Saturn
Spectacular Rings and Mysterious Moons

May 5, 2011, jeb

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

Named for god of agriculture (father of Jupiter) in Roman mythology

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
• 8 major moons of diameters greater than about 300 km, 53 others discovered, mostly since the year 2000.
Rotation:
• Interior: 10h 40m (a little slower than Jupiter)
• Equatorial regions faster -> differential rotation
  • causes 10% equatorial bulge
    • Jupiter's bulge is only 6.5%
    • difference due to Saturn's lower density

The most notable feature of Saturn



History of discovery

1610- Galileo notices feature

1616- Galileo improves rendering

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)
  • detailed exploration of Saturn, and it moons, with the Huygens probe entering Titan's atmosphere

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 10-15 meters
  • diameter of more than 250,000 kilometers
  • (like piece of paper 10 kilometers 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 (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)

• 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 at the same point in its orbit every second time it 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

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

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

Very complex system of rings and moons

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

Elements:
• 92.4% molecular hydrogen {by number} (Jupiter is 86.1%)
• 7.4% helium {by number} (Jupiter is 13.8%)
  • 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 than Jupiter
  • more than 10 times hurricane speeds on Earth
• Large scale flows and small-scale storms powered by
  • convection in interior
  • rapid rotation
Storms
• fewer hurricane like storms and associates spots than on Jupiter

Saturn's Internal Structure

Emits three times as much energy as it receives from the Sun
• -> 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
Similar 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)

Saturn's Magnetosphere

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

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

Moons of Saturn ( caption)

Saturn has 8 major satellites
and 53 other known satellites, for a total of 61 known.
Three groups of satellites
• Small moons (54, one is considered a major moon)
  1. irregularly shaped chunks of ice
  2. all less then 400 km across
  3. complex motion
  4. only largest, Hyperion, listed as major moon
• 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 contrast-enhanced, true-color image from Cassini in 2005
2. about 98% nitrogen (N₂)
3. about 2 percent methane (the source of the complex chemistry and smog)
4. traces of argon and other gases


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



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



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
  
  


Medium-sized Moons
Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus
• 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


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

Other Sources on Saturn

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

Cassini Mission to Saturn and its Moon Titan

Launch: October, 1997
Arrival at Saturn: 2004

Initial Cassini mission: 2004-2008
1st extended mission - Cassini Equinox: 2008-2010
2nd extended mission - Cassini Solstice: 2010-2017
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.