Jovian Planet Moons and Rings

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Jovian Planet Moons and Rings

• Each planet has multiple moons and seem like
  mini-solar systems
• Moons are mostly made of ice, so quite soft and
  can be more active than similar sized rocky
  planets/moons
• Rings are made of individual small particles,
  orbiting like micromoons

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RINGS Saturns First

• Rings as Seen from Earth
• Huygens Galileo's bumps (1610), or ears (1616)
  were rings (1655).
• Main gap, about 2/3rd out, seen by Giovanni
  Cassini in 1675 Cassinis division

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Ring Names

• A ring is outer bright one (with Encke gap in
  it) between 122,000 and 137,000 km
• B ring (inside Cassini division) is brightest
• C ring, inside B, is much fainter (down to 75,000
  km)
• Later, D, inside C then F, G, E (all outside A)
  were discovered

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What are they?

• The rings can't be solid bodies, nor could they
  be liquid or gas --
• solids would be ripped apart, liquids or gases
  would dissipate
• Maxwell suggested many small bodies, each in
  Keplerian orbit (1857)
• different Doppler shifts confirmed hypothesis in
  1895.
• Rings are very thin and let light through
  collisions damp out motions perpendicular to
  plane so they are very narrow (lt100 m thick!).
• High reflectivity implies ice balls, or ice on
  rocks.

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The Roche Limit

• Most ring particles around 10 cm in diameter
  lots of tinier ones and a small number of ones up
  to 10 m or so.
• ROCHE LIMIT is the radius at which tidal stresses
  rip a body apart ---
• no large moons can survive within about 2.8 RS
• So rings probably shredded moons or mashed
  captured asteroids, likely after collisions.
• Estimated lifetime under 107 yr ?temporary.

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Rings as Seen from Voyager

• THOUSANDS OF RINGLETS in known rings
• Due to spiral density waves, produced by small
  moons in and near the rings
• Encke division due to SWEEPING UP'' by Pan, a
  20 km diameter Moon

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Cassini and many other divisions are due to
RESONANCES with Moons (and Moonlets)

• These rhythmic gravitational tugs make it very
  difficult to stay in orbits whose periods are
  fractions of a Moon's period.
• Cassini division is 21 resonance with Mimas
• Even Cassini division not empty --- just many
  fewer particles in that region

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Weird Rings

• Sharp outer edge of A ring is due to little moon
  Atlas, stuck in a 32 resonance with Mimas
• D ring, very thin, inside of C and down to
  cloud-tops
• ? F ring, very narrow and braided, is held by
  SHEPARDING SATELLITES
• E ring, very far out, probably volcanic debris
  from Enceladus
• ? Radial Spokes'' on B ring tiny charged dust
  above ring plane, orbiting w/ Saturn's magnetic
  field

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Last Word on Saturns Rings
Moon repels ring particles as it gives and takes
orbital energy from them sheparding moons force
narrow ring(lets)
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JUPITER HAS RINGS TOO

• Roughly 50,000 km above cloud layers A few
  thousand km wide, most only tens of km thick
• Particles smaller and darker than those of
  Saturn So must be replenished, via collisions of
  asteroids with each other or J's moons.

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ALL OUTER PLANETS HAVE RINGS

• URANUS has 11 sets of rings between 1.45 and 2.00
  planetary radii
• Five densest discovered by stellar occultation in
  1977
• 4 more seen from Earth, 2 more only by Voyager in
  1986
• Much narrower, darker, with wider gaps than
  Saturn's Kept in line by sheparding satellites
• NEPTUNE has 5 sets of rings between 1.65 and 2.54
  planetary radii discovered by Voyager 2 in 1989
• They are incomplete -- more arcs than rings
• Since ring systems are believed to be transitory,
  they must be crated fairly easily and often as we
  have found them around all 4 Jovian planets!

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Stellar Occultation Technique
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Rings of Uranus Neptune

• Uranus Occulted Neptune and Shepards (Cordelia,
  Opehlia) for Uranus epsilon ring

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All Rings Lie Within Roche Limits