Terraforming is the process of transforming a hostile environment into one suitable for human life. Being that Mars is the most Earth-like planet, it is the best candidate for terraforming. Once just the subject of science fiction novels, it is now becoming a viable research area. The famed astronomer and Pulitzer prize winner, Carl Sagan, says that there is enormous promise in the search for ancient life on Mars. If life was once sustainable on Mars, it is important to know what caused Mars to evolve into the cold and lifeless planet it is today. With this knowledge, we can terraform Mars by reversing the process.

- NASA

Artist's conception of a terraformed Mars in three stages of development. (credit: Michael Carroll)

Terraforming (literally, "Earth-shaping") is the process of modifying a planet, moon or other body to a more habitable atmosphere, temperature or ecology. It is a type of planetary engineering. The term is sometimes used very broadly as a synonym for planetary engineering in general; see that article for related information. This article primarily focuses on the modification of atmospheric and thermal conditions.

The concepts of terraforming are rooted both in science fiction, and actual science. 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 Last and First 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.

Since space exploration is in its infancy, a good deal of terraforming remains speculative. 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, however the feasibility of creating an unconstrained planetary biosphere that mimics Earth on another planet has yet to be verified.

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 stand between the present and an active terraforming effort on Mars or any other world. The long 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.

History of scholarly study

Carl Sagan, the astronomer and popularizer of science, proposed the planetary engineering of Venus in a 1961 article published in the journal Science entitled, "The Planet Venus". Sagan imagined seeding the atmosphere of Venus 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 since Venus 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.

Sagan also visualized making Mars habitable for human life in "Planetary Engineering on Mars", a 1973 article published in the journal Icarus. Three years later, NASA officially addressed the issue of planetary engineering in a study, but used the term planetary ecosynthesis instead. The study concluded that there was no known limitiation in the ability to alter Mars to support life and be made into a habitable planet. That same year, in 1976, one of the researchers, Joel Levine, organized the first conference session on terraforming, which at the time was called "Planetary Modeling".

In March 1979, NASA engineer and author James Oberg organized the "First Terraforming Colloquium", a special session on terraforming held at the Lunar and Planetary Science Conference in Houston. Oberg popularized the terraforming concepts discussed at the colloquium to the general public in his 1981 book, New Earths. It wasn't until 1982 that the word terraforming was used in the title of a published journal article. Planetologist Christopher McKay wrote "Terraforming Mars", a paper for the Journal of the British Interplanetary Society. The paper discussed the prospects of a self-regulating Martian biosphere, and McKay's use of the word has since become the preferred term. In 1984, James Lovelock and Michael Allaby published The Greening of Mars. Lovelock's book was one of the first books to describe a novel method of warming Mars, where chlorofluorocarbons are added to the atmosphere. Motivated by Lovelock's book, biophysicist Robert Haynes worked behind the scenes to promote terraforming, and contributed the word ecopoiesis to its lexicon.

Today, Mars seems the most feasible local planet for terraforming. Mars Society founder Robert Zubrin has produced a well-designed and relatively cost-effective plan for a Mars return mission called Mars Direct that would setup a permanent human presence on Mars and steer efforts towards eventual terraformation.

The principal reason given to pursue terraforming is the creation of an ecology to support worlds suitable for habitation by humans. 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 1 g 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

Related article: Environmental ethics

There is a philosophical debate within biology and ecology as to whether terraforming other worlds is an ethical endeavor. On the pro-terraforming side of the argument, there are those like Robert Zubrin and Richard L. S. Taylor who believe that it is humanity's moral obligation to make other worlds suitable for life, as a continuation of the history of life transforming the environments around it on Earth. They also point out that Earth would eventually be destroyed if Nature takes its course, so that humanity faces a very long-term choice between terraforming other worlds or allowing all Earth life to go extinct. Dr. Zubrin further argues that even if native microbes have arisen on Mars, for example, the fact that they have not progressed beyond the microbe stage by this point, halfway through the lifetime of the Sun, is a strong indicator that they never will; and that if microbial life exists on Mars, it is likely related to Earth life through a common origin on one of the two planets, which spread to the other as an example of panspermia. Since Mars life would then not be fundamentally unrelated to Earth life, it would not be as ultimately unique, and competition with such life would not be fundamentally different than competing against microbes on Earth. Taylor exemplified this point of view with the slogan, "move over microbe".

Some critics label this argument as an example of homocentrism. These critics may view the homocentric view as not only geocentric but short-sighted, and tending to favor human interests to the detriment of ecological systems. They argue that a homocentrically driven approach could lead to the extinction of indigenous extraterrestrial life.

Ecocentrists like Christopher McKay recognize the intrinsic value of life, and seek to preserve the existence of native lifeforms. This idea is usually referred to as biocentrism. In response to these objections, moderate homocentrism (weak anthropocentrism) incorporates biocentric ethics, allowing for various degrees of terraforming. James Pollack and Carl Sagan might be described as moderate homocentrists.

On the other hand, for those opposed to terraforming, the impact of the human species on otherwise untouched worlds and the possible interference with or elimination of alien life forms are good reasons to leave these other worlds in their natural states; this is an example of a strong biocentric view, or object-centered ethic. Critics claim this is a form of anti-humanism and they assert that rocks and bacteria can not have rights, nor should the discovery of alien life prevent terraforming from occurring. Since life on Earth will ultimately be destroyed by planetary impacts or the red giant phase of the Sun, all native species will perish if not allowed to move to other objects.

The contrasts between these arguments are fully explored in the field of environmental ethics. Some researchers suggest that both paradigms need to mature into a more complex, cosmocentric ethic which incorporates the (unknown) value of extraterrestrial life with the values of humanity and all things in the universe. Debates often focus on how much time and effort should be expended on investigating the possibility of any microscopic life on a planet before deciding whether to terraform, and what level of sophistication or chances for future development alien life would deserve varying levels of commitment to non-interference. Such debates have been engaged in live, between Zubrin and McKay and others, at various conferences of the Mars Society, which has made written and video records of the debates available.

Theoretical methods of terraforming

Artist's conception of a terraformed Mars. (credit: Michael Carroll)

Artist's conception of a terraformed Mars. (credit: Mathew Crisp)

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 gases to space. The lack of plate tectonics on Mars is another possibility, preventing the recycling of gases 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 unrealised. However, none of these processes are likely to be significant over the typical lifespan of most animal species, or even on the timescale of human civilization, 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 augment 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 decrease 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.

Another possibility to heat the surface of Mars would be to place a microwave array, powered by solar cells, nuclear reactor, or a combination of the two, into geosynchronous orbit. Microwaves of approximately 2.45GHz are used in microwave ovens to cause vibrations in water molecules and produce heat. If microwaves of this frequency with sufficient amplitude were focused onto the surface of Mars it would heat the ice crystals trapped in the soil. A long enough exposure to the microwaves would release the water into the atmosphere and gradually heat the surface of the planet. Several such arrays could be placed in orbit around Mars and designed to gradually sweep the beam across vast areas.

One drastic proposal for adding some heat to Mars is to brake the inner moon, Phobos, so that it crashes into the surface. Apart from the comparatively little heat generated by this, it removes an important danger for future settlements: A thickening atmosphere would slow down Phobos so much that it would crash land within a few hundred years anyway.

Thinking far into the future, some scientists point out that the Sun will eventually grow too hot for Earth to sustain life, even before it becomes a red giant star. All main sequence stars brighten slowly throughout their lifetimes. As a result, Mars will warm up on its own, making terraforming easier.

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, it may be possible to move these and send them into Mars' atmosphere. Since ammonia is NH3 it would also take care of the problem involved in needing a buffer gas in the 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. It may be better to impact several smaller asteroids into the planet, both to build up the planet mass and to add to the atmosphere. Keeping these smaller impacts on their own will eventually build up the temperature as well as mass to both the planet and its atmosphere.

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.

Building a shield against radiation

Another significant, and probably most over-looked aspect of terraforming Mars, would be the lack of a magnetosphere. The magnetosphere deflects most of the hard particulate radiation from the solar wind. Without some form of radiation protection anyone on Mars would have prolonged exposure to an unhealthy amount of radiation every time a serious solar eruption occurred. Terraforming involves making life viable on another world, and so long as that life is going to be exposed to high levels of radiation it will not be desirable. The lack of a magnetosphere is also thought to have caused the Martian atmosphere to become as thin as it is in the first place, the solar wind adding a significant amount of heating to the atmosphere's top layers which enables the atmospheric particles to reach escape velocity and leave Mars (essentially "blowing" the atmosphere away, though that particular word would be innacurate in this case). Indeed, this effect has even been detected by Mars-orbiting probes. Venus, however, shows that the lack of a magnetosphere does not preclude an atmosphere. A thick atmosphere will also provide radiation protection for the surface, as it does at Earth's polar regions where aurorae form, so in the short term the lack of a magnetosphere would not seriously impact the habitability of a terraformed Mars.

On a longer timescale, and with the technology of the future (in perhaps 25-50 years), an artificial magnetosphere seems possible: If the energy of several large fusion-power-stations is used to power large superconducting magnets - the field should be strong enough to protect at least local settlements. However, recent scientific evidence suggest that just a thick enough atmosphere like Earths is enough to create a magnetic shielding in a absence of a magnetosphere. In the past, Earth regularily had periods where the magnetosphere changed direction and collapsed for some time. A phenomeon that didn't have any serious effects on earth. Scientists believe that in the ionosphere, a magnetic shielding was created almost instantly after the magnetosphere collapsed.[1] A principle that applies to Venus as well and would also be the case in every other planet or moon with a large enough atmosphere.

Venus

Artist's conception of a terraformed Venus. (credit: David Nash)

Terraforming Venus requires two major changes; removing most of the planet's dense 9 MPa carbon dioxide atmosphere and reducing the planet's 500 °C (770 K) 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 our current 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 atmosphere had been modified already since Venus' surface is currently completely shrouded by clouds. 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 700 km diameter striking Venus at greater than 20 km/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 4×10¹⁹ kg of hydrogen to convert the whole Venusian 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.

An ingenious method that could convert the atmosphere involves the use of genetically engineered bacteria. Such organisms would be similar to those found in hotsprings or deep ocean vents which are able to survive harsh enviornments like that on Venus. The organisms could convert the CO2 and other elements in the atmosphere in order to transform it into one that is more amenable to terran life forms. Although such a technique might be slow, it would require no technology other than that which we currently posses, and would be very economical. Indeed, most scientists speculate this itself happened on an early earth, transforming a harsh atmosphere into one that allowed more complex eukaryotic photosynthetic life forms to develop.

Cloud-top colonization

Geoffrey A. Landis proposes colonizing the cloud-tops of Venus. Initially, the image of floating cities may seem fanciful, but Landis' proposal points out that a Terran breathable air mixture (21:79 Oxygen-Nitrogen) is a lifting gas in the Venusian atmosphere. In effect, a gasbag full of human-breathable air would sustain itself and extra weight (such as a colony) in midair. At an altitude of 50 kilometers above Venusian surface, the environment is the most Earthlike in the solar system - a pressure of approximately 1 bar and temperatures in the 0-50 Celsius range. Because there is not a significant pressure differential between the inside and the outside of the breathable-air balloon, any rips or tears would not result in an explosive decompression, but rather would only diffuse at normal atmospheric mixing rates, giving time to repair any such defects.

Such colonies could be constructed at any rate desired, allowing a dynamic approach instead of needing any 'fell swoop' solutions. They could be used to gradually transform the Venusian atmosphere, with their impact directly related to the number of colonies in the atmosphere. As the constructed colonies increased, more solar panels could be used to absorb insolation and thus cool Venus; they could also be used to grow plant matter that would reduce the amount of carbon dioxide in the air. In the beginning, any impact on Venus would be insignificant, but as the number of colonies grew, they could transform Venus more and more rapidly.

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 (at least by any current technology). 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, all these bodies come with conditions that make terraforming difficult to imagine.

Most have too little mass to hold an atmosphere (although it is possible, but not certain, that an atmosphere could remain for tens of thousands of years, plenty of time on human timescales). In addition, aside from the Moon, most of these worlds are so far from the Sun that adding sufficient heat would be much more difficult than even Mars would be.

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. Proponents claim worldhouses 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 (a domed city for example), 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. Meteor strikes are a particular concern in the absence of any external atmosphere in which they would burn up before reaching the surface.

Small Worldhouses are often referred to as "Domes".

Popular culture

Main article: Terraforming in popular culture

See also

• Globus Cassus
• Colonization of the Moon
• Colonization of Mars
• Colonization of Venus
• Colonization of Mercury
• Colonization of the outer solar system
• Planetary habitability

References

• Oberg, James Edward (1981). New Earths: Restructuring Earth and Other Planets, Stackpole Books, Harrisburg, PA
• Fogg, Martyn J. (1995). Terraforming: Engineering Planetary Environments, SAE International, Warrendale, PA.
• Landis, Geoffrey A. (2003) Colonization of Venus http://powerweb.grc.nasa.gov/pvsee/publications/venus/VenusColony_STAIF03.pdf

External links

• Red Colony
• Visualizing the steps of solar system terraforming
• Possible Costs of Terraforming
• Research Paper: Technological Requirements for Terraforming Mars
• An approach to terraforming Venus