Planetary engineering, or terraforming as it is sometimes called, is the large-scale modification or manipulation of the environment of a planet to make it more suitable for human habitation. In the case of Mars, human settlers would probably seek to make its atmosphere more dense and breathable by adding more oxygen. Early "Martians" would probably also attempt to alter the planet's harsh temperatures and to modify them to fit a more terrestrial thermal pattern. Venus represents an even larger challenge to the planetary engineer. Its current atmospheric pressure would have to be significantly reduced, its infernolike surface temperatures greatly diminished, the excessive amounts of carbon dioxide in its atmosphere reduced, and—perhaps the biggest task of all—its rotation rate increased to shorten the length of the solar day.
It should now be obvious that when scientists and engineers discuss planetary engineering projects, they are speaking of truly large, long-term projects. Typical time estimates for the total terraforming of a planet such as Mars or Venus range from centuries to a few millennia. However, we can also develop ecologically suitable enclaves or niches, especially on the Moon or Mars. Such localized planetary modification efforts could probably be accomplished within a few decades of project initiation.
Just what are the "tools" of planetary engineering? The planetary pioneers in the latter portions of this century will need at least the following, if they are to convert presently inhospitable worlds into new ecospheres that permit human habitation with little or no personal lifesupport equipment: first, and perhaps the most often overlooked, human ingenuity; second, a thorough knowledge of the physical processes of the particular planet or moon that is undergoing terraforming (especially the existence and location of environmental pressure points at which small modifications of the local energy or material balance can cause global environmental effects); third, the ability to manipulate large quantities
This is an artist's rendering of the domed-habitat approach to planetary engineering—an early space age vision of how to make the Moon and possibly Mars suitable for large-scale settlement by human pioneers decades after the Apollo Project. (NASA)
of energy; fourth, the ability to manipulate the surface or material composition of the planet; and fifth, the ability to move large quantities of extraterrestrial materials (for example, small asteroids, comets, or waterice shipments from the Saturn rings) to any desired locations within heliocentric space.
One frequently suggested approach to planetary engineering is the use of biological techniques and agents to manipulate alien worlds into more desirable ecospheres. For example, scientists have proposed seeding the Venusian atmosphere with special microorganisms (such as genetically engineered algae) that are capable of converting excess carbon dioxide into free oxygen and combined carbon. This biological technique would not only provide a potentially more breathable Venusian atmosphere but it would also help to lower the currently intolerable surface temperatures by reducing the runaway greenhouse effect.
Other individuals have suggested the use of special vegetation (such as genetically engineered lichen, small plants, or scrubs) to help modify the polar regions on Mars. The use of specially engineered, survivable plants would reduce the albedo of these frigid regions by darkening the surface, thereby allowing more incident sunlight to be captured. In time, an increased amount of solar energy absorption would elevate global temperatures and cause melting of the long frozen volatiles, including water. This would raise the atmospheric pressure on Mars and possibly cause a greenhouse effect. With the polar caps melted, large quantities of liquid water would be available for transport to other regions of the planet. Perhaps one of the more interesting Martian projects late this century will be to construct a series of large irrigation canals.
Of course, there are other alternatives to help melt the Martian polar caps. The Martian settlers could decide to construct giant mirrors in orbit above the Red Planet. These mirrors would be used to concentrate and focus raw sunlight directly on the polar regions. Other scientists have suggested dismantling one of the Martian moons (Phobos or Deimos) or perhaps a small dark asteroid and then using its dust to darken the polar regions physically. This action would again lower the albedo and increase the absorption of incident sunlight.
Another approach to terraforming Mars is to use nonbiological replicating systems—that is, robots that can perform work as well as function as self-replicating systems. (The role and function of the self-replicating system in spreading life around in the solar system and beyond is discussed in a later section of this chapter.) These self-replicating machines probably will be able to survive more hostile environmental conditions than will genetically engineered microorganisms or plants.
To examine the scope and magnitude of this type of planetary engineering effort, we first assume that the Martian crust is mainly silicone dioxide (SiO2) and then that a general purpose 100-ton, self-replicating system (SRS) "seed machine" can make a replica of itself on Mars in just one year. This SRS unit initially would make other units like itself, using native Martian raw materials. In the next phase of the planetary engineering project, these SRS units would be used to reduce SiO2 into oxygen that is then released into the Martian atmosphere. In just 36 years from the arrival of the "seed machine," a silicon dioxide reduction capability would be available that could release up to 220,000 tons per second of pure oxygen into the thin atmosphere of the Red Planet. In only 60 years of operation, this array of SRS units would have produced and liberated 8.8 x 1017 pounds (4 x 1017 kg) of oxygen into the Martian environment. Assuming negligible leakage through the Martian exosphere, this is enough "free" oxygen to create a 0.1-bar (10-kPa) pressure, breathable atmosphere across the entire plant. This pressure level is roughly equivalent to the terrestrial atmosphere at an altitude of 9,840 feet (3,000 m).
What would be the environmental impact of all these mining operations on Mars? Scientists estimate that the total amount of material that must be excavated to terraform Mars is on the order of 2.2 x 1018 pounds (1 x 1018 kg) of silicon dioxide. This is enough soil to fill a surface depression 0.6 mile (1 km) deep and about 370 miles (600 km) in diameter. This is approximately the size of the crater Edom near the Martian equator. The future Martians might easily rationalize: Just one small hole for Mars, but a new ecosphere for all of us transplanted humans!
Asteroids can also play an interesting role in planetary engineering scenarios. People have suggested crashing one or two "small" asteroids into depressed areas on Mars (such as the Hellas Basin) to deepen and enlarge the depression instantly. The goal would be individual or multiple (connected) instant depressions about 6 miles (10 km) deep and 62 miles (l00 km) across. These human-caused impact craters would be deep enough to trap a denser atmosphere—allowing a small ecological enclave or niche to develop. Environmental conditions in such enclaves could range from typical polar conditions to perhaps something almost balmy.
Other would-be planetary engineers have suggested crashing asteroids into Venus to help increase its spin. If the asteroid hits Venus's surface at just the right angle and speed, it conceivably could help speed up the planet's rotation rate—greatly assisting any overall planetary engineering project. Unfortunately, if the asteroid is too small or too slow, it will have little or no effect; if it is too large or hits too fast, it could possibly shatter the planet.
It has also been proposed that several large-yield nuclear devices be used to disintegrate one or more small asteroids that had previously been maneuvered into orbits around Venus. This would create a giant dust and debris cloud that would encircle the planet and reduce the amount of incoming sunlight. This, in turn, would lower surface temperatures on the planet and allow the rocks to cool sufficiently to start to absorb carbon dioxide from the dense atmosphere of Venus.
Finally, others who are for large-scale planetary engineering projects have suggested mining the rings of Saturn for frozen volatiles, especially water-ice, and then transporting these large chunks of ice back into the inner solar system for use on Mars, the Moon, or Venus.
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