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Chapter 23
Comparative Planetology
of Jupiter and Saturn
Guidepost
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 challenge—to 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?
Guidepost (continued)
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?
There’s 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 Saturn’s rings. It will
be interesting, but it is no place like home.
Outline
I. A Travel Guide to the Outer Planets
A. The Outer Planets
B. Atmospheres and Interiors
C. Satellite Systems
II. 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
Outline (continued)
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
A Travel Guide to the Outer Planets
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
Jupiter
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: Multicolored cloud belts
Visual image
Infrared falsecolor image
Explored in detail by
several space probes:
Pioneer 10,
Pioneer 11,
Voyager 1,
Voyager 2,
Galileo
The Mass of Jupiter
Mass can be inferred from the orbit of Io,
the innermost of the 4 Galilean Moons:
Using Kepler’s third law  MJupiter = 318 MEarth
Jupiter’s Interior
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.
The Chemical Composition of
Jupiter and Saturn
Jupiter’s Rotation
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.
Jupiter’s Magnetic Field
Discovered through observations of decimeter (radio) radiation
Magnetic field at least
10 times stronger than
Earth’s magnetic field.
Magnetosphere over
100 times larger than
Earth’s.
Extremely intense
radiation belts:
Very high energy
particles can be
trapped; radiation
doses corresponding
to ~ 100 times lethal
doses for humans!
Aurorae on Jupiter
Just like on
Earth, Jupiter’s
magnetosphere
produces aurorae
concentrated in
rings around the
magnetic poles.
~ 1000 times
more powerful
than aurorae
on Earth.
The Io Plasma Torus
Some of the heavier ions originate from Jupiter’s moon Io.
Io flux tube
Visible
Io flux tube
UV
Inclination of Jupiter’s magnetic field against rotation
axis leads to wobbling field structure passing over Io 
Acceleration of particles to high energies.
Jupiter’s Atmosphere
Jupiter’s 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.
Jupiter’s Atmosphere (2): Clouds
Three layers
of clouds:
1. Ammonia
(NH3) crystals
2. Ammonia
hydrosulfide
3. Water
crystals
The Cloud Belts on Jupiter
Dark belts and bright zones.
Zones higher and cooler than belts;
high-pressure regions of rising gas.
The Cloud Belts on Jupiter (2)
Just like on Earth, high-and low-pressure zones
are bounded by high-pressure winds.
Jupiter’s Cloud belt structure has remained
unchanged since humans began mapping them.
The Great Red Spot
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
The Great Red Spot (2)
Structure of Great Red Spot may be determined
by circulation patterns in the liquid interior
Jupiter’s Ring
Galileo spacecraft
image of Jupiter’s
ring, illuminated
from behind
Not only Saturn, but all four
gas giants have rings.
Jupiter’s 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 can’t be old because
radiation pressure and Jupiter’s
magnetic field force dust particles to
spiral down into the planet.
Rings must be constantly
re-supplied with new dust.
Comet Impact on Jupiter
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.
Visual: Impacts
seen for many days
as dark spots
Impacts released energies equivalent to
a few megatons of TNT (Hiroshima
bomb: ~ 0.15 megaton)!
The History of Jupiter
• Formed from
cold gas in the
outer solar
nebula, where
ices could
condense.
• Rapid growth
• Soon able to
trap gas directly
through gravity
• In the interior,
hydrogen becomes
metallic (very good
electrical conductor)
• Rapid rotation 
strong magnetic
field
• Rapid rotation
and large size
 belt-zone
cloud pattern
• Heavy materials • Dust from meteorite impacts onto
sink to the center
inner moons trapped to form ring
Jupiter’s Family of Moons
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.
Callisto: The Ancient Face
Tidally locked to Jupiter, like all of Jupiter’s 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.
Ganymede: A Hidden Past
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
Jupiter’s Influence on its Moons
Presence of Jupiter has at least two effects
on the geology of its moons:
1.
2.
Tidal
effects:
possible
source of
heat for
interior
of Ganymede
Focusing of
meteoroids,
exposing
nearby
satellites to
more
impacts
than those
further out.
Europa: A Hidden Ocean
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.
The Surface of Europa
Cracked surface and high albedo (reflectivity)
provide further evidence for geological activity.
The Interior of Europa
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.
Io: Bursting Energy
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.
Interaction with Jupiter’s Magnetosphere
Io’s volcanoes blow out sulfur-rich gasses
 tenuous
atmosphere, but
gasses can not
be retained by
Io’s gravity
 gasses
escape from Io
and form an ion
torus in
Jupiter’s
magnetosphere
The History of the Galilean Moons
• 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.
Saturn
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.
Saturn’s Magnetosphere
Magnetic field ~ 20 times weaker than Jupiter’s
 weaker
radiation
belts
Magnetic field not
inclined against
rotation axis
 Aurorae
centered
around poles of
rotation
Saturn’s Atmosphere
Cloud-belt
structure, formed
through the same
processes as on
Jupiter,
but not as distinct as
on Jupiter; colder
than on Jupiter.
Saturn’s Atmosphere (2)
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!
Saturn’s Rings
Ring consists of 3
main segments:
A, B, and C Ring
separated by
empty regions:
divisions
Rings can’t have been
formed together with
Saturn because
material would have
been blown away by
particle stream from
hot Saturn at time of
formation.
A Ring
B Ring
C Ring
Cassini
Division
Rings must be
replenished by
fragments of
passing comets
& meteoroids.
Composition of Saturn’s Rings
Rings are
composed of
ice particles
moving at large
velocities around
Saturn, but small
relative velocities
(all moving in the
same direction).
Shepherd Moons
Some moons
on orbits close
to the rings
focus the ring
material,
keeping the
rings confined.
Divisions and Resonances
Moons do not only serve as “Shepherds”.
Where the orbital period of a moon is a
small-number fractional multiple (e.g., 2:3)
of the orbital period of material in the disk
(“resonance”), the material is cleared out
 Divisions
Titan
• About the size of Jupiter’s
moon Ganymede.
• Rocky core, but also large
amount of ice.
• Thick atmosphere, hiding
the surface from direct view.
Titan’s Atmosphere
Explored in detail by the Cassini
spacecraft and the Huygens
probe, which landed on Titan’s
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).
Titan’s Surface
Before Huygens, Titan’s
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
Saturn’s Smaller Moons
Hyperion: Too small to pull
itself into spherical shape.
All other known moons are
large enough to attain a
spherical shape.
Saturn’s Smaller Moons (2)
Saturn’s 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.
The Origin of
Saturn’s Satellites
• No evidence of common
origin, as for Jupiter’s moons.
• Probably captured icy
planetesimals.
• Moons interact
gravitationally, mutually
affecting each other’s 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.