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Chapter 23
Jupiter and Saturn
Guidepost
As we begin this chapter, we leave behind the
psychological security of planetary surfaces. We can
imagine standing on the moon, on Venus, or on Mars,
but Jupiter and Saturn have no surfaces. Thus, we face
a new challenge—to use comparative planetology to
study worlds so unearthly we cannot imagine being
there.
One reason we find the moon and Mars of interest is
that we might go there someday. Humans may become
the first Martians. But the outer solar system seems
much less useful, and that gives us a chance to think
about the cultural value of science.
This chapter begins our journey into the outer solar
system. In the next chapter, we will visit worlds out in
the twilight at the edge of the sun’s family.
Outline
I. 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
II. 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
Outline (continued)
III. Saturn
A. Planet Saturn
B. Saturn's Rings
C. The History of Saturn
IV. Saturn's Moons
A. Titan
B. The Smaller Moons
C. The Origin of Saturn's Satellites
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:
Io
Moon
Earth
Relative sizes, distances,
and times to scale
Jupiter
1 s corresponds to
10 hr in real time.
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.
Explorable Jupiter, Jupiter’s Aurora
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
Planetary Atmospheres
magnetic fields and other
properties of the solar planets
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.
Rings must be constantly
re-supplied with new dust.
Ring material can’t be
old because radiation
pressure and Jupiter’s
magnetic field force dust
particles to spiral down
into the planet.
Planetary atmospheres and
other properties of the solar
planets.
Planet Atmospheres and Magnetic Fields
This data is from the National Space Science Data Center's Fact Sheet site.
Planet
Mercury
Venus
Earth
Mars
Jupiter
g
vesc
distance
albedo
temperature
atm. press.
(* gE)
(km/s)
(A.U.)
(%)
(K)
(* Earth's)
0.378
4.3
0.387
5.6
100 night,
590--725 day
10-15
98% He, 2% H2
58.81 d
0.006
92
96.5% CO2, 3.5%
N2,
0.015% SO2
243.69 d
0.00
78.084% N2,
20.946% O2,
0.934% Ar, 0.035%
23.9345 h
CO2,
H2O highly variable
(< 1%)
1.000
95.32% CO2, 2.7%
N2
1.6% Ar, 0.13% O2,
184--242 day 0.007--0.009
24.623 h
0.08% CO, 0.021%
H2O,
0.01% NO
0.00
0.907
10.36
1.000 11.186
0.377
2.364
5.03
59.5
0.723
1.000
1.524
5.203
72
737
38.5 283--293 day
16
70
165
1.000
> > 100
atm. comp.
rotation
mag. field
(* Earth's)
89% H2, 11% He,
0.2% CH4, 0.02%
NH3
9.925 h
19,519
Saturn
0.916
35.5
9.539
75
134
> > 100
89% H2,
11% He,
0.3% CH4,
0.02% NH3
Uranus
0.889
21.3
19.182
90
76
> > 100
89% H2,
11% He
17.24 h
47.9
Neptune
1.125
23.5
30.06
82
72
> > 100
89% H2,
11% He
16.11 h
27.0
Pluto
0.0675
1.1
39.53
14.5
50
0.003
CH4, N2
6.405 d
0.00
10.50 h
578
Notes: Surface gravity g is given in Earth gravities (1 gE = 9.803 m/s2); escape velocity is vesc; albedo is the percent of ALL of the
Sun's energy hitting the planet that is reflected (100% would be perfect reflection); temperature and surface gravity for Jupiter, Saturn,
Uranus, Neptune are given at a depth where the atmospheric pressure = 1 Earth atmosphere; atmospheric pressure (atm. press.) is at the
surface (> > 100 for the jovian planets); rotation is the sidereal rotation period, h = hours and d = days; magnetic field (mag. field) is the
total strength (NSSDC gives strength in #gauss × Rplanet3, where Rplanet is the radius of the planet and
Earth's strength = 0.3076 gauss × RE3 = 7.981×1010 gauss.
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 were able to
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
geology of its moons:
1. Tidal
effects:
possible
source of
heat for
interior
of Ganymede
2. 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!
Planetary Atmospheres
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
Electromagnetic Phenomena in
Saturn’s Rings
Radial spokes in
the rings rotate with
the rotation period
of Saturn:
Magnetized ring particles
lifted out of the ring plane
and rotating along with
the magnetic-field
structure
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
Because of the thick, hazy
atmosphere, surface
features are only visible in
infrared images.
Many of the organic
compounds in Titan’s
atmosphere may have been
precursors of life on Earth.
Surface pressure: 50% greater
than air pressure on Earth
Surface temperature: 94 K (-290 oF)
 methane and ethane are liquid!
Methane is gradually converted to
ethane in the Atmosphere
 Methane must be constantly
replenished, probably through
breakdown of ammonia (NH3).
Saturn’s Smaller Moons
Saturn’s smaller moons formed of rock
and ice; heavily cratered and appear
geologically dead.
Tethys:
Iapetus:
Enceladus:
Heavily cratered;
marked by 3 km
deep, 1500 km
long crack.
Leading (upper
right) side darker
than rest of
surface because
of dark deposits.
Possibly active; regions
with fewer craters,
containing parallel
grooves, possibly filled
with frozen water.
Saturn’s Smaller Moons (2)
Hyperion: Too small to pull
itself into spherical shape.
All other known moons are large
enough to attain a spherical shape.
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.
Coorbital Moons
New Terms
oblateness
liquid metallic hydrogen
decameter radiation
decimeter radiation
current sheet
Io plasma torus
Io flux tube
critical point
belt
zone
forward scattering
Roche limit
gossamer rings
grooved terrain
tidal heating
shepherd satellite
spoke
Discussion Questions
1. Some astronomers argue that Jupiter and Saturn are
unusual, while other astronomers argue that all solar
systems should contain one or two such giant planets.
What do you think? Support your argument with
evidence.
2. Why don’t the terrestrial planets have rings?
Quiz Questions
1. Jupiter’s mass is approximately 0.001 solar masses. How is
the mass of Jupiter determined?
a. We use Newton’s form of Kepler’s third law.
b. We use the period and semimajor axis of Jupiter’s orbit
around the Sun.
c. We can use the period and semimajor axis of Callisto’s orbit
around Jupiter.
d. Both a and b above.
*e. Both a and c above.
Quiz Questions
2. What evidence do we have that Jupiter is primarily
composed of hydrogen and helium rather than rock?
a. Jupiter has hydrogen and helium lines in its spectrum.
b. The density of Jupiter is 1.3 grams per cubic centimeter.
c. Jupiter’s equatorial diameter is about 6% larger than its polar
diameter.
d. Both a and b above.
*e. All of the above
Quiz Questions
3. What energy source drives the weather that we see on
Jupiter?
*a. Thermal energy escaping from Jupiter’s interior, still hot
from formation.
b. Thermal energy escaping from Jupiter’s interior, created by
nuclear fusion.
c. Sunlight striking the cloud tops warms Jupiter’s atmosphere
from above.
d. Sunlight heats the surface of Jupiter, then the surface
radiates at infrared wavelengths, which warms the atmosphere.
e. Thermal energy escaping from Jupiter’s interior due to the
condensation of helium droplets that sink beneath the less
dense hydrogen.
Quiz Questions
4. What evidence do we have that Jupiter has a very hot
interior?
a. It reflects 170% of the visible light than it receives from the
Sun.
b. It reflects 170% of the infrared light than it receives from the
Sun.
c. It emits 70% more energy at visible wavelengths than it
receives from the Sun.
*d. It emits 70% more energy at infrared wavelengths than it
receives from the Sun.
e. Both a and b above.
Quiz Questions
5. Which method of heat transfer is responsible for Jupiter’s
belts & zones, and the Great Red Spot?
a. Radiation.
*b. Convection
c. Conduction.
d. All of the above.
e. None of the above.
Quiz Questions
6. In the 1950s, radio telescopes first detected synchrotron
radiation from Jupiter. What did this discovery tell us about
Jupiter?
a. Jupiter has three distinct cloud layers in its atmosphere.
b. Jupiter began emitting radio waves in the 1950s.
c. Io orbits around Jupiter once every 1.8 days.
*d. Jupiter has a strong magnetic field.
e. Jupiter rotates rapidly.
Quiz Questions
7. The two requirements for a strong planetary magnetic field
are rapid rotation, and a convective interior zone composed of
an electrically conductive material. Jupiter’s rotational period is
slightly less than 10 hours. What type of matter fulfills the
second requirement?
a. Liquid molecular hydrogen.
b. Molten copper-aluminum.
*c. Liquid metallic hydrogen.
d. A salty subsurface ocean.
e. Molten iron-nickel.
Quiz Questions
8. What is the Roche Limit?
*a. The maximum distance from a planet at which planetary
rings can exist.
b. The maximum distance that a moon can travel from its
planet.
c. The maximum mass of a blattella germanica.
d. The maximum mass of a Terrestrial planet.
e. The minimum mass of a Jovian planet.
Quiz Questions
9. Jupiter’s ring appears dark in back-scattered light, yet
appears bright in forward-scattered light. What does this tell us
about the particles that make up Jupiter’s ring?
a. The average diameter of a particle is a few micrometers.
b. The particles are most likely dust.
c. The particles are most likely ice.
*d. Both a and b above.
e. Both a and c above.
Quiz Questions
10. How does Ganymede differ from Callisto?
a. Ganymede has more impact craters than Callisto.
*b. Ganymede is differentiated, and Callisto is not.
c. Ganymede has a lower density than Callisto.
d. Ganymede is smaller than Callisto.
e. All of the above.
Quiz Questions
11. The density of Callisto is 1.8 grams per cubic centimeter,
and that of Ganymede is 1.9 grams per cubic centimeter. What
does this suggest about these outer two of Jupiter’s four big
moons?
a. These two moons must be made of roughly equal volumes of
rock and iron.
*b. These two moons must be made of roughly equal volumes
of ice and rock.
c. Both moons must be larger than Earth’s moon.
d. Both moons must be smaller than Earth’s moon.
e. Both b and c above.
Quiz Questions
12. What evidence do we have that the surface of Europa is
young and active?
a. Europa has very few impact craters.
b. The icy crust of Europa is highly reflective.
c. Europa is the smallest of Jupiter’s four large moons.
*d. Both a and b above.
e. All of the above.
Quiz Questions
13. What evidence do we have that Io’s crust and lava is mostly
silicate rock rather than sulfur compounds?
a. Some mountains on Io are much higher than any mountains
on Earth.
b. Much of the lava flowing from Io’s volcanoes is hotter than
Earth lava.
c. The various hues of yellow, orange, and red cannot be
explained by sulfur.
*d. Both a and b above.
e. All of the above.
Quiz Questions
14. Why are Europa, Ganymede, and Callisto necessary for the
continued heating of Io?
a. The tidal forces that these moons exert on Io are greater
than the tidal force on Io due to Jupiter.
*b. These moons periodically tug on Io and keep its orbit
elliptical.
c. These moons send incoming comet bodies toward Io.
d. These outer moons disrupt Jupiter’s magnetic field lines,
causing them to twist back and forth across Io.
e. Io is the smallest of these moons and subject to their
influence.
Quiz Questions
15. What evidence supports the model of Jupiter’s Galilean
satellites forming in a mini accretion disk around Jupiter?
a. The density trend of these four moons is highest close to
Jupiter and decreases with distance.
b. The two inner moons are much smaller than the two outer
moons of this group.
c. Moons closer to a large planet have more impact craters on
their surface.
*d. Both a and b above.
e. All of the above.
Quiz Questions
16. In which way does Saturn differ from Jupiter?
a. Saturn is less oblate.
b. Saturn has more mass.
c. Saturn has a stronger magnetic field.
d. Saturn has more distinct belts and zones
*e. Saturn has a smaller zone of liquid metallic hydrogen.
Quiz Questions
17. How do Saturn’s three layers of clouds differ from Jupiter’s
three layers of clouds?
a. Saturn’s three cloud layers have a very different chemical
composition than Jupiter’s.
b. Saturn’s three cloud layers are at much lower temperatures
than Jupiter’s.
c. Saturn’s three cloud layers are located higher up in the
atmosphere than Jupiter’s.
d. All of the above.
*e. None of the above.
Quiz Questions
18. What gives Saturn’s rings their beautiful structure?
a. The gravitational interaction between ring particles.
*b. The gravitational influence of Saturn’s moons on the ring
particles.
c. The gravitational interactions between Saturn and the ring
particles.
d. Both and b above.
e. All of the above.
Quiz Questions
19. How can Titan have a nitrogen-methane atmosphere with a
surface pressure 1.5 times that of Earth’s atmosphere, whereas
the larger and more massive Ganymede has no atmosphere at
all?
a. Titan does not have the strong gravitational field of Jupiter
nearby to break apart atmospheric gas molecules.
b. Titan has a stronger surface gravity and can hold onto gas
molecules more easily than Ganymede.
c. Titan does not have to compete for gases with other large
moons that are located nearby.
d. Titan has a sticky surface that holds onto large gas
molecules.
*e. Titan is farther from the Sun, and thus colder than
Ganymede.
Quiz Questions
20. What causes the leading side of Saturn’s small moon
Iapetus to differ from its trailing side?
*a. The leading side is darker, because it collides with and
captures dark dust.
b. The leading side is brighter, since it collides with and
captures fresh ices.
c. The leading side is darker, as ices melt and condense on the
trailing side.
d. The leading side is darker, eroded as it is by impacts with
solar wind particles.
e. The leading side is brighter, eroded as it is by impacts with
solar wind particles.
Answers
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
e
e
a
d
b
d
c
a
d
b
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
b
d
d
b
d
e
e
b
e
a