Pale Blue Dot - Carl Sagan [121]
Recognizing the speculative nature of the question and the limitations in our knowledge, is it nevertheless possible to envision terraforming the planets?
We need look no further than our own world to see that humans are now able to alter planetary environments in a profound way. Depletion of the ozone layer, global warming from an increased greenhouse effect, and global cooling from nuclear war are all ways in which present technology can significantly alter the environment of our world—and in each case as an inadvertent consequence of doing something else. If we had intended to alter our planetary environment, we would be fully able to generate still greater change. As our technology becomes more powerful, we will be able to work still more profound changes.
But just as (in parallel parking) it’s easier to get out of a parking place than into one, it’s easier to destroy a planetary environment than to move it into a narrowly prescribed range of temperatures, pressures, compositions, and so on. We already know of a multitude of desolate and uninhabitable worlds, and—with very narrow margins—only one green and clement one. This is a major conclusion from early in the era of spacecraft exploration of the Solar System. In altering the Earth, or any world with an atmosphere, we must be very careful about positive feedbacks, where we nudge an environment a little bit and it takes off on its own—a little cooling leading to runaway glaciation, as may have happened on Mars, or a little warming to a runaway greenhouse effect, as happened on Venus. It is not at all clear that our knowledge is sufficient to this purpose.
As far as I know, the first suggestion in the scientific literature about terraforming the planets was made in a 1961 article I wrote about Venus. I was pretty sure then that Venus had a surface temperature well above the normal boiling point of water, produced by a carbon dioxide/water vapor greenhouse effect. I imagined seeding its high clouds with genetically engineered microorganisms that would take CO2, N2, and H2O out of the atmosphere and convert them into organic molecules. The more CO2 removed, the smaller the greenhouse effect and the cooler the surface. The microbes would be carried down through the atmosphere toward the ground, where they would be fried, so water vapor would be returned to the atmosphere; but the carbon from the CO2 would be converted irreversibly by the high temperatures into graphite or some other involatile form of carbon. Eventually, the temperatures would fall below the boiling point and the surface of Venus would become habitable, dotted with pools and lakes of warm water.
The idea was soon taken up by a number of science fiction authors in the continuing dance between science and science fiction—in which the science stimulates the fiction, and the fiction stimulates a new generation of scientists, a process benefiting both genres. But as the next step in the dance, it is now clear that seeding Venus with special photosynthetic microorganisms will not work. Since 1961 we’ve discovered that the clouds of Venus are a concentrated solution of sulfuric acid, which makes the genetic engineering rather more challenging. But that in itself is not a fatal flaw. (There are microorganisms that live out their lives in concentrated solutions of sulfuric acid.) Here’s the fatal flaw: In 1961 I thought the atmospheric pressure at the surface of Venus was a few “bars,” a few times the surface pressure on Earth. We now know it to be 90 bars, so that if the scheme worked, the result would be a surface buried in hundreds of meters of fine graphite, and an atmosphere made of 65 bars of almost pure molecular oxygen. Whether we would first implode under the atmospheric pressure or spontaneously burst into flames in all that oxygen is an open question. However, long before so much oxygen could build up, the graphite would spontaneously burn back into CO2, short-circuiting the process. At best, such a scheme can carry the terraforming of Venus only partway.