Saturn's
Rings
Printable
version
April
18, 2000
Announcements
Saturn's
Rings Preview
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finish
Jupiter's moons
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Large
(Galilean) moons
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Io,
Europa, Ganymede, Callisto
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Callisto
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Ice
rock mix
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surface
not active
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All
of Jupiter's moons
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Seeing
Saturn's rings from Earth
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Now
you see them, now you don't
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A,
B, C, ..., Cassini division
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Composition
of rings
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Structure
of rings
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Effect
of Saturn's inner Satellites
Callisto
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Callisto has craters
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surface no longer active
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Biggest crater named Valhalla
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Callisto has no groves or
any sign of geologic activity
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other than slumping of Valhalla's
rings
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Valhalla crater probably
4 billion years old
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No magnetic field
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Not differentiated
All
of Jupiter's Moons
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Large
(Galilean) moons:
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Io,
Europa, Ganymede, Callisto
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16
moons discovered so far
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Smallest
is Leda (16 km)
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Outer
4 orbit backwards (retrograde)
Seeing
Saturn's Rings from Earth
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Saturn is ~10 AU from Earth
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compare 5 AU for Jupiter
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Saturn looks smaller in telescope
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1610: Galileo saw funny "ears"
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Ears came and went every
couple of years
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(now you see them now you
don't)
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1655: Huygens proposed "ears"
were a thin flat ring
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inclination of the rings
relative to ecliptic accounted for changes
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rings aligned with Saturn's
equator
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In 1675 Giovanni Cassini
first to see gap in rings
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Cassini division
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Rings lettered A, B, C ....
Saturn from a good
Earth telescope
Composition
of Rings
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In 1857 James Clerk Maxwell
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also
famous for: Maxwell's Equations
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Calculated
tidal forces on thin sheet
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Suggested
Saturn's rings must be composed of many small particles
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In
1895 James Keeler measured Doppler shift of reflected light from rings
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Inner
part of rings moves faster
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consistent
with Kepler's 3rd law (P2 = a3)
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confirmed
Maxwell's calculations
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Rings
reflect 80% of sunlight that hits them
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Voyager
fly by measured
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temperature
of rings -180o C to
-200o C
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sizes:
1 cm to 5 m
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average
size 10 cm
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What
are the rings made of?
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Why
didn't Saturn's rings form a moon?
Structure
of Rings
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Why didn't Saturn's rings
for a moon?
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Rings are too close to Saturn
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Large gravitationally bound
structures can't exist inside a critical radius
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Roche limit
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Tidal forces rip them apart
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We exist inside Earth's Roche
limit.
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Why aren't we ripped apart
by tidal forces?
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We
aren't gravitationally bound
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our
bodies are held together by chemical bonds
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Voyager
showed rings are composed of many ringlets
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Rings
are as little as 10 km thick!
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Backlighting
shows gaps have particles not 1 cm to 5 m but 1 mm
(dust/smoke)
Saturn
Jupiter's
ring
Ringlets
seen from Voyager (Sunward side)
F-ring
Rings
backlit by Sun. Cassini division is white,
B-ring
is purple
Effect
of Saturn's Inner Satellites
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Moon Mimas responsible for
Cassini division
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Mimas period 22.6 days
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Cassini division particles
period 11.3 days
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2-to-1 resonance
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Where else do we see this
effect?
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Kirkwood
gaps in Asteroid belt
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What
planet causes Kirkwood gaps?
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Pandora and Prometheus are
shepherd moons for F ring
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Pan causes Enke gap
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Rings themselves are not
uniform all the way around
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See "Structure of Saturn's
Rings movie" on CD ROM
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Radial spokes/shadows might
be caused by Saturn's magnetic field
Saturn's
Rings Summary
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Seeing
Saturn's rings from Earth
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Now
you see them, now you don't
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Saturn's
27 degree inclination
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A,
B, C, ..., Cassini division
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Composition
of rings
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Structure
of rings
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Effect
of Saturn's inner Satellites
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Cassini
probe will orbit Saturn starting in 2004
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More
textbook chapters will be needed!