Editing Terraforming (section)

== 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}}</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}}</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}}</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}}</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}}</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"/>