Editing Galilean moons (section)

== Comparative structure ==
[[File:Jupiter.moons1.jpg|thumb|600px|center|Surface features of the four members at different levels of zoom in each row]]

<div style="float:right; margin:2px;">
{| class=wikitable style="text-align:center; font-size:11px"
|+ Jovian radiation
! Moon !! [[Röntgen equivalent man|rem]]/day
|-
| Io || 3600<ref name="ringwald">{{cite web |date=29 February 2000 |title=SPS 1020 (Introduction to Space Sciences) |publisher=California State University, Fresno |last=Ringwald |first=Frederick A. |url=https://zimmer.csufresno.edu/~fringwal/w08a.jup.txt |url-status=dead |access-date=5 January 2014 |archive-url=https://web.archive.org/web/20080725050708/https://zimmer.csufresno.edu/~fringwal/w08a.jup.txt |archive-date=25 July 2008 }}</ref>
|-
| Europa || 540<ref name="ringwald"/>
|-
|Ganymede || 8<ref name="ringwald"/>
|-
| Callisto || 0.01<ref name="ringwald"/>
|-
! Earth (Max) !!  0.07
|-
! Earth (Avg) !! 0.0007

|}</div>

Fluctuations in the orbits of the moons indicate that their mean density decreases with distance from Jupiter. Callisto, the outermost and least dense of the four, has a density intermediate between ice and rock whereas Io, the innermost and densest moon, has a density intermediate between rock and iron. Callisto has an ancient, heavily cratered and unaltered ice surface and the way it rotates indicates that its density is equally distributed, suggesting that it has no rocky or metallic core but consists of a homogeneous mix of rock and ice. This may well have been the original structure of all the moons. The rotation of the three inner moons, in contrast, indicates differentiation of their interiors with denser matter at the core and lighter matter above. They also reveal significant alteration of the surface. Ganymede reveals past tectonic movement of the ice surface which required partial melting of subsurface layers. Europa reveals more dynamic and recent movement of this nature, suggesting a thinner ice crust. Finally, Io, the innermost moon, has a sulfur surface, active volcanism and no sign of ice. All this evidence suggests that the nearer a moon is to Jupiter the hotter its interior. The current model is that the moons experience tidal heating as a result of the gravitational field of Jupiter in inverse proportion to the square of their distance from the giant planet. In all but Callisto this will have melted the interior ice, allowing rock and iron to sink to the interior and water to cover the surface. In Ganymede a thick and solid ice crust then formed. In warmer Europa a thinner more easily broken crust formed. In Io the heating is so extreme that all the rock has melted and water has long ago boiled out into space.

===Size===

[[File:Moons of solar system v7.jpg|thumb|900px|center|Galilean moons compared with moons of other planets (and with Earth; the scale is changed to 1 pixel = 94 km at this resolution).]]

===Latest flyby===
{{multiple image
| width     = 100
| footer    = <div style="text-align: center;">Jupiter and Galilean moons circa 2007, imaged by ''[[New Horizons]]'' during flyby. (greyscale colour)</div>
| align     = center
| image1    = Jupiter taken by New Horizons probe (2007-01-08).jpg
| alt1      =
| caption1  = [[w:Jupiter|Jupiter]] and [[w:Io (moon)|Io]]
| image2    = NewHorizonsIo.jpg
| alt2      =
| caption2  = [[w:Io (moon)|Io]]
| image3    = PIA09246 Europa.jpg
| alt3      =
| caption3  = [[w:Europa (moon)|Europa]]
| image4    = PIA09245 Ganymede.jpg
| alt4      =
| caption4  = [[w:Ganymede (moon)|Ganymede]]
| image5    = NewHorizonsCallisto.jpg
| alt5      =
| caption5  = [[w:Callisto (moon)|Callisto]]
}}
{{clear}}