Saturn (Outer Planet)

Saturn (Outer Planet)

1. Ring system

   • Brightest among all other outer planet's ring system
   • Main rings
     • A & B rings are separated by the Cassini Division
       • Cassini Division
         • populated by ring material bearing such similarity to the C Ring
     • A &B rings are bright
       • C ring is almost translucent
   • Density waves
     • Rings contain concentric regions of alternating high & low concentrations of ring particles
       • induced by the gravitational perturbation of some nearby moons
     • Ringlets = high-density peaks
   • Thin ring plane
   • rings are composed of a great no. of small water ice particles, all independently orbiting Saturn
     • Water ice = prime constituent of the bright rings
     • reflect sunlight effectively = appears bright
   • Young rings
     • brightness & purity of water ice in the rings
       • Evidence: rings are much younger than Saturn
       • about 150 mn years old
         • Saturn formed 4.6bn y/a

2. Formation of the Rings

   • [Theory] Icy moon of Saturn was hit by a comet & was destroyed
     • Pieces of the moon then pursued their own individual orbits around Saturn
       • eventually spreading all the way around it in the form of rings
   • Saturn's rings have numerous gaps where particle density drops sharply
     • Pan & Daphnis are small moons orbitting w/i Encke Gap & Keeler Gap respectively in Saturn's A ring
       • The gaps are thought to be kept clear by the gravitational action of the small moons
         • such small moons can survive w/i the RL
           • held tgt mostly by IMF not by gravity

3. Tidal Force

   • Pieces that have not aggregated into a moon ever since cuz they lie w/i the Roche limit
     • A planet's Roche Limit = 2.4 times the radius of the planet
       • Inside the RL: Saturn's tidal force is strong enough to tear a moon apart
         • => Prevent formation of any moon
       • Roche Limit
         • a moon is held tgt by its own gravity
           • A planet's tidal force is to stretch a moon
             • creating a tidal bulge
             • The tidal for INCREASES with DECREASING distance from the planet
         • [D] the critical distance inside of which the planet's tidal force becomes greater than the moon's internal gravitational force

4. Moons

   • many known moons
     • 53 of them have formal names
     • 1. Large Moon
       • Titan (only 1)
         • 2nd largest moon in the solar system
         • HAS an atmosphere
           • thick, uniform layer of haze that envelops the moon
             • completely obscures the spacecraft's view of its surface
             • thicker & denser than Earth
               • Atmosphere contains 10 times more gas than Earth's
               • atmospheric pressure at ground level is 60% greater than on Earth
               • far more substantial than any other moons
           • mostly nitrogen (~90%) & argon (at most 10%) with a small % of methane
         • Formation of atmosphere
           • Titan formed at a considerably low temp, with water ice making up the bulk of its interior
             • Water ice could absorb methane & ammonia that were present in abundance at those early times
               • trapped gas -> forming thick ammonia atmos.
                 • Sunlight split NH3 into N & H
                   • H escaped into space
                   • N remained in the atmos.
         • Low Temp
           • distance from the Sun is great
             • Surface temp = 94K
               • methane may behave rather like the water on Earth
                 • Liquid methane
                   • only object besides Earth for which has stable bodies of surface liquid
                     • Suggested that life can exist in liquid methane lakes (just as Earth's org live in water)
         • dense + opaque atmos = prevent observing its surface
           • Cassini-Huygens Mission 2004
             • orbiter & lander
             • Radar Mapping & Direct Images
               • Cassini Orbiter
                 • entered orbit around July 1, 2004
                   • radar image its surface during close fly-bys
               • Huygens lander reached Titan on Jan 13, 2005
                 • obtained the first direct images of Titan's surface
                   • photographed: pale hills with dark "rivers"
                     • periodically rains liquid methane & other dark organic cpd
                     • Cassini spacecraft discovered liquid HC lakes in its polar regions
     • 2. Medium Moons
       • [Cassini SC] water-rich plumes venting from Enceladus' south polar region
         • strong evidence that Enceladus has an ocean with an energy source
       • Saturn has a diffuse E ring which lies beyond the main 3 rings
         • Enceladus orbits in the densest part of the E ring
           • released most of the material making up the E ring
     • 3. Small Moons
       • many small moons
         • irregularly shaped chunks of ice
       • Eg) Prometheus, Pandora
         • aka "Shepherd Satellites"
           • orbit on either side of Saturn's faint, narrow F ring
             • Their gravitational influence on the F-ring particles keeps the ring tightly confined in its narrow orbit

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