Terraforming

Terraforming

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Terraforming of a planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

, moon
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

, or other body is the hypothetical
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...

 process of deliberately modifying its atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

, temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

, surface topography
Topography
Topography is the study of Earth's surface shape and features or those ofplanets, moons, and asteroids...

 or ecology
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...

 to be similar to those of Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, in order to make it habitable
Planetary habitability
Planetary habitability is the measure of a planet's or a natural satellite's potential to sustain life. Life may develop directly on a planet or satellite or be transferred to it from another body, a theoretical process known as panspermia...

 by terrestrial organisms.

The term is sometimes used more generally as a synonym for planetary engineering
Planetary engineering
Planetary engineering is the application of technology for the purpose of influencing the global properties of a planet. The goal of this theoretical task is usually to make other worlds habitable for life....

, although some consider this more general usage an error. The concept of terraforming developed from both science fiction
Science fiction
Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...

 and actual science
Science
Science is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe...

. The term was coined by Jack Williamson
Jack Williamson
John Stewart Williamson , who wrote as Jack Williamson was a U.S. writer often referred to as the "Dean of Science Fiction" following the death in 1988 of Robert A...

 in a science-fiction story ("Collision Orbit") published during 1942 in Astounding Science Fiction, but the concept may pre-date this work
Terraforming in popular culture
Terraforming is well-represented in popular culture, usually in the form of science fiction. Author Jack Williamson is credited with inventing and popularizing the term "terraform". In July 1942, under the pseudonym Will Stewart, Williamson published a science fiction novella entitled "Collision...

.

Based on experiences with Earth, the environment of a planet can be altered deliberately: however the feasibility of creating an unconstrained planetary biosphere that mimics Earth on another planet has yet to be verified. Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

 is considered by many to be the most likely candidate for terraforming. Much study has been done concerning the possibility of heating the planet and altering its atmosphere, and NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 has even hosted debates on the subject. Several potential methods of altering the climate of Mars
Terraforming of Mars
The terraforming of Mars is the hypothetical process by which the climate, surface, and known properties of Mars would be deliberately changed with the goal of making it habitable by humans and other terrestrial life, thus providing the possibility of safe and sustainable colonization of large...

 may fall within humanity's technological capabilities, but at present the economic resources required to do so are far beyond that which any government or society is willing to allocate to the purpose. The long timescales and practicality of terraforming are the subject of debate. Other unanswered questions relate to the ethics
Ethics of terraforming
The ethics of terraforming has constituted a philosophical debate within biology, ecology, and environmental ethics as to whether terraforming other worlds is an ethical endeavor.-Support:...

, logistics
Logistics
Logistics is the management of the flow of goods between the point of origin and the point of destination in order to meet the requirements of customers or corporations. Logistics involves the integration of information, transportation, inventory, warehousing, material handling, and packaging, and...

, economics
Economics
Economics is the social science that analyzes the production, distribution, and consumption of goods and services. The term economics comes from the Ancient Greek from + , hence "rules of the house"...

, politics
Politics
Politics is a process by which groups of people make collective decisions. The term is generally applied to the art or science of running governmental or state affairs, including behavior within civil governments, but also applies to institutions, fields, and special interest groups such as the...

, and methodology
Methodology
Methodology is generally a guideline for solving a problem, with specificcomponents such as phases, tasks, methods, techniques and tools . It can be defined also as follows:...

 of altering the environment of an extraterrestrial world.

History of scholarly study


Carl Sagan
Carl Sagan
Carl Edward Sagan was an American astronomer, astrophysicist, cosmologist, author, science popularizer and science communicator in astronomy and natural sciences. He published more than 600 scientific papers and articles and was author, co-author or editor of more than 20 books...

, an astronomer, proposed the planetary engineering of Venus in an article published in the journal Science
Science (journal)
Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....

in 1961. Sagan imagined seeding the atmosphere of Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

 with algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

, which would convert water, nitrogen and carbon dioxide into organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

s. As this process removed carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 from the atmosphere, the greenhouse effect
Greenhouse effect
The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere...

 would be reduced until surface temperatures dropped to "comfortable" levels. The resulting carbon, Sagan supposed, would be incinerated by the high surface temperatures of Venus, and thus be sequestered in the form of "graphite or some involatile form of carbon" on the planet's surface. However, later discoveries about the conditions on Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

 made this particular approach impossible. One problem is that the clouds of Venus are composed of a highly concentrated sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

 solution. Even if atmospheric algae could thrive in the hostile environment of Venus' upper atmosphere, an even more insurmountable problem is that its atmosphere is simply far too thick—the high atmospheric pressure would result in an "atmosphere of nearly pure molecular oxygen" and cause the planet's surface to be thickly covered in fine graphite powder. This volatile combination could not be sustained through time. Any carbon that was fixed in organic form would be liberated as carbon dioxide again through combustion, "short-circuiting" the terraforming process.

Sagan also visualized making Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

 habitable for human life in "Planetary Engineering on Mars" (1973), an article published in the journal Icarus
Icarus (journal)
Icarus is a premier scientific journal dedicated to the field of planetary science. It is published under the auspices of the American Astronomical Society's Division for Planetary Sciences . The longtime publisher was Academic Press, which is now part of Elsevier...

. Three years later, NASA addressed the issue of planetary engineering officially in a study, but used the term "planetary ecosynthesis" instead. The study concluded that it was possible for Mars to support life and be made into a habitable planet
Planetary habitability
Planetary habitability is the measure of a planet's or a natural satellite's potential to sustain life. Life may develop directly on a planet or satellite or be transferred to it from another body, a theoretical process known as panspermia...

. The first conference session on terraforming, then referred to as "Planetary Modeling", was organized that same year.

In March 1979, NASA engineer and author James Oberg
James Oberg
James Edward Oberg is an American space journalist and historian, regarded as an expert on the Russian space program.-Biography:...

 organized the First Terraforming Colloquium, a special session at the Lunar and Planetary Science Conference in Houston. Oberg popularized the terraforming concepts discussed at the colloquium to the general public in his book New Earths (1981). Not until 1982 was the word terraforming 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
Journal of the British Interplanetary Society
The Journal of the British Interplanetary Society is a technical scientific journal, first published in 1934. JBIS is concerned with space science and space technology...

. 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
James Lovelock
James Lovelock, CH, CBE, FRS is an independent scientist, environmentalist and futurologist who lives in Devon, England. He is best known for proposing the Gaia hypothesis, which postulates that the biosphere is a self-regulating entity with the capacity to keep our planet healthy by controlling...

 and Michael Allaby published The Greening of Mars. Lovelock's book was one of the first to describe a novel method of warming Mars, where chlorofluorocarbon
Chlorofluorocarbon
A chlorofluorocarbon is an organic compound that contains carbon, chlorine, and fluorine, produced as a volatile derivative of methane and ethane. A common subclass are the hydrochlorofluorocarbons , which contain hydrogen, as well. They are also commonly known by the DuPont trade name Freon...

s (CFCs) are added to the atmosphere. Motivated by Lovelock's book, biophysicist Robert Haynes
Robert Haynes
Robert Hall Haynes, OC, FRSC was a Canadian geneticist and biophysicist. He was the Distinguished Research Professor in the Department of Biology at York University...

 worked behind the scenes to promote terraforming, and contributed the word ecopoiesis
Ecopoiesis
Ecopoiesis is a neologism created by Robert Haynes. The word was formed from the Greek, οικος, house, and ποιησις, production. Ecopoiesis refers to the origin of an ecosystem. In the context of space exploration, Haynes describes ecopoiesis as the "fabrication of a sustainable ecosystem on a...

 to its lexicon.

Beginning in 1985, Martyn J. Fogg
Martyn J. Fogg
-Biography:After becoming a dental surgeon, Fogg earned a degree in physics and geology and a master's degree in astrophysics, and is working on a Ph.D. in planetary science. Fogg lives in London.-Contributions to Planetary Engineering:...

 began publishing several articles on terraforming. He also served as editor for a full issue on terraforming for the Journal of the British Interplanetary Society in 1991. In his book Terraforming: Engineering Planetary Environments (1995), Fogg proposed the following definitions for different aspects related to terraforming:
  • Planetary engineering
    Planetary engineering
    Planetary engineering is the application of technology for the purpose of influencing the global properties of a planet. The goal of this theoretical task is usually to make other worlds habitable for life....

    : the application of technology for the purpose of influencing the global properties of a planet.
  • Geoengineering
    Geoengineering
    The concept of Geoengineering refers to the deliberate large-scale engineering and manipulation of the planetary environment to combat or counteract anthropogenic changes in atmospheric chemistry The Intergovernmental Panel on Climate Change concluded in 2007 that geoengineering options, such...

    : planetary engineering applied specifically to the Earth. It includes only those macroengineering concepts that deal with the alteration of some global parameter, such as the greenhouse effect, atmospheric composition, insolation
    Insolation
    Insolation is a measure of solar radiation energy received on a given surface area in a given time. It is commonly expressed as average irradiance in watts per square meter or kilowatt-hours per square meter per day...

     or impact flux.
  • Terraforming: a process of planetary engineering, specifically directed at enhancing the capacity of an extraterrestrial planetary environment to support life as we know it. The ultimate achievement in terraforming would be to create an open planetary biosphere emulating all the functions of the biosphere of the Earth, one that would be fully habitable for human beings.
  • Astrophysical engineering: taken to represent proposed activities, relating to future habitation, that are envisaged to occur on a scale greater than that of "conventional" planetary engineering.


Fogg also devised definitions for candidate planets of varying degrees of human compatibility:
  • Habitable Planet (HP): A world with an environment sufficiently similar to the Earth as to allow comfortable and free human habitation.
  • Biocompatible Planet (BP): A planet possessing the necessary physical parameters for life to flourish on its surface. If initially lifeless, then such a world could host a biosphere of considerable complexity without the need for terraforming.
  • Easily Terraformable Planet (ETP): A planet that might be rendered biocompatible, or possibly habitable, and maintained so by modest planetary engineering techniques and with the limited resources of a starship or robot precursor mission.


Fogg suggests that Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

 was a biologically compatible planet in its youth, but is not now in any of these three categories, since it could only be terraformed with greater difficulty. Mars Society
Mars Society
The Mars Society is an international space advocacy non-profit organization dedicated to promoting the human exploration and settlement of the planet Mars. It was founded by Robert Zubrin and others in 1998 and attracted the support of notable science fiction writers and filmmakers, including Kim...

 founder Robert Zubrin
Robert Zubrin
Robert Zubrin is an American aerospace engineer and author, best known for his advocacy of the manned exploration of Mars. He was the driving force behind Mars Direct—a proposal intended to produce significant reductions in the cost and complexity of such a mission...

 produced a plan for a Mars return mission called Mars Direct
Mars Direct
Mars Direct is a proposal for a manned mission to Mars. Proponents of the scheme have claimed it to be both cost-effective and that it can be conducted with current technology. It was originally detailed in a research paper by NASA engineers Robert Zubrin and David Baker in 1990, and later expanded...

 that would set up a permanent human presence on Mars and steer efforts towards eventual terraformation.

Requirements for sustaining terrestrial life



An absolute requirement for life is an energy source, but the notion of planetary habitability implies that many other geophysical, geochemical, and astrophysical criteria must be met before the surface of an astronomical body is able to support life. Of particular interest is the set of factors that has sustained complex, multicellular animals in addition to simpler organisms on this planet. Research and theory in this regard is a component of planetary science and the emerging discipline of astrobiology.

In its astrobiology roadmap, NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 has defined the principal habitability criteria as "extended regions of liquid water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

, conditions favorable for the assembly of complex organic molecules, and energy sources to sustain metabolism."

Preliminary stages of terraforming



Once conditions become more suitable for life
Life
Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased , or else because they lack such functions and are classified as inanimate...

, the importation of microbial life could begin. As conditions approach that of Earth, plant life could also be brought in. This would accelerate the production of oxygen, which theoretically would make the planet eventually able to support animal and human life.

Prospective planets



Mars


In many respects, Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

 is the most earthlike of all the other planets in our Solar system. Indeed, it is thought that Mars once did have a more Earth-like 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 three mechanisms in particular seem likely: First, whenever surface water is present, carbon dioxide reacts with rocks to form carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....

s, thus drawing atmosphere off and binding it to the planetary surface. On Earth, this process is counteracted when plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...

 works to cause volcanic eruptions that vent carbon dioxide back to the atmosphere. On Mars, the lack of such tectonic activity worked to prevent the recycling of gases locked up in sediments.

Second, the lack of a magnetosphere
Magnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...

 surrounding the entire surface of Mars may have allowed the solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...

 to gradually erode the atmosphere. Convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

 within the core of Mars, which is made mostly of iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

, originally generated a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

. However the dynamo ceased to function long ago, and the magnetic field of Mars has largely disappeared, probably due to "... loss of core heat, solidification of most of the core, and/or changes in the mantle convection regime." Mars does still retain a limited magnetosphere
Magnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...

 that covers approximately 40% of its surface. Rather than uniformly covering and protecting the atmosphere from solar wind, however, the magnetic field takes the form of a collection of smaller, umbrella-shaped fields, mainly clustered together around the planet's southern hemisphere. It is within these regions that chunks of atmosphere are violently "blown away", as astronomer David Brain explains:
The joined fields wrapped themselves around a packet of gas at the top of the Martian atmosphere, forming a magnetic capsule a thousand kilometres wide with ionised air trapped inside... Solar wind pressure caused the capsule to 'pinch off' and it blew away, taking its cargo of air with it.


Finally, between approximately 4.1 and 3.8 billion years ago, asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

 impacts during the Late Heavy Bombardment
Late Heavy Bombardment
The Late Heavy Bombardment is a period of time approximately 4.1 to 3.8 billion years ago during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well...

 caused significant changes to the surface environment of objects in our Solar system. The low gravity of Mars suggests that these impacts could have ejected much of the Martian atmosphere into deep space.

Terraforming Mars would entail two major interlaced changes: building the atmosphere and heating it. A thicker atmosphere of greenhouse gases such as carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 would trap incoming solar radiation. Because the raised temperature would add greenhouse gases to the atmosphere, the two processes would augment each other.


Venus


Terraforming Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

 requires two major changes; removing most of the planet's dense 9 MPa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

 carbon dioxide atmosphere and reducing the planet's 450 °C (723.15 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

) surface temperature. These goals are closely interrelated, since Venus' extreme temperature is thought to be due to the greenhouse effect
Greenhouse effect
The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere...

 caused by its dense atmosphere. Sequestering the atmospheric carbon would likely solve the temperature problem as well.

Europa (moon)


Europa
Europa (moon)
Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and...

, a moon of Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

, is a potential candidate for terraforming. One advantage to Europa is the presence of liquid water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 which could be extremely helpful for the introduction of any form of life. The difficulties are numerous; Europa is near a huge radiation belt around Jupiter. This would require the building of radiation deflectors, which is currently impractical. Additionally, this satellite is covered in ice and would have to be heated, and there would need to be a supply of oxygen, though this could, at sufficient energy cost, be manufactured locally by electrolysis
Electrolysis
In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...

 of the copious water available.

Other planets and solar system entities


Other possible candidates for terraforming (possibly only partial or paraterraforming) include Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....

, Callisto
Callisto (moon)
Callisto named after the Greek mythological figure of Callisto) is a moon of the planet Jupiter. It was discovered in 1610 by Galileo Galilei. It is the third-largest moon in the Solar System and the second largest in the Jovian system, after Ganymede. Callisto has about 99% the diameter of the...

, Ganymede
Ganymede (moon)
Ganymede is a satellite of Jupiter and the largest moon in the Solar System. It is the seventh moon and third Galilean satellite outward from Jupiter. Completing an orbit in roughly seven days, Ganymede participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively...

, the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

, and even Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...

, Saturn's moon Enceladus
Enceladus (moon)
Enceladus is the sixth-largest of the moons of Saturn. It was discovered in 1789 by William Herschel. Until the two Voyager spacecraft passed near it in the early 1980s very little was known about this small moon besides the identification of water ice on its surface...

 and the dwarf planet Ceres. Most, however, have too little mass and gravity to hold an atmosphere indefinitely (although it is possible, but not certain, that an atmosphere could remain for tens of thousands of years or be replenished as needed). In addition, aside from the Moon and Mercury, most of these worlds are so far from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 that adding sufficient heat would be much more difficult than even Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

 would be. Terraforming Mercury would present different challenges, but in certain aspects would be easier than terraforming Venus. Though not widely discussed, the possibility of terraforming Mercury's poles has been presented. Saturn's Titan offers several unique advantages, such as an atmospheric pressure similar to Earth and an abundance of nitrogen and frozen water. Jupiter's moons Europa, Ganymede, and Callisto also have an abundance of water ice.

Paraterraforming


Also known as the "worldhouse" concept, or domes in smaller versions, 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. The Biosphere 2
Biosphere 2
Biosphere 2 is a structure originally built to be an artificial, materially-closed ecological system in Oracle, Arizona by Space Biosphere Ventures, a joint venture whose principal officers were John P. Allen, inventor and Executive Director, and Margret Augustine, CEO...

 project built a dome on Earth that contained a habitable environment. The project encountered difficulties in construction and operation.

Paraterraforming has several advantages over the traditional approach to terraforming. For example, it provides an immediate payback to investors (assuming a capitalistic financing model); the worldhouse starts out small in area (a domed city
Domed city
A domed city is a kind of theoretical or fictional structure that encloses a large urban area under a single roof. In most descriptions, the dome is airtight and pressurized, creating a habitat that can be controlled for air temperature and quality...

 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 to provide Earth-like 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 asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s) 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 massive amounts of construction and maintenance activity. It also would not likely have a completely independent water cycle, as rainfall may be able to develop with a high enough roof, but still probably not efficiently enough for agriculture or a water cycle. The extra cost might be off-set somewhat by automated manufacturing and repair mechanisms. A worldhouse might also be more susceptible to catastrophic failure if a major breach occurred, though this risk might be reduced by compartmentalization and other active safety precautions. Meteor
METEOR
METEOR is a metric for the evaluation of machine translation output. The metric is based on the harmonic mean of unigram precision and recall, with recall weighted higher than precision...

 strikes are a particular concern because without any external atmosphere they would reach the surface before burning up.

Ethical issues


There is a philosophical debate within biology
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...

 and ecology
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...

 as to whether terraforming other worlds is an ethical
Ethics
Ethics, also known as moral philosophy, is a branch of philosophy that addresses questions about morality—that is, concepts such as good and evil, right and wrong, virtue and vice, justice and crime, etc.Major branches of ethics include:...

 endeavor. From the point of view of a cosmocentric ethic
Cosmocentric ethic
A cosmocentric ethic maintains that the natural state of our universe should be preserved, without human attempts to terraform planets for human habitation...

, this involves balancing the need for the preservation of human life against the intrinsic value of existing planetary ecologies.

On the pro-terraforming side of the argument, there are those like Robert Zubrin
Robert Zubrin
Robert Zubrin is an American aerospace engineer and author, best known for his advocacy of the manned exploration of Mars. He was the driving force behind Mars Direct—a proposal intended to produce significant reductions in the cost and complexity of such a mission...

, Martyn J. Fogg
Martyn J. Fogg
-Biography:After becoming a dental surgeon, Fogg earned a degree in physics and geology and a master's degree in astrophysics, and is working on a Ph.D. in planetary science. Fogg lives in London.-Contributions to Planetary Engineering:...

, Richard L. S. Taylor and the late Carl Sagan
Carl Sagan
Carl Edward Sagan was an American astronomer, astrophysicist, cosmologist, author, science popularizer and science communicator in astronomy and natural sciences. He published more than 600 scientific papers and articles and was author, co-author or editor of more than 20 books...

 who believe that it is humanity's moral obligation to make other worlds suitable for life
Life
Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased , or else because they lack such functions and are classified as inanimate...

, 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 terrestrial life to become extinct
Extinction
In biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...

. Terraforming totally barren planets, it is asserted, is not morally wrong as it does not affect any other life.

Others believe terraforming would be an unethical interference in nature
Nature
Nature, in the broadest sense, is equivalent to the natural world, physical world, or material world. "Nature" refers to the phenomena of the physical world, and also to life in general...

, and that given humanity's past treatment of the Earth, other planets may be better off without human interference. Still others strike a middle ground, such as Christopher McKay
Christopher McKay
Christopher P. McKay is a planetary scientist at NASA Ames Research Center, studying planetary atmospheres, astrobiology, and terraforming. McKay received his PhD in astrogeophysics from the University of Colorado in 1982 and his bachelor's degree from Florida Atlantic University.-Overview:McKay...

, who argues that terraforming is ethically sound only once we have completely assured that an alien planet does not harbor life of its own; but that if it does, while we should not try to reshape the planet to our own use, we should engineer the planet's environment to artificially nurture the alien life
Extraterrestrial life
Extraterrestrial life is defined as life that does not originate from Earth...

 and help it thrive and co-evolve, or even co-exist with humans. Even this would be seen as a type of terraforming to the strictest of ecocentrists, who would say that all life has the right, in its home biosphere, to evolve at its own pace as well as its own direction, free of any outside interference, apparently even when it lacks volition of its own, for evolution is not a personally directed process in non-sapient species.

Economic issues


The initial cost of such projects as planetary terraforming would be gargantuan, and the infrastructure of such an enterprise would have to be built from scratch. Such technology
Technology
Technology is the making, usage, and knowledge of tools, machines, techniques, crafts, systems or methods of organization in order to solve a problem or perform a specific function. It can also refer to the collection of such tools, machinery, and procedures. The word technology comes ;...

 is not yet developed, let alone financially feasible at the moment. John Hickman has pointed out that almost none of the current schemes for terraforming incorporate economic strategies
Economy
An economy consists of the economic system of a country or other area; the labor, capital and land resources; and the manufacturing, trade, distribution, and consumption of goods and services of that area...

, and most of their models and expectations seem highly optimistic. Access to the vast resources of space may make such projects more economically feasible, though the initial investment required to enable easy access to space will likely be tremendous (see Asteroid mining
Asteroid mining
Asteroid mining refers to the possibility of exploiting raw materials from asteroids and planetoids in space, including near-Earth objects. Minerals and volatiles could be mined from an asteroid or spent comet to provide space construction material , to extract water and oxygen to sustain the lives...

, solar power satellites, In-Situ Resource Utilization
In-Situ Resource Utilization
In space exploration, in-situ resource utilization describes the proposed use of resources found or manufactured on other astronomical objects to further the goals of a space mission....

, bootstrapping
Bootstrapping
Bootstrapping or booting refers to a group of metaphors that share a common meaning: a self-sustaining process that proceeds without external help....

, space elevator
Space elevator
A space elevator, also known as a geostationary orbital tether or a beanstalk, is a proposed non-rocket spacelaunch structure...

).

Political issues



There are potential political issues arising from terraforming a planet. National pride, rivalries between nations, and the politics of public relations
Public relations
Public relations is the actions of a corporation, store, government, individual, etc., in promoting goodwill between itself and the public, the community, employees, customers, etc....

 have been a primary motivation for shaping space projects.

In popular culture



Terraforming is a common concept in science fiction
Science fiction
Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...

, ranging from television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...

, movies and novels to video games. The concept of changing a planet for habitation precedes the use of the word 'terraforming', with H. G. Wells
H. G. Wells
Herbert George Wells was an English author, now best known for his work in the science fiction genre. He was also a prolific writer in many other genres, including contemporary novels, history, politics and social commentary, even writing text books and rules for war games...

 describing a reverse-terraforming, where aliens in his story The War of the Worlds change Earth for their own benefit. Olaf Stapledon
Olaf Stapledon
William Olaf Stapledon was a British philosopher and author of several influential works of science fiction.-Life:...

's Last and First Men
Last and First Men
Last and First Men: A Story of the Near and Far Future is a "future history" science fiction novel written in 1930 by the British author Olaf Stapledon. A work of unprecedented scale in the genre, it describes the history of humanity from the present onwards across two billion years and eighteen...

(1930) provides the first example in fiction in which Venus is modified, after a long and destructive war with the original inhabitants, who naturally object to the process. The word itself was coined in fiction by Jack Williamson
Jack Williamson
John Stewart Williamson , who wrote as Jack Williamson was a U.S. writer often referred to as the "Dean of Science Fiction" following the death in 1988 of Robert A...

, but features in many other stories of the 1950s & 60s, such Poul Anderson
Poul Anderson
Poul William Anderson was an American science fiction author who began his career during one of the Golden Ages of the genre and continued to write and remain popular into the 21st century. Anderson also authored several works of fantasy, historical novels, and a prodigious number of short stories...

's The Big Rain, and James Blish
James Blish
James Benjamin Blish was an American author of fantasy and science fiction. Blish also wrote literary criticism of science fiction using the pen-name William Atheling, Jr.-Biography:...

's "Pantropy" stories. Recent works involving terraforming of Mars include the Mars trilogy
Mars trilogy
The Mars trilogy is a series of award-winning science fiction novels by Kim Stanley Robinson that chronicles the settlement and terraforming of the planet Mars through the intensely personal and detailed viewpoints of a wide variety of characters spanning almost two centuries...

 by Kim Stanley Robinson
Kim Stanley Robinson
Kim Stanley Robinson is an American science fiction writer known for his award-winning Mars trilogy. His work delves into ecological and sociological themes regularly, and many of his novels appear to be the direct result of his own scientific fascinations, such as the fifteen years of research...

 and The Platform by James Garvey.

Terraforming has also been explored on television and in feature films, including the "Genesis device", developed to quickly terraform barren planets, in the Star Trek
Star Trek
Star Trek is an American science fiction entertainment franchise created by Gene Roddenberry. The core of Star Trek is its six television series: The Original Series, The Animated Series, The Next Generation, Deep Space Nine, Voyager, and Enterprise...

movie The Wrath of Khan. A similar device exists in the animated feature film Titan A.E.
Titan A.E.
Titan A.E. is an American animated post-apocalyptic science fiction film directed by Don Bluth and Gary Goldman released in 2000. The title refers to the spacecraft that is central to the plot, with A.E. meaning "After Earth."...

which depicts the eponymous ship Titan, capable of creating a planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

. The word 'terraforming' was used in James Cameron's Aliens
Aliens (film)
Aliens is a 1986 science fiction action film directed by James Cameron and starring Sigourney Weaver, Carrie Henn, Michael Biehn, Lance Henriksen, William Hope, and Bill Paxton...

. The 2000 movie Red Planet
Red Planet (film)
Red Planet is a 2000 Technicolor science fiction film directed by Antony Hoffman, starring Val Kilmer and Carrie-Anne Moss. It was released on November 10, 2000.-Plot:...

also uses the motif: after humanity faces heavy overpopulation and pollution on Earth, uncrewed space probes loaded with algae are sent to Mars with the aim of terraforming and creating a breathable atmosphere. The television series Firefly
Firefly (TV series)
Firefly is an American space western television series created by writer and director Joss Whedon, under his Mutant Enemy Productions label. Whedon served as executive producer, along with Tim Minear....

 and its cinematic sequel Serenity
Serenity (film)
Serenity is a 2005 space western film written and directed by Joss Whedon. It is a continuation of the short-lived 2002 Fox science fiction television series Firefly, taking place after the events of the final episode. Set in 2518, Serenity is the story of the captain and crew of a cargo ship...

is set in a solar system with about seventy terraformed planets and moons. In the 2008 video game Spore
Spore (2008 video game)
Spore is a multi-genre single-player god game developed by Maxis and designed by Will Wright. The game was released for the Microsoft Windows and Macintosh operating systems in September 2008 as Spore...

, the player is able to terraform any planet by using either terraforming rays or a "Staff of Life" that completely terraforms the planet and fills it with creatures. Doctor Who
Doctor Who
Doctor Who is a British science fiction television programme produced by the BBC. The programme depicts the adventures of a time-travelling humanoid alien known as the Doctor who explores the universe in a sentient time machine called the TARDIS that flies through time and space, whose exterior...

episode "The Doctor's Daughter
The Doctor's Daughter
"The Doctor's Daughter" is the sixth episode of the fourth series of British science fiction television series Doctor Who. It was broadcast on BBC One on 10 May 2008.- Synopsis :...

" also references terraforming, where a glass orb is broken to release gases which terraform the planet the characters are on at the time.

In the video game Halo (2001), the main setting is an ancient ring-shaped structure whose radius is nearly that of Earth; the structure is terraformed to support an Earthlike ecosystem. The rings are created using Forerunner technology, and terraformed during their construction by an extra-galactic construct known as The Ark or Installation 00. Various works of fiction based on Halo also mention the terraforming of planets.

In the computer game Spore, a key concept of the Space Stage is the utilization of terraforming to make planets and moons more productive.

See also

  • Colonization of Mars
    Colonization of Mars
    The colonization of Mars by humans is the focus of speculation and serious study because the surface conditions and availability of water on Mars make it arguably the most hospitable planet in the solar system other than Earth...

  • BIOS-3
    BIOS-3
    BIOS-3 is a closed ecosystem at the Institute of Biophysics in Krasnoyarsk, Russia.Its construction began in 1965, and was completed in 1972. BIOS-3 consists of a 315-cubic-metre habitat suitable for up to three persons, and was initially used for developing closed ecosystems capable of supporting...

  • Domed city
    Domed city
    A domed city is a kind of theoretical or fictional structure that encloses a large urban area under a single roof. In most descriptions, the dome is airtight and pressurized, creating a habitat that can be controlled for air temperature and quality...

  • Extraterrestrial liquid water
    Extraterrestrial liquid water
    Extraterrestrial liquid water, the presence of water in its liquid state, is a subject of wide interest because it is a commonly suggested prerequisite for the emergence of extraterrestrial life....

  • Extraterrestrial real estate
    Extraterrestrial real estate
    Extraterrestrial real estate is land on other planets or natural satellites or parts of space that is sold either through organizations or by individuals. Ownership of extraterrestrial real estate is not recognised by any authority...

  • Floating city (science fiction)
    Floating city (science fiction)
    In science fiction, floating cities are settlements that strictly use buoyancy to remain in the atmosphere of a planet. However the term generally refers to any city that is flying, hovering, or otherwise suspended in the air via any means technological or even magical.-Earth:In Jonathan Swift's...

  • Geoengineering
    Geoengineering
    The concept of Geoengineering refers to the deliberate large-scale engineering and manipulation of the planetary environment to combat or counteract anthropogenic changes in atmospheric chemistry The Intergovernmental Panel on Climate Change concluded in 2007 that geoengineering options, such...

  • Health threat from cosmic rays
    Health threat from cosmic rays
    The health threat from cosmic rays is the danger posed by galactic cosmic rays and solar energetic particles to astronauts on interplanetary missions.Galactic cosmic rays consist of high energy protons and other nuclei with extrasolar origin...

  • Human outpost
    Human outpost
    Human outposts are artificially-created, controlled human habitats located in environments inhospitable for humans, such as on the ocean floor, in space or on another planet....

  • Human adaptation to space
    Human adaptation to space
    Human physiological adaptation to the conditions of space is a challenge faced in the development of human spaceflight.The fundamental engineering problems of escaping Earth's gravity well and developing systems for in-space propulsion have been examined for well over a century, and millions of...

  • In-situ resource utilization
    In-Situ Resource Utilization
    In space exploration, in-situ resource utilization describes the proposed use of resources found or manufactured on other astronomical objects to further the goals of a space mission....

  • Manned mission to Mars
    Manned mission to Mars
    A manned mission to Mars has been the subject of science fiction, engineering, and scientific proposals throughout the 20th century and into the 21st century...

  • Mars Direct
    Mars Direct
    Mars Direct is a proposal for a manned mission to Mars. Proponents of the scheme have claimed it to be both cost-effective and that it can be conducted with current technology. It was originally detailed in a research paper by NASA engineers Robert Zubrin and David Baker in 1990, and later expanded...

  • Mars to Stay
    Mars to Stay
    Mars to Stay missions propose astronauts sent to Mars for the first time should stay there indefinitely, both to reduce cost and to ensure permanent settlement of Mars. Among many notable Mars to Stay advocates, former Apollo astronaut Buzz Aldrin has been particularly outspoken, suggesting in...

  • Planetary habitability
    Planetary habitability
    Planetary habitability is the measure of a planet's or a natural satellite's potential to sustain life. Life may develop directly on a planet or satellite or be transferred to it from another body, a theoretical process known as panspermia...

  • Recycling
    Recycling
    Recycling is processing used materials into new products to prevent waste of potentially useful materials, reduce the consumption of fresh raw materials, reduce energy usage, reduce air pollution and water pollution by reducing the need for "conventional" waste disposal, and lower greenhouse...

  • Renewable energy
    Renewable energy
    Renewable energy is energy which comes from natural resources such as sunlight, wind, rain, tides, and geothermal heat, which are renewable . About 16% of global final energy consumption comes from renewables, with 10% coming from traditional biomass, which is mainly used for heating, and 3.4% from...

  • Solar analog
    Solar analog
    Solar-type, solar analog, and solar twin stars are those stars that are particularly similar to the Sun. The classification is a hierarchy with solar twin being most like the Sun followed by solar analog and then solar-type...

  • Space colonization
    Space colonization
    Space colonization is the concept of permanent human habitation outside of Earth. Although hypothetical at the present time, there are many proposals and speculations about the first space colony...

  • Space habitat
    Space habitat
    A space habitat is a space station intended as a permanent settlement rather than as a simple waystation or other specialized facility...

  • The Case for Mars
    The Case For Mars
    The Case for Mars: The Plan to Settle the Red Planet and Why We Must is a nonfiction science book by Robert Zubrin, first published in 1996....

  • The Millennial Project: Colonizing the Galaxy in Eight Easy Steps
    The Millennial Project: Colonizing the Galaxy in Eight Easy Steps
    The Millennial Project: Colonizing the Galaxy in Eight Easy Steps by Marshall T. Savage is a book in the field of Exploratory engineering that gives a series of concrete stages the author believes will lead to interstellar colonization...

     (Book)
  • Terraforming of Mars
    Terraforming of Mars
    The terraforming of Mars is the hypothetical process by which the climate, surface, and known properties of Mars would be deliberately changed with the goal of making it habitable by humans and other terrestrial life, thus providing the possibility of safe and sustainable colonization of large...

  • Space habitat
    Space habitat
    A space habitat is a space station intended as a permanent settlement rather than as a simple waystation or other specialized facility...

  • Space stations and habitats in popular culture
  • Space weather
    Space weather
    Space weather is the concept of changing environmental conditions in near-Earth space or thespace from the Sun's atmosphere to the Earth's atmosphere. It is distinct from the concept ofweather within the Earth's planetary atmosphere...

  • Underground city
    Underground city
    An Underground city is a series of linked subterranean spaces that may provide a defensive refuge; a place for living, working or shopping; a transit system; mausolea; wine or storage cellars; cisterns or drainage channels; or several of these. The term may also refer to a network of tunnels that...


External links