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Chapter 8
Jupiter and Saturn
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
Explored in detail by
several space probes:
Pioneer 10 and 11,
Voyager 1 and 2,
Galileo,
Juno arrives July ’16
Infrared falsecolor image
The Mass of Jupiter
Mass can be inferred from 2 things: 1)The orbit of Io,
which is the innermost of the 4 Galilean Moons:
Io
Moon
Earth
Jupiter
Relative sizes, and
distances, to scale
2)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 ~ 20,000 K and 1x108
the pressure of Earth
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.
And the rapid spin with
a large metallic
hydrogen interior
creates a HUGE
magnetic field
Jupiter’s Magnetic Field
Discovered through observations of decimeter (radio) radiation
Magnetic field is
nearly 20,000 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 and surround
the rings and inner
moons. That means
radiation  enough to
kill unprotected
humans instantly
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 particles
causing them are
thought to come
from the inner
moons, not the
Sun as 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.
Galileo probe mission
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 basically
unchanged since humans began mapping them in
1600’s.
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 350 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. 3 parts:
Main, Halo(closer to J), and Gossamer
Main ring ~6400 km wide ~123,000 km from J
Halo is ~23,000 km wide ~ 100,000 km from J
Gossamer is ~85,000 km wide outside of the
main ring
Ring material can’t be
old because radiation
pressure and Jupiter’s
magnetic field force dust
Rings must be constantly
particles to spiral down
re-supplied with new dust.
into the planet.
Roche Limit
Image of
Uranus
and its
rings with
moons.
Image credit:
Hubble
The Roche limit is a distance, the minimum distance that a smaller
object (e.g. a moon) can exist, as a body held together by its selfgravity, as it orbits a more massive body (e.g. its parent planet); closer
in, and the smaller body is ripped to pieces by the tidal forces on it.
Read more: http://www.universetoday.com/56538/rochelimit/#ixzz2DXWFSN5Q
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 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 Io and
Europa
2. Focusing
of
meteoroids,
exposing
nearby
satellites to
more
impacts
than those
further out.
Jupiter’s Family of Moons
Over five dozen moons known now; 50 are named,
with another 17 still to be named the latest 2 just
discovered in 2011.
Four largest moons discovered by Galileo in 1610:
They are now called 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: 1.86 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. Largest in SS and
even larger than Mercury
• Av. density = 1.9 g/cm3
• Iron core = Mag. field
• Rocky 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
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, ~32 of
them and they mostly have retrograde motions.
• Galilean and other ‘inner’ moons probably formed
together with Jupiter.
• Densities decreasing outward  Probably formed in a
disk around Jupiter, similar to planets around the sun.
Simulations suggest
earlier generations of
moons around Jupiter
may have been lost
and spiraled into
Jupiter;
Today’s Galilean
moons are possibly a
fifth generation of
moons.
Saturn
Cassini
Division
Av. density: 0.69
g/cm3  Would
float in water!
Most distant of the planets you can see unaided. In 1610
Galileo saw the rings but drew them as handle-like. In 1659
Christian Huygens said it was a thin flat ring that
surrounded Saturn. And in 1675 Dominique Cassini saw
that there was a gap in the rings (we now call it the Cassini
Division) between the A and B set of rings
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
and fainter D,E,F,
and G
separated by
empty regions:
divisions/gaps
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 mainly ice
particles, believed to
be from shattered
comets/asteroids, or
moons.
Moving at large
velocities around
Saturn, but small
relative velocities (all
moving in the same
direction). They vary
in size from tiny dust
sized ice grains to
Mt. sized
Shepherd Moons
Some moons on
orbits close to the
rings focus the ring
material, keeping
the rings confined.
And two moons
even travel in gaps
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 (but slightly
smaller).
• Rocky core, but also large
amount of ice.
• Thick hazy 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: ~60% 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. Over 60 total moons
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.
New Terms
oblateness
liquid metallic hydrogen
Io plasma torus
Io flux tube
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?