Terraforming: Habitability Requirements
edit
Planetary habitability, broadly defined as the capacity for an astronomical body to sustain life, requires that various geophysical, geochemical, and 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 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[2]. Other requirements for habitability have been defined as the presence of raw materials, an energy source, a solvent, and clement conditions[3], or elemental requirements (such as carbon, hydrogen nitrogen, oxygen, phosphorous and sulfur), an energy source, water, and reasonable physiochemical conditions[4]. 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 molecules, and energy sources to sustain metabolism."[5]
Temperature Requirements
editThe general temperature range for all life on earth is -20ºC to 122ºC[2]; this may constitute a bounding range for the development of life on other planets, in the context of terraforming. Much of earth's biomass (~60%) relies on photosynthesis for an energy source, while a further ~40% is chemotropic[2]. For the development of life on other planetary bodies, chemical energy may have been critical[2], while for sustaining life on another planetary body in our solar system, sufficiently high solar energy may also be necessary for phototrophic organisms.
Water Requirements
editAll known life requires water[3]; thus the capacity for planetary body to sustain water is a critical aspect of habitability. The "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[3][6]. 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 CO2 clouds that would increase albedo, setting an outer boundary on stable liquid water[6]. These constraints are applicable in particular to Earth-like planets and would not as easily apply to icy moons like Europa and Enceladus.
Energy Requirements
editOn the most fundamental level, the only absolute requirement of life may be thermodynamic disequilibrium, or the presence of Gibbs Free Energy[3]. 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[3]. 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[7].
Elemental Requirements
editOn earth, life generally requires six elements in high abundance: carbon, hydrogen, nitrogen, oxygen, phosphorous, and sulfur[4]. These elements are considered "essential" for all known life and plentiful within biological systems[8]. Additional elements crucial to life include the cations Mg2+, Ca2+, K+ and Na+ and the anion Cl-[8]. Many of these elements may undergo biologically facilitated oxidation or reduction to produce usable metabolic energy[8][9].
- ^ Hoehler, Tori M. (2007-12-28). "An Energy Balance Concept for Habitability". Astrobiology. 7 (6): 824–838. doi:10.1089/ast.2006.0095. ISSN 1531-1074.
- ^ a b c d Lineweaver, Charles H.; Chopra, Aditya (2012-05-30). "The Habitability of Our Earth and Other Earths: Astrophysical, Geochemical, Geophysical, and Biological Limits on Planet Habitability". Annual Review of Earth and Planetary Sciences. 40 (1): 597–623. doi:10.1146/annurev-earth-042711-105531. ISSN 0084-6597.
- ^ a b c d e Hoehler, Tori M.; Som, Sanjoy M.; Kiang, Nancy Y. (2018), Deeg, Hans J.; Belmonte, Juan Antonio (eds.), "Life's Requirements", Handbook of Exoplanets, Cham: Springer International Publishing, pp. 1–22, doi:10.1007/978-3-319-30648-3_74-1, ISBN 978-3-319-30648-3, retrieved 2023-03-14
- ^ a b Cockell, C.S.; Bush, T.; Bryce, C.; Direito, S.; Fox-Powell, M.; Harrison, J.P.; Lammer, H.; Landenmark, H.; Martin-Torres, J.; Nicholson, N.; Noack, L.; O'Malley-James, J.; Payler, S.J.; Rushby, A.; Samuels, T. (2016-01-20). "Habitability: A Review". Astrobiology. 16 (1): 89–117. doi:10.1089/ast.2015.1295. ISSN 1531-1074.
- ^ "Astrobiology Roadmap". web.archive.org. 2011-01-17. Retrieved 2023-03-17.
- ^ a b Kasting, James F.; Whitmire, Daniel P.; Reynolds, Ray T. (1993-01-01). "Habitable Zones around Main Sequence Stars". Icarus. 101 (1): 108–128. doi:10.1006/icar.1993.1010. ISSN 0019-1035.
- ^ "Cell Energy, Cell Functions | Learn Science at Scitable". www.nature.com. Retrieved 2023-04-13.
- ^ a b c Wackett, Lawrence; Dodge, Anthony; Ellis, Lynda (2004-02-01). "Microbial Genomics and the Periodic Table". Applied and Environmental Microbiology. 70 (2).
- ^ Falkowski, Paul G.; Fenchel, Tom; Delong, Edward F. (2008-05-23). "The Microbial Engines That Drive Earth's Biogeochemical Cycles". Science. 320 (5879): 1034–1039. doi:10.1126/science.1153213. ISSN 0036-8075.