Editing Terraforming (section)

=== 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.