Terraforming
Terraforming (literally, "Earth-shaping") is the process of modifying a planet, moon or other body to a more habitable atmosphere, temperature or ecology.
The concepts of terraforming are rooted both in science fiction, and actual science. Since the human race has no permanent presence on an extraterrestrial world at this point in history, a good deal of terraforming remains speculative. However, based on what we know of our own world it seems possible to affect the environment in a deliberate way in order to change it. The timescales and practicality of terraforming are the subject of debate. Other unanswered questions relate to the ethics, logistics, economics, politics and methodology of altering the environment of an extraterrestrial world.
Mars is considered by many to be the most likely candidate for terraformation. Much study has gone into the possibility of heating the planet and altering its atmosphere. NASA has even hosted debates on the subject. However, a host of obstacles stands between the present and an active terraforming effort on Mars or any other world.
History
In fiction
The term first appeared in a science fiction novel, Seetee Shock (1949) by Jack Williamson, but the actual concept pre-dates this work. Olaf Stapledon's First and Last Men (1930) provides an example in fiction in which Venus is modified, after a long and destructive war with the original inhabitants, who naturally object to the process.
Early fictional accounts of the process are frequently handicapped by the inaccurate contemporary knowledge of the actual conditions, as in the Stapledon example, which had Venus covered in oceans.
A more recent example, using the actual conditions on Mars as revealed by planetary probes to that time, is the Mars trilogy by Kim Stanley Robinson. The three volumes provide a lengthy description of a fictional terraforming of Mars, and very evidently result from a massive amount of research by the author.
Terraforming has been explored in the Star Trek universe. In the Star Trek II: The Wrath of Khan, Federation scientests had developed the Genesis Device. The Genesis Device was supposed to rapidly terraform previously dead planets and make them suitable for settlement. At the end of the film, a Genesis Device was detonated in the Mutara nebula. This resulted in the creation of a main sequence star and a habitable planet. However, in Star Trek III, the process was shown to have been a failure when protomatter was used, and the planet was destroyed. Since then, Trek has further explored terraforming. Terraforming was a process that slowly transformed planets into habitable worlds.
The Star Wars Expanded Universe has explored terraforming as well. When the Yuuzhan Vong captured Coruscant, they made extensive modifications to the planet to transform it into their ideal enviornment.
In the film Aliens, the world LV-426 is the subject of a terraforming effort. The people on the planet are described as being part of a "shake-and-bake" colony. The nickname and the statement that the process "takes decades" implies that the process of making a warm, breathable atmosphere is substantially quicker than current estimates.
Scholarly study
In the 1960s the astronomer and popularizer of science Carl Sagan proposed terraforming the planet Venus by seeding its atmosphere with algae, which would remove carbon dioxide and reduce the greenhouse effect until surface temperatures dropped to "comfortable" levels. Later discoveries about the conditions on Venus made this particular approach impossible, however: Venus simply has too much atmosphere to process and sequester. Even if atmospheric algae could thrive in the hostile and arid environment of Venus' upper atmosphere, any carbon that was fixed in organic form would be liberated as carbon dioxide again as soon as it fell into the hot lower regions.
Today Mars seems the most feasible local planet for terraforming. Robert Zubrin, the founder of the Mars Society, has presented a well-elaborated plan for this. He wants to start with a Mars return mission called Mars Direct.
The principal reason given to pursue terraforming is the creation of worlds suitable for habitation by human beings and of an ecology to support them. However, some researchers believe that space habitats will provide a more economical means for supporting space colonization.
If research in nanotechnology and other advanced chemical processes continues apace, it may become feasible to terraform planets in centuries rather than millennia. On the other hand, it may become reasonable to modify humans so that they don't require an oxygen/nitrogen atmosphere in a 1g gravity field to live comfortably. That would then reduce the need to terraform worlds, or at least the degree to which other worlds' environments would need to be altered.
Ethical issues
There is debate as to whether terraforming other worlds is an ethical endeavor. On the pro-terraforming side of the argument, there are those who say that it is humanity's moral obligation to make as much of the universe suitable for human life as possible. For those opposed to terraforming, the impact of the human race on otherwise untouched worlds and the possible interference or elimination of alien life forms are reasons to leave other worlds in their natural states.
Theoretical methods of terraforming
Mars
There is some scientific debate over whether it would even be possible to terraform Mars, or how stable its climate would be once terraformed. It is possible that over geological timescales - tens or hundreds of millions of years - Mars could lose its water and atmosphere again, possibly to the same processes that reduced it to its current state.
Indeed, it is thought that Mars once did have a relatively Earthlike environment early in its history, with a thicker atmosphere and abundant water that was lost over the course of hundreds of millions of years. The exact mechanism of this loss is still unclear, though several mechanisms have been proposed. The lack of a magnetosphere surrounding Mars may have allowed the solar wind to erode the atmosphere, the relatively low gravity of Mars helping to accelerate the loss of lighter gasses to space. The lack of plate tectonics on Mars is another possibility, preventing the recycling of gasses locked up in sediments back into the atmosphere. The lack of magnetic field and geologic activity may both be a result of Mars' smaller size allowing its interior to cool more quickly than Earth's, though the details of such processes are still sketchy. However, none of these processes are likely to be significant on the timescale of human civilization, or even over the typical lifespan of most animal species, and the slow loss of atmosphere could possibly be counteracted with ongoing low-level artificial terraforming activities.
Terraforming Mars would entail two major interlaced changes: building the atmosphere and heating it. Since a thicker atmosphere of carbon dioxide and/or some other greenhouse gases would trap incoming solar radiation the two processes would build off of one another.
Adding heat
Mirrors made of extremely thin aluminized mylar could be placed in orbit around Mars to increase the total insolation it receives. This would increase the planet's temperature directly, and also vaporize water and carbon dioxide to increase the planet's greenhouse effect.
While producing halocarbons on Mars would contribute to adding mass to the atmosphere, their primary function would be to trap incoming solar radiation. Halocarbons (such as CFCs and PFCs) are powerful greenhouse gases, and are stable for lengthy periods of time in atmospheres. They could be produced by genetically engineered aerobic bacteria or by mechanical contraptions scattered across the planet's surface.
Changing the albedo of the Martian surface would also make more efficient use of incoming sunlight. Altering the color of the surface with dark dust, soot, dark microbial life forms or lichens would transfer a larger amount of incoming solar radiation to the surface as heat before it is reflected off into space again. Using life forms is particularly attractive since they could propagate themselves.
Nuclear bombardment of the crust and the polar caps has been suggested as a quick-and-dirty way of heating up the planet. If detonated on the polar regions, the intense heat would melt vast quantities of water and frozen carbon dioxide. The gases produced would thicken the atmosphere and contribute to the greenhouse effect. Additionally, the dust kicked up by a nuclear explosion would fall on the ice and increase its albedo thus allowing it to melt faster under the sun’s rays. Detonation of nuclear weapons under the surface would heat the crust and help speed outgassing of trapped carbon dioxide. While using nuclear devices is attractive in the sense that it makes use of ageing and dangerous Earth weaponry and adds quick and cheap heat to the planet, it carries the ugly connotations of mass destruction to the native environment and potential harmful effects of nuclear fallout.
Building the atmosphere
Since ammonia is a powerful greenhouse gas, and it is possible that nature has stockpiled large amounts of it in frozen form on asteroidal sized objects orbiting in the outer solar system, we could move these and send them into Mars' atmosphere. Impacting a comet onto the surface of the planet might cause destruction to the point of being counter-productive. Aerobraking, if an option, would allow a comet's frozen mass to outgas and become part of the atmosphere through which it would travel.
The need for a buffer gas is a challenge that will face any potential atmosphere builders. On Earth, nitrogen is the primary atmospheric component making up 77% of the atmosphere. Mars would require a similar buffer gas component although not necessarily as much. Still, obtaining significant quantities of nitrogen, argon or some other non-volatile gas could prove difficult.
Hydrogen importation could also be done for atmospheric and hydrospheric engineering. Depending on the level of carbon dioxide in the atmosphere, importation and reaction of hydrogen would produce heat, water and graphite via the Bosch reaction. Adding water and heat to the environment will be key to making the dry, cold world suitable for Earth life. Alternatively, reacting hydrogen with the carbon dioxide atmosphere via the Sabatier reaction would yield methane and water. The methane could be vented into the atmosphere where it would act to compound the greenhouse effect. Presumably, hydrogen could be gotten in bulk from the gas giants or refined from hydrogen-rich compounds in other outer solar system objects, though the energy required to transport large quantities would be great.
Simply thickening the Martian atmosphere will not make it habitable for Earth life unless it contains the proper mix of gases. Achieving a suitable mixture of buffer gas, oxygen, carbon dioxide, water vapor and trace gases will entail either direct processing of the atmosphere or altering it by means of plant life and other organisms. Genetic engineering would allow such organisms to process the atmosphere more efficiently and survive in the otherwise hostile environment.
Venus
Terraforming Venus requires two major changes; removing most of the planet's dense 9 MPa carbon dioxide atmosphere and reducing the planet's 737K surface temperature. These goals are closely interrelated, since Venus' extreme temperature is due to the greenhouse effect caused by its dense atmosphere.
Solar shades
Solar shades placed in the Sun-Venus L1 point or in a more closely-orbiting ring could be used to reduce the total insolation received by Venus, cooling the planet somewhat. This does not directly deal with the immense atmospheric density of Venus, but could make it easier to do so by other methods. They could also serve double duty as solar power generators.
Construction of a suitably large solar shade is a potentially daunting task. The sheer size of such a structure would necessitate construction in space. There would also be the difficulty of balancing a thin-film shade at the Sun-Venus L1 point with the incoming radiation pressure which would tend to turn the shade into a huge solar sail.
Other proposed cooling solutions involve comets, or creating artificial rings. A comet at the Sun-Venus L1 point could produce a coma which could provide at least temporary shade for the planet, possibly allowing enough time for atmospheric processing to be done. Keeping a continuously decaying comet in a stable position could prove to be a difficult feat. Rings created by putting debris in orbit would provide some shade but to a lesser extent. The inclination of the rings would also need to be such that they present a significant amount of surface area to the Sun.
Space based solar shade techniques are largely speculative due to the fact that they are beyond out technological grasp. The vast sizes require material strengths and construction methods that have not even reached their infancy.
Cooling could be sustained by placing reflectors in the atmosphere or on the surface. Reflective balloons floating in the upper atmosphere could create shade. The number and/or size of the balloons would necessarily be great. Increasing the planet's albedo by deploying light color or reflective material on the surface could help keep the atmosphere cool. The amount would be large and would have to be put in place after the temperature had already cooled somewhat. The advantage of atmospheric and surface cooling solutions is that they take advantage of existing technology.
Removing atmosphere
Removal of Venus' atmosphere could be attempted by a variety of methods, possibly in combination.
Directly lifting atmospheric gas from Venus into space would likely prove very difficult. Venus has sufficiently high escape velocity to make blasting it away with asteroid impacts impractical. Pollack and Sagan calculated in 1993 that an impactor of 700km diameter striking Venus at greater than 20km/s, would eject all the atmosphere above the horizon as seen from the point of impact, but since this is less than a thousandth of the total atmosphere and there would be diminishing returns as the atmosphere's density decreased a very great number of such giant impactors would be required. Smaller objects would not work as well, requiring even more. The violence of the bombardment could well result in significant outgassing that replaces removed atmosphere. Furthermore, most of the ejected atmosphere would go into solar orbit near Venus, eventually to fall right back onto Venus again.
Removal of atmospheric gas in a more controlled manner could also prove difficult. Venus' extremely slow rotation means that space elevators would be impossible to construct, and the very atmosphere to be removed makes mass drivers useless for removing payloads from the planet's surface. Possible workarounds include placing mass drivers on high-altitude balloons or balloon-supported towers extending above the bulk of the atmosphere, using space fountains, or rotovators. Such processes would take a great deal of technical sophistication and time, however, and may not be economically feasible without the use of extensive automation.
Converting atmosphere
Alternatively, Venus' atmosphere could be converted into some other form in situ by reacting it with externally supplied elements.
Bombardment of Venus with refined magnesium and calcium metal from Mercury or some other source, could sequester carbon dioxide in the form of calcium and magnesium carbonates.
Bombardment of Venus with hydrogen, possibly from some outer solar system source and reacting with carbon dioxide could produce elemental carbon (graphite) and water by the Bosch reaction. It would take about 4×1019kg of hydrogen to convert the whole Venutian atmosphere, and the resulting water would cover about 80% of the surface compared to 70% for Earth. The amount of water produced would amount to around 10% of the water found on Earth. A solar shade or equivalent would also be necessary, as water vapor is itself a greenhouse gas. Oceans on Venus would increase the planet’s albedo and allow more incoming solar radiation to be reflected back into space.
Other modifications
Venus' extremely slow rotation rate would result in extremely long days and nights, which could prove difficult for most Earth life to adapt to. Speeding up Venus' rotation would require many orders of magnitude greater amounts of energy than removing its atmosphere would, and so is likely to be infeasible. Instead, a system of orbiting solar mirrors might be used to provide sunlight to the night side of Venus. Alternately, instead of requiring that Venus support life identical to Earth's, Earth life could instead be modified to adapt to the long Venusian day and night.
Venus also lacks a magnetic field. It is thought that this may have contributed greatly to its current uninhabitable state, as the upper atmosphere is exposed to direct erosion by solar wind and has lost most of its original hydrogen to space. However, this process is extremely slow, and so is unlikely to be significant on the timescale of any civilization capable of terraforming the planet in the first place.
Other worlds
Other possible candidates for terraformation include Titan, Mercury, Europa, Ganymede, Io, Callisto, Earth's Moon, and even some of the larger asteroids like Ceres. However, as difficult as terraforming Mars might seem, all these bodies come with conditions that may render them unterraformable even with the most fantastic technologies.
Paraterraforming
Also known as the "worldhouse" concept, paraterraforming involves the construction of a habitable enclosure on a planet which eventually grows to encompass most of the planet's usable area. The enclosure would consist of a transparent roof held one or more kilometers above the surface, pressurized with a breathable atmosphere, and anchored with tension towers and cables at regular intervals. A worldhouse can be constructed with technology known since the 1960s.
Paraterraforming has several advantages over the traditional approach to terraforming. For example, it provides an immediate payback to investors; the worldhouse starts out small in area, but those areas provide habitable space from the start. The paraterraforming approach also allows for a modular approach that can be tailored to the needs of the planet's population, growing only as fast and only in those areas where it is required. Finally, paraterraforming greatly reduces the amount of atmosphere that one would need to add to planets like Mars in order to provide Earthlike atmospheric pressures. By using a solid envelope in this manner, even bodies which would otherwise be unable to retain an atmosphere at all (such as asteroids) could be given a habitable environment. The environment under an artificial worldhouse roof would also likely be more amenable to artificial manipulation.
It has the disadvantage of requiring a great deal of construction and maintenance activity, the cost of which could be ameliorated to some degree through the use of automated manufacturing and repair mechanisms. A worldhouse could also be more susceptible to catastrophic failure in the event of a major breach, though this risk can likely be reduced by compartmentalization and other active safety precautions.
Geoengineering
Geoengineering is the deliberate modification of Earth's environment on a large scale, in a sense "terraforming" Earth itself. Currently a great deal of debate takes place over the notion that human civilization has already inadvertently modified Earth's climate through the industrial emission of greenhouse gases, and proposals have been made to counter any such effects through further deliberate geoengineering. For example, some have proposed to put large mirrors in orbit which would modify the insolation received by Earth - either increasing or decreasing it, as the need arises. Large-scale sequestration of carbon dioxide inside geological formations or ocean sediment, modification of Earth's albedo with reflective or absorbtive materials spread over portions of its surface, or the alteration of rainfall patterns through the creation of artificial seas are other examples which have been seriously considered.