Editing Callisto (moon) (section)

==Origin and evolution==
The partial [[planetary differentiation|differentiation]] of Callisto (inferred e.g. from moment of inertia measurements) means that it has never been heated enough to melt its ice component.<ref name="Spohn 2003"/> Therefore, the most favorable model of its formation is a slow [[accretion (astrophysics)|accretion]] in the low-density Jovian [[solar nebula|subnebula]]—a disk of the gas and dust that existed around Jupiter after its formation.<ref name=Canup2002/> Such a prolonged accretion stage would allow cooling to largely keep up with the heat accumulation caused by impacts, radioactive decay and contraction, thereby preventing melting and fast differentiation.<ref name="Canup2002">{{cite journal |last1=Canup |first1=Robin M. |author-link=Robin Canup |last2=Ward |first2=William R. |year=2002 |title=Formation of the Galilean Satellites: Conditions of Accretion |url=http://www.boulder.swri.edu/~robin/cw02final.pdf |url-status=live |journal=The Astronomical Journal |volume=124 |issue=6 |pages=3404–3423 |bibcode=2002AJ....124.3404C |doi=10.1086/344684 |s2cid=47631608 |archive-url=https://ghostarchive.org/archive/20221009/http://www.boulder.swri.edu/~robin/cw02final.pdf |archive-date=9 October 2022}}</ref> The allowable timescale for the formation of Callisto lies then in the range 0.1&nbsp;million–10&nbsp;million years.<ref name=Canup2002/>

[[File:Jagged Hills PIA03455.jpg|thumb|left|Views of eroding (top) and mostly eroded (bottom) ice knobs (~100&nbsp;m high), possibly formed from the [[Ejecta blanket|ejecta]] of an ancient [[impact crater|impact]]]]
The further evolution of Callisto after [[accretion (astrophysics)|accretion]] was determined by the balance of the [[radioactive]] heating, cooling through [[thermal conduction]] near the surface, and solid state or subsolidus [[convection]] in the interior.<ref name=Freeman2006>{{cite journal|last=Freeman |first=J. |title=Non-Newtonian stagnant lid convection and the thermal evolution of Ganymede and Callisto |year=2006 |volume=54 |issue=1 |pages=2–14 |doi=10.1016/j.pss.2005.10.003 |url=http://bowfell.geol.ucl.ac.uk/~lidunka/EPSS-papers/pete2.pdf |journal=Planetary and Space Science |bibcode=2006P&SS...54....2F |url-status=dead |archive-url=https://web.archive.org/web/20070824155106/http://bowfell.geol.ucl.ac.uk/~lidunka/EPSS-papers/pete2.pdf |archive-date=24 August 2007 }}</ref> Details of the subsolidus convection in the ice is the main source of uncertainty in the models of all [[icy moon]]s. It is known to develop when the temperature is sufficiently close to the [[melting point]], due to the temperature dependence of ice [[viscosity]].<ref name=McKinnon2006/> Subsolidus convection in icy bodies is a slow process with ice motions of the order of 1&nbsp;centimeter per year, but is, in fact, a very effective cooling mechanism on long timescales.<ref name=McKinnon2006>{{cite journal|last=McKinnon|first=William B.|title=On convection in ice I shells of outer Solar System bodies, with detailed application to Callisto|year=2006|volume=183|issue=2|pages=435–450|doi=10.1016/j.icarus.2006.03.004| bibcode=2006Icar..183..435M | journal = Icarus}}</ref> It is thought to proceed in the so-called stagnant lid regime, where a stiff, cold outer layer of Callisto conducts heat without convection, whereas the ice beneath it convects in the subsolidus regime.<ref name="Spohn 2003"/><ref name=McKinnon2006/> For Callisto, the outer conductive layer corresponds to the cold and rigid [[lithosphere]] with a thickness of about 100&nbsp;km. Its presence would explain the lack of any signs of the [[endogenic]] activity on the Callistoan surface.<ref name=McKinnon2006/><ref name=Nagel2004/> The convection in the interior parts of Callisto may be layered, because under the high pressures found there, water [[ice]] exists in different crystalline phases beginning from the [[ice I]] on the surface to [[ice VII]] in the center.<ref name=Freeman2006/> The early onset of subsolidus convection in the Callistoan interior could have prevented large-scale ice melting and any resulting [[planetary differentiation|differentiation]] that would have otherwise formed a large rocky [[core (geology)|core]] and icy [[mantle (geology)|mantle]]. Due to the convection process, however, very slow and partial separation and differentiation of rocks and ices inside Callisto has been proceeding on timescales of billions of years and may be continuing to this day.<ref name=Nagel2004>{{cite journal|last1=Nagel|first1=K.a|last2=Breuer, D. |last3=Spohn, T. |title=A model for the interior structure, evolution, and differentiation of Callisto|year=2004|volume=169|issue=2|pages=402–412|doi=10.1016/j.icarus.2003.12.019| bibcode=2004Icar..169..402N | journal = Icarus}}</ref>

The current understanding of the evolution of Callisto allows for the existence of a layer or "ocean" of liquid water in its interior. This is connected with the anomalous behavior of ice I phase's melting temperature, which decreases with [[pressure]], achieving temperatures as low as 251&nbsp;K at 2,070&nbsp;bar (207&nbsp;[[MPa]]).<ref name="Spohn 2003"/> In all realistic models of Callisto the temperature in the layer between 100 and 200&nbsp;km in depth is very close to, or exceeds slightly, this anomalous melting temperature.<ref name=Freeman2006/><ref name=McKinnon2006/><ref name=Nagel2004/> The presence of even small amounts of [[ammonia]]—about 1–2% by weight—almost guarantees the liquid's existence because ammonia would lower the melting temperature even further.<ref name="Spohn 2003"/>

Although Callisto is very similar in bulk properties to [[Ganymede (moon)|Ganymede]], it apparently had a much simpler [[Historical geology|geological history]]. The surface appears to have been shaped mainly by impacts and other [[exogenic]] forces.<ref name="Greeley 2000"/> Unlike neighboring Ganymede with its grooved terrain, there is little evidence of [[plate tectonics|tectonic]] activity.<ref name=Showman1999/> Explanations that have been proposed for the contrasts in internal heating and consequent differentiation and geologic activity between Callisto and Ganymede include differences in formation conditions,<ref name = "Barr2">{{Cite journal
| last1 = Barr
| first1 = A. C.
|last2=Canup, R. M.
| title = Constraints on gas giant satellite formation from the interior states of partially differentiated satellites
| journal = [[Icarus (journal)|Icarus]]
| volume = 198
| issue = 1
| pages = 163–177
| date = 3 August 2008
| doi = 10.1016/j.icarus.2008.07.004
| bibcode=2008Icar..198..163B}}</ref> the greater tidal heating experienced by Ganymede,<ref name = "Showman2">{{Cite journal
| last1 = Showman
| first1 = A. P.
|last2=Malhotra, R.
| s2cid = 55790129
| title = Tidal evolution into the Laplace resonance and the resurfacing of Ganymede
| journal = [[Icarus (journal)|Icarus]]
| volume = 127
| issue = 1
| pages = 93–111
| date = March 1997
| doi = 10.1006/icar.1996.5669
| bibcode=1997Icar..127...93S}}</ref> and the more numerous and energetic impacts that would have been suffered by Ganymede during the [[Late Heavy Bombardment]].<ref name = "Baldwin">{{cite web
 | last = Baldwin
 | first = E.
 | title = Comet impacts explain Ganymede-Callisto dichotomy
 | website = [[Astronomy Now]]
 | date = 25 January 2010
| url = http://www.astronomynow.com/news/n1001/25galilean/
 | access-date = 1 March 2010
| archive-date = 30 January 2010
| archive-url = https://web.archive.org/web/20100130231918/http://www.astronomynow.com/news/n1001/25galilean/
 | url-status = dead
 }}</ref><ref name="LPI1158">{{Cite conference |last1=Barr |first1=A. C. |last2=Canup |first2=R. M. |date=March 2010 |title=Origin of the Ganymede/Callisto dichotomy by impacts during an outer solar system late heavy bombardment |url=http://www.lpi.usra.edu/meetings/lpsc2010/pdf/1158.pdf |conference= |location=Houston |archive-url=https://web.archive.org/web/20110605044843/http://www.lpi.usra.edu/meetings/lpsc2010/pdf/1158.pdf |archive-date=5 June 2011 |access-date=1 March 2010 |work=41st Lunar and Planetary Science Conference (2010) |url-status=live}}</ref><ref name="Barr">{{Cite journal |last1=Barr |first1=A. C. |last2=Canup |first2=R. M. |date=24 January 2010 |title=Origin of the Ganymede–Callisto dichotomy by impacts during the late heavy bombardment |url=http://www.planetary.brown.edu/pdfs/ab19.pdf |url-status=dead |journal=[[Nature Geoscience]] |volume=3 |issue=March 2010 |pages=164–167 |bibcode=2010NatGe...3..164B |doi=10.1038/NGEO746 |archive-url=https://web.archive.org/web/20210301065853/http://www.planetary.brown.edu/pdfs/ab19.pdf |archive-date=1 March 2021 |access-date=12 April 2020}}</ref> The relatively simple geological history of Callisto provides planetary scientists with a reference point for comparison with other more active and complex worlds.<ref name=Showman1999/>