THE SATURN SYSTEM

SATURN: Ringed and Very Gassy

  • Mass is 95.2 times that of Earth.
  • Radius is 60,268 km or 9.45 times Earth's.
  • Density is only 0.687 g/cm^3 or 0.125 of Earth's.

    As seen from Earth:

  • Most distant naked eye planet.
  • Period of 29.42 years.
  • Semi-major axis of 9.54 AU, e = 0.054.
  • Rings are prominent; tilted at 27 degrees so seen from
    different angles from Earth; nearly invisible when edge-on
  • 9 moons discovered from the ground.
  • No solid surface: differential rotation
  • Magnetic (interior) spin period: 10h 40m
  • Spins so fast and is so gassy that its ellipticity is over 10%

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    Saturn's Atmosphere and Interior are Very Similar to Jupiter's

    BUT the main differences include:

  • Lower density: less mass so less compression
  • Molecular H_2 layer ~= 30,000 km thick
  • Metallic H layer ~= 15,000 km thick
  • Rocky/metallic core, R ~= 15,000 km
  • Central: T = 15,000 K; P = 5 times 10^6 atm
  • Strong magnetic field, but still only 0.05 of Jupiter's.

  • Fewer belts and zones (due to lower T)
  • but faster winds (to 1500 km/h)
  • Thicker cloud layers (lower gravity)
  • Less coloration (probably due to lower T)
  • In atm: H_2 over 92%, He almost 8%;
    He has settled or ``rained'' downwards.
    At the lower T and P of Saturn's atm, He doesn't dissolve in H_2.
    Separation of He and compression of whole planet yields more heat,
    so Saturn's surface T is 97 K, not 74 K.

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    SATURN'S RINGS

    Rings as Seen from Earth

  • Huygens: Galileo's bumps (1610) were rings (1659).
  • Main gap, about 2/3rd out, seen by Giovanni Cassini in 1675.

    RING NAMES:

  • A ring is outermost (with Encke gap in it)
  • B ring (inside Cassini division) is brightest
  • the C ring, inside B, is much fainter.

    The rings can't be solid bodies, nor could they be liquid or gas --
    solids would be ripped apart, liquids or gases would dissipate.

  • James Clerk 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.

  • High reflectivity implies ice balls, or icy mantles on rocks.
  • Most ring particles around 10 cm in diameter; lots of tinier ones
    and a small number of ones up to 10 m or so.

  • The ROCHE LIMIT is the radius at which tidal stresses rip a body apart ---
    no large moons can survive within about 2.8 R_S.
  • So rings are probably shredded Moons or captured asteroids, most likely after collisions.
  • Estimated lifetime under 10^7 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

    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 2:1 resonance with Mimas
  • Even Cassini division not empty --- just many fewer particles in that region
  • Sharp outer edge of A ring is due to little moon Atlas, stuck in a 3:2 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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    SATURN'S MOONS

    TITAN: the biggest by far

  • a = 1,220,000 km; D = 5150 rm; M = 1.83 M_{Moon}
  • rho = 1.9g/cm^3 -- > very thick ice layer OR more likely, a mixture of rock and ice

    An atmosphere thicker and denser than Earth's, w/ P = 1.6 bar;
    over 400 km thick; several haze layers

  • T_surf = 94K
  • Methane and ethane rain or snow. Possible hydrocarbon oceans?
  • There is a slim chance that some very different form of life has evolved in Titan's oceans.
  • The Huygens craft detached from Cassini and landed on Titan in Jan 2005.
    It showed craters, "seas" or at least sludge, and "springs"