Editing Callisto (moon) (section)

===Surface features===
{{See also|List of geological features on Callisto}}
[[File:Cratered plains PIA00745.jpg|thumb|upright|left|''Galileo'' image of cratered plains, illustrating the pervasive local smoothing of Callisto's surface]]
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The ancient surface of Callisto is one of the most heavily cratered in the Solar System.<ref name="Zahnle 1998">{{cite journal|last1=Zahnle |first1=K. |last2=Dones, L. |title=Cratering Rates on the Galilean Satellites |journal=Icarus |year=1998 |volume=136 |issue=2 |pages=202–222 |doi=10.1006/icar.1998.6015 |url=http://lasp.colorado.edu/icymoons/europaclass/Zahnle_etal_1998.pdf |pmid=11878353 |bibcode=1998Icar..136..202Z |last3=Levison |first3=Harold F. |url-status=dead |archive-url=https://web.archive.org/web/20080227015923/http://lasp.colorado.edu/icymoons/europaclass/Zahnle_etal_1998.pdf |archive-date=27 February 2008}}</ref> In fact, the [[impact crater|crater]] density is close to [[wikt:saturation|saturation]]: any new crater will tend to erase an older one. The large-scale [[geology]] is relatively simple; on Callisto there are no large mountains, volcanoes or other [[endogenic]] [[tectonic]] features.<ref name="Bender 1997">{{Cite journal |last1=Bender |first1=K. C. |last2=Rice |first2=J. W. |last3=Wilhelms |first3=D. E. |last4=Greeley |first4=R. |title=Geological map of Callisto |journal=Abstracts of the 25th Lunar and Planetary Science Conference |volume=25 |pages=91 |year=1997 |url=https://astrogeology.usgs.gov/Projects/PlanetaryMapping/DIGGEOL/galsats/callisto/jcglobal.htm |bibcode=1994LPI....25...91B |access-date=28 August 2017 |archive-url=https://web.archive.org/web/20150124085702/http://astrogeology.usgs.gov/Projects/PlanetaryMapping/DIGGEOL/galsats/callisto/jcglobal.htm |archive-date=24 January 2015 |url-status=dead }}</ref> The impact craters and multi-ring structures—together with associated [[fracture (geology)|fractures]], [[escarpment|scarps]] and [[deposit (geology)|deposits]]—are the only large features to be found on the surface.<ref name="Greeley 2000"/><ref name="Bender 1997"/>

Callisto's surface can be divided into several geologically different parts: cratered plains, light plains, bright and dark smooth plains, and various units associated with particular multi-ring structures and impact craters.<ref name="Greeley 2000">{{cite journal|last1=Greeley|first1=R.|last2=Klemaszewski, J. E. |last3=Wagner, L. |title=Galileo views of the geology of Callisto|journal=Planetary and Space Science| year=2000| volume=48| issue=9| pages=829–853| bibcode=2000P&SS...48..829G| doi=10.1016/S0032-0633(00)00050-7| display-authors=etal}}</ref><ref name="Bender 1997"/> The cratered plains make up most of the surface area and represent the ancient lithosphere, a mixture of ice and rocky material. The light plains include bright impact craters like [[Asgard (crater)|Burr]] and [[Lofn (crater)|Lofn]], as well as the effaced remnants of old large craters called [[Palimpsest (planetary astronomy)|palimpsest]]s,{{refn|In the case of icy satellites, palimpsests are defined as bright circular surface features, probably old impact craters<ref name="Greeley 2000"/>|group=lower-alpha}} the central parts of multi-ring structures, and isolated patches in the cratered plains.<ref name="Greeley 2000"/> These light plains are thought to be icy impact deposits. The bright, smooth plains make up a small fraction of Callisto's surface and are found in the ridge and [[trough (geology)|trough]] zones of the [[Valhalla (crater)|Valhalla]] and [[Asgard (crater)|Asgard]] formations and as isolated spots in the cratered plains. They were thought to be connected with endogenic activity, but the high-resolution ''Galileo'' images showed that the bright, smooth plains correlate with heavily fractured and knobby terrain and do not show any signs of resurfacing.<ref name="Greeley 2000"/> The ''Galileo'' images also revealed small, dark, smooth areas with overall coverage less than 10,000&nbsp;km<sup>2</sup>, which appear to embay<ref group=lower-alpha>To ''embay'' means to shut in, or shelter, as in a bay.</ref> the surrounding terrain. They are possible [[cryovolcano|cryovolcanic]] deposits.<ref name="Greeley 2000"/> Both the light and the various smooth plains are somewhat younger and less cratered than the background cratered plains.<ref name="Greeley 2000"/><ref name="Wagner 2001">{{cite conference |last1=Wagner |first1=R. |last2=Neukum, G. |last3=Greeley, R |title=Fractures, Scarps, and Lineaments on Callisto and their Correlation with Surface Degradation |work=32nd Annual Lunar and Planetary Science Conference |date= 12–16 March 2001 |url=http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1838.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1838.pdf |archive-date=9 October 2022 |url-status=live|display-authors=etal}}</ref>

[[File:Callisto Har PIA01054.jpg|thumb|right|Impact crater [[Hár (crater)|Hár]] with a central dome. [[Crater chain|Chains]] of [[secondary crater]]s from formation of the more recent crater [[Tindr (crater)|Tindr]] at upper right crosscut the terrain.]]
Impact crater diameters seen range from 0.1&nbsp;km—a limit defined by the [[image resolution|imaging resolution]]—to over 100&nbsp;km, not counting the multi-ring structures.<ref name="Greeley 2000"/> Small craters, with diameters less than 5&nbsp;km, have simple bowl or flat-floored shapes. Those 5–40&nbsp;km across usually have a central peak. Larger impact features, with diameters in the range 25–100&nbsp;km, have central pits instead of peaks, such as [[Tindr (crater)|Tindr]] crater.<ref name="Greeley 2000"/> The largest craters with diameters over 60&nbsp;km can have central domes, which are thought to result from central [[tectonic uplift]] after an impact;<ref name="Greeley 2000"/> examples include [[Asgard (crater)|Doh]] and [[Hár (crater)|Hár]] craters. A small number of very large—more than 100&nbsp;km in diameter—and bright impact craters show anomalous dome geometry. These are unusually shallow and may be a transitional [[landform]] to the multi-ring structures, as with the [[Lofn (crater)|Lofn]] impact feature.<ref name="Greeley 2000"/> Callisto's craters are generally shallower than those on the [[Moon]].

[[File:Valhalla crater on Callisto.jpg|thumb|''[[Voyager 1]]'' image of [[Valhalla (crater)|Valhalla]], a [[Complex crater|multi-ring impact structure]] 3,800&nbsp;km in diameter]]
The largest impact features on Callisto's surface are multi-ring basins.<ref name="Greeley 2000"/><ref name="Bender 1997"/> Two are enormous. [[Valhalla (crater)|Valhalla]] is the largest, with a bright central region 600&nbsp;km in diameter, and rings extending as far as 1,800&nbsp;km from the center (see figure).<ref name="Map 2002">{{cite map |title=Controlled Photomosaic Map of Callisto JC 15M CMN |publisher=U.S. Geological Survey |edition=2002 |url=http://geopubs.wr.usgs.gov/i-map/i2770/ |access-date=17 April 2007 |archive-date=9 May 2013 |archive-url=https://web.archive.org/web/20130509055309/http://geopubs.wr.usgs.gov/i-map/i2770/ |url-status=live }}</ref> The second largest is [[Asgard (crater)|Asgard]], measuring about 1,600&nbsp;km in diameter.<ref name="Map 2002"/> Multi-ring structures probably originated as a result of a post-impact [[concentric]] fracturing of the lithosphere lying on a layer of soft or liquid material, possibly an ocean.<ref name=Klemaszewski2001/> The catenae—for example [[Gomul Catena]]—are long chains of impact craters lined up in straight lines across the surface. They were probably created by objects that were tidally disrupted as they passed close to Jupiter prior to the impact on Callisto, or by very [[wikt:oblique|oblique]] impacts.<ref name="Greeley 2000"/> A historical example of a disruption was [[Comet Shoemaker–Levy 9]].

As mentioned above, small patches of pure water ice with an [[albedo]] as high as 80% are found on the surface of Callisto, surrounded by much darker material.<ref name=Moore2004/> High-resolution ''[[Galileo (spacecraft)|Galileo]]'' images showed the bright patches to be predominately located on elevated surface features: [[rim (craters)|crater rims]], [[Escarpment|scarps]], ridges and knobs.<ref name=Moore2004/> They are likely to be thin water [[frost]] [[deposit (geology)|deposits]]. Dark material usually lies in the lowlands surrounding and mantling bright features and appears to be smooth. It often forms patches up to 5&nbsp;km across within the crater floors and in the intercrater depressions.<ref name=Moore2004/>

[[File:Landslides and knobs PIA01095.jpg|thumb|upright|left|Two [[landslide]]s 3–3.5&nbsp;km long are visible on the right sides of the floors of the two large craters on the right.]]
On a sub-kilometer scale the surface of Callisto is more degraded than the surfaces of other icy [[Galilean moons]].<ref name=Moore2004/> Typically there is a deficit of small impact craters with diameters less than 1&nbsp;km as compared with, for instance, the dark plains on [[ganymede (moon)|Ganymede]].<ref name="Greeley 2000"/> Instead of small craters, the almost ubiquitous surface features are small knobs and pits.<ref name=Moore2004/> The knobs are thought to represent remnants of crater rims degraded by an as-yet uncertain process.<ref name="Moore1999">{{cite journal |last1=Moore |first1=Jeffrey M. |last2=Asphaug |first2=Erik |last3=Morrison, David |last4=Spencer |first4=John R. |last5=Chapman |first5=Clark R. |last6=Bierhaus |first6=Beau |last7=Sullivan |first7=Robert J. |last8=Chuang |first8=Frank C. |last9=Klemaszewski |first9=James E. |last10=Greeley |first10=Ronald |last11=Bender |first11=Kelly C. |last12=Geissler |first12=Paul E. |last13=Helfenstein |first13=Paul |last14=Pilcher |first14=Carl B. |year=1999 |title=Mass Movement and Landform Degradation on the Icy Galilean Satellites: Results of the Galileo Nominal Mission |url=https://zenodo.org/record/1229836 |url-status=live |journal=Icarus |volume=140 |issue=2 |pages=294–312 |bibcode=1999Icar..140..294M |doi=10.1006/icar.1999.6132 |archive-url=https://web.archive.org/web/20190129011552/https://zenodo.org/record/1229836 |archive-date=29 January 2019 |access-date=26 August 2018}}</ref> The most likely candidate process is the slow [[sublimation (chemistry)|sublimation]] of ice, which is enabled by a temperature of up to 165&nbsp;[[kelvin|K]], reached at a subsolar point.<ref name=Moore2004/> Such sublimation of water or other [[Volatile (astrogeology)|volatiles]] from the dirty ice that is the [[bedrock]] causes its decomposition. The non-ice remnants form [[debris]] avalanches descending from the slopes of the crater walls.<ref name=Moore1999/> Such avalanches are often observed near and inside impact craters and termed "debris aprons".<ref name=Moore2004/><ref name="Greeley 2000"/><ref name=Moore1999/> Sometimes crater walls are cut by sinuous valley-like incisions called "gullies", which resemble certain [[Mars|Martian]] surface features.<ref name=Moore2004/> In the ice sublimation hypothesis, the low-lying dark material is interpreted as a blanket of primarily non-ice debris, which originated from the degraded rims of craters and has covered a predominantly icy bedrock.

The relative ages of the different surface units on Callisto can be determined from the density of impact craters on them. The older the surface, the denser the crater population.<ref name=Chapman1997>{{cite web|last1=Chapman |first1=C.R. |last2=Merline |first2=W.J. |last3=Bierhaus |first3=B.|title= Populations of Small Craters on Europa, Ganymede, and Callisto: Initial Galileo Imaging Results |year=1997|publisher=Lunar and Planetary Science XXXI| url=http://www.lpi.usra.edu/meetings/lpsc97/pdf/1221.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.lpi.usra.edu/meetings/lpsc97/pdf/1221.pdf |archive-date=9 October 2022 |url-status=live| page=1221|display-authors=etal}}</ref> Absolute dating has not been carried out, but based on theoretical considerations, the cratered plains are thought to be ~4.5&nbsp;[[1000000000 (number)|billion]] years old, dating back almost to the formation of the [[Solar System]]. The ages of multi-ring structures and impact craters depend on chosen background cratering rates and are estimated by different authors to vary between 1 and 4&nbsp;billion years.<ref name="Greeley 2000"/><ref name="Zahnle 1998"/>