Editing Galileo project (section)

===Ganymede===
[[File:Ganymede - June 26 1996 (26781123830).jpg|thumb|right|Ganymede, photographed on June 26, 1996|alt=Ganymede looks like the Moon, with craters and darker and lighter grey regions]]
The largest of the Galilean moons with a radius of {{convert|2620|km|sp=us}}, Ganymede is larger than Earth's moon, the [[dwarf planet]] [[Pluto]] or the planet [[Mercury (planet)|Mercury]].{{sfn|Meltzer|2007|pp=267–268}} It is the largest of the moons in the Solar system that are characterized by large amounts of water ice, which also includes Saturn's moon [[Titan (moon)|Titan]], and Neptune's moon [[Triton (moon)|Triton]]. Ganymede has three times as much water for its mass as Earth has.{{Sfn|Stevenson|1996|pp=511–512}}

When ''Galileo'' entered Jovian orbit, it did so at an [[orbital inclination]] to the Jovian equator, and therefore in the orbital plane of the four Galilean moons. To transfer orbit while conserving propellant, two slingshot maneuvers were performed. On G1, the gravity of Ganymede was used to slow the spacecraft's orbital period from 210 to 72 days to allow for more encounters and to take ''Galileo'' out of the more intense regions of radiation. On G2, the gravity assist was employed to put it into a coplanar orbit to permit subsequent encounters with Io, Europa and Callisto.{{sfn|Meltzer|2007|pp=267–268}}

Although the primary purpose of G1 and G2 was navigational, the opportunity to make some observations was not missed. The plasma-wave experiment and the magnetometer detected a magnetic field with a strength of about {{convert|750|nT|nT|lk=on|disp=out|abbr=off}}, more than strong enough to create a separate magnetosphere within that of Jupiter.{{efn|Earth's magnetic field varies from 22,000 to 67,000 nanoteslas.<ref>{{cite web |title=An Overview of the Earth's Magnetic Field |publisher=British Geological Survey |url=http://www.geomag.bgs.ac.uk/education/earthmag.html#_Toc2075549 |access-date=16 April 2024 |archive-date=September 7, 2023 |archive-url=https://web.archive.org/web/20230907044147/http://www.geomag.bgs.ac.uk/education/earthmag.html#_Toc2075549 |url-status=live }}</ref> }} This was the first time that a magnetic field had ever been detected on a moon contained within the magnetosphere of its host planet.{{sfn|Kivelson|Khurana|Russell|Walker|1996|pp=537–541}}<ref>{{cite press release |title=Galileo Makes Discoveries at Ganymede |publisher=NASA/Jet Propulsion Laboratory |date=October 7, 1996 |url=https://www2.jpl.nasa.gov/releases/96/gany1res.html |access-date=December 5, 2020 |archive-url=https://web.archive.org/web/20221202092949/https://www2.jpl.nasa.gov/releases/96/gany1res.html |archive-date=2 December 2022 }}</ref><ref>{{cite press release|title= New Discoveries From Galileo – Big Icy Moon of Jupiter Found to Have a 'Voice' After All; Europa Flyby Next for Galileo |date=December 12, 1996 |first1=Douglas |last1=Isbell |first2=Mary Beth |last2=Murrill |id=96-255|publisher=NASA/Jet Propulsion Laboratory |url=http://www2.jpl.nasa.gov/galileo/status961212.html |archive-url=https://web.archive.org/web/20100602211623/http://www2.jpl.nasa.gov/galileo/status961212.html |archive-date=2010-06-02 |url-status=dead |df=mdy-all}}</ref> This discovery led naturally to questions about its origin. The evidence pointed to an iron or iron sulfide core and [[mantle (geology)|mantle]] {{convert|400|to|1,300|km|sp=us}} below the surface, encased in ice. Margaret Kivelson, the scientist in charge of the magnetometer experiment, contended that the induced magnetic field required an iron core, and speculated that an electrically conductive layer was required, possibly a brine ocean {{convert|200|km|sp=us}} below the surface.{{sfn|Meltzer|2007|pp=270–272}}<ref name="Hidden ocean">{{cite press release |title=Solar System's Moon Likely Has a Hidden Ocean |publisher=NASA |first=Guy |last=Webster |date=December 16, 2000 |url=https://solarsystem.nasa.gov/news/169/solar-systems-moon-likely-has-a-hidden-ocean/ |access-date=December 5, 2020 |archive-date=October 18, 2020 |archive-url=https://web.archive.org/web/20201018122955/https://solarsystem.nasa.gov/news/169/solar-systems-moon-likely-has-a-hidden-ocean/ |url-status=live }}</ref>

[[File:Ganymede diagram.svg|thumb|left|The internal structure of Ganymede]]
''Galileo'' returned to Ganymede on orbits G7 and G9 in April and May 1997, and on G28 and G29 in May and December 2000 on the GMM.{{sfn|Meltzer|2007|pp=268–270}} Images of the surface revealed two types of terrain: highly cratered dark regions and grooved terrain [[Sulcus (geology)|sulcus]]. Images of the Arbela Sulcus taken on G28 made Ganymede look more like Europa, but tidal flexing could not provide sufficient heat to keep water in liquid form on Ganymede, although it may have made a contribution. One possibility was radioactivity, which might provide sufficient heat for liquid water to exist {{convert|50|to|200|km|sp=us}} below the surface.<ref name="Hidden ocean" />{{sfn|Meltzer|2007|pp=271–273}} Another possibility was volcanism. Slushy water or ice reaching the surface would quickly freeze over, creating areas of a relatively smooth surface.<ref>{{cite magazine |last=Cowen |first=Ron |title=Images Suggest Icy Eruptions on Ganymede |magazine=[[Science News]] |issn=0036-8423 |date=March 3, 2001 |volume=159 |issue=9 |page=133 |doi=10.2307/3981750 |jstor=3981750 }}</ref>
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