Changes

== Physical characteristics ==

== Physical characteristics ==

=== Surface ===

=== Surface ===

[[Image:Callisto equatorial.jpg|thumb|150 px|left|Callisto's equatorial surface]]Callisto is one of the most heavily cratered celestial bodies, if not the most heavily cratered body, in the solar system. Because of this cratering, [[uniformitarianism|uniformitarian]] [[astronomy|astronomers]] commonly assume Callisto to be 4.5 billion years old, or not much "younger" than they consider the solar system itself to be.<ref name=darling>"[http://www.daviddarling.info/encyclopedia/C/Callisto.html Callisto (moon of Jupiter)]." ''The Internet Encyclopedia of Science''. Accessed March 1, 2008.</ref><ref name=spires>"[http://science.nasa.gov/headlines/y2001/ast22aug_1.htm The Strange Spires of Callisto]." ''Science@NASA''. Accessed March 1, 2008.</ref>

[[Image:Callisto equatorial.jpg|thumb|150 px|left|Callisto's equatorial surface]]Callisto is one of the most heavily cratered celestial bodies, if not the most heavily cratered body, in the solar system. Because of this cratering, [[uniformitarianism|uniformitarian]] [[astronomy|astronomers]] commonly assume Callisto to be 4.5 billion years old, or not much "younger" than they consider the solar system itself to be.<ref name=darling>"[http://www.daviddarling.info/encyclopedia/C/Callisto.html Callisto (moon of Jupiter)]." ''The Internet Encyclopedia of Science''. Accessed March 1, 2008.</ref><ref name=spires>"[https://science.nasa.gov/headlines/y2001/ast22aug_1.htm The Strange Spires of Callisto]." ''Science@NASA''. Accessed March 1, 2008.</ref>

[[Image:Callisto scarp.jpg|thumb|200 px|right|Escarpment in Valhalla region]]Callisto's craters are remarkably flat, and lack the high ridges, radial rays, and central depressions observed on impact craters on other bodies like the [[Moon]] and [[Mercury]].<ref name=arnett/> Moreover, many of the smaller craters have been obscured, as if by subsequent impacts.<ref name=arnett/> Some authorities hold that this relative flattening and lack of large mountains is due primarily to Callisto having an icy surface.<ref name=solarviews/>

[[Image:Callisto scarp.jpg|thumb|200 px|right|Escarpment in Valhalla region]]Callisto's craters are remarkably flat, and lack the high ridges, radial rays, and central depressions observed on impact craters on other bodies like the [[Moon]] and [[Mercury]].<ref name=arnett/> Moreover, many of the smaller craters have been obscured, as if by subsequent impacts.<ref name=arnett/> Some authorities hold that this relative flattening and lack of large mountains is due primarily to Callisto having an icy surface.<ref name=solarviews/>

Callisto has two very broad basins that most consider the result of repeated impacts. They are named for the two fabled homes of the [[Viking]] gods, [[Valhalla]] and [[Asgard]].<ref name=darling/> Valhalla measures 3,000&nbsp;km across; Asgard 1,600&nbsp;km.<ref name=solarviews/>

Callisto has two very broad basins that most consider the result of repeated impacts. They are named for the two fabled homes of the [[Viking]] gods, [[Valhalla]] and [[Asgard]].<ref name=darling/> Valhalla measures 3,000&nbsp;km across; Asgard 1,600&nbsp;km.<ref name=solarviews/>

Spectroscopy reveals a layer of solid [[carbon dioxide]] ("dry ice") on Callisto's surface and a thin, constantly escaping carbon-dioxide "atmosphere."<ref name=darling/><ref name=britannica>"[http://www.britannica.com/eb/article-9018714 Entry for Callisto]." ''Encyclopædia Britannica'' 2008, Encyclopædia Britannica Online. Accessed March 1, 2008.</ref>

Spectroscopy reveals a layer of solid [[carbon dioxide]] ("dry ice") on Callisto's surface and a thin, constantly escaping carbon-dioxide "atmosphere."<ref name=darling/><ref name=britannica>"[https://www.britannica.com/eb/article-9018714 Entry for Callisto]." ''Encyclopædia Britannica'' 2008, Encyclopædia Britannica Online. Accessed March 1, 2008.</ref>

=== Interior ===

=== Interior ===