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{{short description|Hypothetical planetary engineering process}}
{{about|the hypothetical planetary engineering process||Terraform (disambiguation){{!}}Terraform|section=no}}
{{for|the video game|Syd Mead's TerraForming}}
[[File:MarsTransitionV.jpg|thumb|180px|right|An artist's conception shows a terraformed [[Mars]] in four stages of development.]]

'''Terraforming''' or '''terraformation''' ("Earth-shaping") is the [[hypothesis|hypothetical]] process of deliberately modifying the [[atmosphere]], [[temperature]], surface [[topography]] or [[ecology]] of a [[planet]], [[natural satellite|moon]], or other body to be similar to the environment of [[Earth]] to make it [[planetary habitability|habitable]] for humans to live on.

The concept of terraforming developed from both [[science fiction]] and actual [[science]]. [[Carl Sagan]], an [[astronomer]], proposed the [[planetary engineering]] of [[Venus]] in 1961, which is considered one of the first accounts of the concept.<ref name="The Planet Venus" /> The term was coined by [[Jack Williamson]] in a science-fiction short story ("[[Collision Orbit]]") published in 1942 in ''Astounding Science Fiction''.<ref>{{cite web| url=https://sfdictionary.com/view/125/terraforming| title=Historical Dictionary of Science Fiction: terraforming| access-date=2022-11-14}}</ref>

Even if the environment of a planet could be altered deliberately, the feasibility of creating an unconstrained planetary environment that mimics Earth on another planet has yet to be verified. While Venus and the [[Moon]] have been studied in relation to the subject, Mars is usually considered 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]] has even hosted debates on the subject. Several potential methods for the [[terraforming of Mars]] may be within humanity's technological capabilities, but according to Martin Beech, the economic attitude of preferring short-term profits over long-term investments will not support a terraforming project.<ref name=":0">{{cite book |last1=Beech |first1=Martin |title=Terraforming: The Creating of Habitable Worlds |date=21 April 2009 |publisher=Springer Science & Business Media |isbn=978-0-387-09796-1 |page=17 |url=https://books.google.com/books?id=dwm72BO5zoMC&pg=PA17 |language=en}} "The present economic fashion of favoring short-term gain over long term investment will never be able to support a terraforming project."</ref>

The long timescales and practicality of terraforming are also the subject of debate. As the subject has gained traction, research has expanded to other possibilities including biological terraforming, para-terraforming, and modifying humans to better suit the environments of [[planets]] and [[moons]]. Despite this, questions still remain in areas relating to the [[ethics of terraforming|ethics]], [[logistics]], [[economics]], [[politics]], and [[methodology]] of altering the environment of an extraterrestrial world, presenting issues to the implementation of the concept.

==History of scholarly study==
The astronomer [[Carl Sagan]] proposed the [[planetary engineering]] of Venus in an article published in the journal ''[[Science (journal)|Science]]'' in 1961.<ref name="The Planet Venus">{{cite journal| journal=Science| year=1961|doi=10.1126/science.133.3456.849  |title=The Planet Venus| first=Carl| last=Sagan|bibcode = 1961Sci...133..849S| volume=133| issue=3456| pages=849–58| pmid=17789744 }}</ref> Sagan imagined seeding the atmosphere of [[Venus]] with [[algae]], which would convert water, nitrogen and carbon dioxide into [[organic compound]]s. As this process removed [[carbon dioxide]] from the atmosphere, the [[greenhouse effect]] would be reduced until surface temperatures dropped to "comfortable" levels. The resulting plant matter, Sagan proposed, would be [[Pyrolysis|pyrolyzed]] by the high surface temperatures of Venus, and thus be [[Carbon sequestering|sequestered]] in the form of "graphite or some involatile form of carbon" on the planet's surface.<ref name="Sagan276">Sagan 1997, pp. 276–7.</ref> However, later discoveries about the conditions on [[Venus]] made this particular approach impossible. One problem is that the clouds of Venus are composed of a highly concentrated [[sulfuric acid]] solution. Even if atmospheric algae could thrive in the hostile environment of Venus's upper atmosphere, an even more insurmountable problem is that its atmosphere is simply far too thick: the high atmospheric pressure would result in a "atmosphere of nearly pure molecular oxygen"<ref name="Sagan276"/> at high pressure. This volatile combination could not be sustained through time. Any carbon that had been reduced by photosynthesis would be quickly oxidized in this atmosphere through combustion, "short-circuiting" the terraforming process.<ref name="Sagan276"/>

Sagan also visualized making [[Mars]] habitable for human life in an article published in the journal ''[[Icarus (journal)|Icarus]]'', "Planetary Engineering on Mars" (1973).<ref name=":3">{{cite journal |last1=Sagan |first1=Carl |title=Planetary engineering on Mars |journal=Icarus |date=December 1973 |volume=20 |issue=4 |pages=513–514 |doi=10.1016/0019-1035(73)90026-2 |bibcode=1973Icar...20..513S }}</ref> Three years later, NASA addressed the issue of planetary engineering officially in a study, but used the term "planetary ecosynthesis" instead.{{sfn|Averner|MacElroy|1976|pp=front cover, study results}} The study concluded that it was possible for Mars to support life and be made into a [[Planetary habitability|habitable planet]]. 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]] 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).<ref>{{cite book|last=Oberg| first=James Edward| year=1981| title=New Earths: Restructuring Earth and Other Planets| publisher=Stackpole Books, Harrisburg, Pennsylvania}}</ref> 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]]''.<ref>{{cite journal |last1=McKay |first1=Christopher P. |title=On Terraforming Mars |journal=Extrapolation |date=January 1982 |volume=23 |issue=4 |pages=309–314 |doi=10.3828/extr.1982.23.4.309 }}</ref> The paper discussed the prospects of a self-regulating Martian biosphere, and the word "terraforming" has since become the preferred term.{{citation needed|date=July 2021}}

In 1984, [[James Lovelock]] and Michael Allaby published ''The Greening of Mars''.<ref>{{cite book|year=1984| title=The Greening of Mars|url=https://archive.org/details/greeningofmars00mich|url-access=registration|author1=Lovelock, James  |author2=Allaby, Michael | publisher=Warner Books| isbn=9780446329675|name-list-style=amp }}</ref> Lovelock's book was one of the first to describe a novel method of warming Mars, where [[chlorofluorocarbon]]s (CFCs) are added to the atmosphere to produce a strong greenhouse effect.

Motivated by Lovelock's book, biophysicist [[Robert Haynes (geneticist)|Robert Haynes]] worked behind the scenes{{citation needed|date=December 2016}} to promote terraforming, and contributed the [[neologism]] '''''Ecopoiesis''''',<ref>Haynes, RH (1990), "Ecce Ecopoiesis: Playing God on Mars", in MacNiven, D. (1990-07-13), ''Moral Expertise: studies in practical and professional ethics,'' Routledge. pp. 161–163. {{ISBN|0-415-03576-7}}.</ref> forming the word from the Greek {{lang|grc|οἶκος}}, ''oikos'', "house",<ref>{{LSJ|oi{{=}}kos1|οἶκος|ref}}.</ref> and {{lang|grc|ποίησις}}, ''poiesis'', "production".<ref>{{LSJ|poi/hsis|ποίησις|shortref}}.</ref> 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 currently lifeless, sterile planet". Fogg defines ecopoiesis as a type of [[planetary engineering]] and is one of the first stages of terraformation. This primary stage of ecosystem creation is usually restricted to the initial seeding of microbial life.<ref name="EPE">{{cite book| last=Fogg| first=Martyn J.| year=1995| title=Terraforming: Engineering Planetary Environments| publisher=SAE International, Warrendale, PA.}}</ref> A 2019 opinion piece by Lopez, Peixoto and Rosado has reintroduced microbiology as a necessary component of any possible colonization strategy based on the principles of microbial symbiosis and their beneficial [[ecosystem service]]s.<ref>{{cite journal |last1=Lopez |first1=Jose V |last2=Peixoto |first2=Raquel S |last3=Rosado |first3=Alexandre S |title=Inevitable future: space colonization beyond Earth with microbes first |journal=FEMS Microbiology Ecology |date=22 August 2019 |volume=95 |issue=10 |doi=10.1093/femsec/fiz127 |pmid=31437273 |pmc=6748721 }}</ref> As conditions approach that of Earth, plant life could be brought in, and this will accelerate the production of oxygen, theoretically making the planet eventually able to support animal life.

===Aspects and definitions===
In 1985, [[Martyn Fogg]] started 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 1992. In his book ''Terraforming: Engineering Planetary Environments'' (1995), Fogg proposed the following definitions for different aspects related to terraforming:<ref name="EPE" />

* [[Planetary engineering]]: the application of technology for the purpose of influencing the global properties of a planet.
* [[Climate engineering|Geoengineering]]: planetary engineering applied specifically to Earth. It includes only those macro engineering concepts that deal with the alteration of some global parameter, such as the greenhouse effect, atmospheric composition, [[insolation]] 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 [[ecosystem]] emulating all the functions of the [[biosphere]] of Earth, one that would be fully habitable for human beings.

Fogg also devised definitions for candidate planets of varying degrees of human compatibility:<ref>Fogg, 1996</ref>

* Habitable planet (HP): A world with an environment sufficiently similar to Earth's 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]] was a biologically compatible planet in its youth, but is not now in any of these three categories, because it can only be terraformed with greater difficulty.<ref>{{Cite book|title=Terraforming : engineering planetary environments|last=Fogg|first=Martyn J.|date=1995|publisher=Society of Automotive Engineers|isbn=1560916095|oclc=32348444}}</ref>

== Habitability requirements ==
[[File:Habitability Diagram (2024).png|thumb|300x300px|Necessary conditions for habitability, adapted from Hoehler (2007)<ref>{{Cite journal |last=Hoehler |first=Tori M. |date=2007-12-28 |title=An Energy Balance Concept for Habitability |url=http://www.liebertpub.com/doi/10.1089/ast.2006.0095 |journal=Astrobiology |language=en |volume=7 |issue=6 |pages=824–838 |doi=10.1089/ast.2006.0095 |pmid=18163865 |bibcode=2007AsBio...7..824H |issn=1531-1074|url-access=subscription }}</ref>]]
Planetary habitability, broadly defined as the capacity for an astronomical body to sustain life, requires that various [[Geophysics|geophysical]], [[Geochemistry|geochemical]], and [[Astrophysics|astrophysical]] criteria must be met before the surface of such a body is considered habitable. Modifying a planetary surface such that it is able to sustain life, particularly for humans, is generally the end-goal of the hypothetical process of terraforming. Of particular interest in the context of terraforming is the set of factors that have sustained complex, [[Multicellular organism|multicellular]] animals in addition to simpler organisms on Earth. Research and theory in this regard is a component of [[planetary science]] and the emerging discipline of [[astrobiology]].

Classifications of the criteria of habitability can be varied, but it is generally agreed upon that the presence of water, non-extreme temperatures, and an energy source put broad constraints on habitability.<ref name=":8">{{Cite journal |last1=Lineweaver |first1=Charles H. |last2=Chopra |first2=Aditya |date=2012-05-30 |title=The Habitability of Our Earth and Other Earths: Astrophysical, Geochemical, Geophysical, and Biological Limits on Planet Habitability |url=https://www.annualreviews.org/doi/10.1146/annurev-earth-042711-105531 |journal=Annual Review of Earth and Planetary Sciences |language=en |volume=40 |issue=1 |pages=597–623 |doi=10.1146/annurev-earth-042711-105531 |bibcode=2012AREPS..40..597L |issn=0084-6597|url-access=subscription }}</ref> Other requirements for habitability have been defined as the presence of raw materials, a solvent, and clement conditions,<ref name=":10">{{Citation |last1=Hoehler |first1=Tori M. |title=Life's Requirements |date=2018 |url=http://link.springer.com/10.1007/978-3-319-30648-3_74-1 |work=Handbook of Exoplanets |pages=1–22 |editor-last=Deeg |editor-first=Hans J. |access-date=2023-03-14 |place=Cham |publisher=Springer International Publishing |language=en |doi=10.1007/978-3-319-30648-3_74-1 |isbn=978-3-319-30648-3 |last2=Som |first2=Sanjoy M. |last3=Kiang |first3=Nancy Y. |editor2-last=Belmonte |editor2-first=Juan Antonio|url-access=subscription }}</ref> or [[CHON|elemental requirements]] (such as carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur), and reasonable physiochemical conditions.<ref name=":9">{{Cite journal |last1=Cockell |first1=C.S. |last2=Bush |first2=T. |last3=Bryce |first3=C. |last4=Direito |first4=S. |last5=Fox-Powell |first5=M. |last6=Harrison |first6=J.P. |last7=Lammer |first7=H. |last8=Landenmark |first8=H. |last9=Martin-Torres |first9=J. |last10=Nicholson |first10=N. |last11=Noack |first11=L. |last12=O'Malley-James |first12=J. |last13=Payler |first13=S.J. |last14=Rushby |first14=A. |last15=Samuels |first15=T. |date=2016-01-20 |title=Habitability: A Review |url=http://www.liebertpub.com/doi/10.1089/ast.2015.1295 |journal=Astrobiology |language=en |volume=16 |issue=1 |pages=89–117 |doi=10.1089/ast.2015.1295 |pmid=26741054 |bibcode=2016AsBio..16...89C |issn=1531-1074|url-access=subscription }}</ref> When applied to organisms present on the earth, including humans, these constraints can substantially narrow.

In its astrobiology roadmap, [[NASA]] has defined the principal habitability criteria as "extended regions of liquid water, conditions favorable for the assembly of complex [[organic molecule]]s, and energy sources to sustain [[metabolism]]."<ref>{{Cite web |date=2011-01-17 |title=Astrobiology Roadmap |url=http://astrobiology.arc.nasa.gov/roadmap/g1.html |access-date=2023-03-17 |archive-url=https://web.archive.org/web/20110117011137/http://astrobiology.arc.nasa.gov/roadmap/g1.html |archive-date=2011-01-17 }}</ref>

=== Temperature ===
The general temperature range for all life on Earth is -20&nbsp;°C to 122&nbsp;°C,<ref name=":8" /> set primarily by the ability of water (possibly saline, or under high pressure in the ocean bottom) to be available in liquid form. This may constitute a bounding range for the development of life on other planets, in the context of terraforming. For Earth, the temperature is set by the equilibrium of incident solar radiation absorbed and outgoing infrared radiation, including the effect of [[greenhouse gasses]] in modifying the [[planetary equilibrium temperature]]; terraforming concepts may include modifying temperature by methods including solar reflectors to increase or decrease the amount of solar illumination, and hence modify temperature.

=== Water ===
All known life requires water;<ref name=":10" /> thus the capacity for planetary body to sustain water is a critical aspect of habitability. The [[Circumstellar habitable zone|habitable zone]] of a solar system is generally defined as the region in which stable surface liquid water may be present on a planetary body.<ref name=":10" /><ref name=":11" /> The boundaries of the habitable zone were originally defined by water loss by [[photolysis]] and hydrogen escape, setting a limit on how close a planet may be to its orbited star, and the prevalence of CO<sub>2</sub> clouds that would increase [[albedo]], setting an outer boundary on stable liquid water.<ref name=":11">{{Cite journal |last1=Kasting |first1=James F. |last2=Whitmire |first2=Daniel P. |last3=Reynolds |first3=Ray T. |date=1993-01-01 |title=Habitable Zones around Main Sequence Stars |url=https://www.sciencedirect.com/science/article/pii/S0019103583710109 |journal=Icarus |language=en |volume=101 |issue=1 |pages=108–128 |doi=10.1006/icar.1993.1010 |pmid=11536936 |bibcode=1993Icar..101..108K |issn=0019-1035|url-access=subscription }}</ref> These constraints are applicable in particular to Earth-like planets, and would not as easily apply to moons like [[Europa (moon)|Europa]] and [[Enceladus]] with ice-covered oceans, where the energy source to keep the water liquid is from [[tidal heating]], rather than solar energy.

=== Energy ===
On the most fundamental level, the only absolute requirement of life may be thermodynamic [[Non-equilibrium thermodynamics|disequilibrium]], or the presence of [[Gibbs free energy]].<ref name=":10" /> It has been argued that habitability can be conceived of as a balance between life's demand for energy and the capacity for the environment to provide such energy.<ref name=":10" /> For humans, energy comes in the form of sugars, fats, and proteins provided by consuming plants and animals, necessitating in turn that a habitable planet for humans can sustain such organisms.<ref>{{Cite web |title=Cell Energy, Cell Functions {{!}} Learn Science at Scitable |url=https://www.nature.com/scitable/topicpage/cell-energy-and-cell-functions-14024533/ |access-date=2023-04-13 |website=www.nature.com |language=en}}</ref>

Much of Earth's biomass (~60%) relies on [[photosynthesis]] for an energy source, while a further ~40% is [[Chemotroph|chemotropic]].<ref name=":8" /> For the development of life on other planetary bodies, chemical energy may have been critical,<ref name=":8" /> while for sustaining life on another planetary body in the [[Solar System]], sufficiently high solar energy may also be necessary for phototrophic organisms.

=== Elements ===
On Earth, life generally requires six elements in high abundance: [[carbon]], [[hydrogen]], [[nitrogen]], [[oxygen]], [[Phosphorus cycle|phosphorus]], and [[sulfur]].<ref name=":9" /> These elements are considered "essential" for all known life and plentiful within biological systems.<ref name=":2">{{Cite journal |last1=Falkowski |first1=Paul G. |last2=Fenchel |first2=Tom |last3=Delong |first3=Edward F. |date=2008-05-23 |title=The Microbial Engines That Drive Earth's Biogeochemical Cycles |url=https://www.science.org/doi/10.1126/science.1153213 |journal=Science |language=en |volume=320 |issue=5879 |pages=1034–1039 |doi=10.1126/science.1153213 |pmid=18497287 |bibcode=2008Sci...320.1034F |s2cid=2844984 |issn=0036-8075}}</ref> Additional elements crucial to life include the cations Mg<sup>2+</sup>, Ca<sup>2+</sup>, K<sup>+</sup> and Na<sup>+</sup> and the anion Cl<sup>−</sup>.<ref name=":2" /> Many of these elements may undergo biologically facilitated oxidation or reduction to produce usable metabolic energy.<ref name=":2"/>

==Preliminary stages==
{{Multiple issues|section=yes|{{Expand section|date=December 2022}}}}
Terraforming a planet would involve making it fit the habitability requirements listed in the previous section. For example, a planet may be too cold for liquid water to exist on its surface. Its temperature could be raised by adding greenhouse gases to the atmosphere,<ref>{{cite journal |last1=Gerstell |first1=M. F. |last2=Francisco |first2=J. S. |last3=Yung |first3=Y. L. |last4=Boxe |first4=C. |last5=Aaltonee |first5=E. T. |date=27 February 2001 |title=Keeping Mars warm with new super greenhouse gases |journal=Proceedings of the National Academy of Sciences |volume=98 |issue=5 |pages=2154–2157 |bibcode=2001PNAS...98.2154G |doi=10.1073/pnas.051511598 |pmc=30108 |pmid=11226208 |doi-access=free}}</ref> using orbiting mirrors to reflect more sunlight onto the planet,<ref>{{Citation |last=McInnes |first=Colin R. |title=Mars Climate Engineering Using Orbiting Solar Reflectors |date=2009 |url=http://link.springer.com/10.1007/978-3-642-03629-3_25 |work=Mars |pages=645–659 |editor-last=Badescu |editor-first=Viorel |access-date=2023-12-07 |place=Berlin, Heidelberg |publisher=Springer Berlin Heidelberg |language=en |doi=10.1007/978-3-642-03629-3_25 |isbn=978-3-642-03628-6|url-access=subscription }}</ref> or lowering the [[albedo]] of the planet.<ref name=":3" /> Conversely, a planet too hot for liquid water could be cooled down by removing greenhouse gases (if these are present), placing a sunshade in the [[Lagrange point|L<sub>1</sub> point]] to reduce sunlight reaching the planet, or increasing the albedo.<ref name="Landis2011"/>
Atmospheric pressure is another issue: various celestial bodies including Mars, Mercury and most moons have lower pressure than Earth. At pressures below the [[triple point of water]] (611.7 Pa), water cannot be liquid at any temperature. Human survival requires a still-higher pressure of at least 6.3 kPa, the [[Armstrong limit]]; below this pressure, exposed body fluids boil at body temperature. Furthermore, a thick atmosphere protects the surface from [[cosmic ray]]s.<ref name=":4">{{Cite web |title=Cosmic Radiation |url=https://www.epa.gov/radtown/cosmic-radiation |access-date=December 8, 2023 |website=epa.gov|date=27 November 2018 }}</ref> A thin atmosphere could be thickened using gases produced locally (e.g. the Moon could be given an atmosphere of oxygen by reducing lunar rock<ref name="landis1990" />) or gases could be imported from elsewhere.

Once conditions become more suitable for [[life]] of the [[introduced species]], the importation of [[microbial]] life could begin.<ref name="EPE" /> As conditions approach that of Earth, [[plants|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]] life.

==Prospective targets==

===Mars===
{{main|Terraforming of Mars}}

[[File:TerraformedMars.jpg|thumb|right|210px|Artist's conception of a terraformed Mars]]

In many respects, [[Mars]] is the most Earth-like planet in the Solar System.<ref>Read and Lewis 2004, p.16</ref><ref>Kargel 2004, pp. 185–6.</ref> It is thought that Mars once had 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.<ref>Kargel 2004, 99''ff''</ref>

The exact mechanism of this loss is still unclear, though three mechanisms, in particular, seem likely: First, whenever surface water is present, [[carbon dioxide]] ({{chem|C|O|2}}) reacts with rocks to form [[carbonate]]s, thus drawing atmosphere off and binding it to the planetary surface. On Earth, this process is counteracted when [[plate tectonics]] 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.<ref name="forgetmars81">Forget, Costard &amp; Lognonné 2007, pp. 80–2.</ref>

Second, the lack of a [[magnetosphere]] around Mars may have allowed the [[solar wind]] to gradually erode the atmosphere.<ref name="forgetmars81" /><ref>{{Cite web |archive-url=https://web.archive.org/web/20120427034520/http://www.cosmosmagazine.com/news/2369/solar-wind-ripping-chunks-mars |archive-date=2012-04-27 |title=Solar wind ripping chunks off Mars |work=[[Cosmos (magazine)|Cosmos]] |date=25 November 2008 |url=http://www.cosmosmagazine.com/news/2369/solar-wind-ripping-chunks-mars |access-date=2009-06-18}}</ref> [[Convection]] within the core of Mars, which is made mostly of [[iron]],<ref>{{cite news |author = Dave Jacqué |url = http://www.anl.gov/Media_Center/News/2003/030926mars.htm |title = APS X-rays reveal secrets of Mars' core | publisher = Argonne National Laboratory | date = 2003-09-26 | access-date = 2009-06-10 }}</ref> originally generated a [[magnetic field]]. However the [[dynamo theory|dynamo]] ceased to function long ago,<ref>Schubert, Turcotte &amp; Olson 2001, p. 692</ref> 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."<ref name="Carr & Bell 2007">{{cite book |doi=10.1016/B978-0-12-415845-0.00017-7 |chapter=Mars |title=Encyclopedia of the Solar System |pages=359–377 |year=2014 |last1=Carr |first1=Michael H. |last2=Bell |first2=James F. |isbn=978-0-12-415845-0 }}</ref> Results from the NASA [[MAVEN]] mission show that the atmosphere is removed primarily due to [[coronal mass ejection]] events, where outbursts of high-velocity protons from the Sun impact the atmosphere. Mars does still retain a limited [[magnetosphere]] 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.<ref name="SOLWIND">Solar Wind, 2008</ref>

Finally, between approximately 4.1 and 3.8 billion years ago, [[asteroid]] impacts during the [[Late Heavy Bombardment]] caused significant changes to the surface environment of objects in the Solar System. The low [[gravity]] of Mars suggests that these impacts could have ejected much of the Martian atmosphere into deep space.<ref name="forgetmars80">Forget, Costard &amp; Lognonné 2007, pp. 80.</ref>

Terraforming Mars would entail two major interlaced changes: building the atmosphere and heating it.<ref>Faure &amp; Mensing 2007, p. 252.</ref> A thicker atmosphere of greenhouse gases such as [[carbon dioxide]] would trap incoming [[solar radiation]]. Because the raised temperature would add greenhouse gases to the atmosphere, the two processes would augment each other.<ref>{{cite book |doi=10.2514/6.1993-2005 |chapter=Technological requirements for terraforming Mars |title=29th Joint Propulsion Conference and Exhibit |year=1993 |last1=Zubrin |first1=Robert |last2=McKay |first2=Christopher }}</ref> Carbon dioxide alone would not suffice to sustain a temperature above the freezing point of water, so a mixture of specialized greenhouse molecules might be manufactured.<ref>{{cite journal |last1=Gerstell |first1=M. F. |last2=Francisco |first2=J. S. |last3=Yung |first3=Y. L. |last4=Boxe |first4=C. |last5=Aaltonee |first5=E. T. |title=Keeping Mars warm with new super greenhouse gases |journal=Proceedings of the National Academy of Sciences |date=27 February 2001 |volume=98 |issue=5 |pages=2154–2157 |doi=10.1073/pnas.051511598 |pmid=11226208 |pmc=30108 |bibcode=2001PNAS...98.2154G |doi-access=free }}</ref>

===Venus===
{{main|Terraforming of Venus}}
[[File:Venus-real color.jpg|thumb|True-color image of Venus. To terraform, this dense atmosphere will need to be removed.|left|200x200px]]Terraforming [[Venus]] requires two major changes: removing most of the planet's dense {{cvt|9|MPa|psi atm}} carbon dioxide atmosphere, and reducing the planet's {{cvt|450|C}} surface temperature.<ref name="fogg1987">{{cite journal |bibcode=1987JBIS...40..551F |title=The terraforming of Venus |journal=Journal of the British Interplanetary Society |volume=40 |pages=551 |last1=Fogg |first1=M. J. |year=1987 }}</ref><ref name="Landis2011">{{cite book |doi=10.2514/6.2011-7215 |chapter=Terraforming Venus: A Challenging Project for Future Colonization |title=AIAA SPACE 2011 Conference & Exposition |year=2011 |last1=Landis |first1=Geoffrey |isbn=978-1-60086-953-2 }}</ref> These goals are closely interrelated because Venus's extreme temperature may result from the [[greenhouse effect]] caused by its dense atmosphere. 
[[File:TerraformedVenus.jpg|thumb|right|210px|Artist's conception of a terraformed [[Venus]]]]

Venus's atmosphere currently contains little oxygen, so an additional step would be to inject breathable O<sub>2</sub> into the atmosphere. An early proposal for such a process comes from [[Carl Sagan]], who suggested the injection of floating, [[Photosynthesis|photosynthetic]] bacteria into the Venusian atmosphere to reduce CO<sub>2</sub> to [[Organic carbon|organic form]], and increase the atmospheric concentration of O<sub>2</sub> in the atmosphere.<ref name="The Planet Venus" /> This concept, however, was based in a flawed 1960s understanding of Venus's atmosphere as much lower pressure; in reality, the Venusian atmospheric pressure (9300 kPa) is far higher than early estimates. Sagan's idea is therefore untenable, as he later conceded.<ref>{{Cite book |last=Sagan |first=Carl |url=https://www.worldcat.org/oclc/30736355 |title=Pale blue dot : a vision of the human future in space |date=1994 |isbn=0-679-76486-0 |edition=First |location=New York |oclc=30736355}}</ref>

An additional step noted by Martin Beech includes the injection of water and/or hydrogen into the planetary atmosphere;<ref name=":0" /> this step follows after sequestering CO<sub>2</sub> and reducing the mass of the atmosphere. In order to combine hydrogen with O<sub>2</sub> produced by other means, an estimated 4×10<sup>19</sup> kg of hydrogen is necessary; this may need to be mined from another source, such as Uranus or Neptune.<ref name=":0" />

=== Moon ===
{{Main|Colonization of the Moon}}
[[File:TerraformedMoonFromEarth.jpg|thumb|right|210px|Artist's conception of the [[Moon]] terraformed as seen from Earth]]
Although the gravity on Earth's [[Moon]] is too low to hold an atmosphere for geological spans of time, if given one, it would retain it for spans of time that are long compared to human lifespans.<ref>{{cite web |last1=Oberg |first1=James |title=New Earths |url=http://www.jamesoberg.com/earth.html |website=jamesoberg.com}}</ref><ref name= "landis1990" /> Landis<ref name="landis1990">{{cite magazine |last=Landis |first=Geoffrey |date=June 1990 |title=Air Pollution on the Moon |url=https://www.geoffreylandis.com/moonair.html |magazine=Analog}}</ref> and others<ref name= "benford2014">{{cite web |last=Benford |first=Greg |date=July 14, 2014 |url=https://slate.com/technology/2014/07/terraforming-the-moon-it-would-be-a-lot-like-florida.html |title=How to Terraform the Moon |work=Slate |access-date=30 January 2017}}</ref><ref>{{cite news |last=Williams |first=Matt |date=March 31, 2016 |url=http://www.universetoday.com/121140/could-we-terraform-the-moon/ |title=How Do We Terraform the Moon? |work=Universe Today |access-date=30 January 2017}}</ref> have thus proposed that it could be feasible to terraform the Moon, although not all agree with that proposal.<ref>{{cite news |last=Dorminey |first=Bruce |url=https://www.forbes.com/sites/brucedorminey/2016/07/27/why-the-moon-should-never-be-terraformed |title=Why The Moon Should Never Be Terraformed |work=Forbes |date=July 27, 2016 |access-date=30 January 2017}}</ref> Landis estimates that a 6.89 kPa atmosphere of pure oxygen on the Moon would require on the order of two hundred trillion tons of oxygen, and suggests it could be produced by reducing the oxygen from an amount of lunar rock equivalent to a cube about fifty kilometers on an edge. Alternatively, he suggests that the water content of "fifty to a hundred comets" the size of [[Halley's Comet]] would do the job, "assuming that the water doesn't splash away when the comets hit the moon."<ref name= "landis1990" /> Likewise, Benford calculates that terraforming the moon would require "about 100 comets the size of Halley's."<ref name= "benford2014" />

=== Mercury ===
{{More citations needed|section|date=March 2021}}
[[File:TerraformedMercuryGlobe.jpg|thumb|An artist's conception of the terraformed Mercury]]

[[Mercury (planet)|Mercury]] would be difficult to terraform. Beech<ref name= "Beech-Mercury">Beech, Martin. ''[https://books.google.com/books?id=dwm72BO5zoMC&dq=terraforming+mercury&pg=PA219 Terraforming: The Creating of Habitable Worlds],'' pp. 217-219. Springer. Retrieved 29 Dec. 2023.</ref> states "There seems little prospect of terraforming Mercury such that any animals or plants might exist there," and suggests that its primary use in a terraforming project would be as a mining source for minerals. Nevertheless, terraforming has been considered.<ref name="Roy"/> Mercury's magnetic field is only 1.1% that of Earth's, and, being closer to the Sun, any atmosphere would be stripped rapidly unless it can be protected from the solar wind. It is conjectured that Mercury's magnetic field should be much stronger, up to 30% of Earth's, if it weren't being suppressed by certain solar wind feedback effects.<ref>{{Cite journal|last1=Gómez-Pérez|first1=Natalia|last2=Solomon|first2=Sean C.|date=2010|title=Mercury's weak magnetic field: A result of magnetospheric feedback?|journal=Geophysical Research Letters|language=en|volume=37|issue=20|pages=n/a|doi=10.1029/2010GL044533|bibcode=2010GeoRL..3720204G|issn=1944-8007|doi-access=free}}</ref> If some means of shielding Mercury from solar wind by placing an artificial [[magnetic shielding|magnetic shield]] at Mercury-Sun [[Lagrange point#L1|L<sub>1</sub>]] (similar to the proposal for Mars), then Mercury's magnetic field could possibly grow in intensity to a point where Mercury's magnetic field could be self-sustaining provided the field wasn't made to "stall" by another solar event.{{citation needed|date=March 2021}}

Despite being much smaller than Mars, Mercury has an [[escape velocity]] only slightly less than that of Mars due to its higher density and could, if a [[magnetosphere]] prevents atmospheric stripping, hold a [[nitrogen]]/[[oxygen]] [[atmosphere]] for millions of years.

To provide one atmosphere of pressure, roughly 1.1×10<sup>18</sup> [[kilogram]]s of gas would be required;<ref name="Roy">Roy, Kenneth (2015). "Terraforming Mercury". In ''Inner Solar System: Prospective Energy and Material Resources,'' pp. 421-435. Springer International Publishing. [https://www.researchgate.net/profile/Kenneth-Roy-2/publication/285603153_Terraforming_Mercury/links/5ffc8c8292851c13fe0692b4/Terraforming-Mercury.pdf pdf] retrieved 29 Dec. 2023.</ref> or a somewhat lower amount if lower pressure is acceptable. Water could be delivered from the outer [[Solar System]]. Once this water has been delivered, it would split the water into its constituent [[oxygen]] and [[hydrogen]] molecules, possibly using a photo-catalytic dust, with the hydrogen rapidly being lost to space. At an oxygen pressure of 20-30 kPa, the atmosphere would be breathable and [[nitrogen]] may be added as required to allow for plant growth in the presence of [[nitrate]]s.

Temperature management would be required, due to the equilibrium average temperature of ~159°C. However, millions of square kilometers at the poles have an average temperature of 0-50°C (''i.e.,'' an area the size of Mexico at each pole with habitable temperatures). The total habitable area could be even larger if the planetary albedo were increased from 0.12 to ~0.6, potentially increasing the habitable area. Roy proposes that the temperature could be further managed by decreasing the solar flux at Mercury to near the terrestrial value by solar sails reflecting sunlight. He calculates that 16 to 17 million sails, each with an area of one square kilometer would be needed.<ref name= "Roy" />

===Earth===
{{main|Climate change mitigation}}
It has been recently proposed{{when|date=July 2020}} that due to the effects of [[climate change]], an interventionist program might be designed to return [[Earth]] to pre-industrial climate parameters. In order to achieve this, multiple approaches have been proposed, such as the management of [[Solar radiation management|solar radiation]], the [[carbon sequestration|sequestration of carbon]] dioxide, and the design and release of climate altering [[Genetically modified organism|genetically engineered]] organisms.<ref>{{cite journal |last1=Solé |first1=Ricard V. |last2=Montañez |first2=Raúl |last3=Duran-Nebreda |first3=Salva |title=Synthetic circuit designs for earth terraformation |journal=Biology Direct |date=18 July 2015 |volume=10 |issue=1 |pages=37 |doi=10.1186/s13062-015-0064-7 |pmid=26187273 |pmc=4506446 |bibcode=2015arXiv150305043S |arxiv=1503.05043 |doi-access=free }}</ref><ref>{{cite journal |last1=Solé |first1=Ricard V. |last2=Montañez |first2=Raúl |last3=Duran-Nebreda |first3=Salva |last4=Rodriguez-Amor |first4=Daniel |last5=Vidiella |first5=Blai |last6=Sardanyés |first6=Josep |title=Population dynamics of synthetic terraformation motifs |journal=Royal Society Open Science |date=4 July 2018 |volume=5 |issue=7 |pages=180121 |doi=10.1098/rsos.180121 |pmid=30109068 |pmc=6083676 |bibcode=2018RSOS....580121S }}</ref> These are typically referred to as geoengineering or [[climate engineering]], rather than terraforming.

===Other bodies in the Solar System===
{{see also|Colonization of the asteroid belt|Colonization of Europa (moon)|label 2=Colonization of Europa|Colonization of Callisto|Colonization of Titan|Colonization of Ceres|Colonization of trans-Neptunian objects|Colonization of the outer Solar System}}

Other possible candidates for terraforming (possibly only partial or paraterraforming) include large moons of Jupiter or Saturn ([[Europa (moon)|Europa]], [[Ganymede (moon)|Ganymede]], [[Callisto (moon)|Callisto]], [[Enceladus]], [[Titan (moon)|Titan]]), and the dwarf planet [[Ceres (dwarf planet)|Ceres]].

The moons are covered in ice, so heating them would make some of this ice sublimate into an atmosphere of water vapour, ammonia and other gases.<ref name=":02">{{Cite web |last=Williams |first=Matt |date=2016-04-22 |title=How Do We Terraform Jupiter's Moons? |url=https://www.universetoday.com/128530/how-do-we-terraform-jupiters-moons/ |access-date=2023-12-08 |website=Universe Today |language=en-US}}</ref><ref name=":5">{{Cite web |last=Williams |first=Matt |date=2016-04-27 |title=How Do We Terraform Saturn's Moons? |url=https://www.universetoday.com/128532/terraform-saturns-moons/ |access-date=2023-12-12 |website=Universe Today |language=en-US}}</ref> For Jupiter's moons, the intense radiation around Jupiter would cause [[radiolysis]] of water vapour, splitting it into hydrogen and oxygen.<ref name=":02" /> The former would be rapidly lost to space, leaving behind the oxygen (this already occurs on the moons to a minor extent, giving them thin atmospheres of oxygen).<ref name=":02" /> For Saturn's moons, the water vapour could be split by using orbital mirrors to focus sunlight, causing [[photolysis]].<ref name=":5" /> The ammonia could be converted to nitrogen by introducing bacteria such as ''[[Nitrosomonas]]'', ''[[Pseudomonas]]'' and ''[[Clostridium]]'', resulting in an Earth-like nitrogen-oxygen atmosphere.<ref name=":02" /><ref name=":5" /> This atmosphere would protect the surface from Jupiter's radiation,<ref name=":4" /> but it would also be possible to clear said radiation using orbiting tethers<ref name="mirnov1996">{{cite journal |last1=Mirnov |first1=Vladimir |last2=Üçer |first2=Defne |last3=Danilov |first3=Valentin |author-link3=Valentin Danilov |date=November 10–15, 1996 |title=High-Voltage Tethers For Enhanced Particle Scattering In Van Allen Belts |journal=APS Division of Plasma Physics Meeting Abstracts |volume=38 |pages=7 |bibcode=1996APS..DPP..7E06M |oclc=205379064 |id=Abstract #7E.06}}</ref> or radio waves.<ref>{{Cite web |title=NASA Finds Lightning Clears Safe Zone in Earth's Radiation Belt - NASA |url=https://www.nasa.gov/news-release/nasa-finds-lightning-clears-safe-zone-in-earths-radiation-belt/ |access-date=2023-12-11 |language=en-US}}</ref>

Challenges to terraforming the moons include their high amounts of ice and their low gravity.<ref name=":02" /><ref name=":5" /> If all of the ice were fully melted, it would result in deep moon-spanning oceans, meaning any settlements would have to be floating (unless some of the ice was allowed to remain, to serve as land).<ref name=":02" /><ref name=":5" /> Low gravity would cause [[atmospheric escape]] over time and may cause [[Effect of spaceflight on the human body#Weightlessness|problems for human health]]. However, atmospheric escape would take place over spans of time that are long compared to human lifespans, as with the Moon.<ref name="landis1990" />

One proposal for terraforming Ceres would involve heating it (using orbital mirrors, detonating thermonuclear devices or colliding small asteroids with Ceres), creating an atmosphere and deep ocean.<ref>{{Cite web |last=Williams |first=Matt |date=2016-05-05 |title=How Do We Terraform Ceres? |url=https://www.universetoday.com/128711/how-do-we-terraform-ceres/ |access-date=2023-12-12 |website=Universe Today |language=en-US}}</ref> However, this appears to be based on a misconception that Ceres' surface is icy in a similar way to the gas giant moons. In reality, Ceres' surface is "a layer of mixed ice, silicates and light strong phases best matched by hydrated salts and clathrates".<ref name="EPSC">{{Cite book |last1=Raymond |first1=C. |title=European Planetary Science Congress |last2=Castillo-Rogez |first2=J. C. |last3=Park |first3=R. S. |last4=Ermakov |first4=A. |last5=Bland |first5=M. T. |last6=Marchi |first6=S. |last7=Prettyman |first7=T. |last8=Ammannito |first8=E. |last9=De Sanctis |first9=M. C. |date=September 2018 |volume=12 |chapter=Dawn Data Reveal Ceres' Complex Crustal Evolution |display-authors=4 |access-date=19 July 2020 |chapter-url=https://meetingorganizer.copernicus.org/EPSC2018/EPSC2018-645-1.pdf |archive-url=https://web.archive.org/web/20200130111631/https://meetingorganizer.copernicus.org/EPSC2018/EPSC2018-645-1.pdf |archive-date=30 January 2020 |url-status=live |author10=Russell, C.T.}}</ref> It is unclear what the result of heating this up would be.

==Other possibilities==

===Biological terraforming===
{{See also|Interplanetary contamination}}

Many proposals for planetary engineering involve the use of genetically engineered bacteria.<ref>{{cite journal |last1=Hiscox |first1=Juliana A. |last2=Thomas |first2=David J. |title=Genetic modification and selection of microorganisms for growth on Mars. |journal=Journal of the British Interplanetary Society |date=October 1995 |volume=48 |issue=10 |pages=419–26 |pmid=11541203 }}</ref><ref>{{cite journal|title=Mercury|journal=The Society|year=2000|volume=29|url=https://books.google.com/books?id=WkNBAQAAIAAJ|access-date=10 January 2017|language=en}}</ref>

As [[synthetic biology]] matures over the coming decades it may become possible to build designer organisms from scratch that directly manufacture desired products efficiently.<ref>{{cite journal |last1=Menezes |first1=Amor A. |last2=Cumbers |first2=John |last3=Hogan |first3=John A. |last4=Arkin |first4=Adam P. |title=Towards synthetic biological approaches to resource utilization on space missions |journal=Journal of the Royal Society Interface |date=6 January 2015 |volume=12 |issue=102 |pages=20140715 |doi=10.1098/rsif.2014.0715 |pmid=25376875 |pmc=4277073 }}</ref> Lisa Nip, Ph.D. candidate at the [[MIT Media Lab]]'s Molecular Machines group, said that by synthetic biology, scientists could genetically engineer humans, plants and bacteria to create Earth-like conditions on another planet.<ref>{{cite web|title=Video: Humans Could Engineer Themselves for Long-Term Space Travel|date=7 April 2016|url=http://www.livescience.com/54341-synthetic-biology-for-space-exploration.html|publisher=Live Science|access-date=10 January 2017}}</ref><!--engineering bacteria--><ref>{{cite web|title=You Can Now Play God From The Comfort Of Your Garage|url=http://fusion.net/story/285454/diy-crispr-biohackers-garage-labs/|publisher=Fusion |date=29 March 2016 |first=Kristen V. |last=Brown |access-date=10 January 2017 |archive-url=https://web.archive.org/web/20160402053819/http://fusion.net/story/285454/diy-crispr-biohackers-garage-labs/ |archive-date=2016-04-02}}</ref>

Gary King, microbiologist at [[Louisiana State University]] studying the most extreme organisms on Earth, notes that "synthetic biology has given us a remarkable toolkit that can be used to manufacture new kinds of organisms specially suited for the systems we want to plan for" and outlines the prospects for terraforming, saying "we'll want to investigate our chosen microbes, find the genes that code for the survival and terraforming properties that we want (like radiation and [[drought tolerance|drought resistance]]), and then use that knowledge to genetically engineer specifically Martian-designed microbes". He sees the project's biggest bottleneck in the ability to genetically tweak and tailor the right microbes, estimating that this could take "a decade or more" to be solved<!--grammar?-->. He also notes that it would be best to develop "not a single kind of microbe but a suite of several that work together".<ref>{{cite magazine|title=Here's How We'll Terraform Mars With Microbes|url=http://www.popularmechanics.com/space/moon-mars/a15410/terraform-mars-with-microbes/|magazine=Popular Mechanics|access-date=10 January 2017|date=7 May 2015 |first=William |last=Herkewitz}}</ref>

[[DARPA]] is researching the use of photosynthesizing plants, bacteria, and algae grown directly on the Mars surface that could warm up and thicken its atmosphere. In 2015 the agency and some of its research partners created a software called [[DTA GView]], in which [[Genome|genomes]] of several organisms can be pulled up on the program to immediately show a list of known genes and where they are located in the genome. According to Alicia Jackson, deputy director of [[Biological Technologies Office (DARPA)|DARPA's Biological Technologies Office]], they have developed a "technological toolkit to transform not just hostile places here on Earth, but to go into space not just to visit, but to stay".<ref>{{cite news|title=Will tweaked microbes make Mars Earth-like? |url=http://timesofindia.indiatimes.com/home/science/Will-tweaked-microbes-make-Mars-Earth-like/articleshow/47858464.cms|newspaper=The Times of India|access-date=10 January 2017 |date=29 June 2015}}</ref><ref>{{cite web|title=DARPA: We Are Engineering the Organisms That Will Terraform Mars|url=https://www.vice.com/en/article/darpa-we-are-engineering-the-organisms-that-will-terraform-mars/ |first=Jason |last=Koebler |date=24 June 2015 |publisher=Vice Motherboard|access-date=10 January 2017|language=en-us}}</ref><ref>{{cite web|last1=Smith|first1=Chris|title=We Definitely Want to Live on Mars – Here's How We Plan to Tame the Red Planet|url=https://bgr.com/2015/06/25/mars-darpa-terraforming-project/|publisher=BGR|access-date=10 January 2017|date=25 June 2015}}</ref><ref>{{cite web|title=DARPA Wants To Use Genetically Engineered Organisms To Make Mars More Earth-Like|url=http://www.techtimes.com/articles/63927/20150627/darpa-wants-to-use-genetically-engineered-organisms-to-turn-make-mars-more-earth-like.htm|publisher=Tech Times|access-date=10 January 2017|date=27 June 2015 |first=Dianne |last=Depra}}</ref>

===Paraterraforming===
{{See also|Westland (region), Netherlands}}
Also known as the "world house" concept, para-terraforming involves the construction of a habitable enclosure on a planet that encompasses most of the planet's usable area.<ref>Taylor, 1992</ref> 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. The world house concept is similar to the concept of a [[Domed city|domed habitat]], but one which covers all (or most) of the planet.

Potential targets for paraterraforming include Mercury, the Moon, Ceres and the gas giant moons.<ref>{{Cite web |last=Williams |first=Matthew S. |date=2019-08-09 |title=Making a Greenhouse on Another World: Paraterraforming in Our Solar System |url=https://interestingengineering.com/science/making-a-greenhouse-on-another-world-where-can-we-paraterraform-in-our-solar-system |access-date=2023-12-12 |website=interestingengineering.com |language=en-US}}</ref>

===Adapting humans===
It has also been suggested that instead of or in addition to terraforming a hostile environment humans might adapt to these places by the use of [[genetic engineering]], [[biotechnology]] and [[Cyborg|cybernetic enhancements]].<ref>{{cite conference |first1=Aaron |last1=Gronstal |first2=Julio Aprea |last2=Perez |first3=Tobias |last3=Bittner |first4=Erik |last4=Clacey |first5=Angelo |last5=Grubisic |first6=Damian |last6=Rogers |year=2005 |title=Bioforming and terraforming: A balance of methods for feasible space colonization |conference=56th International Astronautical Congress |url=http://iafastro.directory/iac/archive/browse/IAC-05/A5/1/2157/ }}</ref><ref>{{cite book|last1=Lunan|first1=Duncan|title=Man and the Planets: The Resources of the Solar System|publisher=Ashgrove Press|isbn=9780906798171|url=https://books.google.com/books?id=St8JAAAAMAAJ|access-date=10 January 2017|language=en|date=January 1983}}{{page needed|date=October 2019}}</ref><ref>{{cite book|last1=Spitzmiller|first1=Ted|title=Astronautics: A Historical Perspective of Mankind's Efforts to Conquer the Cosmos|publisher=Apogee Books|isbn=9781894959667|url=https://books.google.com/books?id=kjVGAAAAYAAJ|access-date=10 January 2017|language=en|year=2007}}{{page needed|date=October 2019}}</ref><ref>{{cite news|title=Could We Marsiform Ourselves? |url=http://www.universetoday.com/132783/could-we-marsiform-ourselves/|work=Universe Today|access-date=10 January 2017|date=10 January 2017 |first=Fraser |last=Cain}}</ref><ref name=motherboard1>{{cite web|title=Be Your Own Spaceship: How We Can Adapt Human Bodies for Alien Worlds|url=https://www.vice.com/en/article/be-your-own-spaceship-how-we-can-adapt-human-bodies-for-alien-worlds/|publisher=Vice Motherboard|access-date=10 January 2017|language=en-us|url-status=live|archive-url=https://web.archive.org/web/20170113041207/https://motherboard.vice.com/blog/be-your-own-spaceship-how-we-can-adapt-human-bodies-for-alien-worlds|archive-date=13 January 2017 |date=29 July 2013 |first=Becky |last=Ferreira}}</ref> This is known as [[pantropy]].
<!--Examples of such adjustments include the making organs fit for low gravity, increasing [[lung volumes]] for atmospheres with low oxygen levels and [[Powered exoskeleton|exoskeletons]]<ref>{{cite web|title=Humanity and Space|url=https://web.wpi.edu/Pubs/E-project/Available/E-project-031015-113556/unrestricted/Humanity_and_Space_IQP.pdf|access-date=10 January 2017}}</ref><ref name=motherboard1/> for high pressure ratios.-->

==Issues==

===Ethical issues===
{{main|Ethics of terraforming}}

There is a philosophical debate within [[biology]] and [[ecology]] as to whether terraforming other worlds is an [[ethics|ethical]] endeavor. From the point of view of a [[cosmocentric ethic]], this involves balancing the need for the preservation of human life against the intrinsic value of existing planetary ecologies.<ref>MacNiven 1995</ref> [[Lucianne Walkowicz]] has even called terraforming a "planetary-scale [[strip mining]] operation".<ref name="Mandelbaum 2018">{{cite web | last=Mandelbaum | first=Ryan F. | title=Decolonizing Mars: Are We Thinking About Space Exploration All Wrong? | website=Gizmodo | date=November 20, 2018 | url=https://gizmodo.com/decolonizing-mars-are-we-thinking-about-space-explorat-1830348568 | access-date=October 31, 2021}}</ref>

On the pro-terraforming side of the argument, there are those like [[Robert Zubrin]], [[Martyn J. Fogg]], Richard L. S. Taylor, and the late [[Carl Sagan]] who believe that it is humanity's moral obligation to make other worlds suitable for human [[life]], as a continuation of the history of life-transforming the environments around it on Earth.<ref name="zoob1">Robert Zubrin, ''[[The Case for Mars|The Case for Mars: The Plan to Settle the Red Planet and Why We Must]]'', pp. 248–249, Simon & Schuster/Touchstone, 1996, {{ISBN|0-684-83550-9}}</ref><ref>Fogg 2000</ref> They also point out that [[Future of the Earth|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 [[extinction|extinct]]. Terraforming totally barren planets, it is asserted, is not morally wrong as it does not affect any other life.

The opposing argument posits that terraforming would be an unethical interference in [[nature]], and that given humanity's past treatment of Earth, other planets may be better off without human interference.{{citation needed|date=July 2021}} Still others strike a middle ground, such as [[Christopher McKay]], who argues that terraforming is ethically sound only once it is completely certain that an alien planet does not harbor life of its own; but that if it does, it should not try be reshaped to fit humans' own use, but rather to engineer its environment to artificially nurture the [[Extraterrestrial life|alien life]] and help it thrive and co-evolve, or even co-exist with humans.<ref name="ontomars">Christopher McKay and Robert Zubrin, "Do Indigenous Martian Bacteria have Precedence over Human Exploration?", pp. 177–182, in ''On to Mars: Colonizing a New World'', Apogee Books Space Series, 2002, {{ISBN|1-896522-90-4}}</ref> 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 without outside interference.

===Economic issues===
The initial cost of such projects as planetary terraforming would be massive, and the infrastructure of such an enterprise would have to be built from scratch. Such [[technology]] has not yet been developed, let alone financially feasible at the moment. John Hickman has pointed out that almost none of the current schemes for terraforming incorporate [[economy|economic strategies]], and most of their models and expectations seem highly optimistic.<ref>{{cite journal| url=http://www.jetpress.org/volume4/space.htm |title=The Political Economy of Very Large Space Projects |first=John |last=Hickman |access-date=2006-04-28 |journal=Journal of Evolution and Technology |volume=4 |pages=1–14|date=November 1999}}</ref>

==In popular culture==
{{main|Terraforming in popular culture}}
Terraforming is a common concept in [[science fiction]], ranging from [[science fiction television|television]], [[science fiction film|movies]] and [[science fiction novels|novels]] to [[video games]].<ref>{{Cite web |title=SFE: Terraforming |url=https://sf-encyclopedia.com/entry/terraforming |access-date=2022-07-28 |website=sf-encyclopedia.com}}</ref>

A related concept from science fiction is xenoforming – a process in which [[Extraterrestrial life|aliens]] change the Earth or other planets to suit their own needs, already suggested in the classic ''[[The War of the Worlds]]'' (1898) of [[H. G. Wells|H.G. Wells]].<ref>{{Cite web |title=Themes : Xenoforming : SFE : Science Fiction Encyclopedia |url=http://www.sf-encyclopedia.com/entry/xenoforming |access-date=2021-08-24 |website=www.sf-encyclopedia.com}}</ref>

==See also==
{{div col}}
* {{annotated link|Artificial planet}}
* {{annotated link|Astrobotany}}
* {{annotated link|Climate engineering|aka=Geoengineering}}
* {{annotated link|Colonization of Mars}}
* {{annotated link|Colonization of Venus}}
* {{annotated link|Desert greening}}
* {{annotated link|Effect of spaceflight on the human body}}
* {{annotated link|Extraterrestrial liquid water}}
* {{annotated link|Health threat from cosmic rays}}
* {{annotated link|In situ resource utilization}}
* {{annotated link|Pantropy}}
* {{annotated link|Planetary engineering}}
* {{annotated link|Planetary habitability}}
* {{annotated link|Space colonization}}
* {{annotated link|Terraforming of Mars}}
* {{annotated link|Terraforming of Venus}}
{{div col end}}

== Notes ==
{{reflist|30em}}

== References ==
* {{cite book |editor1-last=Averner |editor1-first=M. M. |editor2-last=MacElroy |editor2-first=R. D. |year=1976 |title=On the Habitability of Mars: An Approach to Planetary Ecosynthesis |url=https://ntrs.nasa.gov/search.jsp?R=19770005775|publisher=NASA|access-date=April 4, 2023}}
* {{cite book |last1=Beech |first1=Martin |title=Terraforming: The Creating of Habitable Worlds |date=21 April 2009 |publisher=Springer Science & Business Media |isbn=978-0-387-09796-1 |url=https://books.google.com/books?id=dwm72BO5zoMC&pg=PA17 |language=en}}
* Dalrymple, G. Brent (2004). ''Ancient Earth, ancient skies: the age of Earth and its cosmic surroundings''. [[Stanford University Press]]. {{ISBN|0-8047-4933-7}}
* Faure, Gunter & Mensing, Teresa M. (2007). ''Introduction to planetary science: the geological perspective''. Springer. {{ISBN|1-4020-5233-2}}.
* {{cite book|last=Fogg| first=Martyn J.| year=1995| title=Terraforming: Engineering Planetary Environments| publisher=SAE International, Warrendale, PA|isbn=1-56091-609-5}}
* {{cite book
|title=Islands in the Sky
|chapter=A Planet Dweller's Dream
|last=Fogg
|first=Martyn J.
|editor1-last=Schmidt
|editor1-first=Stanley
|editor2-last=Zubrin
|editor2-first=Robert
|publisher=Wiley
|place=New York
|year=1996
|pages=143–67
}}
* {{cite journal |last=Fogg |first=Martyn J. |author-link=Martyn J. Fogg |title=Terraforming Mars: A Review of Current Research | doi=10.1016/S0273-1177(98)00166-5  |journal=Advances in Space Research |volume=2 |issue=3 |pages=415–420 |url=http://www.users.globalnet.co.uk/~mfogg/fogg1998.pdf |publisher=[[Committee on Space Research]] |year=1998 |bibcode = 1998AdSpR..22..415F }}
* Fogg, Martyn J. (2000). [http://www.users.globalnet.co.uk/~mfogg/EthicsDTP.pdf The Ethical Dimensions of Space Settlement] (PDF format). ''Space Policy'', 16, 205–211. Also presented (1999) at the 50th [[International Astronautical Congress]], [[Amsterdam]] (IAA-99-IAA.7.1.07).
* Forget, François; Costard, François & Lognonné, Philippe (2007). ''Planet Mars: Story of Another World''. Springer. {{ISBN|0-387-48925-8}}.
* Kargel, Jeffrey Stuart (2004). ''Mars: a warmer, wetter planet''. Springer. {{ISBN|1-85233-568-8}}.
* {{cite book
|title=The cyanobacteria: molecular biology, genomics, and evolution
|chapter=Cyanobacteria and earth history
|last=Knoll
|first=Andrew H.
|editor1-last=Herrero
|editor1-first=Antonia
|editor2-first=Enrique
|editor2-last=Flores
|publisher=Horizon Scientific Press
|year=2008
|pages=1–20
|isbn=978-1-904455-15-8}}
* {{cite journal | last1 = MacNiven | first1 = D. | year = 1995 | title = Environmental Ethics and Planetary Engineering | journal = [[Journal of the British Interplanetary Society]] | volume = 48 | pages = 441–44 }}
* McKay Christopher P. & Haynes, Robert H. (1997). "Implanting Life on Mars as a Long Term Goal for Mars Exploration", in ''The Case for Mars IV: Considerations for Sending Humans'', ed. Thomas R. Meyer (San Diego, California: American Astronautical Society/Univelt), Pp.&nbsp;209–15.
* Read, Peter L.; Lewis, Stephen R. (2004). ''The Martian climate revisited: atmosphere and environment of a desert planet''. Springer. {{ISBN|3-540-40743-X}}.
* [[Carl Sagan|Sagan, Carl]] & Druyan, Ann (1997). ''Pale Blue Dot: A Vision of the Human Future in Space''. Ballantine Books. {{ISBN|0-345-37659-5}}.
* Schubert, Gerald; Turcotte, Donald L.; Olson, Peter. (2001). ''Mantle convection in the Earth and planets''. Cambridge University Press. {{ISBN|0-521-79836-1}}.
* Taylor, Richard L. S. (1992). "Paraterraforming – The world house concept". ''[[Journal of the British Interplanetary Society]]'', vol. 45, no. 8, pp.&nbsp;341–352. {{ISSN|0007-084X}}. {{Bibcode|1992JBIS...45..341T}}.
* Thompson, J. M. T. (2001). ''Visions of the future: astronomy and Earth science''. [[Cambridge University Press]]. {{ISBN|0-521-80537-6}}.

==External links==
{{Commons category}}
* [http://www.newmars.com/forums/ New Mars forum]
* [https://web.archive.org/web/20110228204329/http://www.terraformers.ca/ Terraformers Society of Canada]
* [http://www.hudsonfla.com/spaceviewinner.htm Visualizing the steps of Solar System terraforming]
* [http://www.users.globalnet.co.uk/~mfogg/zubrin.htm  Research Paper: Technological Requirements for Terraforming Mars]
* {{usurped|1=[https://web.archive.org/web/20120530035251/http://www.terraformers.info/ Terraformers Australia]}}
* [https://web.archive.org/web/20100803052204/http://terraformers.org.uk/ Terraformers UK]
* [http://www.nexialquest.com/The%20Terraformation%20of%20Worlds.pdf The Terraformation of Worlds] {{Webarchive|url=https://web.archive.org/web/20190609033202/http://www.nexialquest.com/The%20Terraformation%20of%20Worlds.pdf |date=2019-06-09 }}
* [https://web.archive.org/web/20091229045854/http://nintenball.ifrance.com/telechargements/tpe/la-terraformation-de-mars_v3.01.pdf Terraformation de Mars]
* Fogg, Martyn J. [http://www.users.globalnet.co.uk/~mfogg/ The Terraforming Information Pages]
* [https://www.bbc.co.uk/news/science-environment-11137903 BBC article on Charles Darwin's and Joseph Hooker's artificial ecosystem on Ascension Island that may be of interest to terraforming projects]
* {{cite web |url=https://www.scientificamerican.com/article/bugs-in-space/ |title=Bugs in Space: Microscopic miners could help humans thrive on other planets |work=[[Scientific American]] |first=Charles Q. |last=Choi |date=November 1, 2010}}
* [http://www.niac.usra.edu/files/studies/final_report/884Todd.pdf Robotic Lunar Ecopoiesis Test Bed] Principal Investigator: Paul Todd (2004)

{{Science fiction}}
{{Space colonization}}

[[Category:1940s neologisms]]
[[Category:Ecosystems]]
[[Category:Open problems]] <!-- As this article says, "has yet to be verified" -->
[[Category:Planetary engineering]]
[[Category:Science fiction themes]]
[[Category:Space colonization]]
[[Category:Terraforming| ]]