by Carl Sagan
The observed average temperature of about 15°C on the Earth today can be explained quite well by the observed intensity of sunlight, global albedo, the tilt of the rotational axis and the greenhouse effect. But all of these parameters can, in principle, vary; and past or future climatic change can be attributed to changes in any of them. In fact, there have been almost a hundred different theories of climatic change on Earth, and even today the subject is hardly marked by unanimity of opinion. This is not because climatologists are by nature ignorant or contentious, but rather because the subject is exceedingly complex.
Both negative and positive feedback mechanisms probably exist. Suppose, for example, there were a decrease of a few degrees in the Earth’s temperature. The amount of water vapor in the atmosphere is determined almost entirely by temperature and declines by snowing out as the temperature declines. Less water in the atmosphere implies a smaller greenhouse effect and a further lowering of the temperature, which may result in even less atmospheric water vapor, and so on. Likewise, a decline in temperature may increase the amount of polar ice, increasing the albedo of the Earth and decreasing the temperature still further. On the other hand, a decline in temperature may decrease the amount of cloudiness, which will decrease the average albedo of the Earth and increase the temperature—perhaps enough to undo the initial temperature decrease. And it has been proposed recently that the biology of the planet Earth acts as a kind of thermostat to prevent too extreme excursions in temperature which might have deleterious global biological consequences. For example, a decline in temperature may cause an increase of a species of hardy plants that has extensive ground cover and low albedo.
Three of the more fashionable and more interesting theories of climatic change should be mentioned. The first involves a change in celestial mechanical variables: the shape of the Earth’s orbit, the tilt of its axis of rotation, and the precession of that axis all vary over long periods of time because of the interaction of the Earth with other nearby celestial objects. Detailed calculations of the extent of such variations show that they can be responsible for at least a few degrees of temperature variation, and with the possibility of positive feedbacks this might, by itself, be adequate to explain major climatic variations.
A second class of theories involves albedo variations. One of the more striking causes for such variations is the injection into the Earth’s atmosphere of massive amounts of dust—for example, from a volcanic explosion such as Kiakatoa’s in 1883. While there has been some debate on whether such dust heats or cools the Earth, the bulk of present calculations shows that the fine particulates, very slowly falling out of Earth’s stratosphere, increase the Earth’s albedo and therefore cool it. There is recent sedimentological evidence that past epochs of extensive production of volcanic particulates correspond in time to past epochs of glaciation and low temperatures. In addition, episodes of mountain building and the creation of land surface on the Earth increase the global albedo because the land is brighter than the water.
Finally, there is the possibility of variations in the brightness of the Sun. We know—from theories of solar evolution—that over many billions of years the Sun has been getting steadily brighter. This immediately poses a problem for the most ancient climatology of the Earth, because the Sun should have been 30 or 40 percent dimmer some 3 or 4 billion years ago; and this is enough, even with the greenhouse effect, to have resulted in global temperatures well below the freezing point of seawater. Yet there is extensive geological evidence—for example, underwater ripple marks, pillow lavas produced by the quenching of magma in the ocean, and fossil stromatolites produced by oceanic algae—that there was ample water then available. One proposed way out of this quandary is the possibility that there were additional greenhouse gases in the early atmosphere of the Earth—especially ammonia—which produced the required temperature increment. But apart from this very slow evolution of the brightness of the Sun, is it possible that shorter-term fluctuations occur? This is an important and unsolved problem, but recent difficulties in finding neutrinos—which should, according to current theories, be emitted from the interior of the Sun—have led to the suggestion that the Sun is today in an anomalously dim period.
The inability to distinguish between the various alternative models of climatic change might appear to be nothing more than an unusually annoying intellectual problem—except for the fact that there appear to be certain practical and immediate consequences of climatic change. Some evidence on the trend of global temperature seems to show a very slow increase from the beginning of the industrial revolution to about 1940, and an alarmingly steep decline in global temperature thereafter. This pattern has been attributed to the burning of fossil fuels, which has two consequences—the liberation of carbon dioxide, a greenhouse gas, into the atmosphere, and the simultaneous injection into the atmosphere of fine particles, from the incomplete burning of the fuel. The carbon dioxide heats the Earth; the fine particles, through their higher albedo, cool it. It may be that until 1940 the greenhouse effect was winning, and since then the increased albedo is winning.
The ominous possibility that human activities may cause inadvertent climate modification makes the interest in planetary climatology rather important. There are worrisome positive feedback possibilities on a planet with declining temperatures. For example, an increased burning of fossil fuels in a short-term attempt to stay warm can result in more rapid long-term cooling. We live on a planet in which agricultural technology is responsible for the food of more than a billion people. The crops have not been bred for hardiness against climatic variations. Human beings can no longer undertake great migrations in response to climatic change, or at least it is more difficult on a planet controlled by nation-states. It is becoming imperative to understand the causes of climatic variations and to develop the possibility of performing climatic re-engineering of the Earth.
Oddly enough, some of the most interesting hints on the nature of such climatic changes appear to be coming from studies not of the Earth at all, but of Mars. Mariner 9 was injected into Martian orbit on November 14, 1971. It had a useful scientific lifetime of a full terrestrial year and procured 7,200 photographs, covering the planet from pole to pole, as well as tens of thousands of spectra and other scientific information. As we saw earlier, when Mariner 9 arrived at Mars there was virtually no detail whatever to be seen on the surface because the planet was in the throes of a great global dust storm. It was readily observed that the atmospheric temperatures increased, but the surface temperatures decreased during the dust storm, and this simple observation immediately provides at least one clear case of the cooling of a planet by the massive injection of dust into its atmosphere. Calculations have been performed that use precisely the same physics for both the Earth and Mars and treat them as two different examples of the general problem of the climatic effects of massive dust injection into a planetary atmosphere.
There was another and entirely unexpected climatological finding by Mariner 9—the discovery of numerous sinuous channels, replete with tributaries, covering the equatorial and mid-latitudes of Mars. In all cases where relevant data exist, the channels are going in the proper direction—downhill. Some of them show braided patterns, sand bars, slumping of the banks, streamlined teardrop-shaped interior “islands” and other characteristic morphological signs of terrestrial river valleys.
But there is a great problem with the interpretation of the Martian channels as dry riverbeds, or arroyos: liquid water apparently cannot exist on Mars today. The pressures are simply too low. Carbon dioxide on Earth is known as both a solid and a gas, but never as a liquid (except in high-pressure storage tanks). In the same way, water on Mars can exist as a solid (ice or snow) or as vapor, but not as a liquid. For this reason some geologists are reluctant to accept the theory that at one time the channels contained liquid water. Yet they are dead ringers for terrestrial rivers, and at least many of them have forms inconsistent with other possible structures such as collapsed
lava tubes, which may be responsible for sinuous valleys on the Moon.
Furthermore, there is an apparent concentration of such channels toward the Martian equator. The one striking fact about the equatorial regions of Mars is that they are the only places on the planet where the average daytime temperature is above the freezing point of water. And no other liquid is simultaneously cosmically abundant, of low viscosity, and with a freezing point below Martian equatorial temperatures.
If, then, the channels were made by running water on Mars, that water apparently must have run at a time when the Martian environment was significantly different from what it is today. Today Mars has a thin atmosphere, low temperatures and no liquid water. At some time in the past, it may have had higher pressures, perhaps somewhat higher temperatures and extensive running water. Such an environment appears to be more hospitable to forms of life based on familiar terrestrial biochemical principles than the present Martian environment.
A detailed study of the possible causes of such major climatic changes on Mars has laid stress on a feedback mechanism known as advective instability. The Martian atmosphere is composed primarily of carbon dioxide. There seem to be large repositories of frozen CO2 in at least one of the two polar caps. The pressure of CO2 in the Martian atmosphere is quite close to the pressure of CO2 expected in equilibrium with frozen carbon dioxide at the temperature of the cold Martian pole. This is a situation quite similar to the pressure in a laboratory vacuum system determined by the temperature of a “cold finger” in the system. At the present time the Martian atmosphere is so thin that hot air, rising from the equator and settling at the poles, plays a very small role in heating the high latitudes. But let us imagine that the temperature in the polar regions is somehow slightly increased. The total atmospheric pressure increases, the efficiency of heat transport by advection from equator to pole also increases, polar temperatures increase still further, and we see the possibility of a runaway to high temperatures. Likewise a decrease in temperature, from whatever cause, could bring about a runaway toward a lower temperature. The physics of this Martian situation is easier to work out than the comparable case on Earth, because on Earth the major atmospheric constituents, oxygen and nitrogen, are not condensable at the poles.
For a major increase in pressure to occur on Mars, the amount of heat absorbed in the polar regions of the planet must be increased by some 15 or 20 percent for a period of at least a century. Three possible sources of variation in the heating of the cap have been identified, and they are, interestingly enough, very similar to the three fashionable models of terrestrial climatic change discussed above. In the first, variations of the tilt of the Martian rotational axis toward the Sun are invoked. Such variations are much more striking than for the Earth, because Mars is close to Jupiter, the most massive planet in the solar system, and the gravitational perturbations by Jupiter are pronounced. Here variations in global pressure and temperature will occur on hundred thousand to million year time scales.
Secondly, a variation in the albedo of the polar regions can cause major climatic variations. We can already see substantial sand and dust storms on Mars, because of which the polar caps seasonally darken and brighten. There has been one suggestion that the climate of Mars may be made more hospitable if a hardy species of polar plant can be developed that will lower the albedo of the Martian polar regions.
Finally, there is the possibility of variations in the luminosity of the Sun. Some of the channels on Mars have an occasional impact crater in them, and crude dating of the channels from the frequency of impacts from interplanetary space shows that some of them must be about a billion years old. This is reminiscent of the last epoch of high global temperatures on the planet Earth and raises the captivating possibility of synchronous major variations in climate between the Earth and Mars.
The subsequent Viking missions to Mars have increased our knowledge about the channels in a major way, have provided quite independent evidence for a dense earlier atmosphere and have demonstrated a great repository of frozen carbon dioxide in the polar ice. When the Viking results are fully assimilated, they promise to add greatly to our knowledge of the present environment as well as the past history of the planet, and of the comparison between the climates of the Earth and Mars.
When scientists are faced with extremely difficult theoretical problems, there is always the possibility of performing experiments. In studies of the climate of an entire planet, however, experiments are expensive and difficult to perform, and have potentially awkward social consequences. By the greatest good fortune, nature has come to our aid by providing us with nearby planets with significantly different climates and significantly different physical variables. Perhaps the sharpest test of theories of climatology is that they be able to explain the climates of all the nearby planets, Earth, Mars and Venus. Insights gained from the study of one planet will inevitably aid the study of the others. Comparative planetary climatology appears to be a discipline, just in the process of birth, with major intellectual interest and practical applications.
CHAPTER 15
KALLIOPE AND THE KAABA
We imagine them
flitting
cheek to jowl,
these driftrocks
of cosmic ash
thousandfold afloat
between Jupiter and Mars.
Frigga,
Fanny,
Adelheid
Lacrimosa.
Names to conjure with,
Dakotan black hills,
a light-opera
staged on a barrier reef.
And swarm they may have,
crumbly as blue-cheese,
that ur-moment
when the solar system
broke wind.
But now
they lumber
so wide apart
from each
to its neighbor’s
pinprick-glow
slant millions
and millions
of watertight miles.
Only in the longest view
do they graze
like one herd
on a breathless tundra.
DIANE ACKERMAN,
The Planets (New York, Morrow, 1976)
ONE OF THE seven wonders of the ancient world was the Temple of Diana at Ephesus, in Asia Minor, an exquisite example of Greek monumental architecture. The Holy of Holies in this temple was a great black rock, probably metallic, that had fallen from the skies, a sign from the gods, perhaps an arrowhead shot from the crescent moon, the symbol of Diana the Huntress.
Not many centuries later—perhaps even at the same time—another great black rock, according to the belief of many, fell out of the sky onto the Arabian Peninsula. There, in pre-Islamic times, it was emplaced in a Meccan temple, the Kaaba, and offered something akin to worship. Then, in the seventh and eight centuries A.D., came the stunning success of Islam, founded by Muhammed, who lived out most of his days not far from this large dark stone, the presence of which might conceivably have influenced his choice of career. The earlier worship of the stone was incorporated into Islam, and today a principal focus of every pilgrimage to Mecca is that same stone—often called the Kaaba after the temple that enshrines it. (All religions have shamelessly coopted their predecessors—e.g., consider the Christian festival of Easter, where the ancient fertility rites of the spring equinox are today cunningly disguised as eggs and baby animals. Indeed the very name Easter is, according to some etymologies, a corruption of the name of the great Near Eastern Earth mother goddess, Astarte. The Diana of Ephesus is a later and Hellenized version of Astarte and Cybelle.)
In primitive times, a great boulder falling out of a clear blue sky must have provided onlookers with a memorable experience. But it had a greater importance: at the dawn of metallurgy, iron from the skies was, in many parts of the world, the purest available form of this metal. The military significance of iron swords and the agricultural significance of iron plowshares made meta
l from the sky a concern of practical men.
Rocks still fall from the skies; farmers still occasionally break their plows on them; museums still pay a bounty for them; and, very rarely, one falls through the eaves of a house, narrowly missing a family in its evening hypnogogic ritual before the television set. We call these objects meteorites. But naming them is not the same as understanding them. Where, in fact, do meteorites come from?
Between the orbits of Mars and Jupiter are thousands of irregularly shaped, tumbling little worlds called asteroids or planetoids. “Asteroid” is not a good term for them because they are not like stars. “Planetoid” is much better because they are like planets, only smaller, but “asteroid” is the more widely used term by far. Ceres, the first asteroid to be found, was discovered* telescopically on January 1, 1801—an auspicious finding on the first day of the nineteenth century—by G. Piazzi, an Italian monk. Ceres is about 1,000 kilometers in diameter and is by far the largest asteroid. (By comparison, the diameter of the Moon is 3,464 kilometers.) Since then, more than two thousand asteroids have been discovered. Asteroids are given a number indicating their order of discovery. But following Piazzi’s lead, a great effort was also made to give them names—female names, preferably from Greek mythology. However, two thousand asteroids is a great many, and the nomenclature becomes a little ragged toward the end. We find 1 Ceres, 2 Pallas, 3 Juno, 4 Vesta, 16 Psyche, 22 Kalliope, 34 Circe, 55 Pandora, 80 Sappho, 232 Russia, 324 Bamberga, 433 Eros, 710 Gertrud, 739 Mandeville, 747 Winchester, 904 Rockefelleria, 916 America, 1121 Natasha, 1224 Fantasia, 1279 Uganda, 1556 Icarus, 1620 Geographos, 1685 Toro, and 694 Ekard (Drake [University] spelled backwards). 1984 Orwell is, unfortunately, a lost opportunity.