# Icy moon
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{{Short description|Natural satellite with a surface mainly composed of ice}}
[[File:PIA07763 Rhea full globe5.jpg|thumb|[Rhea](/source/Rhea_(moon)), the second-largest moon of Saturn and the ninth-largest in the Solar System, is around 3/4 ice by mass.[{{cite journal |last1= Anderson|first1=J. D.|last2= Schubert|first2= G.|title= Saturn's satellite Rhea is a homogeneous mix of rock and ice|journal= Geophysical Research Letters |volume= 34|issue= 2|pages=L02202|year= 2007|doi= 10.1029/2006GL028100|bibcode= 2007GeoRL..34.2202A|doi-access= free}}]]]
'''Icy moons''' are a class of [natural satellite](/source/natural_satellite)s with surfaces composed mostly of [ice](/source/ice). An icy moon may harbor an ocean underneath the surface, and possibly include a rocky core of silicate or metallic rocks.[{{cite journal | title=The Physical Oceanography of Ice-Covered Moons | display-authors=1 | last1=Soderlund | first1=Krista M. | last2=Rovira-Navarro | first2=Marc | last3=Le Bars | first3=Michael | last4=Schmidt | first4=Britney E. | last5=Gerkema | first5=Theo | journal=Annual Review of Marine Science | volume=16 | pages=25–53 | date=January 2023 | doi=10.1146/annurev-marine-040323-101355 | pmid=37669566 | bibcode=2023ARMS...16...25S | doi-access=free }}] It is thought that they may be composed of [ice II](/source/ice_II) or other [polymorph](/source/Polymorphism_(materials_science)) of water ice.[{{cite web |url=http://www.lsbu.ac.uk/water/ice_ii.html |title=Ice-two structure |accessdate=2008-01-02 |author=Chaplin, Martin |date=2007-10-26 |work=Water Structure and Science |archive-date=2012-04-27 |archive-url=https://web.archive.org/web/20120427030534/http://www.lsbu.ac.uk/water/ice_ii.html |url-status=dead }}] The prime example of this class of object is [Europa](/source/Europa_(moon)).
Icy moons warmed by [tide](/source/tide)s may be the most common type of [celestial body](/source/celestial_body) in the galaxy to have liquid [water](/source/water),[{{Cite web |date=2024-10-12 |title=Can Life Exist on an Icy Moon? NASA's Europa Clipper Aims to Find Out – NASA |url=https://www.nasa.gov/missions/europa-clipper/can-life-exist-on-an-icy-moon-nasas-europa-clipper-aims-to-find-out/ |access-date=2024-11-18 |language=en-US}}] and thus the most likely type of object to possibly have [water](/source/water)-based [life](/source/extraterrestrial_life).
Some icy moons exhibit [cryovolcanism](/source/Cryovolcano), as well as geysers. The best studied example is [Saturn's](/source/Saturn) [Enceladus](/source/Enceladus).
==Orbits==
Most known large icy moons belong to [giant planet](/source/giant_planet)s, whose orbits lie beyond the Solar System's [frost line](/source/Frost_line_(astrophysics)); the remainder (such as [Charon](/source/Charon_(moon)) and [Dysnomia](/source/Dysnomia_(moon))) formed around dwarf planets such as [Pluto](/source/Pluto) and {{dp|Eris}}, typically in large impacts not unlike the impact thought to have formed Earth's moon. In the case of icy gas giant satellites, an additional requirement is that a moon did not form in the inner region of a proto-satellite disk, which is too warm for ices to condense.
[Europa](/source/Europa_(moon)) is thought to contain 8% ice and water by mass with the remainder rock.[{{cite book |arxiv= 0812.4995 |author1= Canup, Robin M. |author1-link=Robin Canup |author2= Ward, William R. |authorlink2=William Ward (astronomer) |title= Origin of Europa and the Galilean Satellites |year=2008 |bibcode= 2009euro.book...59C |page= (page needed) }}] Jupiter's outer two [Galilean moons](/source/Galilean_moons) [Ganymede](/source/Ganymede_(moon)) and [Callisto](/source/Callisto_(moon)) contain more ice since they formed further from the hot proto-Jupiter.
Saturn's moon [Titan](/source/Titan_(moon)) looks and behaves more like Earth than any other body in the Solar System.[{{cite web
|date=2009-08-06
|title=Surface features on Titan form like Earth's, but with a frigid twist
|publisher=[IAU](/source/IAU)
|author1=Rosaly Lopes
|author1-link=Rosaly Lopes
|author2=Robert M. Nelson
|url=http://www.iau.org/public_press/news/detail/iau0915/
|accessdate=2009-12-21
|archive-date=2009-09-26
|archive-url=https://web.archive.org/web/20090926080025/http://www.iau.org/public_press/news/detail/iau0915/
|url-status=dead
}}] Titan is known to have stable pools of [liquid methane](/source/liquid_methane) on the surface.