Broca's Brain: The Romance of Science

CHAPTER 10

IMAGINE THE EARTH scrutinized by some very careful and extremely patient extraterrestrial observer: 4.6 billion years ago the planet is observed to complete its condensation out of interstellar gas and dust, the final planetesimals falling in to make the Earth produce enormous impact craters; the planet heats internally from the gravitational potential energy of accretion and from radioactive decay, differentiating the liquid iron core from the silicate mantle and crust; hydrogen-rich gases and condensible water are released from the interior of the planet to the surface; a rather humdrum cosmic organic chemistry yields complex molecules, which lead to extremely simple self-replicating molecular systems-the first terrestrial organisms; as the supply of impacting interplanetary boulders dwindles, running water, mountain building and other geological processes wipe out the scars attendant to the Earth’s origin; a vast planetary convection engine is established which carries mantle material up at the ocean floors and subducts it down at the continental margins, the collision of the moving plates producing the great folded mountain chains and the general configuration of land and ocean, glaciated and tropical terrain varies continuously. Meanwhile, natural selection extracts out from a wide range of alternatives those varieties of self-replicating molecular systems best suited to the changing environments; plants evolve that use visible light to break down water into hydrogen and oxygen, and the hydrogen escapes to space, changing the chemical composition of the atmosphere from reducing to oxidizing; organisms of fair complexity and middling intelligence eventually arise.

Yet in all the 4.6 billion years our hypothetical observer is struck by the isolation of the Earth. It receives sunlight and cosmic rays-both important for biology-and occasional impact of interplanetary debris. But nothing in all those eons of time leaves the planet. And then the planet suddenly begins to fire tiny dispersules throughout the inner solar system, first in orbit around the Earth, then to the planet’s blasted and lifeless natural satellite, the Moon. Six capsules-small, but larger than the rest-set down on the Moon, and from each, two tiny bipeds can be discerned, briefly exploring their surroundings and then hotfooting it back to the Earth, having extended tentatively a toe into the cosmic ocean. Eleven little spacecraft enter the atmosphere of Venus, a searing hellhole of a world, and six of them survive some tens of minutes on the surface before being fried. Eight spacecraft are sent to Mars. Three successfully orbit the planet for years; another flies past Venus to encounter Mercury, on a trajectory obviously chosen intentionally to pass by the innermost planet many times. Four others successfully traverse the asteroid belt, fly close to Jupiter and are there ejected by the gravity of the largest planet into interstellar space. It is clear that something interesting is happening lately on the planet Earth.

If the 4.6 billion years of the Earth history were compressed into a single year, this flurry of space exploration would have occupied the last tenth of a second, and the fundamental changes in attitude and knowledge responsible for this remarkable transformation would fill only the last few seconds. The seventeenth century saw the first widespread application of simple lenses and mirrors for astronomical purposes. With the first astronomical telescope Galileo was astounded and delighted to see Venus as a crescent, and the mountains and the craters of the Moon. Johannes Kepler thought that the craters were constructions of intelligent beings inhabiting that world. But the seventeenth-century Dutch physicist Christianus Huygens disagreed. He suggested that the effort involved in constructing the lunar craters would be unreasonably great, and also thought that he could see alternative explanations for these circular depressions.

Huygens exemplified the synthesis of advancing technology, experimental skills, a reasonable, hard-nosed and skeptical mind, and an openness to new ideas. He was the first to suggest that we are looking at atmosphere and clouds on Venus; the first to understand something of the true nature of the rings of Saturn (which had seemed to Galileo as two “ears” enveloping the planet); the first to draw a picture of a recognizable marking on the Martian surface (Syrtis Major); and the second, after Robert Hooke, to draw the Great Red Spot of Jupiter. These last two observations are still of scientific importance because they establish the permanence at least for three centuries of these features. Huygens was of course not a thoroughly modern astronomer. He could not entirely escape the fashions of belief of his time. For example, he presented a curious argument from which we could deduce the presence of hemp on Jupiter: Galileo had observed that Jupiter has four moons. Huygens asked a question few modern planetary astronomers would ask: Why does Jupiter have four moons? An insight into this question, he thought, could be garnered by asking the same question of the Earth’s single moon, whose function, apart from giving a little light at night and raising the tides, was to provide a navigational aid to mariners. If Jupiter has four moons, there must be many mariners on that planet. But mariners imply boats; boats imply sails; sails imply ropes; and, I suppose, ropes imply hemp. I wonder how many of our present highly prized scientific arguments will seem equally suspect from the vantage point of three centuries.

A useful index of our knowledge about a planet is the number of bits of information necessary to characterize our understanding of its surface. We can think of this as the number of black and white dots in the equivalent of a newspaper wirephoto which, held at arm’s length, would summarize all existing imagery. Back in Huygens’ day, about ten bits of information, all obtained by brief glimpses through telescopes, would have covered our knowledge of the surface of Mars. By the time of the close approach of Mars to Earth in the year 1877, this number had risen to perhaps a few thousand, if we exclude a large amount of erroneous information-for example, drawings of the “canals,” which we now know to be entirely illusory. With further visual observations and the development of ground-based astronomical photography, the amount of information grew slowly until a dramatic upturn in the curve occurred, corresponding to the advent of space-vehicle exploration of the planet.

The twenty photographs obtained in 1965 by the Mariner 4 fly-by comprised five million bits of information, roughly comparable to all previous photographic knowledge about the planet. The coverage was still only a tiny fraction of the planet. The dual fly-by mission, Mariner 6 and 7 in 1969, increased this number by a factor of 100, and the Mariner 9 orbiter in 1971 and 1972 increased it by another factor of 100. The Mariner 9 photographic results from Mars correspond roughly to 10,000 times the total previous photographic knowledge of Mars obtained over the history of mankind. Comparable improvements apply to the infrared and ultraviolet spectroscopic data obtained by Mariner 9, compared with the best previous ground-based data.

Going hand in hand with the improvement in the quantity of our information is the spectacular improvement in its quality. Prior to Mariner 4, the smallest feature reliably detected on the surface of Mars was several hundred kilometers across. After Mariner 9, several percent of the planet had been viewed at an effective resolution of 100 meters, an improvement in resolution of a factor of 1,000 in the last ten years, and a factor of 10,000 since Huygens’ time. Still further improvements were provided by Viking. It is only because of this improvement in resolution that we today know of vast volcanoes, polar laminae, sinuous tributaried channels, great rift valleys, dune fields, crater-associated dust streaks, and many other features, instructive and mysterious, of the Martian environment.

Both resolution and coverage are required to understand a newly explored planet. For example, even with their superior resolution, by an unlucky coincidence the Mariner 4, 6 and 7 spacecraft observed the old, cratered and relatively uninteresting part of Mars and gave no hint of the young and geologically active third of the planet revealed by Mariner 9.

LIFE ON EARTH is wholly undetectable by orbital photography until about 100-meter resolution is achieved, at which point the urban and agricultural geometrizing of our technological civilization becomes strikingly evident. Had there been a civilization on Mars of comparable extent and level of development, it would not have been detected photographically until the Mariner 9 and Viking missions. There is no reason to expect such civilizations on the nearby planets, but the comparison strikingly illustrates that we are just beginning an adequate reconnaissance of neighboring worlds.

THERE IS NO question that astonishments and delights await us as both resolution and coverage are dramatically improved in photography, and comparable improvements are secured in spectroscopic and other methods.

The largest professional organization of planetary scientists in the world is the Division for Planetary Sciences of the American Astronomical Society. The vigor of this burgeoning science is apparent in the meetings of the society. In the 1975 annual meeting, for example, there were announcements of the discovery of water vapor in the atmosphere of Jupiter, ethane on Saturn, possible hydrocarbons on the asteroid Vesta, an atmospheric pressure approaching that of the Earth on the Saturnian moon Titan, decameter-wavelength radio bursts from Saturn, the radar detection of the Jovian moon Ganymede, the elaboration of the radio emission spectrum of the Jovian moon Callisto, to say nothing of the spectacular views of Mercury and Jupiter (and their magnetospheres) presented by the Mariner 10 and Pioneer 11 experiments. Comparable advances were reported in subsequent meetings.

In all the flurry and excitement of recent discoveries, no general view of the origin and evolution of the planets has yet emerged, but the subject is now very rich in provocative hints and clever surmises. It is becoming clear that the study of any planet illuminates our knowledge of the rest, and if we are to understand Earth thoroughly, we must have a comprehensive knowledge of the other planets. For example, one now fashionable suggestion, which I first proposed in 1960, is that the high temperatures on the surface of Venus are due to a runaway greenhouse effect in which water and carbon dioxide in a planetary atmosphere impede the emission of thermal infrared radiation from the surface to space; the surface temperature then rises to achieve equilibrium between the visible sunlight arriving at the surface and the infrared radiation leaving it; this higher surface temperature results in a higher vapor pressure of the greenhouse gases, carbon dioxide and water; and so on, until all the carbon dioxide and water vapor is in the vapor phase, producing a planet with high atmospheric pressure and high surface temperature.

Now, the reason that Venus has such an atmosphere and Earth does not seems to be a relatively small increment of sunlight. Were the Sun to grow brighter or Earth’s surface and clouds to grow darker, could Earth become a replica of the classical vision of Hell? Venus may be a cautionary tale for our technical civilization, which has the capability to alter profoundly the environment of Earth.

Despite the expectation of almost all planetary scientists, Mars turns out to be covered with thousands of sinuous tributaried channels probably several billion years old. Whether formed by running water or running CO₂, many such channels probably could not be carved under present atmospheric conditions; they require much higher pressures and probably higher polar temperatures. Thus the channels-as well as the polar laminated terrain on Mars-may bear witness to at least one, and perhaps many, previous epochs of much more clement conditions, implying major climatic variations during the history of the planet. We do not know if such variations are internally or externally caused. If internally, it will be of interest to see whether the Earth might, through the activities of man, experience a Martian degree of climatic excursions-something much greater than the Earth seems to have experienced at least recently. If the Martian climatic variations are externally produced-for example, by variations in solar luminosity-then a correlation of Martian and terrestrial paleoclimatology would appear extremely promising.

Mariner 9 arrived at Mars in the midst of a great global dust storm, and the Mariner 9 data permit an observational test of whether such storms heat or cool a planetary surface. Any theory with pretensions to predicting the climatic consequences of increased aerosols in the Earth’s atmosphere had better be able to provide the correct answer for the global dust storm observed by Mariner 9. Drawing upon our Mariner 9 experience, James Pollack of NASA Ames Research Center, Brian Toon of Cornell and I have calculated the effects of single and multiple volcanic explosions on the Earth’s climate and have been able to reproduce, within experimental error, the observed climatic effects after major explosions on our planet. The perspective of planetary astronomy, which permits us to view a planet as a whole, seems to be very good training for studies of the Earth. As another example of this feedback from planetary studies on terrestrial observations, one of the major groups studying the effect on the Earth’s ozonosphere of the use of halocarbon propellants from aerosol cans is headed by M. B. McElroy at Harvard University-a group that cut its teeth for this problem on the aeronomy of the atmosphere of Venus.

We now know from space-vehicle observations something of the surface density of impact craters of different sizes for Mercury, the Moon, Mars and its satellites; radar studies are beginning to provide such information for Venus, and although it is heavily eroded by running water and tectonic activity, we have some information about craters on the surface of the Earth. If the population of objects producing such impacts were the same for all these planets, it might then be possible to establish both an absolute and a relative chronology of cratered surfaces. But we do not yet know whether the populations of impacting objects are common-all derived from the asteroid belt, for example-or local; for example, the sweeping up of rings of debris involved in the final stages of planetary accretion.

The heavily cratered lunar highlands speak to us of an early epoch in the history of the solar system when cratering was much more common than it is today; the present population of interplanetary debris fails by a large factor to account for the abundance of the highland craters. On the other hand, the lunar maria have a much lower crater abundance, which can be explained by the present population of interplanetary debris, largely asteroids and possibly dead comets. It is possible to determine, for planetary surfaces that are not so heavily cratered, something of the absolute age, a great deal about the relative age, and in some cases, even something about the distribution of sizes in the population of objects that produced the craters. On Mars, for example, we find the flanks of the large volcanic mountains are almost free of impact craters, implying their comparative youth; they were not around long enough to accumulate very much in the way of impact scars. This is the basis for the contention that volcanoes on Mars are a comparatively recent phenomenon.

The ultimate objective of comparative planetology is, I suppose, something like a vast computer program into which we put a few input parameters-perhaps the initial mass, composition, angular momentum and population of neighboring impacting objects-and out comes the time evolution of the planet. We are very far from having such a deep understanding of planetary evolution at the present time, but we are much closer than would have been thought possible only a few decades ago.

Every new set of discoveries raises a host of questions which we were never before wise enough even to ask. I will mention just a few of them. It is now becoming possible to compare the compositions of asteroids with the compositions of meteorites on Earth (see Chapter 15). Asteroids seem to divide neatly into silicate-rich and organic-matter-rich objects. One immediate consequence appears to be that the asteroid Ceres is apparently undifferentiated, while the less massive asteroid Vesta is differentiated. But our present understanding is that planetary differentiation occurs above a certain critical mass. Could Vesta be the remnant of a much larger parent body now gone from the solar system? The initial radar glimpse of the craters of Venus shows them to be extremely shallow. Yet there is no liquid water to erode the Venus surface, and the lower atmosphere of Venus seems to be so slow-moving that dust may not be able to fill the craters. Could the source of the filling of the craters of Venus be a slow molasseslike collapse of a very slightly molten surface?

The most popular theory on the generation of planetary magnetic fields invokes rotation-driven convection currents in a conducting planetary core. Mercury, which rotates once every fifty-nine days, was expected in this scheme to have no detectable magnetic field. Yet such a field is manifestly there, and a serious reappraisal of theories of planetary magnetism is in order. Only Saturn and Uranus have rings. Why? There is on Mars an exquisite array of longitudinal sand dunes nestling against the interior ramparts of a large eroded crater. There is in the Great Sand Dunes National Monument near Alamosa, Colorado, a very similar set of sand dunes nestling in the curve of the Sangre de Cristo mountains. The Martian and the terrestrial sand dunes have the same total extent, the same dune-to-dune spacing and the same dune heights. Yet the Martian atmospheric pressure is 1/200 that on Earth, the winds necessary to initiate the saltation of sand grains are ten times that for Earth, and the particle-size distribution may be different on the two planets. How, then, can the dune fields produced by windblown sand be so similar? What are the sources of the decameter radio emission on Jupiter, each less than 100 kilometers across, fixed on the Jovian surface, which intermittently radiate to space?

Mariner 9 observations imply that the winds on Mars at least occasionally exceed half the local speed of sound. Are the winds ever much larger? What is the nature of a transonic meteorology? There are pyramids on Mars about 3 kilometers across at the base and 1 kilometer high. They are unlikely to have been constructed by Martian pharaohs. The rate of sandblasting by wind-transported grains on Mars is at least 10,000 times that on Earth because of the greater speeds necessary to move particles in the thinner Martian atmosphere. Could the facets of the Martian pyramids have been eroded by millions of years of such sandblasting from more than one prevailing wind direction?

The moons in the outer solar system are almost certainly not replicas of our own, rather dull satellite. Many of them have such low densities that they must be composed largely of methane, ammonia or water ices. What will their surfaces look like close up? How will impact craters erode on an icy surface? Might there be volcanoes of solid ammonia with a lava of liquid NH₃ trickling down the sides? Why is Io, the innermost large satellite of Jupiter, enveloped in a cloud of gaseous sodium? How does Io help to modulate the synchrotron emission from the Jovian radiation belt in which it lives? Why is one side of Iapetus, a moon of Saturn, six times brighter than the other? Because of a particlesize difference? A chemical difference? How did such differences become established? Why on Iapetus and nowhere else in the solar system in so symmetrical a way?

The gravity of the solar system’s largest moon, Titan, is so low and the temperature of its upper atmosphere sufficiently high that hydrogen should escape into space extremely rapidly in a process known as blow-off. But the spectroscopic evidence suggests that there is a substantial quantity of hydrogen on Titan. The atmosphere of Titan is a mystery. And if we go beyond the Saturnian system, we approach a region in the solar system about which we know almost nothing. Our feeble telescopes have not even reliably determined the periods of rotation of Uranus, Neptune and Pluto, much less the character of their clouds and atmospheres, and the nature of their satellite systems. The poet Diane Ackerman of Cornell University writes: “Neptune/is/elusive as a dappled horse in fog. Pulpy?/Belted? Vapory? Frost-bitten? What we know/wouldn’t/fill/a lemur’s fist.”

One of the most tantalizing issues that we are just beginning to approach seriously is the question of organic chemistry and biology elsewhere in the solar system. The Martian environment is by no means so hostile as to exclude life, nor do we know enough about the origin and evolution of life to guarantee its presence there or anywhere else. The question of organisms both large and small on Mars is entirely open, even after the Viking missions.

The hydrogen-rich atmospheres of places such as Jupiter, Saturn, Uranus and Titan are in significant respects similar to the atmosphere of the early Earth at the time of the origin of life. From laboratory simulation experiments we know that organic molecules are produced in high yield under such conditions. In the atmospheres of Jupiter and Saturn the molecules will be convected to pyrolytic depths. But even there the steady-state concentration of organic molecules can be significant. In all simulation experiments the application of energy to such atmospheres produces a brownish polymeric material, which in many significant respects resembles the brownish coloring material in their clouds. Titan may be completely covered with a brownish, organic material. It is possible that the next few years will witness major and unexpected discoveries in the infant science of exobiology.

The principal means for the continued exploration of the solar system over the next decade or two will surely be unmanned planetary missions. Scientific space vehicles have now been launched successfully to all the planets known to the ancients. There is a range of unapproved proposed missions that have been studied in some detail. (See Chapter 16.) If most of these missions are actually implemented, it is clear that the present age of planetary exploration will continue brilliantly. But it is by no means clear that these splendid voyages of discovery will be continued, at least by the United States. Only one major planetary mission, the Galileo project to Jupiter, has been approved in the last seven years-and even it is in jeopardy.

Even a preliminary reconnaissance of the entire solar system out to Pluto and a more detailed exploration of a few planets by, for example, Mars rovers and Jupiter entry probes will not solve the fundamental problem of solar system origins; what we need is the discovery of other solar systems. Advances in ground-based and spaceborne techniques in the next two decades might be capable of detecting dozens of planetary systems orbiting nearby single stars. Recent observational studies of multiple-star systems by Helmut Abt and Saul Levy, both of Kitt Peak National Observatory, suggest that as many as one-third of the stars in the sky may have planetary companions. We do not know whether such other planetary systems will be like ours or built on very different principles.

We have entered, almost without noticing, an age of exploration and discovery unparalleled since the Renaissance. It seems to me that the practical benefits of comparative planetology for Earthbound sciences; the sense of adventure imparted by the exploration of other worlds to a society that has almost lost the opportunity for adventure; the philosophical implications of the search for a cosmic perspective-these are what will in the long run mark our time. Centuries hence, when our very real political and social problems may be as remote as the very real problems of the War of the Austrian Succession seem to us, our time may be remembered chiefly for one fact: this was the age when the inhabitants of the Earth first made contact with the cosmos around them.

CHAPTER 11

A PLANET NAMED GEORGE

CHAPTER 12

MORE THAN three hundred years ago, Anton van Leeuwenhoek of Delft explored a new world. With the first microscope he viewed a stagnant infusion of hay and was astounded to find it swarming with small creatures:

On April 24th, 1676, observing this water by chance, I saw therein with great wonder unbelievably very many small animalcules of various sorts; among others, some that were three to four times as long as broad. Their entire thickness was, in my judgement, not much thicker than one of the little hairs that cover the body of a louse. These creatures had very short, thin legs in front of the head (although I can recognize no head, I speak of the head for the reason that this part always went forward during movement)… Close to the hindmost part lay a clear globule; and I judged that the very hindmost part was slightly cleft. These animalcules are very cute while moving about, oftentimes tumbling all over.

These tiny “animalcules” had never before been seen by any human being. Yet Leeuwenhoek had no difficulty in recognizing them as alive.

Two centuries later Louis Pasteur developed the germ theory of disease from Leeuwenhoek’s discovery and laid the foundation for much of modern medicine. Leeuwenhoek’s objectives were not practical at all, but exploratory and adventuresome. He himself never guessed the future practical applications of his work.

In May of 1974 the Royal Society of Great Britain held a discussion meeting on “The Recognition of Alien Life.” Life on Earth has developed by a slow, tortuous step-by-step progression known as evolution by natural selection. Random factors play a critical role in this process-as, for example, which gene at what time will be mutated or changed by an ultraviolet photon or a cosmic ray from space. All the organisms on Earth are exquisitely adapted to the vagaries of their natural environments. On some other planet, with different random factors operating and extremely exotic environments, life may have evolved very differently. If we landed a spacecraft on the planet Mars, for example, would we even be able to recognize the local life forms as alive?

One theme which was stressed at the Royal Society discussion was that life elsewhere should be recognizable by its improbability. Take trees, for example. Trees are long skinny structures, above ground fatter at the top than at the bottom. It is easy to see that after millennia of rubbing by wind and water, most trees should have fallen down. They are in mechanical disequilibrium. They are unlikely structures. Not all top-heavy structures are produced by biology. There are, for example, pedestal rocks in deserts. But were we to see a great many top-heavy structures, all closely similar, we could make a reasonable guess that they were of biological origin. Likewise for Leeuwenhoek’s animalcules. There are many of them, closely similar, highly complex and improbable in the extreme. Without ever having seen them before, we correctly guess they are biological.

There have been elaborate debates on the nature and definition of life. The most successful definitions invoke the evolutionary process. But we do not land on another planet and wait to see if any nearby objects evolve. We do not have the time. The search for life then takes on a much more practical aspect. This point was brought out with some finesse at the Royal Society discussion when, after an exchange remarkable for its rambling metaphysical vagueness, Sir Peter Medawar rose to his feet and said, “Gentlemen, everyone in this room knows the difference between a live horse and a dead horse. Pray, therefore, let us cease flogging the latter.” Medawar and Leeuwenhoek would have seen eye to eye.

But are there trees or animalcules on the other worlds of our solar system? The simple answer is that no one yet knows. From the vantage point of the nearest planets, it would be impossible to detect photographically the presence of life on our own planet. Even from the closest orbital observations of Mars made to date, from the American spacecraft Mariner 9 and Viking 1 and 2, details on Mars much smaller than 100 meters across have remained invisible. Since even the most ardent enthusiasts of extraterrestrial life do not anticipate Martian elephants 100 meters long, many important tests have not yet been performed.

At the present time we can only assess the physical environments of the other planets, determine whether they are so severe as to exclude life-even forms rather different from those we know on Earth-and in the case of the more clement environments perhaps speculate on the life forms that might be present. The one exception is the Viking lander results, briefly discussed below.

A place may be too hot or too cold for life. If the temperatures are very high-say, several thousands of degrees Centigrade-then the molecules that make up the organism will fall to pieces. Thus it is customary to exclude the Sun as an abode of life. On the other hand, if the temperatures are too low, then the chemical reactions that drive the internal metabolism of the organism will proceed at too ponderous a pace. For this reason the frigid wastes of Pluto are customarily excluded as an abode of life. However, there may be chemical reactions which proceed at respectable rates at low temperatures but which are unexplored here on Earth, where chemists dislike working in laboratories at −230°C. We must be careful not to take too chauvinistic a view of the matter.

The giant outer planets of the solar system, Jupiter, Saturn, Uranus, and Neptune, are sometimes excluded from biological considerations because their temperatures are very low. But these temperatures are the temperatures of their upper clouds. Deeper down in the atmospheres of such planets, as in the atmosphere of the Earth, much more clement conditions are to be encountered. And they appear to be rich in organic molecules. By no means can they be excluded.

While we human beings enjoy oxygen, this is hardly a recommendation for it, since there are many organisms that are poisoned by it. If the thin protective ozone layer in our atmosphere, made by sunlight from oxygen, did not exist, we would rapidly be fried by ultraviolet light from the Sun. But on other worlds, ultraviolet sunshades or biological molecules impervious to near-ultraviolet radiation can readily be imagined. Such considerations merely underline our ignorance.

An important distinction among the other worlds of our solar system is the thickness of their atmospheres. In the total absence of an atmosphere it is very difficult to conceive of life. As on Earth, the biology on other planets must, we think, be driven by sunlight. On our planet, the plants eat the sunlight and the animals eat the plants. Were all the organisms on Earth forced (by some unimaginable catastrophe) into a subterranean existence, life would cease as soon as accumulated food stores were exhausted. The plants, the fundamental organisms on any planet, must see the Sun. But if a planet has no atmosphere, not only ultraviolet radiation but X-rays and gamma rays and charged particles from the solar wind will fall unimpeded on the planetary surface and frizzle the plants.

Furthermore, an atmosphere is necessary for exchange of materials so that the basic molecules for biology are not all used up. On Earth, for example, green plants give off oxygen-a waste product-into the atmosphere. Many respiring animals, like human beings, breathe the oxygen and give off carbon dioxide, which the plants in turn imbibe. Without this clever (and painfully evolved) equilibrium between plants and animals, we would rapidly run out of oxygen or carbon dioxide. For these two reasons-radiation protection and molecular exchange-an atmosphere seems required for life.

Some of the worlds in our solar system have exceedingly thin atmospheres. Our Moon, for example, has at its surface less than one million millionth the atmospheric pressure on Earth. Six places on the near side of the Moon were examined by Apollo astronauts. No top-heavy structures, no lumbering beasts were found. Nearly four hundred kilograms of samples have been returned from the Moon and meticulously examined in terrestrial laboratories. There were no animalcules, no microbes, almost no organic chemicals, or even any water. We expected the Moon to be lifeless, and apparently it is. Mercury, the closest planet to the Sun, resembles the Moon. Its atmosphere is exceedingly thin, and it ought not to support life. In the outer solar system there are many large satellites the size of Mercury or our own Moon, composed of some mix of rock (like the Moon and Mercury) and ices. Io, the second moon of Jupiter, falls into this category. Its surface seems to be covered with a kind of reddish salt deposit. We are very ignorant about it. But because of its very low atmospheric pressure, we do not expect life on it.

Then there are planets with moderate atmospheres. Earth is the most familiar example. Here life has played a major role in determining the composition of our atmosphere. The oxygen is, of course, produced by green-plant photosynthesis, but even the nitrogen is thought to be made by bacteria. Oxygen and nitrogen together comprise 99 percent of our atmosphere, which has evidently been reworked on a massive scale by the life on our planet.

The total pressure on Mars is about one half of one percent that on Earth, but the atmosphere there is composed largely of carbon dioxide. There are small quantities of oxygen, water vapor, nitrogen and other gases. The Martian atmosphere has not obviously been reworked by biology, but we do not know Mars well enough to exclude life there. It has congenial temperatures at some times and places, a dense enough atmosphere, and abundant water locked away in the ground and polar caps. Even some varieties of terrestrial microorganisms can survive there very well. Mariner 9 and Viking found hundreds of dry riverbeds, apparently Indicating a time in the recent geological history of the planet when abundant liquid water flowed. It is a world awaiting exploration.

A third and less familiar example of places with moderate atmospheres is Titan, the largest moon of Saturn. Titan appears to have an atmosphere with a density between that of Mars and Earth. This atmosphere is, however, composed largely of hydrogen and methane, and is surmounted by an unbroken layer of reddish clouds-probably complex organic molecules. Because of its remoteness, Titan has attracted the interest of exobiologists only recently, but it holds the promise of a long-term fascination.

The planets with very dense atmospheres present a special problem. Like Earth, their atmospheres are cold at the top and warmer at the bottom. But when the atmosphere is very thick, the temperatures at the bottom become too hot for biology. In the case of Venus, the surface temperatures are about 480°C; for the Jovian planets, many thousands of degrees Centigrade. All these atmospheres, we think, are convective, with vertical winds vigorously carrying materials both up and down. Life probably cannot be imagined on their surfaces because of the high temperatures. The cloud environments are perfectly clement, but convection will carry hypothetical cloud organisms down to the depth and fry them there. There are two obvious solutions. There might be small organisms that reproduce as fast as they are carried down to the planetary skillet or the organisms might be buoyant. Fish on Earth have float bladders for a similar purpose, and both on Venus and on the Jovian planets, organisms that are essentially hydrogen-filled balloons can be envisioned. For them to float at modest temperatures on Venus, they need to be at least a few centimeters across, but for the same purpose on Jupiter, they must be at least meters across-the size of ping-pong balls and meteorological balloons, respectively. We do not know that such beasts exist, but it is of some little interest to see that they can be envisioned without doing violence to what is known of physics, chemistry or biology.

Our profound ignorance of whether other planets harbor life may end within this century. Plans are now afoot for the chemical and biological examination of many of these candidate worlds. The first step was the American Viking missions, which landed two sophisticated automatic laboratories on Mars in the summer of 1976, almost three hundred years to the month of Leeuwenhoek’s discovery of hay infusoria. Viking found no curious structures nearby (or sauntering by) which were top-heavy, and no detectable organic molecules. Of three experiments in microbial metabolism, two in both landing sites repeatedly gave what seemed to be positive results. The implications are still under vigorous debate. In addition, we must remember that the two Viking landers examined closely, even with photography, less than one millionth of the surface area of the planet. More observations-particularly with more sophisticated instrumentation (including microscopes) and with roving vehicles-are needed. But despite the ambiguous nature of the Viking results, these missions represent the first time in the history of the human species that another world has been seriously examined for life.

In the following decades it is likely that there will be buoyant probes into the atmospheres of Venus, Jupiter and Saturn, and landers on Titan, as well as more detailed studies of the surface of Mars. A new age of planetary exploration and exobiology dawned in the seventh decade of the twentieth century. We live in a time of adventure and high intellectual excitement; but also-as the step from Leeuwenhoek to Pasteur shows-in the midst of an endeavor which promises great practical benefits.

CHAPTER 13

TITAN IS NOT a household word, or world. We do not usually think of it when we run through a list of familiar objects in the solar system. But in the last few years this satellite of Saturn has emerged as a place of extraordinary interest and prime significance for future exploration. Our most recent studies of Titan have revealed that it has an atmosphere more like the Earth’s-at least in terms of density-than any other object in the solar system. This fact alone gives it new significance as the exploration of other worlds begins in earnest.

Besides being the largest satellite of Saturn, Titan is also, according to recent work by Joseph Veverka, James Elliot and others at Cornell University, the largest satellite in the solar system-about 5,800 kilometers (3,600 miles) in diameter. Titan is larger than Mercury and nearly as large as Mars. And yet there it is in orbit around Saturn.

We might obtain some clues about the nature of Titan by examining the two major worlds in the outer solar system-Jupiter and Saturn. Both have a general reddish or brownish coloration. That is, the upper layer of clouds that we see from the Earth has this hue primarily. Something in the atmosphere and clouds of these planets is strongly absorbing blue and ultraviolet light, so that the light that is reflected back to us is primarily red. The outer solar system, in fact, has a number of objects that are remarkably red. Although we have no color photographs of Titan because it is 800 million miles away and has an angular size smaller than the Galilean satellites of Jupiter, photoelectric studies reveal that it is, in fact, very red. Astronomers who thought about the problem once believed that Titan was red for the same reason that Mars is red: a rusty surface. But then the reason for Titan’s red color would be different from the reason for Jupiter’s and Saturn’s, because we do not see to a solid surface on those planets.

In 1944 Gerard Kuiper detected spectroscopically an atmosphere of methane around Titan-the first satellite found to have an atmosphere. Since then, the methane observations have been confirmed, and at least moderately suggestive evidence for the presence of molecular hydrogen has been provided by Lawrence Trafton of the University of Texas.

Since we know the amount of gas necessary to produce the observed spectral absorption features, and we know from its mass and radius the surface gravity of Titan, we can deduce the minimum atmospheric pressure. We find it is something like 10 millibars, about one percent of the Earth’s atmospheric pressure-a pressure that exceeds that of Mars. Titan has the most Earth-like atmospheric pressure in the solar system.

Not only the best, but the only visual telescopic observations of Titan have been made by Audouin Dollfus at the Meudon Observatory in France. These are hand drawings done at the telescope during moments of atmospheric steadiness. From the variable patches that he observed, Dollfus concluded that things are happening on Titan that do not correlate with the satellite’s rotation period. (Titan is thought always to face Saturn, as our Moon does the Earth.) Dollfus guessed that there might be clouds, at least of a patchy sort, on Titan.

Our knowledge of Titan has made a number of substantial quantum jumps forward in recent years. Astronomers have successfully obtained the polarization curve of small objects. The idea is that initially unpolarized sunlight falls on Titan, say, and is polarized on reflection. The polarization is detected by a device similar in principle to, but more sophisticated and sensitive than, “polaroid” sunglasses. The amount of polarization is measured as Titan goes through a small range of phases-between “full” Titan and slightly “gibbous” Titan. The resulting polarization curve, when compared to laboratory polarization curves, gives information on the size and composition of the material responsible for the polarization.

The first polarization observations of Titan, made by Joseph Veverka, indicated that the sunlight reflected back from Titan is most likely reflected off clouds and not off a solid surface. Apparently there is on Titan a surface and a lower atmosphere that we do not see; an opaque cloud deck and an overlying atmosphere, both of which we do see; and an occasional patchy cloud above that. Since Titan appears red, and we view it at the cloud deck, there must, according to this argument, be red clouds on Titan.

Additional support for this concept comes from the extremely low amount of ultraviolet light reflected from Titan, as measured by the Orbiting Astronomical Observatory. The only way to keep Titan’s ultraviolet brightness small is to have the ultraviolet absorbing stuff high up in the atmosphere. Otherwise Rayleigh scattering by the atmospheric molecules themselves would make Titan bright in the ultraviolet. (Rayleigh scattering is the preferential scattering of blue rather than red light, which is responsible for blue skies on Earth.)

But material that absorbs in the ultraviolet and violet appears red in reflected light. So there are two separate lines of evidence (or three, if we believe the hand drawings) for an extensive cloud cover on Titan. What do we mean by extensive? More than 90 percent of Titan must be cloaked in clouds to match the polarization data. Titan seems to be covered by dense red clouds.

A second astonishing development was inaugurated in 1971 when D. A. Allen of Cambridge University and T. L. Murdock of the University of Minnesota found that the observed infrared emission from Titan at a wavelength of 10 to 14 microns is more than twice what is expected from solar heating. Titan is too small to have a significant internal energy source like Jupiter or Saturn. The only explanation seemed to be the greenhouse effect in which the surface temperature rises until the infrared radiation trickling out just balances the absorbed visible radiation coming in. It is the greenhouse effect that keeps the surface temperature of the Earth above freezing and the temperature of Venus at 480°C.

But what could cause a Titanian greenhouse effect? It is unlikely to be carbon dioxide and water vapor as on Earth and Venus, because these gases should be largely frozen out on Titan. I have calculated that a few hundred millibars of hydrogen (1,000 millibars is the total sea-level atmospheric pressure on Earth) would provide an adequate greenhouse effect. Since this is more than the amount of hydrogen observed, the clouds would have to be opaque at certain short wavelengths and more nearly transparent at certain longer wavelengths. James Pollack, at NASA’s Ames Research Center, has calculated that a few hundred millibars of methane might also be adequate and, moreover, might explain some of the details of the infrared emission spectrum of Titan. This large amount of methane would also have to hide under the clouds. Both greenhouse models have the virtue of invoking only gases thought to exist on Titan; of course, both gases might play a role.

An alternative model of the Titan atmosphere was proposed by the late Robert Danielson and his colleagues at Princeton University. They suggest that small quantities of simple hydrocarbons-such as ethane, ethylene and acetylene-which have been observed in the upper atmosphere of Titan absorb ultraviolet light from the Sun and heat the upper atmosphere. It is then the hot upper atmosphere and not the surface that we see in the infrared. On this model there need be no enigmatically warm surface, no greenhouse effect, and no atmospheric pressure of hundreds of millibars.

Which view is correct? At the present time no one knows. The situation is reminiscent of studies of Venus in the early 1960s when the planet’s radio-brightness temperature was known to be high, but whether the emission was from a hot surface or a hot region of the atmosphere was (appropriately) hotly debated. Since radio waves pass through all but the densest atmospheres and clouds, the Titan problem might be resolved if we had a reliable measure of the radio-brightness temperature of the satellite. The first such measurement was performed by Frank Briggs of Cornell with the giant interferometer of the National Radio Astronomy Observatory in Green Bank, West Virginia. Briggs finds a surface temperature of Titan of −140°C with an uncertainty of 45°. The temperature in the absence of a greenhouse effect is expected to be about −185°C. Briggs’s observations therefore seem to suggest a fairly sizable greenhouse effect and a dense atmosphere, but the probable error of the measurements is still so large as to permit the zero greenhouse case.

Subsequent observations by two other radio astronomical groups give values both higher and lower than Briggs’s results. The higher range of temperatures, astonishingly, even approaches temperatures in cold regions of the Earth. The observational situation, like the atmosphere of Titan, seems very murky. The problem could be resolved if we could measure the size of the solid surface of Titan by radar (optical measurements give us the distance from cloudtop to cloudtop). The problem may have to await studies by the Voyager mission, which is scheduled to send two sophisticated spacecraft by Titan-one very close to it-in 1981.

Whichever model we select is consistent with the red clouds. But what are they made of? If we take an atmosphere of methane and hydrogen and supply energy to it, we will make a range of organic compounds, both simple hydrocarbons (like the sort that are needed to make Danielson’s inversion layer in the upper atmosphere) and complex ones. In our laboratory at Cornell, Bishun Khare and I have simulated the kinds of atmospheres that exist in the outer solar system. The complex organic molecules we synthesize in them have optical properties similar to those of the Titanian clouds. We think there is strong evidence for abundant organic compounds on Titan, both simple gases in the atmosphere and more complex organics in the clouds and on the surface.

One problem with an extensive Titanian atmosphere is that the light gas hydrogen should be gushing away because of the low gravity. The only way that I can explain this situation is that the hydrogen is in a “steady state.” That is, it escapes but is replenished from some internal source-volcanoes, most likely. The density of Titan is so low that its interior must be almost entirely composed of ices. We can think of it as a giant comet made of methane, ammonia and water ices. There must also be a small admixture of radioactive elements which, while decaying, will heat their surroundings. The heat conduction problem has been worked out by John Lewis, of MIT, and it is clear that the near-surface interior of Titan will be slushy. Methane, ammonia and water vapor should be outgassed from the interior and broken down by ultraviolet sunlight, producing atmospheric hydrogen and cloud organic compounds at the same time. There may be surface volcanoes made of ice instead of rock, spewing out in occasional eruptions not liquid rock but liquid ice-a lava of running methane, ammonia and perhaps water.

There is another consequence of the escape of all this hydrogen. An atmospheric molecule that achieves escape velocity from Titan generally does not have escape velocity from Saturn. Thus, as Thomas McDonough and the late Neil Brice of Cornell have pointed out, the hydrogen that is being lost from Titan will form a diffuse toroid, or doughnut, of hydrogen gas around Saturn. This is a very interesting prediction, first made for Titan but possibly relevant for other satellites as well. Pioneer 10 has detected such a hydrogen toroid around Jupiter in the vicinity of Io. As Pioneer 11 and Voyager 1 and 2 fly near Titan, they may be able to detect the Titan toroid.

Titan will be the easiest object to explore in the outer solar system. Nearly atmosphereless worlds such as Io or the asteroids present a landing problem because we cannot use atmospheric braking. Giant worlds such as Jupiter and Saturn have the opposite problem: the acceleration due to gravity is so large and the increase in atmospheric density is so rapid that it is difficult to devise an atmospheric probe that will not burn up on entry. Titan, however, has a dense enough atmosphere and a low enough gravity. If it were a little closer, we probably would be launching entry probes there today.

Titan is a lovely, baffling and instructive world which we suddenly realize is accessible for exploration: by fly-bys to determine the gross global parameters and to search for breaks in the clouds; by entry probes to sample the red clouds and unknown atmosphere; and by landers to examine a surface like none we know. Titan provides a remarkable opportunity to study the kinds of organic chemistry that on Earth may have led to the origin of life. Despite the low temperatures, it is by no means impossible that there is a Titanian biology. The geology of the surface may be unique in all the solar system. Titan is waiting…

CHAPTER 14

BETWEEN 30 and 10 million years ago, it is thought, temperatures on Earth slowly declined, by just a few Centigrade degrees. But many plants and animals have their life cycles sensitively attuned to the temperature, and vast forests receded toward more tropical latitudes. The retreat of the forests slowly removed the habitats of small furry binocular creatures, weighing only a few pounds, which had lived out their days brachiating from branch to branch. With the forests gone, only those furry creatures able to survive on the grassy savannas were to be found. Some tens of millions of years later, those creatures left two groups of descendants: one which includes the baboons and the other called humans. We may owe our very existence to climatic changes that on the average amount to only a few degrees. Such changes have brought some species into being and extinguished others. The character of life on our planet has been powerfully influenced by such variations, and it is becoming increasingly clear that the climate is continuing to change today.

There are many indications of past climatic changes. Some methods reach far into the past, others have only a limited applicability. The reliability of the methods also differs. One approach, which may be valid for a million years back in time, is based on the ratio of the isotopes oxygen 18 to oxygen 16 in the carbonates of shells of fossil foraminifera. These shells, belonging to species very similar to some that can be studied today, vary the oxygen 16/oxygen 18 ratio according to the temperature of the water in which they grew. Somewhat similar to the oxygen-isotope method is one based upon the ratio of the isotopes sulfur 34 to sulfur 32. There are other, more direct fossil indicators; for example, the widespread presence of corals, figs and palms denotes high temperatures, and the abundant remains of large hairy beasts, such as mammoths, indicate cold temperatures. The geological record is replete with extensive evidence of glaciation-great moving sheets of ice that leave characteristic boulders and erosional traces. There is also clear geological evidence for beds of evaporites-regions where briny water has evaporated leaving behind the salts. Such evaporation occurs preferentially in warm climates.

When this range of climatic information is put together, a complex pattern of temperature variation emerges. At no time, for example, is the average temperature of the Earth below the freezing point of water, and at no time does it even approach the normal boiling point of water. But variations of several degrees are common, and even variations of twenty or thirty degrees may have occurred at least locally. Fluctuations of a few degrees Centigrade happen over characteristic times of tens of thousands of years, and the recent succession of glacial and interglacial periods has this timing and temperature amplitude. But there are climatic fluctuations over much longer periods, the longest being on the order of a few hundred million years. Warm periods appear to have occurred about 650 million years ago and 270 million years ago. By the standards of past climatic fluctuations, we are now in the midst of an ice age. For most of the Earth’s history, there were no “permanent” ice caps, as in the Arctic and Antarctic today. We have, over the past few hundred years, made a partial emergence from our ice age caused by some as yet unexplained minor climatic variation; and there are certain signs that we may plunge back into the global cold temperatures characteristic of our epoch as seen from the perspective of the immense vistas of geological time. It is a sobering fact that 2 million years ago the site of the city of Chicago was buried under a mile of ice.

What determines the temperature of Earth? As seen from space, it is a rotating blue ball streaked with varying cloud patches, reddish-brown deserts and brilliant white polar caps. The energy for heating the Earth comes almost exclusively from sunlight, the energy conducted up from the hot interior of the Earth amounting to less than one thousandth of one percent of that arriving in the form of visible light from the Sun. But not all the sunlight is absorbed by the Earth. Some is reflected back to space by polar ice, clouds, and the rocks and water on the surface of the Earth. The average reflectivity, or albedo, of the Earth, as measured directly from satellites and indirectly from Earthshine reflected off the dark side of the Moon, is about 35 percent. The 65 percent of sunlight that is absorbed by the Earth heats it to a temperature which can readily be calculated. This temperature is about −18°C, below the freezing point of seawater and some 30°C colder than the measured average temperature of the Earth.

The discrepancy is due to the fact that this calculation neglects the so-called greenhouse effect. Visible light from the Sun enters the Earth’s clear atmosphere and is transmitted through to the surface. The surface, however, in attempting to radiate back into space, is constrained by the laws of physics to do so in the infrared. The atmosphere is not so transparent in the infrared, and at some wavelengths of infrared radiation-such as 6.2 microns or 15 microns-radiation would travel only a few centimeters before being absorbed by atmospheric gases. Since the Earth’s atmosphere is murky and absorbing at many wavelengths in the infrared, the thermal radiation given off by the surface of the Earth is impeded in escaping to space. In order to have a close equality between the radiation received by the Earth from the Sun and the radiation emitted by the Earth to space, the surface temperature of the Earth must then rise. The greenhouse effect is due not to the major atmospheric constituents of the Earth, such as oxygen and nitrogen, but almost exclusively to the minor constituents, especially carbon dioxide and water vapor.

As we have seen, the planet Venus is probably a case where the massive injection of carbon dioxide and smaller amounts of water vapor into a planetary atmosphere has led to such a large greenhouse effect that water cannot be maintained on the surface in the liquid state; hence, the planetary temperature runs away to some extremely high value-in the case of Venus, 480°C.

We have so far been talking about average temperatures. The temperature of the Earth varies from place to place. It is colder at the poles than at the equator because, in general, sunlight falls directly on the equator and obliquely on the poles. The tendency for the temperatures to be very different between equator and poles on Earth is moderated by atmospheric circulation. Hot air rises at the equator and moves at high altitudes to the poles, where it settles and returns to the surface; it then retraces its path, but at low altitudes, from pole back to equator. This general motion-complicated by the rotation of the Earth, its topography and the phase changes of water-is responsible for weather.

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

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 metal 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 [11] 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.

Many asteroids have orbits that are highly elliptical or stretched-out, not at all like the almost perfectly circular orbits of Earth or Venus. Some asteroids have their far points from the Sun beyond the orbit of Saturn; some have their near points to the Sun close to the orbit of Mercury; some, like 1685 Toro, live out their days between the orbits of Earth and Venus. Since there are so many asteroids on very elliptical orbits, collisions are inevitable over the lifetime of the solar system. Most collisions will be of the overtaking variety, one asteroid nudging up to another, making a soft splintering crash. Since the asteroids are so small, their gravity is low and the collision fragments will be splayed out into space into slightly different orbits from those of the parent asteroids. It can be calculated that such collisions will produce, on occasion, fragments that by accident intercept the Earth, fall through its atmosphere, survive the ablation of entry, and land at the feet of a quite properly astonished itinerant tribesman.

The few meteorites that have been tracked as they enter the Earth’s atmosphere originated back in the main asteroid belt, between Mars and Jupiter. Laboratory studies of the physical properties of some meteorites show them to have originated where the temperatures are those of the main asteroid belt. The evidence is clear: the meteorites ensconced in our museums are fragments of asteroids. We have on our shelves pieces of cosmic objects!

But which meteorites come from which asteroids? Until the last few years, answering this question was beyond the powers of planetary scientists. Recently, however, it has become possible to perform spectrophotometry of asteroids in visible and near-infrared radiation; to examine the polarization of sunlight reflected off asteroids as the geometry of the asteroid, the Sun and Earth changes; and to examine the middle-infrared emission of the asteroids. These asteroid observations, and comparable studies of meteorites and other minerals in the laboratory, have provided the first fascinating hints on the correlation between specific asteroids and specific meteorites. More than 90 percent of the asteroids studied fall into one of two composition groups: stony-iron or carbonaceous. Only a few percent of the meteorites on Earth are carbonaceous, but carbonaceous meteorites are very friable and rapidly weather to powder under typical terrestrial conditions. They probably also fragment more readily upon entry into the Earth’s atmosphere. Since stony-iron meteorites are much hardier, they are disproportionately represented in our museum collections of meteorites. The carbonaceous meteorites are rich in organic compounds, including amino acids (the building blocks of proteins), and may be representative of the materials from which the solar system was formed some 4.6 billion years ago.

Among the asteroids which appear to be carbonaceous are 1 Ceres, 2 Pallas, 19 Fortuna, 324 Bamberga and 654 Zelinda. If asteroids that are carbonaceous on the outside are also carbonaceous on the inside, then most of the asteroidal material is carbonaceous. They are generally dark objects, reflecting only a small percent of the light shining on them. Recent evidence suggests that Phobos and Deimos, the two moons of Mars, may also be carbonaceous, and are perhaps carbonaceous asteroids that have been captured by Martian gravity.

Typical asteroids showing properties of stony-iron meteorites are 3 Juno, 8 Flora, 12 Victoria, 89 Julia and 433 Eros. Several asteroids fit into some other category: 4 Vesta resembles a kind of meteorite called a basaltic achondrite, while 16 Psyche and 22 Kalliope appear to be largely iron.

The iron asteroids are interesting because geophysicists believe that the parent body of an object greatly enriched in iron must have been molten so as to differentiate, to separate out the iron from the silicates in the initial chaotic jumble of the elements in primordial times. On the other hand, for the organic molecules in carbonaceous meteorites to have survived at all they must never have been raised to temperatures hot enough to melt rock or iron. Thus, different histories are implied for different asteroids.

From the comparison of asteroidal and meteoritic properties, from laboratory studies of meteorites and computer projections back in time of asteroidal motions, it may one day be possible to reconstruct asteroid histories. Today we do not even know whether they represent a planet that was prevented from forming because of the powerful gravitational perturbations of nearby Jupiter, or whether they are the remnants of a fully formed planet that somehow exploded. Most students of the subject incline to the former hypothesis because no one can figure out how to blow up a planet-which is just as well. Eventually we may be able to piece together the whole story.

There may also be in hand meteorites which do not come from asteroids. Perhaps there are fragments of young comets, or of the moons of Mars, or of the surface of Mercury, or of the satellites of Jupiter, sitting dusty and ignored in some obscure museum. But it is clear that the true picture of the origin of the meteorites is beginning to emerge.

The Holy of Holies in the Temple of Diana at Ephesus has been destroyed. But the Kaaba has been carefully preserved, although there seems never to have been a true scientific examination of it. There are some who believe it to be a dark, stony rather than metallic meteorite. Recently two geologists have suggested, on admittedly quite fragmentary evidence, that it is instead an agate. Some Muslim writers believe that the color of the Kaaba was originally white, not black, and that the present color is due to its repeated handling. The official view of the Keeper of the Black Stone is that it was placed in its present position by the patriarch Abraham and fell from a religious rather than an astronomical heaven-so that no conceivable physical test of the object could be a test of Islamic doctrine. It would nevertheless be of great interest to examine, with the full armory of modern laboratory techniques, a small fragment of the Kaaba. Its composition could be determined with precision. If it is a meteorite, its cosmic-ray-exposure age-the time spent from fragmentation to arrival on Earth-could be established. And it would be possible to test hypotheses of origin: such as, for example, the idea that some 5 million years ago, about the time of the origin of the horninids, the Kaaba was chipped off an asteroid named 22 Kalliope, orbited the Sun for ages of geological time, and then accidentally encountered the Arabian Peninsula 2,500 years ago.

CHAPTER 16

MUCH OF HUMAN HISTORY can, I think, be described as a gradual and sometimes painful liberation from provincialism, the emerging awareness that there is more to the world than was generally believed by our ancestors. With awesome ethnocentrism, tribes all over the Earth called themselves “the people” or “all men,” relegating other groups of humans with comparable accomplishments to subhuman status. The high civilization of ancient Greece divided the human community into Hellenes and barbarians, the latter named after an uncharitable imitation of the languages of non-Greeks (“Bar Bar…”). That same classical civilization, which in so many respects is the antecedent of our own, called its small inland sea the Mediterranean-which means the middle of the Earth. For thousands of years China called itself the Middle Kingdom, and the meaning was the same: China was at the center of the universe and the barbarians lived in outer darkness.

Such views or their equivalent are only slowly changing, and it is possible to see some of the roots of racism and nationalism in their pervasive early acceptance by virtually all human communities. But we live in an extraordinary time, when technological advances and cultural relativism have made such ethnocentrism much more difficult to sustain. The view is emerging that we all share a common life raft in a cosmic ocean, that the Earth is, after all, a small place with limited resources, that our technology has now attained such powers that we are able to affect profoundly the environment of our tiny planet. This deprovincialization of mankind has been aided powerfully, I believe, by space exploration-by exquisite photographs of the Earth taken from a great distance, showing a cloudy, blue, spinning ball set like a sapphire in the endless velvet of space; but also by the exploration of other worlds, which have revealed both their similarities and their differences to this home of mankind.

We still talk of “the” world, as if there were no others, just as we talk about “the” Sun and “the” Moon. But there are many others. Every star in the sky is a sun. The rings of Uranus represent millions of previously unsuspected satellites orbiting Uranus, the seventh planet. And, as space vehicles have demonstrated so dramatically in the last decade and a half, there are other worlds-nearby, relatively accessible, profoundly interesting, and not a one closely similar to ours. As these planetary differences, and the Darwinian insight that life elsewhere is likely to be fundamentally different from life here, become more generally perceived, I believe they will provide a cohesive and unifying influence on the human family, which inhabits, for a time, this unprepossessing world among an immensity of others.

Planetary exploration has many virtues. It permits us to refine insights derived from such Earth-bound sciences as meteorology, climatology, geology and biology, to broaden their powers and improve their practical applications here on Earth. It provides cautionary tales on the alternative fates of worlds. It is an aperture to future high technologies important for life here on Earth. It provides an outlet for the traditional human zest for exploration and discovery, our passion to find out, which has been to a very large degree responsible for our success as a species. And it permits us, for the first time in history, to approach with rigor, with a significant chance of finding out the true answers, questions on the origins and destinies of worlds, the beginnings and ends of life, and the possibility of other beings who live in the skies-questions as basic to the human enterprise as thinking is, as natural as breathing.

Interplanetary unmanned spacecraft of the modern generation extend the human presence to bizarre and exotic landscapes far stranger than any in myth or legend. Propelled to escape velocity near the Earth, they adjust their trajectories with small rocket motors and tiny puffs of gas. They power themselves with sunlight and with nuclear energy. Some take only a few days to traverse the lake of space between Earth and Moon; others may take a year to Mars, four years to Saturn, or a decade to traverse the inland sea between us and distant Uranus. They float serenely on pathways predetermined by Newtonian gravitation and rocket technology, their bright metal gleaming, awash in the sunlight which fills the spaces between the worlds. When they arrive at their destinations, some will fly by, garnering a brief glimpse of an alien planet, perhaps with a retinue of moons, before continuing on farther into the depths of space. Others insert themselves into orbit about another world to examine it at close range, perhaps for years, before some essential component runs down or wears out. Some spacecraft will make landfall on another world, decelerating by atmospheric friction or parachute drag or the precision firing of retrorockets before gently setting down somewhere else. Some landers are stationary, condemned to examine a single spot on a world awaiting exploration. Others are self-propelled, slowly wandering to a distant horizon which holds no man knows what. And still others are capable of remotely acquiring rock and soil-a sample of another world-and returning it to the Earth.

All these spacecraft have sensors that extend astonishingly the range of human perception. There are devices that can determine the distribution of radioactivity over another planet from orbit; that can feel from the surface the faint rumble of a distant planetquake deep below; that can obtain three-dimensional color or infrared images of a landscape like none ever seen on Earth. These machines are, at least to a limited degree, intelligent. They can make choices on the basis of information they themselves receive. They can remember with great accuracy a detailed set of instructions which, if written out in English, would fill a good-sized book. They are obedient and can be reinstructed by radio messages sent to them from human controllers on Earth. And they have returned, mostly by radio, a rich and varied harvest of information on the nature of the solar system we inhabit. There have been fly-bys, crash-landers, soft-landers, orbiters, automated roving vehicles, and unmanned returned sample missions from our nearest celestial neighbor, the Moon-as well as, of course, six successful and heroic manned expeditions in the Apollo series. There has been a fly-by of Mercury; orbiters, entry probes and landers on Venus; fly-bys, orbiters and landers to Mars; and fly-bys of Jupiter and Saturn. Phobos and Deimos, the two small moons of Mars, have been examined close up, and tantalizing images have been obtained of a few of the moons of Jupiter.

We have caught our first glimpses of the ammonia clouds and great storm systems of Jupiter; the cold, salt-covered surface of its moon, Io; the desolate, crater-pocked, ancient and broiling Mercurian wasteland; and the wild and eerie landscape of our nearest planetary neighbor, Venus, where the clouds are composed of an acid rain that falls continuously but never patters the surface because that hilly landscape, illuminated by sunlight diffusing through the perpetual cloud layer, is everywhere at 900°F. And Mars: What a puzzle, what a joy, enigma and delight is Mars, with ancient river bottoms; immense, sculpted polar terraces; a volcano almost 80,000 feet high; raging windstorms; balmy afternoons; and an apparent initial defeat of our first pioneering effort to answer the question of questions-whether the planet harbors, now or ever, a home-grown form of life.

There are on Earth only two spacefaring nations, only two powers so far able to send machines much beyond the Earth’s atmosphere-the United States and the Soviet Union. The United States has accomplished the only manned missions to another body, the only successful Mars landers and the only expeditions to Mercury, Jupiter and Saturn. The Soviet Union has pioneered the automated exploration of the Moon, including the only unmanned rovers and return sample missions on any celestial objects, and the first entry probes and landers on Venus. Since the end of the Apollo program, Venus and the Moon have become, to a certain degree, Russian turf, and the rest of the solar system visited only by American space vehicles. While there is a certain degree of scientific cooperation between the two spacefaring nations, this planetary territoriality has come about by default rather than by agreement. There have in recent years been a set of very ambitious but unsuccessful Soviet missions to Mars, and the United States launched a modest but successful set of Venus orbiters and entry probes in 1978. The solar system is very large and there is much to explore. Even tiny Mars has a surface area comparable to the land area of the Earth. For practical reasons it is much easier to organize separate but coordinated missions launched by two or more nations than cooperative multinational ventures. In the sixteenth and seventeenth centuries, England, France, Spain, Portugal and Holland each organized on a grand scale missions of global exploration and discovery in vigorous competition. But the economic and religious motives of exploratory competition then do not seem to have their counterparts today. And there is every reason to think that national competition in the exploration of the planets will, at least for the foreseeable future, be peaceful.

THE LEAD TIMES for planetary missions are very long. The design, fabrication, testing, integration and launch of a typical planetary mission takes many years. A systematic program of planetary exploration requires a continuing commitment. The most celebrated American achievements on the Moon and planets-Apollo, Pioneer, Mariner and Viking-were initiated in the 1960s. At least until recently, the United States has made only one major commitment to planetary exploration in the whole of the decade of the 1970s-the Voyager missions, launched in the summer of 1977, to make the first systematic fly-by examination of Jupiter, Saturn, their twenty-five or so moons and the spectacular rings of the latter.

This absence of new starts has produced a real crisis in the community of American scientists and engineers responsible for the succession of engineering successes and high scientific discovery that began in 1962 with the Mariner 2 fly-by of Venus. There has been an interruption in the pace of exploration. Workers have been laid off and drifted to quite different jobs, and there is a real problem in providing continuity to the next generation of planetary exploration. For example, the earliest likely response to the spectacularly successful and historic Viking exploration of Mars will be a mission that does not even arrive at the Red Planet before 1985-a gap in Martian exploration of almost a decade. And there is not the slightest guarantee that there will be a mission even then. This trend-a little like dismissing most of the shipwrights, sail weavers and navigators of Spain in the early sixteenth century-shows some slight signs of reversal. Recently approved was Project Galileo, a middle-1980s mission to perform the first orbital reconnaissance of Jupiter and to drop the first probe into its atmosphere-which may contain organic molecules synthesized in a manner analogous to the chemical events which on Earth led to the origin of life. But the following year Congress so reduced the funds available for Galileo that it is, at the present writing, teetering on the brink of disaster.

In recent years the entire NASA budget has been well below one percent of the federal budget. The funds spent on planetary exploration have been less than 15 percent of that. Requests by the planetary science community for new missions have been repeatedly rejected-as one senator explained to me, the public has not, despite Star Wars and Star Trek, written to Congress in support of planetary missions, and scientists do not constitute a powerful lobby. And yet, there are a set of missions on the horizon that combine extraordinary scientific opportunity with remarkable popular appeal:

Solar Sailing and Comet Rendezvous. In ordinary interplanetary missions, spacecraft are obliged to follow trajectories that require a minimum expenditure of energy. The rockets burn for short periods of time in the vicinity of Earth, and the spacecraft mainly coast for the rest of the journey. We have done as well as we have not because of enormous booster capability, but because of great skill with severely constrained systems. As a result, we must accept small payloads, long mission times and little choice of departure or arrival dates. But just as on Earth we are considering moving from fossil fuels to solar power, so it is in space. Sunlight exerts a small but palpable force called radiation pressure. A sail-like structure with a very large area for its mass can use radiation pressure for propulsion. By positioning the sail properly, we can be carried by sunlight both inwards toward and outwards away from the Sun. With a square sail about half a mile on each side, but thinner than the thinnest Mylar, interplanetary missions can be accomplished more efficiently than with conventional rocket propulsion. The sail would be launched into Earth orbit by the manned Shuttle craft, unfurled and strutted. It would be an extraordinary sight, easily visible to the naked eye as a bright point of light. With a pair of binoculars, detail on such a sail could be made out-perhaps even what on seventeenth-century sailing ships was called the “device,” some appropriate graphic symbol, perhaps a representation of the planet Earth. Attached to the sail would be a scientific spacecraft designed for a particular application.

One of the first and most exciting applications being discussed is a comet-rendezvous mission, perhaps a rendezvous with Halley’s comet in 1986. Comets spend most of their time in interstellar space and should provide major clues on the early history of the solar system and the nature of the matter between the stars. Solar sailing to Halley’s comet might not only provide close-up pictures of the interior of a comet-about which we now know close to nothing-but also, astonishingly, return a piece of a comet to the planet Earth. The practical advantages and the romance of solar sailing are both evident in this example, and it is clear that it represents not just a new mission but a new interplanetary technology. Because the development of solar-sailing technology is behind that of ion propulsion, it is the latter that may propel us on our first missions to the comets. Both propulsion mechanisms have their place in future interplanetary travel. But in the long term I believe solar sailing will make the greater impact. Perhaps by the early twenty-first century there will be interplanetary regattas competing for the fastest time from Earth to Mars.

Mars Rovers. Before the Viking mission, no terrestrial spacecraft had successfully landed on Mars. There had been several Soviet failures, including at least one which was quite mysterious and possibly attributable to the hazardous nature of the Martian landscape. Thus, both Viking 1 and Viking 2 were, after painstaking efforts, successfully landed in two of the dullest places we could find on the Martian surface. The lander stereo cameras showed distant valleys and other inaccessible vistas. The orbital cameras showed an extraordinarily varied and geologically exuberant landscape which we could not examine close up with the stationary Viking lander. Further Martian exploration, both geological and biological, cries out for roving vehicles capable of landing in the safe but dull places and wandering hundreds or thousands of kilometers to the exciting places. Such a rover would be able to wander to its own horizon every day and produce a continuous stream of photographs of new landscapes, new phenomena and very likely major surprises on Mars. Its importance would be improved still further if it operated in tandem with a Mars polar orbiter which would geochemically map the planet, or with an unmanned Martian aircraft which would photograph the surface from very low altitudes.

Titan Lander. Titan is the largest moon of Saturn and the largest satellite in the solar system (see Chapter 13). It is remarkable for having an atmosphere denser than that of Mars and is probably covered with a layer of brownish clouds composed of organic molecules. Unlike Jupiter and Saturn, it has a surface on which we can land, and its deep atmosphere is not so hot as to destroy the organic molecules. A Titan entry-probe and lander mission would probably be part of a Saturn orbital mission, which might also include a Saturn entry probe.

Venus Orbital Imaging Radar. The Soviet Venera 9 and 10 missions have returned the first close-up photographs of the surface of Venus. Because of the permanent cloud pall, the surface features of Venus are not visible through Earth-bound optical telescopes. However, Earth-based radar and the radar system aboard the small Pioneer Venus orbiter have now begun to map Venus surface features, and have revealed mountains and craters and volcanoes as well as stranger morphology. A proposed Venus orbital imaging radar would provide pole-to-pole radar pictures of Venus with much higher detail than can be achieved from the surface of the Earth, and would permit a preliminary reconnaissance of the Venus surface comparable to that achieved for Mars in 1971-72 by Mariner 9.

Solar Probe. The Sun is the nearest star, the only one we are likely to be able to examine close up, at least for many decades. A near approach to the Sun would be of great interest, would help in understanding its influence on Earth, and would also provide vital additional tests of such theories of gravitation as Einstein’s General Theory of Relativity. A solar probe mission is difficult for two reasons: the energy required to undo the Earth’s (and the probe’s) motion around the Sun so it can fall into the Sun, and the intolerable heating as the probe approaches the Sun. The first problem can be solved by launching the spacecraft out to Jupiter and then using Jupiter’s gravitation to fling it into the Sun. Since there are many asteroids interior to Jupiter’s orbit, this might possibly be a useful mission for studying asteroids as well. An approach to the second problem, at first sight remarkable for its naïveté, is to fly into the Sun at night. On Earth, nighttime is of course merely the interposition of the solid body of the Earth between us and the Sun. Likewise for a solar probe. There are some asteroids that come rather close to the Sun. A solar probe would approach the Sun in the shadow of a Sun-grazing asteroid (meanwhile making observations of the asteroid as well). Near the point of closest approach of the asteroid to the Sun, the probe would emerge from the asteroidal shadow and plunge, filled with a fluid that resists heating, as deeply into the atmosphere of the Sun as it could until it melted and vaporized-atoms from the Earth added to the nearest star.

Manned Missions. As a rule of thumb, a manned mission costs from fifty to a hundred times more than a comparable unmanned mission. Thus, for scientific exploration alone, unmanned missions, employing machine intelligence, are preferred. However, there may well be reasons other than scientific for exploring space-social, economic, political, cultural or historical. The manned missions most frequently talked about are space stations orbiting the Earth (and perhaps devoted to harvesting sunlight and transmitting it in microwave beams down to an energy-starved Earth), and a permanent lunar base. Also being discussed are rather grand schemes for the construction of permanent space cities in Earth orbit, constructed from lunar or asteroidal materials. The cost of transporting materials from such low-gravity worlds as the Moon or an asteroid to Earth orbit is much less than transporting the same materials from our high-gravity planet. Such space cities might ultimately be self-propagating-new ones constructed by older ones. The costs of these large manned stations have not yet been estimated reliably, but it seems likely that all of them-as well as a manned mission to Mars-would cost in the $100 billion to $200 billion range. Perhaps such schemes will one day be implemented; there is much that is far-reaching and historically significant in them. But those of us who have fought for years to organize space ventures costing less than one percent as much may be forgiven for wondering whether the required funds will be allocated, and whether such expenditures are socially responsible.

However, for substantially less money, an important expedition that is preparatory for each of these manned ventures could be mustered-an expedition to an Earth-crossing carbonaceous asteroid. The asteroids occur mostly between the orbits of Mars and Jupiter. A small fraction of them have trajectories that carry them across Earth’s orbit and occasionally within a few million miles of the Earth. Many asteroids are mainly carbonaceous-with large quantities of organic materials and chemically bound water. The organic matter is thought to have condensed in the very earliest stages of the formation of the solar system from interstellar gas and dust, some 4.6 billion years ago, and their study and comparison with cometary samples would be of extraordinary scientific interest. I do not think that materials from a carbonaceous asteroid are likely to be criticized in the same way that the Apollo returned lunar samples were-as being “only” rocks. Moreover, a manned landing on such an object would be an excellent preparation for the eventual exploitation of resources in space. And finally, landing on such an object would be fun: because the gravity field is so low, it would be possible for an astronaut to do a standing high jump of about ten kilometers. These Earth-crossing objects, which are being discovered at a rapidly increasing pace, are called-by a name selected long before manned spaceflight-the Apollo objects. They may or may not be the dead husks of comets. But whatever their origin, they are of great interest. Some of them are the easiest objects in space for humans to get to, using only the Shuttle technology, which will be available in another few years.

THE SORTS of missions I have outlined are well within our technological capability and require a NASA budget not much larger than the present one. They combine scientific and public interest, which very often share coincident objectives. Were such a program carried out, we would have made a preliminary reconnaissance of all the planets and most of the moons from Mercury to Uranus, made a representative sampling of asteroids and comets, and discovered the boundaries and contents of our local swimming hole in space. As the finding of rings around Uranus reminds us, major and unexpected discoveries are waiting for us. Such a program would also have made the first halting steps in the utilization of the solar system by our species, tapping the resources on other worlds, arranging for human habitation in space, and ultimately reworking or terraforming the environments of other planets so that human beings can live there with minimal inconvenience. Human beings will have become a multi-planet species.

The transitional character of these few decades is evident. Unless we destroy ourselves, it is clear that humanity will never again be restricted to a single world. Indeed, the ultimate existence of cities in space and the presence of human colonies on other worlds will make it far more difficult for the human species to self-destruct. It is clear that we have entered, almost without noticing it, a golden age of planetary exploration. As in many comparable cases in human history, the opening of horizons through exploration is accompanied by an opening of artistic and cultural horizons. I do not imagine that many people in the fifteenth century ever wondered if they were living in the Italian Renaissance. But the hopefulness, the exhilaration, the opening of new ways of thought, the technological developments, the goods from abroad, and the deprovincialization of that age were then apparent to thoughtful men and women. We have the ability and the means and-I very much hope-the will for a comparable endeavor today. For the first time in human history, it is within the power of this generation to extend the human presence to the other worlds of the solar system-with awe for their wonders, and a thirst for what they have to teach us.