Editing Europa (moon) (section)

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

Europa orbits Jupiter in roughly 3.55 days, with an orbital radius of about 670,900&nbsp;km. With an [[orbital eccentricity]] of only 0.009, the orbit itself is nearly circular, and the [[orbital inclination]] relative to Jupiter's [[equatorial plane]] is small, at 0.470°.<ref name="datasheet">{{cite web|url=http://www2.jpl.nasa.gov/galileo/europa/#overview|archive-url=https://web.archive.org/web/19970105180851/http://www.jpl.nasa.gov/galileo/europa/|url-status=dead|archive-date=5 January 1997|title=Europa, a Continuing Story of Discovery|access-date=9 August 2007|work=Project Galileo|publisher=NASA, [[Jet Propulsion Laboratory]]}}</ref> Like its fellow [[Galilean satellites]], Europa is [[tidal locking|tidally locked]] to Jupiter, with one hemisphere of Europa constantly facing Jupiter. Because of this, there is a [[Poles of astronomical bodies|sub-Jovian point]] on Europa's surface, from which Jupiter would appear to hang directly overhead. Europa's [[prime meridian]] is a line passing through this point.<ref>{{cite web|title=Planetographic Coordinates|publisher=Wolfram Research|url=http://documents.wolfram.com/applications/astronomer/AdditionalInformation/PlanetographicCoordinates.html|date=2010|access-date=29 March 2010|url-status=dead|archive-url=https://web.archive.org/web/20090301191456/http://documents.wolfram.com/applications/astronomer/AdditionalInformation/PlanetographicCoordinates.html|archive-date=1 March 2009}}</ref> Research suggests that tidal locking may not be full, as a [[non-synchronous rotation]] has been proposed: Europa spins faster than it orbits, or at least did so in the past. This suggests an asymmetry in internal mass distribution and that a layer of subsurface liquid separates the icy crust from the rocky interior.<ref name=Geissler>{{cite journal|last1=Geissler|first1=P. E.|last2=Greenberg|first2=R.|last3=Hoppa|first3=G.|last4=Helfenstein|first4=P.|last5=McEwen|first5=A.|last6=Pappalardo|first6=R.|last7=Tufts|first7=R.|last8=Ockert-Bell|first8=M.|last9=Sullivan|first9=R.|last10=Greeley|first10=R.|last11=Belton|first11=M. J. S.|last12=Denk|first12=T.|last13=Clark|first13=B. E.|last14=Burns|first14=J.|last15=Veverka|first15=J.|date=1998|title=Evidence for non-synchronous rotation of Europa|journal=[[Nature (journal)|Nature]]|volume=391|pages=368–70|bibcode=1998Natur.391..368G|doi=10.1038/34869|pmid=9450751|issue=6665|s2cid=4426840}}</ref>

The slight eccentricity of Europa's orbit, maintained by gravitational disturbances from the other Galileans, causes Europa's sub-Jovian point to oscillate around a mean position. As Europa comes slightly nearer to Jupiter, Jupiter's gravitational attraction increases, causing Europa to elongate towards and away from it. As Europa moves slightly away from Jupiter, Jupiter's gravitational force decreases, causing Europa to relax back into a more spherical shape, and creating tides in its ocean. The orbital eccentricity of Europa is continuously pumped by its [[orbital resonance|mean-motion resonance]] with Io.<ref name="Showman1997">{{cite journal |last1=Showman |first1=Adam P. |last2=Malhotra |first2=Renu |s2cid=55790129 |title=Tidal Evolution into the Laplace Resonance and the Resurfacing of Ganymede |journal=Icarus |date=May 1997 |volume=127 |issue=1 |pages=93–111 |doi=10.1006/icar.1996.5669 |bibcode=1997Icar..127...93S }}</ref> Thus, the [[tidal flexing]] kneads Europa's interior and gives it a source of heat, possibly allowing its ocean to stay liquid while driving subsurface geological processes.<ref name="geology" /><ref name="Showman1997" /> The ultimate source of this energy is Jupiter's rotation, which is tapped by Io through the tides it raises on Jupiter and is transferred to Europa and Ganymede by the orbital resonance.<ref name="Showman1997" /><ref name="Moore2003">{{cite journal |last1=Moore |first1=W. B. |title=Tidal heating and convection in Io |journal=Journal of Geophysical Research |date=2003 |volume=108 |issue=E8 |pages=5096 |doi=10.1029/2002JE001943 |bibcode=2003JGRE..108.5096M |citeseerx=10.1.1.558.6924 }}</ref>

Analysis of the unique cracks lining Europa yielded evidence that it likely spun around a tilted axis at some point in time. If correct, this would explain many of Europa's features. Europa's immense network of crisscrossing cracks serves as a record of the stresses caused by massive tides in its global ocean. Europa's tilt could influence calculations of how much of its history is recorded in its frozen shell, how much heat is generated by tides in its ocean, and even how long the ocean has been liquid. Its ice layer must stretch to accommodate these changes. When there is too much stress, it cracks. A tilt in Europa's axis could suggest that its cracks may be much more recent than previously thought. The reason for this is that the direction of the spin pole may change by as much as a few degrees per day, completing one precession period over several months. A tilt could also affect estimates of the age of Europa's ocean. Tidal forces are thought to generate the heat that keeps Europa's ocean liquid, and a tilt in the spin axis would cause more heat to be generated by tidal forces. Such additional heat would have allowed the ocean to remain liquid for a longer time. However, it has not yet been determined when this hypothesized shift in the spin axis might have occurred.<ref>Cook, Jia-Rui C. (18 September 2013) [http://www.jpl.nasa.gov/news/news.php?release=2013-283 Long-stressed Europa Likely Off-kilter at One Time] {{Webarchive|url=https://web.archive.org/web/20140817113356/http://www.jpl.nasa.gov/news/news.php?release=2013-283 |date=17 August 2014 }}. jpl.nasa.gov</ref>