Both subtly and profoundly, the activities of life have affected the environment of our planet. Our atmosphere is composed of 20 percent oxygen and 80 percent nitrogen. The oxygen is produced almost entirely by green-plant photosynthesis. Similarly, the most recent evidence suggests that nitrogen is almost entirely a product of the biological activity of soil micro-organisms, which convert nitrates and ammonia into the gas N2, molecular nitrogen. Not only are the principal constituents of our atmosphere closely controlled by biological activities, but the minor constituents are as well. To a significant extent, carbon dioxide is also buffered by the photosynthesis/respiration feedback loop. Even so minor a constituent of the Earth's atmosphere as methane, CH4, is of biological origin.
In fact, life on Earth, invisible to photography, could be detectable with a small telescope and an infrared spectrometer from the vantage point of Mars. The Martians, if any, could easily observe, at a wavelength of 3.33 microns in the infrared, a strong absorption feature that straightforward analysis would reveal to be due to one part per million of methane in the terrestrial atmosphere. It should not be difficult to deduce that the methane is probably of biological origin. Methane is chemically unstable in an excess of oxygen. It is oxidized rapidly to carbon dioxide:
The amount of methane that would be in equilibrium with the great excess of oxygen in our atmosphere is less than one billion billion billionth the amount actually observed. How can this be? Methane must be produced at a rate so rapid that there is not time enough for oxygen to reduce its abundance to the equilibrium amount. It might be that there are massive outpourings of methane from ancient petroleum fields on Earth. But because of the huge output required, this is a very unlikely hypothesis. It is far more likely that methane is produced by a biological process.
And this is indeed the case. There seems to be a debate in the ecological literature on two possible sources of this methane. One source is methane bacteria, which live in swamps and marshes - hence the term "marsh gas" to refer to methane. The principal other habitat of methane bacteria is in the rumens of ungulates. There is at least one school of ecological thought that believes that more methane is produced from the latter source than from the former. This means that bovine flatulence - the intimate intestinal activities of cows, reindeer, elephants, and elk - is detectable over interplanetary distances, while the bulk of the activities of mankind are invisible. We would not ordinarily consider the flatulence of cattle as a dominant manifestation of life on Earth, but there it is.
Inadvertently, with no conscious effort by mankind, life on Earth has reworked the environment in a major way. Through the effect of atmospheric pressure and composition on the climate, there is a feedback loop in which the climate itself may to some degree be controlled by the gas exchange reactions in which the life forms on Earth engage. In a way, life on Earth has terraformed Terra. It has to some extent made the Earth the way it is.
Is it possible that at some time in the future we might be able similarly to terraform other planets, to convert a Mars or Venus, today inhospitable to Man, into a clement and habitable environment? Such a change, if possible at all, should be done only after the most careful and responsible examination of the consequences. We would first want to understand thoroughly the present environment of the planet before altering it. We must scrupulously guarantee that any indigenous organisms on the planet would not be disrupted by terraforming. If Mars, for example, has a population of indigenous organisms that would be extinguished by terraforming, we should never perform such terraforming. But if the planet is lifeless, or if the organisms survive better under conditions closer to our own, it might be reasonable at some time in the future to consider such an alteration of a planetary environment.
Our motivations for planetary re-engineering must be clear. This is not a solution to the overpopulation problem. Several hundred thousand people are born every day on Earth. There is certainly no prospect in the immediate future of transshipping hundreds of thousands of people to other planets each day. In its entire history mankind has managed to launch one dozen people to another celestial body. Nor are we likely to see in the immediate future a thriving mining industry in which ores are extracted from another planet and transshipped to Earth: The freightage would be prohibitive.
And yet the human spirit is expansive; the urge to colonize new environments lies deep within many of us. Such activities can be performed without cosmic imperialism, without the kind of arrogance that characterized the European colonization of the New World, or the encroachment on the Indians in the settling by whites of the American West. Interplanetary colonization can be consistent with the highest aspirations and goals of mankind.
How would we do it? In the case of Venus, as we saw in Chapter 12, there is a crushing atmosphere, composed largely of carbon dioxide, and a searing surface temperature in excess of 900 degrees F. It would seem to be a formidable task indeed to convert this environment into one in which men could live and work without enormous technological assistance. But there is a bare possibility of reengineering Venus into a quite Earth-like place, a possibility I suggested with some caution in 1961. The method assumes that the high surface temperature is produced by a greenhouse effect involving carbon dioxide and water, a conjecture that is much more plausible now than it was then. The idea is simply to seed the clouds of Venus with a hardy variety of algae - a genus called Nostocacae was suggested - which would perform photosynthesis in the vicinity of the clouds. Carbon dioxide and water would be converted into organic compounds, largely carbohydrates, and oxygen. The algae would, however, be carried by the atmospheric circulation down to deeper and hotter levels in the Venus atmosphere, where they would be fried. Frying an alga releases simple carbon compounds, carbon, and water into the atmosphere. The water content of the atmosphere thus remains fixed, and the net result is the conversion of carbon dioxide into carbon and oxygen.
The present greenhouse effect on Venus is due largely to carbon dioxide and water. The present total pressure on Venus is about ninety times that on the surface of Earth. The Venus atmosphere is largely composed of carbon dioxide. As the carbon dioxide is converted into carbon and oxygen, and the oxygen is chemically combined with the crust of Venus, the total pressure would decline, decreasing atmospheric infrared absorption, reducing the greenhouse effect, and lowering the temperature.
It is possible, therefore, that the injection of appropriately grown algae into the clouds of Venus, algae able to reproduce there faster than they are fried, would in time convert the present extremely hostile environment of Venus into one much more pleasant for human beings.
The amount of water vapor in the Venus atmosphere, if condensed on the surface of the planet, would give a layer of water about one foot high - not an ocean, but enough to do irrigation and to provide for other human needs. It is also possible that water is available bound to the rocks on the surface of the planet.
No one can estimate whether this is a very likely scenario, or how long it would take to re-engineer the second planet from the Sun. It is perfectly possible that there is some flaw in the idea. For example, the high surface temperature may not be due to a greenhouse effect, but I think this is unlikely.
In any case, I think terraforming Venus is not impossible. The Nostoc scheme is an example of how human technology and science may, in periods quite short compared to geological time, rework the environment of another planet.
For Mars, as we saw in Chapter 18, there is now evidence that in comparatively recent times conditions on that planet were much more Earth-like than they are today. We mentioned the likelihood that enormous quantities of carbon dioxide and water are locked in the Martian polar caps, trapped as permafrost and chemically bound to the surface material elsewhere on the planet. Much of this CO2 and H2O may be released from the polar caps into the atmosphere twice each precessional cycle of fifty thousand years. Drs. Joseph Burns and Martin Harwit of Cornell University have considered a variety of technological schemes to induce more clement conditions on Mars hundreds of years from now, rather than thousands. These schemes involve alteration of the orbits of the Martian satellites or of a nearby asteroid to change the precessional motion of the planet, or the installation of an enormous orbiting mirror over the polar cap to melt the material frozen there. Even easier, however, might be to sprinkle carbon black over the caps, heat up the poles, increase the atmospheric pressure, and warm the planet.
Again, we do not know that such schemes will work, but they do not seem extremely impractical. It may very well be that on time scales of hundreds of years we will have the capability of converting Mars into a much more Earth-like planet than it would otherwise be.
The Moon and the asteroids are much less hospitable than Mars and Venus. They are so much less able to retain an atmosphere that the terraforming schemes we have been discussing are inapplicable to them. But even on airless worlds, the establishment of human colonies on their surfaces or even - in the case of small asteroids - in their interiors seems a possible future project for mankind. Such colonies would be much more constrained than those on a re-engineered Mars or Venus, and would require much greater attention to the husbanding of scarce resources.
Such colonies would be tenable only if significant natural resources -particularly frozen or chemically bound water - were to be found. In the case of the very surface of the Moon, the samples returned by Apollo astronauts showed virtually no such water at all. But it is entirely possible that large stores of water exist in cold, shadowed regions near the lunar poles or at substantial depths beneath the lunar surface.
It is not unlikely that on a time scale of a few centuries there will be extensive human colonies throughout the inner part of the Solar System and on some of the major satellites of the Jovian planets. The prospect is, of course, a difficult one; the engineering tasks are immense and the need to retain ecological respect for other environments pervasive. The danger of both forward and backward biological contamination must always be examined scrupulously.
There may even come a day when we shall be called to account for our stewardship of the Solar System. From that vantage point our own epoch will be viewed as a moment when we first left the cradle of our species and began, in a groping and tentative way, to explore and transform the space surrounding us.
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