Editing Callisto (moon) (section)

===Composition===
[[File:Callisto, Earth & Moon size comparison.jpg|thumb|left|Size comparison of [[Earth]], [[Moon]] and Callisto]]
[[File:PIA00844 NIMS spectra.gif|thumb|right|[[Galileo (spacecraft)#Near-Infrared Mapping Spectrometer (NIMS)|Near-IR spectra]] of dark cratered plains (red) and the [[Asgard (crater)|Asgard impact structure]] (blue), showing the presence of more water ice ([[Water absorption|absorption bands]] from 1 to 2 [[micrometer (unit)|μm]])<ref name="Clark">{{Cite journal | last = Clark | first = R. N. | title = Water frost and ice: the near-infrared spectral reflectance 0.65–2.5 μm | journal = [[Journal of Geophysical Research]] | volume = 86 | issue = B4 | pages = 3087–3096 | date = 10 April 1981 | url = http://www.agu.org/pubs/crossref/1981/JB086iB04p03087.shtml | doi = 10.1029/JB086iB04p03087 | access-date = 3 March 2010 | bibcode = 1981JGR....86.3087C | archive-date = 6 June 2011 | archive-url = https://web.archive.org/web/20110606002239/http://www.agu.org/pubs/crossref/1981/JB086iB04p03087.shtml | url-status = dead }}</ref> and less rocky material within Asgard.]]
The average [[density]] of Callisto, 1.83&nbsp;g/cm<sup>3</sup>,<ref name="Anderson 2001"/> suggests a composition of approximately equal parts of rocky material and [[ice|water ice]], with some additional volatile ices such as [[ammonia]].<ref name=Kuskov2005>{{cite journal| last1=Kuskov|first1=O.L.|last2=Kronrod, V.A.|title=Internal structure of Europa and Callisto| year=2005|volume=177| issue=2|pages=550–369|doi=10.1016/j.icarus.2005.04.014| bibcode=2005Icar..177..550K| journal = Icarus}}</ref> The mass fraction of ices is 49–55%.<ref name=Kuskov2005/><ref name="Spohn 2003"/> The exact composition of Callisto's [[Rock (geology)|rock]] component is not known, but is probably close to the composition of L/LL type [[ordinary chondrite]]s,<ref name=Kuskov2005/> which are characterized by less total [[iron]], less metallic iron and more [[iron oxide]] than [[H chondrite]]s. The weight ratio of iron to [[silicon]] is 0.9–1.3 in Callisto, whereas the [[Sun|solar ratio]] is around 1:8.<ref name=Kuskov2005/>

Callisto's surface has an [[albedo]] of about 20%.<ref name=Moore2004/> Its surface composition is thought to be broadly similar to its composition as a whole. Near-infrared [[spectroscopy]] has revealed the presence of water ice [[absorption band]]s at wavelengths of 1.04, 1.25, 1.5, 2.0 and 3.0 micrometers.<ref name=Moore2004/> Water ice seems to be ubiquitous on the surface of Callisto, with a mass fraction of 25–50%.<ref name=Showman1999/> The analysis of high-resolution, [[near-infrared]] and [[Ultraviolet|UV]] [[spectrum|spectra]] obtained by the ''[[Galileo (spacecraft)|Galileo]]'' spacecraft and from the ground has revealed various non-ice materials: [[magnesium]]- and [[iron]]-bearing hydrated [[silicates]],<ref name=Moore2004/> [[carbon dioxide]],<ref name=Brown2003/> [[sulfur dioxide]],<ref name=Noll1996>{{cite web|last=Noll|first=K.S.|title=Detection of SO<sub>2</sub> on Callisto with the Hubble Space Telescope|year=1996|publisher=Lunar and Planetary Science XXXI|url=http://www.lpi.usra.edu/meetings/lpsc97/pdf/1852.PDF|page=1852|access-date=25 July 2007|archive-url=https://web.archive.org/web/20160604011832/http://www.lpi.usra.edu/meetings/lpsc97/pdf/1852.PDF|archive-date=4 June 2016|url-status=dead}}</ref> and possibly [[ammonia]] and various [[organic compounds]].<ref name=Showman1999/><ref name=Moore2004/> Spectral data indicate that Callisto's surface is extremely heterogeneous at the small scale. Small, bright patches of pure water ice are intermixed with patches of a rock–ice mixture and extended dark areas made of a non-ice material.<ref name=Moore2004/><ref name="Greeley 2000"/>

The Callistoan surface is asymmetric: the leading hemisphere<ref group=lower-alpha name=footnote2>The leading hemisphere is the hemisphere facing the direction of the orbital motion; the trailing hemisphere faces the reverse direction.</ref> is darker than the trailing one. This is different from other [[Galilean satellites]], where the reverse is true.<ref name=Moore2004/> The trailing hemisphere<ref group=lower-alpha name=footnote2/> of Callisto appears to be enriched in [[carbon dioxide]], whereas the leading hemisphere has more [[sulfur dioxide]].<ref name=Hibbitts1998>{{cite web|last1=Hibbitts|first1=C.A.|last2=McCord, T. B.|last3=Hansen, G.B.|title=Distributions of CO<sub>2</sub> and SO<sub>2</sub> on the Surface of Callisto|year=1998|publisher=Lunar and Planetary Science XXXI|url=http://www.lpi.usra.edu/meetings/lpsc2000/pdf/1908.pdf|page=1908|access-date=10 July 2007|archive-url=https://web.archive.org/web/20160604011832/http://www.lpi.usra.edu/meetings/lpsc2000/pdf/1908.pdf|archive-date=4 June 2016|url-status=dead}}</ref> Many fresh [[impact crater]]s like [[Lofn (crater)|Lofn]] also show enrichment in carbon dioxide.<ref name=Hibbitts1998/> Overall, the chemical composition of the surface, especially in the dark areas, may be close to that seen on [[D-type asteroid]]s,<ref name="Greeley 2000"/> whose surfaces are made of [[carbon]]aceous material.