Editing Callisto (moon) (section)

==Physical characteristics==

===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.

===Internal structure===
[[File:Callisto diagram.svg|thumb|upright=2|Model of Callisto's internal structure showing a surface ice layer, a possible liquid water layer, and an ice–rock interior]]
Callisto's battered surface lies on top of a cold, stiff and icy [[lithosphere]] that is between 80 and 150&nbsp;km thick.<ref name=Kuskov2005/><ref name="Spohn 2003"/> A salty ocean 150–200&nbsp;km deep may lie beneath the [[crust (geology)|crust]],<ref name=Kuskov2005/><ref name="Spohn 2003"/> indicated by studies of the [[magnetic field]]s around Jupiter and its moons.<ref name="Khurana 2000">{{cite journal |last1=Khurana|first1=K. K.|title=Induced magnetic fields as evidence for subsurface oceans in Europa and Callisto| journal=Nature| year=1998|volume=395|pages=777–780|doi=10.1038/27394| url=http://www.igpp.ucla.edu/people/mkivelson/Publications/N395777.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.igpp.ucla.edu/people/mkivelson/Publications/N395777.pdf |archive-date=9 October 2022 |url-status=live|pmid=9796812 | issue=6704| bibcode = 1998Natur.395..777K |last2=Kivelson|first2=M. G.| last3=Stevenson| first3=D. J.| last4=Schubert|first4=G.|last5=Russell|first5=C. T.|last6=Walker|first6=R. J.| last7=Polanskey| first7=C.|s2cid=4424606}}</ref><ref name="Zimmer 2000">{{cite journal| last1=Zimmer| first1=C.|last2=Khurana, K. K.|title=Subsurface Oceans on Europa and Callisto: Constraints from Galileo Magnetometer Observations|journal=Icarus|year=2000|volume=147|issue=2|pages=329–347|doi=10.1006/icar.2000.6456| url=http://www.igpp.ucla.edu/people/mkivelson/Publications/ICRUS147329.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.igpp.ucla.edu/people/mkivelson/Publications/ICRUS147329.pdf |archive-date=9 October 2022 |url-status=live| bibcode=2000Icar..147..329Z| last3=Kivelson| first3=Margaret G.| citeseerx=10.1.1.366.7700}}</ref> It was found that Callisto responds to Jupiter's varying background magnetic field like a perfectly [[electrical conductivity|conducting]] sphere; that is, the field cannot penetrate inside Callisto, suggesting a layer of highly conductive fluid within it with a thickness of at least 10&nbsp;km.<ref name="Zimmer 2000"/> The existence of an ocean is more likely if water contains a small amount of [[ammonia]] or other [[antifreeze]], up to 5% by weight.<ref name="Spohn 2003">{{cite journal |last1=Spohn|first1=T.|last2=Schubert, G.|title=Oceans in the icy Galilean satellites of Jupiter?|journal=Icarus|year=2003|volume=161 |issue=2|pages=456–467|doi=10.1016/S0019-1035(02)00048-9| url=http://www.igpp.ucla.edu/public/mkivelso/refs/PUBLICATIONS/SpohnSchubrt03GLLsats.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.igpp.ucla.edu/public/mkivelso/refs/PUBLICATIONS/SpohnSchubrt03GLLsats.pdf |archive-date=9 October 2022 |url-status=live|bibcode=2003Icar..161..456S}}</ref> In this case the water+ice layer can be as thick as 250–300&nbsp;km.<ref name=Kuskov2005/> Failing an ocean, the icy lithosphere may be somewhat thicker, up to about 300&nbsp;km.

Beneath the lithosphere and putative ocean, Callisto's interior appears to be neither entirely uniform nor particularly variable. ''[[Galileo (spacecraft)|Galileo]]'' orbiter data<ref name="Anderson 2001"/> (especially the dimensionless [[moment of inertia]]<ref group="lower-alpha">The dimensionless moment of inertia referred to is <math>I / (mr^2)</math>, where {{var|I}} is the moment of inertia, {{var|m}} the mass, and {{var|r}} the maximal radius. It is 0.4 for a homogenous spherical body, but less than 0.4 if density increases with depth.</ref>—0.3549&nbsp;±&nbsp;0.0042—determined during close flybys) suggest that, if Callisto is in [[hydrostatic equilibrium]], its interior is composed of compressed [[Rock (geology)|rocks]] and [[ice]]s, with the amount of rock increasing with depth due to partial settling of its constituents.<ref name=Kuskov2005/><ref name="Anderson 1998">{{cite journal|last1=Anderson|first1=J. D.|last2=Schubert, G.|last3=Jacobson, R. A.|title=Distribution of Rock, Metals and Ices in Callisto|journal=Science|year=1998|volume=280|pages=1573–1576|doi=10.1126/science.280.5369.1573|url=http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/19178/1/98-0442.pdf|pmid=9616114|issue=5369|bibcode=1998Sci...280.1573A|last4=Lau|first4=E. L.|last5=Moore|first5=W. B.|last6=Sjo Gren|first6=W. L.|url-status=dead|archive-url=https://web.archive.org/web/20070926195310/http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/19178/1/98-0442.pdf|archive-date=26 September 2007}}</ref> In other words, Callisto may be only partially [[planetary differentiation|differentiated]]. The density and moment of inertia for an equilibrium Callisto are compatible with the existence of a small [[silicate]] core in the center of Callisto. The radius of any such core cannot exceed 600&nbsp;km, and the density may lie between 3.1 and 3.6&nbsp;g/cm<sup>3</sup>.<ref name="Anderson 2001"/><ref name=Kuskov2005/> In this case, Callisto's interior would be in stark contrast to [[Ganymede (moon)#Internal structure|that of Ganymede]], which appears to be fully differentiated.<ref name=Showman1999/><ref name="Sohl2002">{{cite journal |last1=Sohl |first1=F. |last2=Spohn |first2=T. |last3=Breuer |first3=D. |last4=Nagel |first4=K. |year=2002 |title=Implications from Galileo Observations on the Interior Structure and Chemistry of the Galilean Satellites |journal=Icarus |volume=157 |issue=1 |pages=104–119 |bibcode=2002Icar..157..104S |doi=10.1006/icar.2002.6828}}</ref>

However, a 2011 reanalysis of Galileo data suggests that Callisto is not in hydrostatic equilibrium.<ref name=Monteux2014>{{cite journal |last1=Monteux |first1=J. |last2=Tobie |first2=G. |last3=Choblet |first3=G. |last4=Le Feuvre |first4=M. |title=Can large icy moons accrete undifferentiated? |journal=Icarus |year= 2014 |volume=237 |pages=377–387 |doi=10.1016/j.icarus.2014.04.041|bibcode=2014Icar..237..377M |s2cid=46172826 |url=https://hal.uca.fr/hal-01636068/file/Monteux-Icarus-V3-1-Final-2014.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://hal.uca.fr/hal-01636068/file/Monteux-Icarus-V3-1-Final-2014.pdf |archive-date=9 October 2022 |url-status=live }}</ref> In that case, the gravity data may be more consistent with a more thoroughly differentiated Callisto with a hydrated silicate core.<ref name="Castillo-Rogez2011">{{cite journal |last1=Castillo-Rogez |first1=J. C.|display-authors=etal |title=How differentiated is Callisto |journal=42nd Lunar and Planetary Science Conference |date=2011 |issue=1608 |pages=2580 |bibcode=2011LPI....42.2580C |url=https://www.lpi.usra.edu/meetings/lpsc2011/pdf/2580.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.lpi.usra.edu/meetings/lpsc2011/pdf/2580.pdf |archive-date=9 October 2022 |url-status=live |access-date=2 January 2020}}</ref>

===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"/>

===Atmosphere and ionosphere===
[[File:Callisto field.svg|thumb|right|Induced magnetic field around Callisto]]
Callisto has a very tenuous atmosphere composed of [[carbon dioxide]]<ref name="Carlson 1999">{{cite journal|last=Carlson|first=R. W.|title=A Tenuous Carbon Dioxide Atmosphere on Jupiter's Moon Callisto|journal=Science|year=1999|volume=283|pages=820–821|doi=10.1126/science.283.5403.820|url=http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/16785/1/99-0186.pdf|pmid=9933159|issue=5403|bibcode=1999Sci...283..820C|display-authors=etal|citeseerx=10.1.1.620.9273|access-date=10 July 2007|archive-date=3 October 2008|archive-url=https://web.archive.org/web/20081003231710/http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/16785/1/99-0186.pdf|url-status=dead}}</ref> and probably oxygen. It was detected by the ''Galileo'' Near Infrared Mapping Spectrometer (NIMS) from its absorption feature near the wavelength 4.2&nbsp;[[micrometers]]. The surface pressure is estimated to be 7.5 pico[[bar (unit)|bar]] (0.75 [[micropascal|μPa]]) and particle density 4{{E-sp|8}}&nbsp;cm<sup>−3</sup>. Because such a thin atmosphere would be lost in only about four years though [[atmospheric escape]], it must be constantly replenished, possibly by slow sublimation of carbon dioxide ice from Callisto's icy crust,<ref name="Carlson 1999"/> which would be compatible with the sublimation–degradation hypothesis for the formation of the surface knobs.

Callisto's ionosphere was first detected during ''Galileo'' flybys;<ref name="Kliore 2002">{{cite journal |last1=Kliore|first1=A. J. |last2=Anabtawi, A. |last3=Herrera, R. G. |title=Ionosphere of Callisto from Galileo radio occultation observations |journal=Journal of Geophysical Research|year=2002|volume=107 |issue=A11|page=1407|doi=10.1029/2002JA009365| bibcode=2002JGRA..107.1407K|display-authors=etal|url=https://deepblue.lib.umich.edu/bitstream/2027.42/95670/1/jgra16576.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://deepblue.lib.umich.edu/bitstream/2027.42/95670/1/jgra16576.pdf |archive-date=9 October 2022 |url-status=live|hdl=2027.42/95670 |doi-access=free}}</ref> its high electron density of 7–17{{E-sp|4}}&nbsp;cm<sup>−3</sup> cannot be explained by the photoionization of the atmospheric [[carbon dioxide]] alone. Hence, it is suspected that the atmosphere of Callisto is actually dominated by [[molecular oxygen]] (in amounts 10–100 times greater than {{chem|CO|2}}).<ref name="Liang 2005">{{cite journal|last1=Liang|first1=M. C.|last2=Lane, B. F.|last3=Pappalardo, R. T.|title=Atmosphere of Callisto|journal=Journal of Geophysical Research|year=2005|volume=110|issue=E2|pages=E02003|doi=10.1029/2004JE002322|bibcode= 2005JGRE..110.2003L|display-authors=etal|doi-access=free}}</ref> However, [[oxygen]] has not yet been directly detected in the atmosphere of Callisto. Observations with the [[Hubble Space Telescope]] (HST) placed an upper limit on its possible concentration in the atmosphere, based on lack of detection, which is still compatible with the ionospheric measurements.<ref name=Strobel2002>{{cite journal|last1=Strobel|first1=Darrell F.|last2=Saur, Joachim |last3=Feldman, Paul D. |title=Hubble Space Telescope Space Telescope Imaging Spectrograph Search for an Atmosphere on Callisto: a Jovian Unipolar Inductor| year=2002| volume=581| issue=1| pages=L51–L54|doi=10.1086/345803|bibcode=2002ApJ...581L..51S | journal = The Astrophysical Journal|display-authors=etal|doi-access=free}}</ref> At the same time, HST was able to detect [[condensation|condensed]] oxygen trapped on the surface of Callisto.<ref name="Spencer2002">{{cite journal|last1= Spencer|first1=John R.|last2=Calvin, Wendy M.|title=Condensed O2 on Europa and Callisto|year=2002|volume=124|issue= 6|pages=3400–3403| doi=10.1086/344307|url=http://www.boulder.swri.edu/~spencer/o2europa.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.boulder.swri.edu/~spencer/o2europa.pdf |archive-date=9 October 2022 |url-status=live| journal = The Astronomical Journal|bibcode=2002AJ....124.3400S|s2cid=51792560 }}</ref>

Atomic hydrogen has also been detected in Callisto's atmosphere via recent analysis of 2001 Hubble Space Telescope data.<ref name=":0">{{Cite journal|last=Roth|first=Lorenz|display-authors=et al|date=27 May 2017|title=Detection of a hydrogen corona at Callisto|journal=Journal of Geophysical Research: Planets|volume=122|issue=5|pages=1046–1055|doi=10.1002/2017JE005294|bibcode=2017JGRE..122.1046R|s2cid=125830948 }}</ref> Spectral images taken on 15 and 24 December 2001 were re-examined, revealing a faint signal of scattered light that indicates a hydrogen corona. The observed brightness from the scattered sunlight in Callisto's hydrogen corona is approximately two times larger when the leading hemisphere is observed. This asymmetry may originate from a different hydrogen abundance in both the leading and trailing hemispheres. However, this hemispheric difference in Callisto's hydrogen corona brightness is likely to originate from the extinction of the signal in Earth's [[geocorona]], which is greater when the trailing hemisphere is observed.<ref>{{Cite journal|date=15 November 2017|title=New constraints on Ganymede's hydrogen corona: Analysis of Lyman-α emissions observed by HST/STIS between 1998 and 2014|journal=Planetary and Space Science|volume=148|pages=35–44|doi=10.1016/j.pss.2017.10.006|issn=0032-0633|last1=Alday|first1=Juan|last2=Roth|first2=Lorenz|last3=Ivchenko|first3=Nickolay|last4=Retherford|first4=Kurt D|last5=Becker|first5=Tracy M|last6=Molyneux|first6=Philippa|last7=Saur|first7=Joachim|bibcode=2017P&SS..148...35A}}</ref>