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[[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 km across; Asgard 1,600 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 km across; Asgard 1,600 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>"[http://www.britannica.com/eb/article-9018714 Entry for Callisto]." ''Encyclopædia Britannica'' 2008, Encyclopædia Britannica Online. Accessed March 1, 2008.</ref>
=== Interior ===
=== Interior ===
[[Image:Callisto interior.jpg|thumb|250 px|left|Callisto cutaway]]Callisto is remarkably lightweight (1.8% of earth's mass), and its density, at 1,834 kg/m³, is consistent with an interior that has almost as much water as it has ice. Until the [[Galileo Project]], Callisto was considered to have an undifferentiated interior. Detailed examination by the Galileo orbiter suggests an interior composed of 40% ice and 60% rock, with the percentage of rock gradually increasing with increasing depth. Unique among the Galilean moons, Callisto does ''not'' have a metallic core.
[[Image:Callisto interior.jpg|thumb|250 px|left|Callisto cutaway]]Callisto is remarkably lightweight (1.8% of earth's mass), and its density, at 1,834 kg/m³, is consistent with an interior that has almost as much water as it has ice. Until the [[Galileo Project]], Callisto was considered to have an undifferentiated interior. Detailed examination by the Galileo orbiter suggests an interior composed of 40% ice and 60% rock, with the percentage of rock gradually increasing with increasing depth. Unique among the Galilean moons, Callisto does ''not'' have a metallic core.
Callisto has a very weak magnetic field that causes definite perturbations in the magnetic field of Jupiter as it passes in its orbit. Magnetic fields require currents, and current in turn require either metal or an electrolytic solution.<ref name=solarviews/> The most likely explanation is that Callisto has a saltwater ocean, which according to current models lies 200 km deep to the surface and is 10 km deep.<ref name=interior>"[http://photojournal.jpl.nasa.gov/catalog/PIA01478 Catalog page for Image PIA01478]." [[Jet Propulsion Laboratory]], [[NASA]], October 21, 1998. Accessed March 1, 2008.</ref> Further evidence in favor of this saltwater ocean lies in the relatively flat nature of the features of the Valhalla basin. Dr. David A. Williams of [[Arizona State University]] has long held that such an ocean must exist and help absorb the shock of meteoric impacts; the recent photographs of the region support this hypothesis.<ref name=Williams>"[http://www.jpl.nasa.gov/releases/2001/release_2001_230.html Ocean Inside Jupiter's Moon Callisto May Have Cushioned Big Impact]." [[NASA]] news release, November 29, 2001. Accessed March 1, 2008.</ref>
Callisto has a very weak magnetic field that causes definite perturbations in the magnetic field of Jupiter as it passes in its orbit. Magnetic fields require currents, and current in turn require either metal or an electrolytic solution.<ref name=solarviews/> The most likely explanation is that Callisto has a saltwater ocean, which according to current models lies 200 km deep to the surface and is 10 km deep.<ref name=interior>"[http://photojournal.jpl.nasa.gov/catalog/PIA01478 Catalog page for Image PIA01478]." [[Jet Propulsion Laboratory]], [[NASA]], October 21, 1998. Accessed March 1, 2008.</ref> Further evidence in favor of this saltwater ocean lies in the relatively flat nature of the features of the Valhalla basin. Dr. David A. Williams of [[Arizona State University]] has long held that such an ocean must exist and help absorb the shock of meteoric impacts; the recent photographs of the region support this hypothesis.<ref name=Williams>"[http://www.jpl.nasa.gov/releases/2001/release_2001_230.html Ocean Inside Jupiter's Moon Callisto May Have Cushioned Big Impact]." [[NASA]] news release, November 29, 2001. Accessed March 1, 2008.</ref>
== Problems for Uniformitarian Theory ==
== Problems for Uniformitarian Theory ==