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Habitability of natural satellites
Natural satellite habitability is the measure of a natural satellite's potential to sustain life. It is an emerging study which is considered important to astrobiology for several reasons, foremost being that natural satellites are predicted to greatly outnumber planets and that it is hypothesized that habitability factors are likely to be similar to those of planets. There are, however, key environmental differences which have a bearing on moons as potential sites for extraterrestrial life.
The strongest candidates for natural satellite habitability are currently icy satellites such as those of Jupiter and Saturn—Europa and Enceladus respectively, although if life exists in either place, it would probably be confined to subsurface habitats. Historically, life on Earth was thought to be strictly a surface phenomenon, but recent studies have shown that up to half of Earth life could live below the surface.〔BOYD, ROBERT S.; . Buried alive: Half of Earth\'s life may lie below land, sea. McClatchy DC. 2014-04-24. URL:http://www.mcclatchydc.com/2010/03/08/90020/buried-alive-half-of-earths-life.html. Accessed: 2014-04-24.(Archived by WebCite® at http://www.webcitation.org/6P5GIqXem)〕 Europa and Enceladus exist outside the circumstellar habitable zone which has historically defined the limits of life within the Solar System as the zone in which water can exist as liquid at the surface. In the Solar System's habitable zone there are only three natural satellites—the Moon, and Mars's moons Phobos and Deimos (although some estimates show Mars and its moons to be slightly outside the habitable zone)〔University of Arizona; NASA; NASA Jet Propulsion Laboratory. Phoenix Mars Mission – Habitability and Biology. University of Arizona. 2014-04-24. URL:http://phoenix.lpl.arizona.edu/mars141.php#1. Accessed: 2014-04-24.(Archived by WebCite® at http://www.webcitation.org/6P58Tsw9q)〕—none of which sustain an atmosphere or water in liquid form. Tidal forces are likely to play as significant a role as stellar radiation in the potential habitability of natural satellites.
Extrasolar moons are not yet confirmed to exist. Detecting them is extremely difficult, because current methods are limited to transit timing. It is possible that some of their attributes could be determined by similar methods as those of transiting planets. Despite this some scientists estimate that there are as many habitable exomoons as habitable exoplanets.〔
==Presumed conditions==
The conditions of habitability for natural satellites are similar to those of planetary habitability. However, there are several factors which differentiate natural satellite habitability and additionally extend their habitability outside the planetary habitable zone.〔Scharf, Caleb (Exomoons Ever Closer ) Scientific American. October 4, 2011〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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