Saturn

Feb. 2, 2005, jeb

OUTLINE
• Basic Properties of Saturn
• Notable Characteristics
• Saturn's Rings
• Saturn's Atmosphere
• Saturn's Internal Structure
• Saturn's Magnetosphere
• Moons of Saturn
• Cassini Mission to Saturn and its Moon Titan
• Saturn's Basic Properties
• Saturn's Golden Globe Awards
• Other sources on Saturn

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Named for god of agriculture in Roman mythology

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Basic Properties of Saturn

Orbital period Semi-major axis Mass Radius Density

29.4 years 9.54 AU 95 x Earth's mass (0.03% of S.S.) 9.4 x Earth's radius 687 kg/m3

Not nearly as bright as Jupiter in the Earth's sky

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Notable Characteristics:

Surface

• Featureless yellowish and tan cloud belts
• No prominent spots as seen on Jupiter, but faint, long-lived ovals are seen

Spectacular ring system

• Due to tilt of rotation axis (27^o) Saturn's appearance on Earth changes every few years.
  • Even appearing edge-on, as in this Hubble Space Telescope image of 1995

Moons:

• Titan discovered in 1655
• 18 moons of diameters greater than 10 km, 13 others discovered

Rotation:

• Interior: 10h 40m (a little slower than Jupiter)
• Equatorial regions faster -> differential rotation
  • causes 10% equatorial bulge

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Saturn's Rings

The most notable feature of Saturn



History of discovery

1610- Galileo notices feature

1655- Huygens realizes Saturn is encircled by flat ring

1675- Cassini discovers dark band above 2/3 out from inner edge

• Cassini Division

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Spacecraft Visits:

• Pioneer 11 (1979), Voyager I + II (1980-81), Cassini (2004)

Rings have been labeled in order of discovery

from the inside out 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 less than two kilometers
  • diameter of more than 250,000 kilometers
  • (like piece of paper 100 meters long)

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 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)

• Cassini Division starts at 117,000 km
• Mimas orbits at 186,000 km

  Kepler's Law tells us that the period squared is proportional to the semi-major axis cubed.
  

  (117,000)³ = 1,600,000,000,000,000

  (186,000)³ = 6,400,000,000,000,000

  A ratio of 4 in the cubes of the semi-major axes means a ratio of two in the periods.

• This resonance condition means anything in the Cassini Division gets a tug from Mimas every second time is comes around Saturn. This is a resonance which over time will "clean out" the region.

Other resonances:

• Outer edge of A ring -> 3:2 resonance with Mimas (Atlas)
• others

Shepherd Satellites

F Ring - narrow and braid-like appearance

Two small satellites (Prometheus and Pandora) orbit on either side

• Shepherding the F ring

D Ring

Faint rings inside the inner edge of the C ring

stretches almost to Saturn's cloud tops

so faint it is completely invisible from Earth

E Ring

Faint ring outside the Roche Limit

associated with volcanism on the moon Enceladus

Spokes

dust hovering above B ring creating shadow

Very complex system of rings and moons

Age of the rings

Dynamic activity suggest fairly young -> 50,000,000 years

Are they being replenished,

or are they left over from a cataclysmic event, destined to vanish before the Solar System?


Ringlets (thousands) resolved by Voyager 2

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Saturn's Atmosphere

Elements:

• 92.4% molecular hydrogen {by number}
• 7.4% helium {by number}
  • missing helium (compare to Jupiter)
  • sank toward center (differentiation)?
• 0.2% methane
• 0.02% ammonia
• little of the early atmosphere escaped due to the strong gravity (large escape velocity)
  

Clouds:

• Three layers
• Thicker than Jupiter - blocking view into different layers
  • explanation for bland appearance of Saturn's surface

Zonal Flow

• As on Jupiter (east to west dominates) although much faster flow rates
  • more than 10 times hurricane speeds on Earth
• Large scale flows and small-scale storms powered by
  • convection in interior
  • rapid rotation

Spots

• fewer hurricane like storms than on Jupiter

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Saturn's Internal Structure

• -> internal heat source
  • remember, since Saturn is about twice as far from the Sun as Jupiter, it receives about 1/4 as much energy from the Sun
• Heat generation mechanism much different from Jupiter
• Helium precipitation
• Saturn in process of differentiating

• Saturn gets denser and denser towards its interior
  • liquid molecular hydrogen
  • liquid metallic hydrogen (generates large magnetic field)
  • rocky core (about 20 x's Earth's mass)

Saturn's Magnetosphere

Intrinsic strength of magnetic field about 1,000 times Earth
  (about 1/20 of Jupiter)

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

Magnetosphere encompasses most of Saturn's moons, with the largest moon (Titan) passing in and out of it

Moons of Saturn ( caption)

Saturn has 18 major satellites
and 13 other known satellites, for a total of 31 known.
Three groups of major satellites
• Small moons (11)
  1. irregularly shaped chunks of ice
  2. all less then 300 km across
  3. complex motion
• Medium-sized moons (6)
  1. spherical
  2. 400-1500 km in diameter
• Titan (single large moon)
  1. second largest satellite in the solar system
  2. atmosphere denser than the Earth
  3. possibly conducive to life

Table of major moons


Titan
Featureless appearance in visible light (upper cloud deck)
Large-scale features visible in infrared
Single large moon of Saturn

second largest moon of Solar System Discovered in 1655 by Christian Huygens

Atmosphere denser than Earth


Very active chemistry
1. surface covered by uniform smog-like haze
   • haze layers (blue) shown in false color image from Voyager 1
2. about 90% nitrogen
3. less than 10% argon
4. few percent methane (the source of the complex chemistry and smog)


Dense atmosphere results from distance from Sun and cold evolutionary conditions.



Internal structure of Titan:
• rocky core
• thick mantle of water ice
• density of 1900 kg/m³
  
  European Space Agency (ESA) reported on first Huygens results on Titan on January 21, 2005


Medium-sized Moons
Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus
• All densities about 1200 kg/m³
• largely rock and water ice, like Titan
• Interesting feature: Herschel crater on Mimas
• Water volcanism on Enceladus
• cratered ice on Rhea


Small Moons

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

Launch: October, 1997
Arrival at Saturn: 2004
Transmissions from Saturn: 2004-2008
Spacecraft:
two-story-tall, 5650 kg (over half propellant) robotic spacecraft consisting of
• an orbiter
• and ESA's Huygens Titan probe.
  • Huygens landed on Titan on January 14, 2005
  • Images came back for analysis
Mission: To explore the most distant planet that can easily be seen by the unaided human eye.
• Saturn's interesting atmosphere and interior
• the most spectacular of the four planetary ring systems
• numerous icy satellites with a variety of unique surface features
• a huge magnetosphere teeming with particles that interact with the rings and moons
• the intriguing moon Titan, slightly larger than the planet Mercury with a hazy atmosphere denser than that of Earth.
Many revealing photos have been taken
Where is Cassini now?

Other Sources on Saturn

StarDate Guide to Saturn
Welcome to the Planets - Saturn
Views of the Solar System - Saturn
The Nine Planets - Saturn