Editing Ganymede (moon) (section)

===Atmosphere and ionosphere===
In 1972, a team of Indian, British and American astronomers working in [[Java]], [[Indonesia]] and [[Kavalur]], India claimed that they had detected a thin atmosphere during an [[occultation]], when it and Jupiter passed in front of a [[star]].<ref name="Carlson1973">{{cite journal |last1=Carlson |first1=R.W. |last2=Bhattacharyya |first2=J. C. |author-link2=J. C. Bhattacharyya |last3=Smith |first3=B.A. |last4=Johnson |first4=T. V. |last5=Hidayat |first5=B. |last6=Smith |first6=S. A. |last7=Taylor |first7=G. E. |last8=O'Leary |first8=B. |last9=Brinkmann |first9=R. T. |display-authors=2 |title=Atmosphere of Ganymede from its occultation of SAO 186800 on 7 June 1972 |journal=Science |date=1973 |volume=182 |bibcode=1973Sci...182...53C |doi=10.1126/science.182.4107.53 |issue=4107 |pmid=17829812 |pages=53–5 |s2cid=33370778 |url=http://authors.library.caltech.edu/61963/1/1736235.pdf |access-date=April 20, 2018 |archive-date=December 2, 2017 |archive-url=https://web.archive.org/web/20171202104832/https://authors.library.caltech.edu/61963/1/1736235.pdf |url-status=live }}</ref> They estimated that the surface pressure was around 0.1 [[Pascal (unit)|Pa]] (1 microbar).<ref name="Carlson1973" /> However, in 1979, ''[[Voyager 1]]'' observed an occultation of the star [[Kappa Centauri|κ Centauri]] during its flyby of Jupiter, with differing results.<ref name="Broadfoot1981">{{cite journal |last1=Broadfoot |first1=A.L. |last2=Sandel |first2=B.R. |last3=Shemansky |first3=D.E. |last4=McConnell |first4=J. C. |last5=Smith |first5=G. R. |last6=Holberg |first6=J. B. |last7=Atreya |first7=S. K. |last8=Donahue |first8=T. M. |last9=Strobel |first9=D. F. |display-authors=2 |title=Overview of the Voyager Ultraviolet Spectrometry Results through Jupiter Encounter |journal=Journal of Geophysical Research |date=1981 |volume=86 |issue=A10 |pages=8259–8284 |url=http://www-personal.umich.edu/~atreya/Articles/1981_Overview_Voyager.pdf |bibcode=1981JGR....86.8259B |doi=10.1029/JA086iA10p08259 |access-date=January 16, 2008 |archive-date=March 27, 2009 |archive-url=https://web.archive.org/web/20090327050807/http://www-personal.umich.edu/~atreya/Articles/1981_Overview_Voyager.pdf |url-status=live }}</ref> The occultation measurements were conducted in the [[far-ultraviolet]] spectrum at [[wavelength]]s shorter than 200 [[nanometre|nm]], which were much more sensitive to the presence of gases than the 1972 measurements made in the [[visible spectrum]]. No atmosphere was revealed by the ''Voyager'' data. The upper limit on the surface particle [[number density]] was found to be {{nowrap|1.5{{E|9}} cm<sup>−3</sup>}}, which corresponds to a surface pressure of less than 2.5 μPa (25 picobar).<ref name="Broadfoot1981" /> The latter value is almost five orders of magnitude less than the 1972 estimate.<ref name="Broadfoot1981" />
[[Image:Map of temparatureof ganymede.jpg|thumb|left|False-color temperature map of Ganymede]]
Despite the ''Voyager'' data, evidence for a tenuous oxygen atmosphere ([[exosphere]]) on Ganymede, very similar to the one found on Europa, was found by the [[Hubble Space Telescope]] (HST) in 1995.<ref name="Hall1998" /><ref name="JPLAtmosphere">{{cite web |archive-url= https://web.archive.org/web/20090504072525/http://www2.jpl.nasa.gov/galileo/hst7.html |url= http://www2.jpl.nasa.gov/galileo/hst7.html|title=Hubble Finds Thin Oxygen Atmosphere on Ganymede |work=Jet Propulsion Laboratory |publisher=NASA |date=October 23, 1996 |access-date= February 17, 2017|url-status= dead |archive-date= May 4, 2009}}</ref> HST actually observed [[airglow]] of atomic oxygen in the far-ultraviolet at the wavelengths 130.4&nbsp;nm and 135.6&nbsp;nm. Such an airglow is excited when [[molecular oxygen]] is [[dissociation (chemistry)|dissociated]] by electron impacts,<ref name="Hall1998" /> which is evidence of a significant neutral atmosphere composed predominantly of O<sub>2</sub> molecules. The surface number density probably lies in the {{nowrap |(1.2–7){{E|8}} cm<sup>−3</sup>}} range, corresponding to the surface pressure of {{nowrap |0.2–1.2 μPa}}.<ref name="Hall1998" /><ref name="surfacedensitynumber" group=lower-alpha /> These values are in agreement with ''Voyager''<nowiki>'</nowiki>s upper limit set in 1981. The oxygen is not evidence of life; it is thought to be produced when water ice on Ganymede's surface is split into [[hydrogen]] and oxygen by radiation, with the hydrogen then being more rapidly lost due to its low atomic mass.<ref name="JPLAtmosphere" /> The airglow observed over Ganymede is not spatially homogeneous like that observed over Europa. HST observed two bright spots located in the northern and southern hemispheres, near ± 50° latitude, which is exactly the boundary between the open and closed field lines of the Ganymedian magnetosphere (see below).<ref name="Feldman2000">{{cite journal |last1=Feldman |first1=Paul D. |last2=McGrath |first2=Melissa A. |last3=Strobell |first3=Darrell F. |last4=Moos |first4=H. Warren |last5=Retherford |first5=Kurt D. |last6=Wolven |first6=Brian C. |display-authors=2 |title=HST/STIS Ultraviolet Imaging of Polar Aurora on Ganymede |journal=The Astrophysical Journal |date=2000 |volume=535 |issue=2 |pages=1085–1090 |doi=10.1086/308889 |bibcode=2000ApJ...535.1085F |arxiv=astro-ph/0003486 |s2cid=15558538 }}</ref> The bright spots are probably polar [[aurora (astronomy)|auroras]], caused by plasma precipitation along the open field lines.<ref name="Johnson1997">{{cite journal |last=Johnson |first=R.E. |date=1997 |title=Polar "Caps" on Ganymede and Io Revisited |journal=Icarus |volume=128 |issue=2 |pages=469–471 |bibcode=1997Icar..128..469J |doi=10.1006/icar.1997.5746 }}</ref>

The existence of a neutral atmosphere implies that an [[ionosphere]] should exist, because oxygen molecules are ionized by the impacts of the energetic [[electron]]s coming from the magnetosphere<ref name="Paranicas1999">{{cite journal |last1=Paranicas |first1=C. |last2=Paterson |first2=W. R. |last3=Cheng |first3=A. F. |last4=Mauk |first4=B. H. |last5=McEntire |first5=R. W. |last6=Frank |first6=L. A. |last7=Williams |first7=D. J. |display-authors=2 |title=Energetic particles observations near Ganymede |journal=J. Geophys. Res. |date=1999 |volume=104 |issue=A8 |pages=17,459–17,469 |doi=10.1029/1999JA900199 |bibcode=1999JGR...10417459P }}</ref> and by solar [[Extreme ultraviolet|EUV]] radiation.<ref name="Eviatar2001" /> However, the nature of the Ganymedian ionosphere is as controversial as the nature of the atmosphere. Some ''Galileo'' measurements found an elevated electron density near Ganymede, suggesting an ionosphere, whereas others failed to detect anything.<ref name="Eviatar2001" /> The electron density near the surface is estimated by different sources to lie in the range 400–2,500&nbsp;cm<sup>−3</sup>.<ref name="Eviatar2001" /> As of 2008, the parameters of the ionosphere of Ganymede were not well constrained.

Additional evidence of the oxygen atmosphere comes from spectral detection of gases trapped in the ice at the surface of Ganymede. The detection of [[ozone]] (O<sub>3</sub>) bands was announced in 1996.<ref name="Noll1996">{{cite journal |last1=Noll |first1=Keith S. |last2=Johnson |first2=Robert E. |last3=Domingue |first3=D. L. |last4=Weaver |first4=H. A. |display-authors=2 |date=July 1996 |title=Detection of Ozone on Ganymede |journal=Science |volume=273 |issue=5273 |pages=341–343 |doi=10.1126/science.273.5273.341 |pmid=8662517 |bibcode=1996Sci...273..341N |s2cid=32074586 }}</ref> In 1997 spectroscopic analysis revealed the [[Dimer (chemistry)|dimer]] (or [[diatomic]]) absorption features of molecular oxygen. Such an absorption can arise only if the oxygen is in a dense phase. The best candidate is molecular oxygen trapped in ice. The depth of the dimer absorption bands depends on [[latitude]] and [[longitude]], rather than on surface albedo—they tend to decrease with increasing latitude on Ganymede, whereas O<sub>3</sub> shows an opposite trend.<ref name="Oxygen97">{{cite journal |last1=Calvin |first1=Wendy M. |last2=Spencer |first2=John R. |date=December 1997 |title=Latitudinal Distribution of O<sub>2</sub> on Ganymede: Observations with the Hubble Space Telescope |journal=Icarus |volume=130 |issue=2 |pages=505–516 |doi=10.1006/icar.1997.5842 |bibcode=1997Icar..130..505C |url=https://zenodo.org/record/1229830 |access-date=July 13, 2019 |archive-date=December 2, 2020 |archive-url=https://web.archive.org/web/20201202001450/https://zenodo.org/record/1229830 |url-status=live }}</ref> Laboratory work has found that O<sub>2</sub> would not cluster or bubble but would dissolve in ice at Ganymede's relatively warm surface temperature of 100 K (−173.15&nbsp;°C).<ref name="sci.5320">{{cite journal |last1=Vidal |first1=R. A. |last2=Bahr |first2=D. |s2cid=27378519 |date=1997 |title=Oxygen on Ganymede: Laboratory Studies |journal=Science |volume=276 |issue=5320 |pages=1839–1842 |bibcode=1997Sci...276.1839V |doi=10.1126/science.276.5320.1839 |pmid=9188525 |display-authors=1 }}</ref>

A search for [[sodium]] in the atmosphere, just after such a finding on Europa, turned up nothing in 1997. Sodium is at least 13 times less abundant around Ganymede than around Europa, possibly because of a relative deficiency at the surface or because the magnetosphere fends off energetic particles.<ref name="ic.126.1">{{cite journal |last=Brown |first=Michael E. |date=1997 |title=A Search for a Sodium Atmosphere around Ganymede |journal=Icarus |volume=126 |issue=1 |pages=236–238  |bibcode=1997Icar..126..236B |doi=10.1006/icar.1996.5675 |citeseerx=10.1.1.24.7010 }}</ref> Another minor constituent of the Ganymedian atmosphere is [[atomic hydrogen]]. Hydrogen atoms were observed as far as 3,000&nbsp;km from Ganymede's surface. Their density on the surface is about {{nowrap |1.5{{E|4}} cm<sup>−3</sup>}}.<ref name="Barth1997">{{cite journal |last1=Barth |first1=C.A. |last2=Hord |first2=C.W. |last3=Stewart |first3=A.I. |last4=Pryor |first4=W. R. |last5=Simmons |first5=K. E. |last6=McClintock |first6=W. E. |last7=Ajello |first7=J. M. |last8=Naviaux |first8=K. L. |last9=Aiello |first9=J. J. |s2cid=123038216 |display-authors=2 |title=Galileo ultraviolet spectrometer observations of atomic hydrogen in the atmosphere of Ganymede |journal=Geophys. Res. Lett. |date=1997 |volume=24 |issue=17 |pages=2147–2150 |bibcode=1997GeoRL..24.2147B |doi=10.1029/97GL01927 |doi-access=free }}</ref>

In 2021, water vapour was detected in the atmosphere of Ganymede.<ref>[https://www.space.com/jupiter-moon-ganymede-water-vapor-discovery Water vapor detected on huge Jupiter moon Ganymede for 1st time] {{Webarchive|url=https://web.archive.org/web/20210806162110/https://www.space.com/jupiter-moon-ganymede-water-vapor-discovery |date=August 6, 2021 }}, Space.com</ref>