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Comparative Planetology of Jupiter and Saturn
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• Chapter 23

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Guidepost
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• As we begin this chapter, we leave behind the
  psychological security of planetary surfaces. You
  can imagine standing on the moon, on Mars, or
  even on Venus, but Jupiter and Saturn have no
  surfaces. Here you face a new challengeto use
  comparative planetology to study worlds so
  unearthly we cannot imagine being there. As you
  study these worlds you will find answers to five
  essential questions
• How do the outer planets compare with the inner
  planets?
• How is Jupiter different from Earth?
• How did Jupiter and its system of moons and
  rings form and evolve?
• How is Saturn different from Jupiter?
• How did Saturn and its system of moons and rings
  form and evolve?

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Guidepost (continued)
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• Your study of Jupiter and Saturn will give you a
  chance to answer two important questions about
  science.
• How Do We Know? How is science different from
  technology?
• How Do We Know? Who pays to gather scientific
  knowledge?
• Theres no place to stand on Jupiter or Saturn,
  but be sure to bring your spacesuit. Both planets
  have big systems of moons, and when you visit
  them you will be able to watch erupting
  volcanoes, stroll through a methane rain storm,
  and swim in Saturns rings. It will be
  interesting, but it is no place like home.

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Outline
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• A Travel Guide to the Outer Planets
• The Outer Planets
• Atmospheres and Interiors
• Satellite Systems
• Jupiter
• A. Surveying Jupiter
• B. Jupiter's Magnetic Fields
• C. Jupiter's Atmosphere
• D. Jupiter's Ring
• E. Comet Impact on Jupiter
• F. The History of Jupiter

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Outline (continued)
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III. Jupiter's Family of Moons A. Callisto
The Ancient Face B. Ganymede A Hidden Past
C. Europa A Hidden Ocean D. Io
Bursting Energy E. The History of the
Galilean Moons IV. Saturn A. Planet Saturn B.
Saturn's Rings C. The History of Saturn V.
Saturn's Moons A. Titan B. The Smaller
Moons C. The Origin of Saturn's Moons
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A Travel Guide to the Outer Planets
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All are much larger than the terrestrial planets,
have thick atmospheres, but are very
inhospitable
Visual image

• No solid surfaces

• Cloud belt patterns

• Mostly Hydrogen and Helium

• Rings

• Strong atmospheric circulation (winds/storms)

• Multiple moons

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Jupiter
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Largest and most massive planet in the solar
system
Contains almost 3/4 of all planetary matter in
the solar system.
Most striking features visible from Earth
Multi-colored cloud belts
Explored in detail by several space probes
Visual image
Pioneer 10, Pioneer 11, Voyager 1, Voyager 2,
Galileo
Infrared false-color image
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The Mass of Jupiter
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Mass can be inferred from the orbit of Io, the
innermost of the 4 Galilean Moons
Using Keplers third law ? MJupiter 318 MEarth
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Jupiters Interior
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From radius and mass ? Average density of Jupiter
1.34 g/cm3
? Jupiter can not be made mostly of rock, like
earthlike planets.
? Jupiter consists mostly of hydrogen and helium.
T 30,000 K
Due to the high pressure, hydrogen is compressed
into a liquid, and even metallic state.
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The Chemical Composition of Jupiter and Saturn
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Jupiters Rotation
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Jupiter is the most rapidly rotating planet in
the solar system
Rotation period slightly less than 10 hr.
Centrifugal forces stretch Jupiter into a
markedly oblate shape.
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Jupiters Magnetic Field
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Discovered through observations of decimeter
(radio) radiation
Magnetic field at least 10 times stronger than
Earths magnetic field.
Magnetosphere over 100 times larger than Earths.
Extremely intense radiation belts
Very high energy particles can be trapped
radiation doses corresponding to 100 times
lethal doses for humans!
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Aurorae on Jupiter
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Just like on Earth, Jupiters magnetosphere
produces aurorae concentrated in rings around the
magnetic poles.
1000 times more powerful than aurorae on Earth.
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The Io Plasma Torus
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Some of the heavier ions originate from Jupiters
moon Io.
Io flux tube
Io flux tube
Visible
UV
Inclination of Jupiters magnetic field against
rotation axis leads to wobbling field structure
passing over Io ? Acceleration of particles to
high energies.
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Jupiters Atmosphere
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Jupiters liquid hydrogen ocean has no surface
Gradual transition from gaseous to liquid phases
as temperature and pressure combine to exceed the
critical point.
Jupiter shows limb darkening ? hydrogen
atmosphere above cloud layers.
Only very thin atmosphere above cloud layers
transition to liquid hydrogen zone 1000 km
below clouds.
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Jupiters Atmosphere (2) Clouds
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Three layers of clouds
1. Ammonia (NH3) crystals
2. Ammonia hydrosulfide
3. Water crystals
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The Cloud Belts on Jupiter
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Dark belts and bright zones.
Zones higher and cooler than belts high-pressure
regions of rising gas.
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The Cloud Belts on Jupiter (2)
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Just like on Earth, high-and low-pressure zones
are bounded by high-pressure winds.
Jupiters Cloud belt structure has remained
unchanged since humans began mapping them.
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The Great Red Spot
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Several bright and dark spots mixed in with cloud
structure.
Largest and most prominent The
Great Red Spot.
Has been visible for over 330 years.
Formed by rising gas carrying heat from below the
clouds, creating a vast, rotating storm.
2 DEarth
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The Great Red Spot (2)
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Structure of Great Red Spot may be determined by
circulation patterns in the liquid interior
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Jupiters Ring
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Not only Saturn, but all four gas giants have
rings.
Galileo spacecraft image of Jupiters ring,
illuminated from behind
Jupiters ring dark and reddish only discovered
by Voyager 1 spacecraft.
Composed of microscopic particles of rocky
material
Location Inside Roche limit, where larger bodies
(moons) would be destroyed by tidal forces.
Ring material cant be old because radiation
pressure and Jupiters magnetic field force dust
particles to spiral down into the planet.
Rings must be constantly re-supplied with new
dust.
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Comet Impact on Jupiter
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Impact of 21 fragments of comet SL-9 in 1994
Impacts occurred just behind the horizon as seen
from Earth, but came into view about 15 min.
later.
Impact sites appeared very bright in the infrared.
Impacts released energies equivalent to a few
megatons of TNT (Hiroshima bomb 0.15 megaton)!
Visual Impacts seen for many days as dark spots
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The History of Jupiter
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• Formed from cold gas in the outer solar nebula,
  where ices could condense.

• In the interior, hydrogen becomes metallic (very
  good electrical conductor)

• Rapid rotation ? strong magnetic field

• Rapid growth

• Soon able to trap gas directly through gravity

• Rapid rotation and large size ? belt-zone cloud
  pattern

• Heavy materials sink to the center

• Dust from meteorite impacts onto inner moons
  trapped to form ring

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Jupiters Family of Moons
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Over two dozen moons known now new ones are
still being discovered.
Four largest moons already discovered by Galileo
The Galilean moons
Io
Europa
Ganymede
Callisto
Interesting and diverse individual geologies.
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Callisto The Ancient Face
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Tidally locked to Jupiter, like all of Jupiters
moons.
Av. density 1.79 g/cm3
? composition mixture of ice and rocks
Dark surface, heavily pocked with craters.
No metallic core Callisto never differentiated
to form core and mantle.
? No magnetic field.
Layer of liquid water, 10 km thick, 100 km
below surface, probably heated by radioactive
decay.
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Ganymede A Hidden Past
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Largest of the 4 Galilean moons.

• Av. density 1.9 g/cm3

• Rocky core

• Ice-rich mantle

• Crust of ice

1/3 of surface old, dark, cratered
rest bright, young, grooved terrain
Bright terrain probably formed through flooding
when surface broke
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Jupiters Influence on its Moons
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Presence of Jupiter has at least two effects on
the geology of its moons
2. Focusing of meteoroids, exposing nearby
satellites to more impacts than those further out.
1. Tidal effects possible source of heat for
interior of Gany-mede
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Europa A Hidden Ocean
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Av. density 3 g/cm3
? composition mostly rock and metal icy surface.
Close to Jupiter ? should be hit by many
meteoroid impacts but few craters visible.
? Active surface impact craters rapidly erased.
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The Surface of Europa
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Cracked surface and high albedo (reflectivity)
provide further evidence for geological activity.
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The Interior of Europa
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Europa is too small to retain its internal heat ?
Heating mostly from tidal interaction with
Jupiter.
Core not molten ? No magnetic field.
Europa has a liquid water ocean 15 km below the
icy surface.
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Io Bursting Energy
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Most active of all Galilean moons no impact
craters visible at all.
Over 100 active volcanoes!
Activity powered by tidal interactions with
Jupiter.
Av. density 3.55 g/cm3 ? Interior is mostly
rock.
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Interaction with Jupiters Magnetosphere
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Ios volcanoes blow out sulfur-rich gasses
? tenuous atmosphere, but gasses can not be
retained by Ios gravity
? gasses escape from Io and form an ion torus in
Jupiters magnetosphere
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The History of the Galilean Moons
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• Minor moons are probably captured asteroids

• Galilean moons probably formed together with
  Jupiter.

• Densities decreasing outward ? Probably formed
  in a disk around Jupiter, similar to planets
  around the sun.

Earliest generation of moons around Jupiter may
have been lost and spiraled into Jupiter
Galilean moons are probably a second generation
of moons.
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Saturn
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Mass 1/3 of mass of Jupiter
Radius 16 smaller than Jupiter
Av. density 0.69 g/cm3 ? Would float in water!
Rotates about as fast as Jupiter, but is twice as
oblate ? No large core of heavy elements.
Mostly hydrogen and helium liquid hydrogen core.
Saturn radiates 1.8 times the energy received
from the sun.
Probably heated by liquid helium droplets falling
towards center.
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Saturns Magnetosphere
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Magnetic field 20 times weaker than Jupiters
? weaker radiation belts
Magnetic field not inclined against rotation axis
? Aurorae centered around poles of rotation
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Saturns Atmosphere
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Cloud-belt structure, formed through the same
processes as on Jupiter,
but not as distinct as on Jupiter colder than on
Jupiter.
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Saturns Atmosphere (2)
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Three-layered cloud structure, just like on
Jupiter
Main difference to Jupiter
Fewer wind zones, but much stronger winds than on
Jupiter
Winds up to 500 m/s near the equator!
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Saturns Rings
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A Ring
Ring consists of 3 main segments A, B, and C Ring
B Ring
C Ring
separated by empty regions divisions
Cassini Division
Rings cant have been formed together with Saturn
because material would have been blown away by
particle stream from hot Saturn at time of
formation.
Rings must be replenished by fragments of passing
comets meteoroids.
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Composition of Saturns Rings
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Rings are composed of ice particles
moving at large velocities around Saturn, but
small relative velocities (all moving in the same
direction).
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Shepherd Moons
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Some moons on orbits close to the rings focus the
ring material, keeping the rings confined.
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Divisions and Resonances
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Moons do not only serve as Shepherds.
Where the orbital period of a moon is a
small-number fractional multiple (e.g., 23) of
the orbital period of material in the disk
(resonance), the material is cleared out
? Divisions
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Titan
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• About the size of Jupiters moon Ganymede.
• Rocky core, but also large amount of ice.
• Thick atmosphere, hiding the surface from direct
  view.

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Titans Atmosphere
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Explored in detail by the Cassini spacecraft and
the Huygens probe, which landed on Titans
surface in 2005.

• Atmosphere consists mostly of nitrogen, methane
  and ethane

• Surface pressure 50 greater than air pressure
  on Earth

• Surface temperature 94 K (-290 oF)

? methane and ethane can condense and lead to
rain of methane and ethane
Methane is gradually converted to ethane in the
Atmosphere
? Methane must be constantly replenished,
probably through breakdown of ammonia (NH3).
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Titans Surface
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Before Huygens, Titans surface could only be
probed with infrared cameras.

• Revealed bright and dark regions on the surface

• Bright regions possibly a result of methane and
  ethane ice

Huygens discovered outflow channels, possibly of
liquid methane
and grapefruit sized rocks on the surface
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Saturns Smaller Moons
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Hyperion Too small to pull itself into spherical
shape.
All other known moons are large enough to attain
a spherical shape.
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Saturns Smaller Moons (2)
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Saturns smaller moons are formed of rock and
ice heavily cratered and appear geologically
dead.
Iapetus Leading (upper right) side darker than
rest of surface because of dark deposits.
Phoebe and Tethys Heavily cratered, ancient
surfaces
Enceladus Possibly active regions with fewer
craters, containing parallel grooves, possibly
filled with frozen water.
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The Origin of Saturns Satellites
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• No evidence of common origin, as for Jupiters
  moons.

• Probably captured icy planetesimals.

• Moons interact gravitationally, mutually
  affecting each others orbits.

• Co-orbital moons (orbits separated by only 100
  km) periodically exchange orbits!

• Small moons are also trapped in Lagrange points
  of larger moons Dione and Tethys.