lecture on Saturn
lecture on Uranus and Neptune

• Saturn is a bright, easily visible object in the sky, and was known as a wanderer to early humans
• Uranus (1781) discovered by William Herschel, British Astronomer, by chance while viewing sky with telescope
• Neptune (1846) discovered when deviations (or perturbations) in orbit of Uranus led to prediction of Neptune's presence by two astronomers: (Adams and Leverrier ) and later observed (where predicted) by Galle

• Pioneer 11
  • Launched on 5 April 1973
  • used Jupiter's gravitational field ("sling-shot") to radically alter trajectory
  • first probe to explore Saturn (1979) and rings
  • passed close to Saturn and then followed trajectory to escape solar system
  • mission ended in September 1995 when electrical power source was exhausted
• Voyager 1 - launched September 5, 1977
  • flew by Saturn November 1980
• Voyager 2 - launched August 20, 1977
  • flew by Saturn August 1981
  • flew by Uranus on January 1986
    • pole was facing Sun
  • flew by Neptune on August 1989
    • most detailed knowledge of Uranus and Neptune comes from Voyager 2
• Cassini -launched October 1997
  • detailed exploration of Saturn beginning in 2004
  • tour of Saturn's moons with flybys for close inspection
  • Huygens probe dropped into Titan's atmosphere in January 2005
  • continues today to explore Saturn and its moons

• Saturn
  • bland appearance
    • thicker atmosphere than Jupiter blocks view into different layers
  • dquatorial regions rotate faster -> differential rotation
  • rapid rotation of gaseous structure causes 10% equatorial bulge
  • density (690 kg/m³) less than water ("floats")
    
    
  • Spectacular ring system
    • Due to tilt of rotation axis (27^o) appearance on Earth of Saturn's rings changes every few years
      • Even almost vanishing when edge-on, as in this Hubble Space Telescope image of 1995
    
    
  • Moons:
    • one large moon, Titan, discovered in 1655
    • 8 major moons of diameters greater than about 300 km, 53 others discovered, mostly since the year 2000.
  
  
• Uranus
  • Rotation and Tilt of Uranus
    • Uranus' rotation axis tilted 98^o relative to orbital plane
      • north pole points directly at the Sun at one point in orbit
        • south total darkness
      • south pole points directly at the Sun 1/2 uranian year later
        • north total darkness
    • Very strange seasons on Uranus
      • poles have summers with Sun directly overhead
        • day on pole is 42 years long
      • winter on poles with Sun never appearing
        • night on pole is 42 years long
      • Origin of Tilt?
        • Catastrophic event? Grazing collison with another planet sized body?
  
  
• Neptune
  • surface temperature (59 K) is higher than surface temperature of Uranus (58 K)
    • Uranus has no significant internal energy source
    • Neptune radiates 2.7 times more heat than it receives from Sun
  • Largest moon of Neptune (Triton) has Nitrogen atmosphere and active geysers

• Atmospheres of Saturn, Uranus and Neptune
  
  Elements
  • molecular hydrogen, H₂ (Saturn is 92%, Uranus and Neptune 84%)
  • 14% helium (Saturn is 7%, Uranus and Neptune 14%)
    • helium is precipitating out and sinking into Saturn
    • Saturn is actively differentiating
  • methane, CH₄ (0.2% on Saturn, 2% on Uranus and 3% on Neptune)
  • ammonia, NH₃, is 0.02% on Saturn, but very little on Uranus and Neptune
    • ammonia frozen out on Uranus and Neptune due to extreme cold
  
  
  
• Color
  • Saturn is featureless with yellowish and tan cloud belts
    • small-scale storms powered by convection and rapid rotation
  • Uranus and Neptune are a bluish color (Uranus blue-green)
    • due to methane in the atmosphere
    • methane absorbs out the red
    • Neptune is distantly more blue than Uranus due to more methane
• Weather
  • Saturn
    • No prominent spots as seen on Jupiter, but faint, long-lived ovals are seen
      • convection and rapid rotation
  • Uranus
    • Feature-less appearance
      • no internal heat source
    • Flows below surface create very efficient energy transfer
  • Neptune
    • Cloud and band structure more easily seen
      • internal heat source -> convection
    • Cloud streaks
    • Upper atmosphere slightly warmer than Uranus (59 K vs. 58 K)
    • Great Dark Spot
      • discovered by Voyager 2 (1989)
      • 1995 - Hubble Space Telescope finds

• Rings of Saturn
  • Rings are the most notable feature of Saturn
  • Rings have been labeled in order of discovery
  D,C,B,A,F,E,...
  • Table of Rings
  • Structure of Rings
    • Large collection of particles
      • ranging in size from fraction of millimeter to tens of meters
      • water ice is prime constituent
        • most particles like large snowballs, with rocky cores
        • particles are highly reflective, giving brightness to the rings
    • Very thin structure:
      • thickness of 10-15 meters
      • diameter of more than 250,000 kilometers
      • (like piece of paper 10 kilometers long)
  • Cassini Division
    • gap in ring system
    • discovered by Cassini in 1675
  • The Roche Limit
    • Tidal forces will create a tidal bulge
      • At some close point to the planet the tidal forces will overcome the internal strength of the object, ripping it apart.
      • Occurs at about 2.4 times the planet's radius
      • The rings of all the jovian planets lie within (or very close to) the Roche Limit
        • Small moons can survive inside Roche limit because they are held together by inter-atomic forces (in addition to gravity)
  • Orbital Resonances
    Cassini Division due to 2:1 resonance with Mimas (the first medium-sized moon)

    • Outer edge of A ring -> 3:2 resonance with Mimas (Atlas)
  • Ringlets (thousands) resolved by Voyager 2
  • Shepherd Satellites
    • F Ring - narrow and braid-like appearance
    • Two small satellites (Prometheus and Pandora) orbit on either side
      • Shepherding the F ring
  • Spokes
    • dust hovering above B ring creating shadow
  • Age of the rings
    • Dynamic activity suggest fairly young -> 50,000,000 years
      • are they left over from a cataclysmic event, destined to vanish before the Solar System?
    • Other evidence suggests they being replenished, and billions of years old
    • Origin of Saturn's ring system not well understood

• Rings of Uranus
  • 11 Rings are known
    • Nine rings discovered from Earth (1977)
      • stellar occultation
  • dark, narrow, widely spaced
  • nearly aligned with equator of Uranus
  • shepherd satellites
  • all within the Roche Limit

• Rings of Neptune
  • 5 known rings
    • stellar occultation evidence confirmed by Voyager 2
  • nearly aligned with equator of Neptune
  • Outer ring (Adams) clumps into arcs
  • all within the Roche Limit

• General Theory of Rings of the Jovian Planets
  • All jovian worlds have rings
  • Rings are relatively short-lived (millions of years)
  • Therefore: formation is a common event
    • otherwise we would not see rings on all 4 planets
  • Local environment important:
    • Moons, catastrophic events, etc.

• Saturn
  Emits three times as much energy as it receives from the Sun
  • -> internal heat source
  • Heat generation mechanism much different from Jupiter
  • Helium precipitation ("helium rain")
  • Saturn in process of differentiating
  Similar internal structure to Jupiter
  
  • Saturn gets denser and denser towards its interior
    • liquid molecular hydrogen
    • liquid metallic hydrogen (generates large magnetic field)
    • rocky core (about 20 times Earth's mass)
• Uranus and Neptune
  • Rocky cores
    • about 10 times the Earth's mass and about the same size
  • no metallic hydrogen (which is found in Jupiter and Saturn)
    • different source of magnetic field from Jupiter and Saturn
      probably related to ionic ammonia ("slush")

Magnetospheres

• Saturn
  Intrinsic strength of magnetic field about 1,000 times Earth
    (about 1/20 of Jupiter)
  
  Magnetic field at Saturn's cloud tops about the same as Earth's surface
  • roughly 10 Earth radii from Saturn's center
  
  Field direction reversed relative to the Earth (just as with Jupiter)
  
  • The magnetic axis is nearly aligned with spin axis ( unlike Earth)
  
  Auroras appear at poles, just as on Earth and Jupiter
• Uranus and Neptune
  • internal magnetic fields 100 x's Earth
  • Uranus and Neptune magnetic fields are tilted to large angles relative to the rotation axis
    • Uranus is 60°
    • Neptune is 46°
  • Uranus and Neptune magnetic fields are not centered on center of planet

• Moons of Saturn
  • Saturn has 8 major satellites, dominated by Titan
    and 53 other known satellites, for a total of 61 known.
    
    Three groups of satellites
    
    
  • Small moons (54, one is considered a major moon in table)
    • irregularly shaped chunks of ice
    • all less then 400 km across
    • complex motion
      • Co-orbital satellites (Janus and Epimetheus)
        • Share one orbit - exhanging inner and outer paths every 4 years
      • Two (Telesto and Calypso) share the orbit of Tethys (a medium sized moon) -
        • Telesto 60 degrees ahead of Tethys and Calypso 60 degrees behind
          • Lagrange Points
      • Helene is tied to Lagrange Point of Dione (medium-sized moon)
      • Hyperion - chaotic rotation - unpredictable tumbling

        • influence of Titan and Saturn
    • only largest of small moons, Hyperion, listed as major moon
    
    
  • Medium-sized moons (6)
    Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus
    • spherical
    • 400-1500 km in diameter
    • All densities between 1000 and 1400 kg/m³
    • largely rock and water ice, like Titan
    • Interesting feature: Herschel crater on Mimas
    • Water volcanism on Enceladus
    • cratered ice on Rhea
    • asymmetric dark markings on Iapetus result from recently discovered diffuse ring
    
    
  • Titan (single dominate large moon of Saturn)
    • second largest satellite in the solar system
      • discovered in 1655 by Christian Huygens
    • Featureless appearance in visible light (upper cloud deck)
      
    • Large-scale features visible in infrared
    • atmosphere denser than the Earth
      • dense atmosphere results from distance from Sun and cold evolutionary conditions
      • very active chemistry
      • possibly conducive to life
    • surface covered by uniform smog-like haze
      • haze layers (blue) shown in contrast-enhanced, true-color image from Cassini in 2005
    • atmosphere about 98% nitrogen (N₂)
      • about 2 percent methane (the source of the complex chemistry and smog)
      • traces of argon and other gases
    • Surface of Titan:
      • Cassini's infrared instruments penetrated the atmosphere to reveal details
        • feature thought to be an icy volcano
        • water ice on Titan plays the role of rocks on Earth
        • liquid water on Titan plays the role of lava on Earth
        • image showing lakes on surface of Titan
        • lakes are composed of liquid hydrocarbons (including ethane - C₂H₆), as confirmed by Cassini data
      • Huygens probe dropped from Cassini through the atmosphere to the surface
        • photograph from altitude of 8 km, showing river-like channels
        • Huygens's view of landing site (showing sedentary rocks - of ice)
    • Internal structure of Titan:
      • rocky core
      • thick mantle of water ice
      • density of 1900 kg/m³
      • thick layer of liquid water a few kilometers below the surface

• Moons of Uranus
  • 27 known moons - major moons
  • 3 classes
    • 11 small, inner moons, mostly discovered by Voyager
    • 5 medium sized moons
      • image shows five largest moons of Uranus and two midsize moons of Neptune(Proteus and Nereid)
    • 11 newly discovered outer moons
  • many of the moons (the earliest discovered) revolve in Uranus skewed equatorial plane
    • the inner moons are in circular, tidally locked orbits(very regular orbits, excepting for tilt)
  • no large moons
    • Only the 5 medium sized moons
  • strangest - Miranda -jagged and fractured

• Moons of Neptune
  • 13 known moons - major moons
  • not a very regular moon system
    • no moons on roughly circular, equatorial, prograde orbits
      • unlike the regular moon system of Uranus
  • Outer moon - Nereid
    • Elongated orbit
    • 1.4 million km -> 9.7 million km
  • Triton
    • 7th largest moon in Solar System
    • retrograde orbit (going around the "wrong" way)
    • 37K surface temperature
    • one of three objects in Solar System know to have a nitrogen dominated atmosphere
      • The other two are Earth, and Titan (Saturn)
    • geysers
      • vaporized nitrogen erupting from below the surface
    • surface covered by few meters of clear, frozen nitrogen
    • origin of Triton?
    • future of Triton
      • spiraling toward Neptune (retrograde orbit)
      • tidal gravitation of Neptune will rip Triton apart
        • perhaps in 100,000,000 years
        • spectacular Rings?