedo patterns. At the end of the major dust storm season (the southern hemisphere spring and summer), the surface features often display decreased contrast due to the continued presence of fine dust in the atmosphere and the deposition of a thin coat of dust on the surface. By the middle of the southern autumn (northern spring), the atmosphere has cleared again, and classical albedo features such as Syrtis Major---always faint immediately after a global dust storm---have fully redeveloped. Most of the dramatic changes in individual features can be explained thus; for example, marked changes in Solis Lacus, the albedo feature associated with the Solis Planum region, were observed after the 1956 and 1973 storms.
For a time it seemed that a major dust storm must develop every time Mars came to perihelion, but we now know that things are not so simple---indeed, there has not been a planet-encircling storm since 1982, although there have been regional obscurations. It is significant that in 1969, the residual carbon dioxide cap around the south pole seems to have disappeared completely, and large amounts of water vapor were detected during the southern hemisphere summer.12 These unusually warm conditions preceded the development of the great storms of 1971 and 1973. During the 1980s and 1990s, by contrast, Mars has apparently been much colder; the Hubble Space Telescope showed thin cirrus clouds over extensive areas of the planet in 1995, but virtually no dust. It may well be that the great storms of 1956 and 1971 were highly anomalous events, and that the usual Martian conditions are more like those seen in recent years.13
Mars's asymmetric polar caps and cycle of southern hemisphere spring and summer dust storms reflect the current position of its axis and the eccentricity of its orbit. At present, Mars's axial tilt, or obliquity---25.2° from the perpendicular---is very nearly the same as the Earth's (23.5°). The current agreement, however, is a sheer coincidence. Both Earth and Mars bulge slightly at the equator because of the centrifugal force of their rotation. The gravitational pull of the Sun on these equatorial bulges causes the axial tilts of both Earth and Mars to vary over time. Earth's axial tilt is largely stabilized by the presence of the Moon, and so ranges through only four degrees. Mars, which lacks a large and massive satellite, wobbles in a much more extreme fashion---at the current epoch, its axial tilt ranges between extremes of 15° and 35° over a period of 120,000 years, with the present value lying close to the mean.14 The spin axis also wobbles, or precesses, just like a top slowing down, with a period of 173,000 years (compared with 25,800 years for the Earth). This is the effect that on Earth gives rise to the well-known precession of the equinoxes.
The planetary orbits themselves rotate slowly in space, resulting in a gradual shift in the position of the perihelion. As a combined effect of the precession of the spin axis and the advance of the perihelion, alternate poles of Mars tilt toward the Sun at perihelion every 25,500 years---that is, on a 51,000-year cycle. The orbits also change shape over time, and again the more extreme changes belong to Mars---its orbital eccentricity (now at 0.093) ranges between 0.00 and 0.13 over a period of 2 million years, while that of the Earth (now 0.017) never exceeds 0.05.15
Periodic oscillations in the obliquity of Earth's axis and the eccentricity and precession of its orbit give rise to the so-called Milankovitch cycles, named for the Serbian astronomer M. Milankovitch, who in 1938 proposed that such cycles may partly explain the Ice Ages. We know, for example, that during the past 3 million years, much of the Northern Hemisphere has been covered with ice, with the last glacial maximum occurring 18,000 years ago---indeed, we may not have fully emerged from it. There have been similar episodes throughout much of the Earth's history. Though other factors may also play a role---for instance, the drift of a continental mass over a pole seems to be a necessary precondition of extensive glaciation, and large impacts too may be important because they can raise large quantities of dust and thus reduce the incident solar radiation---it is generally agreed that the effects of the Milankovitch cycles on Earth's climate are far from negligible. On Mars, which lacks the moderating effect of oceans and suffers much more extreme variations in its axial tilt and orbital eccentricity, they may be even more decisive.
On Mars, the most important cycle of climate change is the 51,000-year cycle caused by the combined effect of the precession of its axis and the advance of its perihelion. Although at present the southern hemisphere is tilted toward the Sun at perihelion, in 25,500 years it will be the northern hemisphere instead. The northern hemisphere will then have the short, hot summers, and since the occurrence of the major dust storms is clearly related to Mars's arrival at perihelion, dust storm activity will presumably shift mainly to the northern hemisphere's spring and summer. The large amounts of fine dust currently deposited in the northern hemisphere in regions such as Tharsis, Arabia, and Elysium will be redistributed to the southern hemisphere, and dust accumulation at the south polar cap will exceed that at the north. The asymmetry of the caps will be completely reversed, and the large seasonal carbon dioxide frost cap will form over the north pole instead of the south pole.
Indeed, there is unequivocal evidence for such climatic cycles on Mars in the layered deposits, or laminated terrain, of the polar regions, discovered by Mariner 9 in the case of the south pole and well documented in the Viking 2 orbiter images of the north pole. This laminated terrain covers most of the area beyond the 80° circles of latitude---in the southern hemisphere, it is in a region of ancient cratered terrain; in the northern hemisphere, in an area of smooth plains. The layers are thought to consist of alternate strata of dust and ice (or of dust and ice in varying proportions). Presumably these layers record changes in water and dust transport or removal to the polar regions during different periods.16
The most interesting climatic conditions are found at the extreme values of the obliquity. Detailed calculations have shown that the obliquity of Mars has ranged between a low of 13° and a high of 47° over the past ten million years (since the obliquity is chaotic, it is inherently unpredictable over significantly longer periods). At the minimum value, 13°, permanent caps of carbon dioxide must form over both poles, and the planet must go into deep-freeze. With most of the atmosphere taken out of circulation, the surface pressure must drop to less than a millibar, in which case there would be too little air to support dust, and dust storm activity must cease altogether. At the maximum value, 47°, both poles would lose their carbon dioxide ice caps each summer. This, obviously, is the most interesting case, since the polar temperatures, no longer buffered to the carbon dioxide frost point, would rise appreciably. More water vapor would be released into the atmosphere, thereby producing still further warming owing to the greenhouse effect.
The greenhouse effect is a result of the fact that certain gases, such as carbon dioxide and water vapor, are transparent to visible light but absorb strongly in the infrared. Thus the light is able to penetrate through the atmosphere but is trapped when it is reradiated as heat from the ground. This produces a net warming of the planet. On the Earth, greenhouse gases are present in very small quantities---carbon dioxide, for example, represents only 0.0003 of the Earth's total atmospheric mass. However, despite the scarcity of these gases, they play the major role in determining the Earth's temperature. At present the average global temperature is about 15°C, around thirty-five degrees warmer that it would be without the effect of greenhouse warming; though this sounds modest, it is very significant, since without it the Earth's oceans would freeze over. Obviously one can get too much of a good thing---on Venus, for instance, which has a massive atmosphere made up 97 percent of carbon dioxide, the greenhouse effect has proceeded with sinister efficiency; the global temperature is 500°C, so the surface is easily hot enough to melt lead. (This may present a cautionary tale to the Earth, where these gases have been building up gradually over the last hundred years or so owing to human activities on the planet. We certainly don't want to end up like Venus!) On Mars, with its very thin atmosphere, the greenhouse effect (even with the help of dust) has increased the global temperature by a mere seven degrees, to about -50°C, which is far too cold for liquid water to form anywhere on the surface.
When the obliquity of Mars reaches its greatest value, the carbon dioxide polar caps may sublimate completely. At such times, Mars's atmosphere would be much more massive than it is now, so that greenhouse warming might increase the global temperature another 30°C---still not enough to allow liquid water to exist on the surface. And yet, water obviously has flowed on the surface of Mars, probably repeatedly, so some other mechanism must be involved. Recall that the valley networks are found almost entirely in the old, heavily cratered Noachian terrain, but the outflow channels are younger, being of Hesperian and Amazonian age. The largest of them debouch onto the wide northern plains, where ponded sediments, shorelines, and other features associated with standing water have been tentatively identified. Apparently the northern plains have been periodically inundated by a great ocean, the Oceanus Borealis.17 Since changes in Mars's atmosphere don't seem to be sufficient to explain these flooding episodes, there can be little doubt that they have been driven by hydrothermal processes---in other words, by heat generated within Mars itself.
We now know a good deal about current conditions on Mars (see appendix 3), and what we know suggests that it is probably not alive. Its environment is harsh in the extreme: cold, all but airless, dry beyond the driest deserts of the Earth. But of all the obstacles to life as we know it, it seems that the lack of water is probably the most important.
And yet Mars has not always been as dry as it is now, and there is at least the chance that life might have gotten a start during the more benign periods of its history. Though it is all but certain that Mars is not now the abode of life, the questions of its past still haunt us and are much further from solution.
The details remain uncertain; what is clear is that there do seem to have been intervals in which the planet has been warmer and wetter than the usual cold, dry post-Noachian conditions---brief oasislike interludes in which life itself might have gotten a foothold. The most recent flooding episodes, indeed, may have occurred within the past several hundred million years. Inevitably, alas, the water seeped back into the regolith again, to be trapped as groundwater and permafrost; the carbon dioxide, too, precipitated out, and once more cold, dry conditions prevailed.
© 1996 The Arizona Board of Regents