Editing Ganymede (moon) (section)

==Orbit and rotation==
[[File:Galilean moon Laplace resonance animation 2.gif|thumb|right|[[Laplace resonance]] of Ganymede, [[Europa (moon)|Europa]], and [[Io (moon)|Io]] (conjunctions are highlighted by color changes)]]

Ganymede [[orbit]]s Jupiter at a distance of {{convert|1070400|km|mi}}, third among the Galilean satellites,<ref name="Planetary Society" /> and completes a revolution every seven days and three hours (7.155 days<ref>{{Cite web |title=Ganymede: Facts – NASA Science |url=https://science.nasa.gov/jupiter/moons/ganymede/facts/ |access-date=November 18, 2023 |website=science.nasa.gov |language=en}}</ref>). Like most known moons, Ganymede is [[tidal locking|tidally locked]], with one side always facing toward the planet, hence its day is also seven days and three hours.<ref name="The Grand Tour">{{cite book |title=The Grand Tour: A Traveler's Guide to the Solar System |last1=Miller |first1=Ron |author-link=Ron Miller (artist and author) |first2=William K. |last2=Hartmann |date=May 2005 |pages=108–114 |publisher=Workman Publishing |location=Thailand |edition=3rd |isbn=978-0-7611-3547-0 }}</ref> Its orbit is very slightly eccentric and inclined to the Jovian [[equator]], with the [[orbital eccentricity|eccentricity]] and [[inclination]] changing [[almost periodic function|quasi-periodically]] due to solar and planetary gravitational [[perturbation (astronomy)|perturbations]] on a timescale of centuries. The ranges of change are 0.0009–0.0022 and 0.05–0.32°, respectively.<ref name="Musotto2002">{{cite journal |last1=Musotto |first1=Susanna |last2=Varadi |first2=Ferenc |last3=Moore |first3=William |last4=Schubert |first4=Gerald |title=Numerical Simulations of the Orbits of the Galilean Satellites |date=2002 |volume=159 |issue=2 |pages=500–504 |doi=10.1006/icar.2002.6939 |bibcode=2002Icar..159..500M |journal=Icarus }}</ref> These orbital variations cause the [[axial tilt]] (the angle between the rotational and orbital axes) to vary between 0 and 0.33°.<ref name="Bills2005" />

Ganymede participates in [[orbital resonance]]s with Europa and Io: for every orbit of Ganymede, Europa orbits twice and Io orbits four times.<ref name="Musotto2002" /><ref name="SPACE.com">{{cite web |url=http://www.space.com/searchforlife/seti_tidal_europa_021003.html |title=High Tide on Europa |first=Cynthia |last=Phillips|author-link=Cynthia B. Phillips |work=SPACE.com |date=October 3, 2002 |url-status=dead |archive-date=October 17, 2002 |archive-url=https://web.archive.org/web/20021017211633/http://space.com/searchforlife/seti_tidal_europa_021003.html }}</ref> [[Astronomical conjunction|Conjunctions]] (alignment on the same side of Jupiter) between Io and Europa occur when Io is at [[periapsis]] and Europa at [[apoapsis]]. Conjunctions between Europa and Ganymede occur when Europa is at periapsis.<ref name="Musotto2002" /> The longitudes of the Io–Europa and Europa–Ganymede conjunctions change at the same rate, making triple conjunctions impossible. Such a complicated resonance is called the [[Laplace resonance]].<ref name="Showman1997a">{{cite journal |last1=Showman |first1=Adam P. |last2=Malhotra |first2=Renu |title=Tidal Evolution into the Laplace Resonance and the Resurfacing of Ganymede |journal=Icarus |date=1997 |volume=127 |issue=1 |pages=93–111 |doi=10.1006/icar.1996.5669 |url=http://www.lpl.arizona.edu/~showman/publications/showman-malhotra-1997.pdf |bibcode=1997Icar..127...93S |access-date=January 22, 2008 |archive-date=May 14, 2011 |archive-url=https://web.archive.org/web/20110514231007/http://www.lpl.arizona.edu/~showman/publications/showman-malhotra-1997.pdf |url-status=live }}</ref>
The current Laplace resonance is unable to pump the orbital eccentricity of Ganymede to a higher value.<ref name="Showman1997a" /> The value of about 0.0013 is probably a remnant from a previous epoch, when such pumping was possible.<ref name="SPACE.com" /> The Ganymedian orbital eccentricity is somewhat puzzling; if it is not pumped now it should have decayed long ago due to the tidal [[dissipation]] in the interior of Ganymede.<ref name="Showman1997a" /> This means that the last episode of the eccentricity excitation happened only several hundred million years ago.<ref name="Showman1997a" /> Because Ganymede's orbital eccentricity is relatively low—on average 0.0015<ref name="SPACE.com" />—tidal heating is negligible now.<ref name="Showman1997a" /> However, in the past Ganymede may have passed through one or more Laplace-like resonances<ref name="laplaceres" group=lower-alpha /> that were able to pump the orbital eccentricity to a value as high as 0.01–0.02.<ref name="Showman1999" /><ref name="Showman1997a" /> This probably caused a significant tidal heating of the interior of Ganymede; the formation of the grooved terrain may be a result of one or more heating episodes.<ref name="Showman1999" /><ref name="Showman1997a" />

There are two hypotheses for the origin of the Laplace resonance among Io, Europa, and Ganymede: that it is primordial and has existed from the beginning of the Solar System;<ref name="Peale2002">{{cite journal |last1=Peale |first1=S.J. |last2=Lee |first2=Man Hoi |title=A Primordial Origin of the Laplace Relation Among the Galilean Satellites |journal=Science |date=2002 |volume=298 |pages=593–597 |doi=10.1126/science.1076557 |bibcode=2002Sci...298..593P |pmid=12386333 |issue=5593 |arxiv=astro-ph/0210589 |s2cid=18590436 }}</ref> or that it developed after the [[Formation and evolution of the Solar System|formation of the Solar System]]. A possible sequence of events for the latter scenario is as follows: Io raised tides on Jupiter, causing Io's orbit to expand (due to conservation of momentum) until it encountered the 2:1 resonance with Europa; after that, the expansion continued, but some of the angular [[moment (physics)|moment]] was transferred to Europa as the resonance caused its orbit to expand as well; the process continued until Europa encountered the 2:1 resonance with Ganymede.<ref name="Showman1997a" /> Eventually the drift rates of conjunctions between all three moons were synchronized and locked in the Laplace resonance.<ref name="Showman1997a" />