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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.
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.
The Interiors of the Galilean Moons
The inner moons are denser than the outer moons (like the planets).
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.
Volcanoes on Io
Europa
Europa 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 color
False color
An Internal Ocean on Europa?
Some Moons of Saturn
Iapetus – a dark
leading surface.
Mimas – the
“bullseye” moon.
Enceladus – the
bright moon.
Miranda – the “messed-up” moon of Uranus
The Rings of
Saturn
Galileo 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 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).
Jupiter’s Ring
Uranus’ Rings
Very dark, originally discovered by “stellar
occultations from Earth, imaged by Voyager.
Neptune’s “Arc” Rings
Sheparded “braided” ring.