Moons of the Outer PlanetsThe Galilean MoonsThe Interiors of the Galilean MoonsIo – the volcanic moonVolcanoes on IoEuropaAn Internal Ocean on Europa?Some Moons of SaturnMiranda – the “messed-up” moon of UranusThe Rings of SaturnRings and the “Roche Limit”Ring StructuresJupiter’s RingUranus’ RingsNeptune’s “Arc” RingsMoons of the Outer PlanetsThe outer planets have large retinues of moons, including objects as big as Mercury (Ganymede,Titan) and with an atmosphere (Titan). There are also numerous little moons, some of which are captured asteriods. The major moons formed in a disk around the planet, like a little solar system.The Galilean MoonsGalileo saw 4 major moons around Jupiter. Each of them is rather different. The surfaces of Io and Europa are rather young, Ganymede and Callisto have lots of craters. The outer 3 have icy surfaces. The moons keep one face towards Jupiter due to tides.The Interiors of the Galilean MoonsThe inner moons are denser than the outer moons (like the planets).Io – the volcanic moonIo has a 4 day period around Jupiter. Its surface is covered with volcanoes and lava flows, and changes as we watch it. The yellow color is due to sulfur, which keeps erupting. The interior of Io is heated by tidal stresses, as Europa keeps trying to disturb its tidal locking with Jupiter.Volcanoes on IoEuropaEuropa is an ice-covered rocky moon with a 3.5 day period (just twice Io’s). This “orbital resonance” keeps both Io and Europa internally heated. The stresses are apparent in the huge surface cracks. There are very few craters, indicating the surface has been continually reworked.Natural color False colorAn Internal Ocean on Europa?False colorSome Moons of SaturnMimas – the “bullseye” moon.Enceladus – the bright moon.Iapetus – a dark leading surface.Miranda – the “messed-up” moon of UranusThe Rings of SaturnGalileo noticed these rings, although he couldn’t quite see what they were. Since they are thousands of miles across and only 20 meters thick, they are much relatively thinner than paper (and you can see through them. They are composed of billions of tiny bodies, from dust to boulder sized. We now think that rings only last for millions of years, so we are privileged to see them so nice. [Maybe 50 million years from now, Mars will be better.]Rings and the “Roche Limit”Inside the Roche limit, the tidal stresses on a body (which grow with its size and closeness) are stronger than its internal strength (basically gravity, in the case of moons). This is generally at 2.5 planetary radii or so.Ring StructuresThe rings show ephemeral “spokes”, probably due to magnetic interactions (movie).The primary structures are due to “orbital resonances with the moons, and the thousands of ringlets are “density waves”. Moonlets also play a role in sheparding rings and clearing gaps.Jupiter’s RingUranus’ RingsVery dark, originally discovered by “stellar occultations from Earth, imaged by Voyager.Neptune’s “Arc” RingsSheparded “braided”
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