Cosmos (21 page)

Recent experiments suggest that this may indeed be the case. In the great Martian dust storm of 1971, spectral features of the dust were obtained by the Mariner 9 infrared spectrometer. In analyzing these spectra, O. B. Toon, J. B. Pollack and I found that certain features seem best accounted for by montmorillonite
and other kinds of clay. Subsequent observations by the Viking lander support the identification of windblown clays on Mars. Now, A. Banin and J. Rishpon have found that they can reproduce some of the key features—those resembling photosynthesis as well as those resembling respiration—of the “successful” Viking microbiology experiments if in laboratory experiments they substitute such clays for the Martian soil. The clays have a complex active surface, given to adsorbing and releasing gases and to catalyzing chemical reactions. It is too soon to say that all the Viking microbiology results can be explained by inorganic chemistry, but such a result would no longer be surprising. The clay hypothesis hardly excludes life on Mars, but it certainly carries us far enough to say that there is no compelling evidence for microbiology on Mars.

Even so, the results of Banin and Rishpon are of great biological importance because they show that in the absence of life there can be a kind of soil chemistry that does some of the same things life does. On the Earth before life, there may already have been chemical processes resembling respiration and photosynthesis cycling in the soil, perhaps to be incorporated by life once it arose. In addition, we know that montmorillonite clays are a potent catalyst for combining amino acids into longer chain molecules resembling proteins. The clays of the primitive Earth may have been the forge of life, and the chemistry of contemporary Mars may provide essential clues to the origin and early history of life on our planet.

The Martian surface exhibits many impact craters, each named after a person, usually a scientist. Crater Vishniac lies appropriately in the Antarctic region of Mars. Vishniac did not claim that there had to be life on Mars, merely that it was possible, and that it was extraordinarily important to know if it was there. If life on Mars exists, we will have a unique opportunity to test the generality of our kind of life. And if there is no life on Mars, a planet rather like the Earth, we must understand why—because in that case, as Vishniac stressed, we have the classic scientific confrontation of the experiment and the control.

The finding that the Viking microbiology results can be explained by clays, that they need not imply life, helps to resolve another mystery: the Viking organic chemistry experiment showed not a hint of organic matter in the Martian soil. If there is life on Mars, where are the dead bodies? No organic molecules could be found—no building blocks of proteins and nucleic acids, no simple hydrocarbons, nothing of the stuff of life on Earth. This is not
necessarily a contradiction, because the Viking microbiology experiments are a thousand times more sensitive (per equivalent carbon atom) than the Viking chemistry experiments, and seem to detect organic matter synthesized in the Martian soil. But this does not leave much margin. Terrestrial soil is loaded with the organic remains of once-living organisms; Martian soil has less organic matter than the surface of the Moon. If we held to the life hypothesis, we might suppose that the dead bodies have been destroyed by the chemically reactive, oxidizing surface of Mars—like a germ in a bottle of hydrogen peroxide; or that there is life, but of a kind in which organic chemistry plays a less central role than it does in life on Earth.

But this last alternative seems to me to be special pleading: I am, reluctantly, a self-confessed carbon chauvinist. Carbon is abundant in the Cosmos. It makes marvelously complex molecules, good for life. I am also a water chauvinist. Water makes an ideal solvent system for organic chemistry to work in and stays liquid over a wide range of temperatures. But sometimes I wonder. Could my fondness for materials have something to do with the fact that I am made chiefly of them? Are we carbon- and water-based because those materials were abundant on the Earth at the time of the origin of life? Could life elsewhere—on Mars, say—be built of different stuff?

I am a collection of water, calcium and organic molecules called Carl Sagan. You are a collection of almost identical molecules with a different collective label. But is that all? Is there nothing in here but molecules? Some people find this idea somehow demeaning to human dignity. For myself, I find it elevating that our universe permits the evolution of molecular machines as intricate and subtle as we.

But the essence of life is not so much the atoms and simple molecules that make us up as the way in which they are put together. Every now and then we read that the chemicals which constitute the human body cost ninety-seven cents or ten dollars or some such figure; it is a little depressing to find our bodies valued so little. However, these estimates are for human beings reduced to our simplest possible components. We are made mostly of water, which costs almost nothing; the carbon is costed in the form of coal; the calcium in our bones as chalk; the nitrogen in our proteins as air (cheap also); the iron in our blood as rusty nails. If we did not know better, we might be tempted to take all the atoms that make us up, mix them together in a big container
and stir. We can do this as much as we want. But in the end all we have is a tedious mixture of atoms. How could we have expected anything else?

Harold Morowitz has calculated what it would cost to put together the correct
molecular
constitutents that make up a human being by buying the molecules from chemical supply houses. The answer turns out to be about ten million dollars, which should make us all feel a little better. But even then we could not mix those chemicals together and have a human being emerge from the jar. That is far beyond our capability and will probably be so for a very long period of time. Fortunately, there are other less expensive but still highly reliable methods of making human beings.

I think the lifeforms on many worlds will consist, by and large, of the same atoms we have here, perhaps even many of the same basic molecules, such as proteins and nucleic acids—but put together in unfamiliar ways. Perhaps organisms that float in dense planetary atmospheres will be very much like us in their atomic composition, except they might not have bones and therefore not need much calcium. Perhaps elsewhere some solvent other than water is used. Hydrofluoric acid might serve rather well, although there is not a great deal of fluorine in the Cosmos; hydrofluoric acid would do a great deal of damage to the kind of molecules that make us up, but other organic molecules, paraffin waxes, for example, are perfectly stable in its presence. Liquid ammonia would make an even better solvent system, because ammonia is very abundant in the Cosmos. But it is liquid only on worlds much colder than the Earth or Mars. Ammonia is ordinarily a gas on Earth, as water is on Venus. Or perhaps there are living things that do not have a solvent system at all—solid-state life, where there are electrical signals propagating rather than molecules floating about.

But these ideas do not rescue the notion that the Viking lander experiments indicate life on Mars. On that rather Earth-like world, with abundant carbon and water, life, if it exists, should be based on organic chemistry. The organic chemistry results, like the imaging and microbiology results, are all consistent with no life in the fine particles of Chryse and Utopia in the late 1970’s. Perhaps some millimeters beneath the rocks (as in the Antarctic dry valleys), or elsewhere on the planet, or in some earlier, more clement time. But not where and when we looked.

The Viking exploration of Mars is a mission of major historical importance, the first serious search for what other kinds of life may be, the first survival of a functioning spacecraft for more than
an hour or so on any other planet (Viking 1 has survived for years), the source of a rich harvest of data on the geology, seismology, mineralogy, meteorology and half a dozen other sciences of another world. How should we follow up on these spectacular advances? Some scientists want to send an automatic device that would land, acquire soil samples, and return them to Earth, where they could be examined in great detail in the large sophisticated laboratories of Earth rather than in the limited microminiaturized laboratories that we are able to send to Mars. In this way most of the ambiguities of the Viking microbiology experiments could be resolved. The chemistry and mineralogy of the soil could be determined; rocks could be broken open to search for subsurface life; hundreds of tests for organic chemistry and life could be performed, including direct microscopic examination, under a wide range of conditions. We could even use Vishniac’s scoring techniques. Although it would be fairly expensive, such a mission is probably within our technological capability.

However, it carries with it a novel danger: back-contamination. If we wish on Earth to examine samples of Martian soil for microbes, we must, of course, not sterilize the samples beforehand. The point of the expedition is to bring them back alive. But what then? Might Martian microorganisms returned to Earth pose a public health hazard? The Martians of H. G. Wells and Orson Welles, preoccupied with the suppression of Bournemouth and Jersey City, never noticed until too late that their immunological defenses were unavailing against the microbes of Earth. Is the converse possible? This is a serious and difficult issue. There may be no micromartians. If they exist, perhaps we can eat a kilogram of them with no ill effects. But we are not sure, and the stakes are high. If we wish to return unsterilized Martian samples to Earth, we must have a containment procedure that is stupefyingly reliable. There are nations that develop and stockpile bacteriological weapons. They seem to have an occasional accident, but they have not yet, so far as I know, produced global pandemics. Perhaps Martian samples
can
be safely returned to Earth. But I would want to be very sure before considering a returned-sample mission.

There is another way to investigate Mars and the full range of delights and discoveries this heterogeneous planet holds for us. My most persistent emotion in working with the Viking lander pictures was frustration at our immobility. I found myself unconsciously urging the spacecraft at least to stand on its tiptoes, as if this laboratory, designed for immobility, were perversely refusing to manage even a little hop. How we longed to poke that dune
with the sample arm, look for life beneath that rock, see if that distant ridge was a crater rampart. And not so very far to the southeast, I knew, were the four sinuous channels of Chryse. For all the tantalizing and provocative character of the Viking results, I know a hundred places on Mars which are far more interesting than our landing sites. The ideal tool is a roving vehicle carrying on advanced experiments, particularly in imaging, chemistry and biology. Prototypes of such rovers are under development by NASA. They know on their own how to go over rocks, how not to fall down ravines, how to get out of tight spots. It is within our capability to land a rover on Mars that could scan its surroundings, see the most interesting place in its field of view and, by the same time tomorrow, be there. Every day a new place, a complex, winding traverse over the varied topography of this appealing planet.

Such a mission would reap enormous scientific benefits, even if there is no life on Mars. We could wander down the ancient river valleys, up the slopes of one of the great volcanic mountains, along the strange stepped terrain of the icy polar terraces, or muster a close approach to the beckoning pyramids of Mars.
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Public interest in such a mission would be sizable. Every day a new set of vistas would arrive on our home television screens. We could trace the route, ponder the findings, suggest new destinations. The journey would be long, the rover obedient to radio commands from Earth. There would be plenty of time for good new ideas to be incorporated into the mission plan. A billion people could participate in the exploration of another world.

The surface area of Mars is exactly as large as the land area of the Earth. A thorough reconnaissance will clearly occupy us for centuries. But there will be a time when Mars is all explored; a time after robot aircraft have mapped it from aloft, a time after rovers have combed the surface, a time after samples have been returned safely to Earth, a time after human beings have walked the sands of Mars. What then? What shall we do with Mars?

There are so many examples of human misuse of the Earth that even phrasing this question chills me. If mere is life on Mars, I believe we should do nothing with Mars. Mars then belongs to the Martians, even if the Martians are only microbes. The existence
of an independent biology on a nearby planet is a treasure beyond assessing, and the preservation of that life must, I think, supersede any other possible use of Mars. However, suppose Mars is lifeless. It is not a plausible source of raw materials: the freightage from Mars to Earth would be too expensive for many centuries to come. But might we be able to live on Mars? Could we in some sense make Mars habitable?

A lovely world, surely, but there is—from our parochial point of view—much wrong with Mars, chiefly the low oxygen abundance, the absence of liquid water, and the high ultraviolet flux. (The low temperatures do not pose an insuperable obstacle, as the year-round scientific stations in Antarctica demonstrate.) All of these problems could be solved if we could make more air. With higher atmospheric pressures, liquid water would be possible. With more oxygen we might breathe the atmosphere, and ozone would form to shield the surface from solar ultraviolet radiation. The sinuous channels, stacked polar plates and other evidence suggest that Mars once had such a denser atmosphere. Those gases are unlikely to have escaped from Mars. They are, therefore, on the planet somewhere. Some are chemically combined with the surface rocks. Some are in subsurface ice. But most may be in the present polar ice caps.

To vaporize the caps, we must heat them; perhaps we could dust them with a dark powder, heating them by absorbing more sunlight, the opposite of what we do to the Earth when we destroy forests and grasslands. But the surface area of the caps is very large. The necessary dust would require 1,200 Saturn 5 rocket boosters to be transported from Earth to Mars; even then, the winds might blow the dust off the polar caps. A better way would be to devise some dark material able to make copies of itself, a little dusky machine which we deliver to Mars and which then goes about reproducing itself from indigenous materials all over the polar caps. There is a category of such machines. We call them plants. Some are very hardy and resilient. We know that at least some terrestrial microbes can survive on Mars. What is necessary is a program of artificial selection and genetic engineering of dark plants—perhaps lichens—that could survive the much more severe Martian environment. If such plants could be bred, we might imagine them being seeded on the vast expanse of the Martian polar ice caps, taking root, spreading, blackening the ice caps, absorbing sunlight, heating the ice, and releasing the ancient Martian atmosphere from its long captivity. We might even imagine a kind of Martian Johnny Appleseed, robot or human, roaming
the frozen polar wastes in an endeavor that benefits only the generations of humans to come.