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Definition of Habitability
TEDx on Habitability

Habitability is commonly understood as “the potential of an environment (past or present) to support life of any kind” (Steele et al., 2005). Based on the only known example of Earth, the concept refers to whether environmental conditions are available that could eventually support life, even if life does not currently exist (Javaux and Dehant, 2010, Astron. Astrophys. Rev., 18, 383-416, DOI: 10.1007/s00159-010-0030-4). To be meaningful, this concept requires an unambiguous definition of life, which is currently lacking (Tsokolov, 2010, Astrobiology, 10(10), 1031-1042, DOI: 10.1089/ast.2010.0532). Life includes properties such as consuming nutrients and producing waste, the ability to reproduce and grow, pass on genetic information, evolve, and adapt to the varying conditions on a planet (Sagan, 1970, Encyclopedia Britannica, 22, 964-981). Terrestrial life requires liquid water. The stability of liquid water at the surface of a planet defines a habitable zone (HZ) around a star. In the Solar System, it stretches between Venus and Mars, but excludes these two planets. If the greenhouse effect is taken into account, the habitable zone may have included early Mars while the case for Venus is still debated.

This simple definition neglects other important requirements for life such as a supply of biogenic elements (C, H, O, N, P, S) and an energy source to drive biochemical reactions. Also, liquid water may exist in oceans covered by ice shells for example in the icy satellites of Jupiter (Schubert et al., 2004, in: Jupiter: The planet, satellites and magnetosphere, Cambridge University Press, Cambridge, 281-306), which are located well outside the conventional habitable zone of the Solar System. On Earth where life mostly depends directly or indirectly on solar energy, habitats exist deep in the oceans in eternal darkness thriving on geothermal energy or deep in the crust (Chyba and Hand, 2001, Nature, 355, 125-132). In glacial areas, interfacial water between ice crystals may remain liquid at temperatures far below 0°C and concentrate nutrients (Petzold and Aguilera, 2009, J. Food Biophysics, 4(4), 378-396, DOI: 10.1007/s11483-009-9136-5). The common view, however, is that extraterrestrial life would probably be based on organic chemistry in a water solvent although alternative biochemistries have been hypothesized. Indeed, organic molecules (molecules composed of at least carbon and hydrogen) are common in the universe, in interstellar medium or in meteorites. Moreover, carbon (C) builds complex molecules with high information content. Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), the building blocks of life, are among the first elements to form in stellar environments, and are ubiquitous in the universe. The origin of life on Earth is not constrained, and several hypotheses involving Panspermia (hypothesis that life exists throughout the Universe), hydrothermal or lukewarm origins exist. Considering the evolution of terrestrial life, the most probable form of life that could exist beyond Earth would be microbial (unicellular microorganisms) (Javaux and Dehant, 2010, Astron. Astrophys. Rev., 18, 383-416, DOI: 10.1007/s00159-010-0030-4).

As envisaged in Javaux and Dehant (2010), this IAP develops and closely integrates the geophysical, geological, and biological aspects of habitability with a particular focus on Earth neighboring planets, Mars and Venus. Important geodynamic processes affect the habitability conditions of a planet. The dynamic processes, e.g. internal dynamo, magnetic field, atmosphere, plate tectonics, mantle convection, volcanism, thermo-tectonic evolution, meteorite impacts, and erosion, modify the planetary surface, the possibility to have liquid water, the thermal state, the energy budget and the availability of nutrients. Shortly after formation (Hadean 4.4-4.0 Ga (billion years)), evidence supports the presence of a liquid ocean and continental crust on Earth (Wilde et al., 2001, Nature, 409, 175-178), Earth may thus have been habitable very early on. The origin of life is not understood yet but the oldest putative traces of life occur in the early Archaean (~3.5 Ga). Studies of early Earth habitats documented in rock containing traces of fossil life provide information about environmental conditions suitable for life beyond Earth, as well as methodologies for their identification and analyses. The extreme values of environmental conditions in which life thrives today can also be used to characterize the “envelope" of the existence of life and the range of potential extraterrestrial habitats. The requirement of nutrients for biosynthesis, growth, and reproduction suggest that a tectonically active planet, with liquid water is required to replenish nutrients and sustain life (as currently known). These dynamic processes play a key role in the apparition and persistence of life.