Moons%20of%20the%20Outer%20Planets

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Moons of the Outer Planets
The 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.
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The Galilean Moons
Galileo 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.
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The Interiors of the Galilean Moons
The inner moons are denser than the outer moons
(like the planets).
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Io the volcanic moon
Io 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.
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Volcanoes on Io
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Europa
Europa is an ice-covered rocky moon with a 3.5
day period (just twice Ios). 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 color
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An Internal Ocean on Europa?
False color
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Some Moons of Saturn
Mimas the bullseye moon.
Iapetus a dark leading surface.
Enceladus the bright moon.
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Miranda the messed-up moon of Uranus
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The Rings of Saturn
Galileo noticed these rings, although he couldnt
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.
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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.
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Ring Structures
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.
The rings show ephemeral spokes, probably due
to magnetic interactions (movie).
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Jupiters Ring
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Uranus Rings
Very dark, originally discovered by stellar
occultations from Earth, imaged by Voyager.
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Neptunes Arc Rings
Sheparded braided ring.