PES 105        Fall 2001
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General Astronomy I
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Lecture Notes:

Saturn

text: Chapter 9: Section 9.2

Introduction and General Properties

Here is a good link for information about the Saturn: http://seds.lpl.arizona.edu/billa/tnp/saturn.html

Here is a good link for information, pictures and animations about Saturn: http://www.planetscapes.com/solar/eng/saturn.htm

Here is a link with pictures of Saturn: http://antwrp.gsfc.nasa.gov/apod/index/Saturn.html

 

Saturn is a large planet

mass = 95 x Earth

volume = 380 x Earth

albedo = 0.47 . . . . cloud covered

density = 0.7 g/cm3 . . . lowest of all planets

=> more like gas or liquid

Motions

orbit:

  • orbital period = 29.5 years

rotation:

  • rotation period = 10.5 hours
    • very fast
  • equator rotates faster than poles
    • "differential rotation"
  • fast rotation flattens out the sphere at the poles - bulges at equator

Interior

observe strong magnetic field

since it rotates quickly, expect liquid inside

magnetic axis is aligned with rotation axis

core of Saturn cross section

higly differentiated

Figure 9.11 also show a cross section of Saturn. [Link to Figure 9.11]

source of heat inside

  • planet emits 2 1/2 times the energy received from the Sun
  • should have cooled faster than Jupiter
    • original heat should be gone
  • heat from helium condensing to liquid and raining down inside planet
    • only happens at cooler temperatures
    • may start to happen soon on Jupiter

Atmosphere

Composition: (from spectroscopy)

  • hydrogen - 88 %
  • helium - 11 %
  • others - < 1 %

similar to composition of original cloud that solar system formed from

=> very little change in atmosphere

 

atmosphere of Saturn

cloud region is more spread out on Saturn (from weaker gravity)

Ciculation

from:
  • convection
    • mainly internal heat rather than Sun
  • rotation

see belts and zones as on Jupiter

not as much color variation - so harder to see

Figure 9.10 shows the belts and zones. [Figure 9.10]

History

basically the same as Jupiter !

Moons:

18 moons

Figure 9.15 shows pictures of some of the moons discussed here. [Link to Figure 9.15]

  • mainly ice
  • all in synchronous orbit [except Hyperion (#16)]

    Moons of Saturn

Smaller moons:

Shepherding moons
  • Prometheus (3) and Pandora (4)
  • inside rings
  • two closely spaced moons with a ring trapped between them . . . . . shepherding moons
    • outer moon slows particles down
    • inner moon speeds them up

    co-orbital moons

    numbers 9,10,11

    • co-orbital moons around Saturn

    balance of gravity from Saturn and Tethys keeps small moons in place

    Phoebe (#18)

    retrograde orbit

    probably captured asteroid

Intermediate size satellites (7 of them):

mainly ice with some rock

several have evidence for major collisions (fractures, large craters)

Enceladus (#8)

albedo almost = 1

  • very clean surface
  • maybe ice volcanoes
    • from tidal forces (like Io around Jupiter)
    • orbital period of Dione (#12) is about 2x Enceladus

    Iapetus (#17)

  • one side very dark
  • other side very bright

    Iapetus

    Figure 9.16 is a picture of Iapetus. [Link to Figure 9.16]

One large satellite: Titan (#15):

  • 2nd largest in Solar System
  • bigger than Mercury
  • very thick atmosphere
    • 5 x denser than Earth
    • pressure at surface = 1.6 x Earth
    • unusual (only one other moon has an atmosphere - Triton around Neptune)

    Do we expect atmosphere ?

    • escape velocity ?
  • gravity weak (not ideal)
  • temperature - very cold (95 K) (good)

    probably could hold heavy gasses

    composition:

  • nitrogen - 97 %
  • methane - 2 %
  • others - 1 %
    • including complex molecules

    mainly nitrogen ! . . . . life ?

    • probably from volcanoes (possibly still active)

      too cold for life as we know it

    small greenhouse effect (about 12 degrees)

    Surface:

  • too cloudy to see
  • may have oceans of ethane
  • possible continents - from radar
  • covered with hydrocarbons ?

Rings

  • orbiting debris
  • each particle orbits independently (obey Kepler's 3rd law)
  • particles are bright - mainly ice
  • total mass of all particles is small
    • about a small moon
  • ring system is very wide but very thin

    dimensions of Saturn's rings

  • OPTIONAL MATERIAL:

    rings orbit about the planet's equator (which is tilted 27 degrees)

    view from Earth changes depending on how Saturn is tilted relative to Earth

    tilt of Saturn's rings

Detail of ring structure:

detail of Saturn's rings

Gaps in rings come mainly from orbital resonances with moons

example: Cassini Division
orbital period in gap = 1/2 orbital period of Mimas

every 2 orbits particles in gap and Mimas line up

extra tug of gravity pulls particles out

Figure 9.13 shows some of the ring detail. [Link to Figure 9.13]

Origin of rings:

3 possibilities:

  • rings are debris that never formed a moon
    • too close to planet
    • tidal forces rip it apart again
  • rings are debris from a moon ripped apart by tidal forces
  • rings are debris from a moon broken up by collisions

Rings should not be stable

  • collisions with gas around planet should cause orbits to decay
  • new material must be added (from moons?)

Now we have completed the solar system known from ancient times to 1781.

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