Editing Callisto (moon) (section)

===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>