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 are the
multi-colored cloud belts
Visual image
Explored in detail by
several space probes:
Pioneer 10 (1973),
Pioneer 11 (1974),
Voyager 1 (1979),
Voyager 2 (1979),
Galileo (1995-2003)
Infrared false-color image
The Mass of Jupiter
Mass can be determined from the orbit of any of
the innermost four Galilean moons
Io
Moon
1.8 day
orbital period
27.3 day
orbital period
Jupiter
Earth
Relative sizes and
distances are to scale
click for orbital
animation
Using Kepler’s third law: MJupiter = 318 MEarth
Jupiter’s Interior
From volume and mass, average density of Jupiter is 1.34 g/cm3
[compare to Mercury (5.44), Venus (5.24), Earth (5.50), Mars (3.94)]
Therefore, Jupiter cannot be made mostly of rock, like earthlike
planets, but consists mostly of hydrogen and helium.
T = 30,000 K
(hotter than sun’s surface!)
Due to the high pressure,
hydrogen is compressed into a
liquid, and even metallic state.
The Chemical Composition of
Jupiter and Saturn
Hydrogen gas
Helium gas
Water/ice
Methane
Ammonia
Jupiter’s Rotation
Jupiter is the most
rapidly rotating
planet in the solar
system:
Rotation period slightly
less than 10 hours.
Centrifugal forces
stretch Jupiter into
a oblate shape
(like an M&M).
Jupiter’s Magnetic Field
Discovered by observations of radio waves and microwaves
Magnetic field at least
10 times stronger than
Earth’s magnetic field.
Magnetosphere over
100 times larger than
Earth’s.
Intense radiation belts
trap very high energy
particles (electrons
and protons).
Radiation doses are
100 times lethal
amount for humans!
Auroras on Jupiter
Just like on
Earth, Jupiter’s
magnetosphere
produces auroras
concentrated in
rings around the
magnetic poles.
They are 1000
times more
powerful than
auroras on
Earth!
Explorable Jupiter
(SLIDESHOW MODE ONLY)
The Io Plasma Torus
Some of the heavier ions originate from Jupiter’s moon Io.
plasma torus
flux tube
Jupiter’s magnetic field sweeps past Io, creating a
donut-shaped plasma torus (donut-shaped ring of ions).
Electrical current (over 1 million amps!) flows along the
flux tube, creating bright spots in the aurora.
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, so hydrogen
atmosphere exists
above cloud layers.
Only very thin atmosphere
above cloud layers.
Transition to liquid
hydrogen zone about
1000 km below clouds.
Jupiter’s Atmosphere (2): Clouds
Three layers
of clouds:
1. Ammonia
(NH3) crystals
2. Ammonia
hydrosulfide
(NH4SH)
3. Water
crystals
Planetary Atmospheres
(SLIDESHOW MODE ONLY)
The Cloud Belts on Jupiter
Dark belts and bright zones.
Zones are higher and cooler than belts since
they are 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 is the
The Great Red Spot.
It 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.
Roche Limit
(SLIDESHOW MODE ONLY)
Comet Impact on Jupiter
Impact of
21
fragments
of comet
SL-9 in
1994
video clip
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 was 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
video clip
• In the interior,
hydrogen becomes
metallic (very good
electrical conductor)
• Rapid rotation
causes strong
magnetic field
• Rapid rotation
and large size
cause 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 five dozen moons known now and
new ones are still being discovered!
Four largest moons discovered by Galileo in
1610 are called the Galilean moons
Io
Europa
Ganymede
Each moon has interesting and
diverse individual geologies.
Callisto
Callisto: The Ancient Face
Tidally locked to Jupiter, like all of Jupiter’s moons.
Density is 1.8 g/cm3
Composition is mixture
of ice and rocks
Dark surface, heavily
pocked with craters.
No metallic core
because it never
differentiated to form
core and mantle.
No magnetic field.
Layer of liquid water, about 10 km thick, about 100 km
below surface, probably heated by radioactive decay.
Ganymede: A Hidden Past
Largest of the all moons in the solar system.
• Density is 1.9 g/cm3
• Rocky core
• Ice-rich mantle
• Crust of ice
1/3 of surface old, dark, cratered.
2/3 is 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. Pull of
gravity on
meteoroids,
exposing
nearby
satellites to
more
impacts
than those
further out.
Europa: A Hidden Ocean
Density is 3,0 g/cm3
Composition is mostly rock
and metal with icy surface.
Close to Jupiter so should be hit
by many meteoroid impacts, but
few craters visible. Why?
It has an active surface, so
impact craters rapidly erased.
The Surface of Europa
Cracked surface and high albedo (reflectivity)
provides 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 so
No magnetic field.
Liquid water ocean 15 km
below the icy surface.
Io: Bursting Energy
Most active of all Galilean moons,
with no impact craters visible.
Over 100 active
volcanoes!
Geologic activity
powered by tidal
interactions
(heating) with
Jupiter.
Density is 3.6 g/cm3, so
interior is mostly rock.
Interaction with Jupiter’s Magnetosphere
Io’s volcanoes blow out sulfur-rich gasses
Io has a weak
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.
• Moon densities decreasing outward – moons probably
formed in a “mini solar nebular disk around Jupiter,
similar to how the planets formed around the sun.
Galilean moons are
probably a second
generation of moons
(earlier moons
spiraled into Jupiter.
Io, Europa, and
Ganymede are in
orbital resonance with
1:2:4 ratio of periods
orbit animation
Saturn
Mass is 1/3 of mass of Jupiter
Radius is 16 % smaller than Jupiter
Density: 0.69 g/cm3
So low it would
float in water!
• Rotates about as fast as Jupiter, in 10 hr 40 min, but is twice
as oblate since it has no large core of heavy elements.
• Mostly hydrogen and helium with liquid hydrogen core.
• Saturn radiates 1.8 times the energy received from the sun.
• Probably heated by liquid helium droplets falling towards center,
similar to how sun heats while it contracts.
Saturn’s Magnetosphere
Saturn’s magnetic field:
• driven by
dynamo effect
• is 20 times weaker
than Jupiter’s
• has weaker
radiation belts
• not inclined (tilted)
to rotation axis
• Auroras are
centered around
poles of rotation
Saturn’s Atmosphere
Has zone-belt structure, formed through
the same processes as on Jupiter,
but not as distinct and colder
than on Jupiter since Saturn is
twice as far from the Sun.
Saturn’s Atmosphere (2)
Three-layered
cloud structure,
just like on
Jupiter
Main
difference
to Jupiter is
fewer wind
zones, but
much
stronger
winds than
on Jupiter. Winds up to 1100 mph near the equator!
Saturn’s Rings
Ring consists of 3
main segments:
A, B, and C ring
separated by
empty regions
called divisions.
Rings did not
form with Saturn
because ice
material would
have been
heated at the
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 but equal
speeds around
Saturn, so the
astronaut shown
here could “swim”
through the ring.
Shepherd Moons
Some moons on
orbits close to the
rings focus the ring
material, keeping
the rings confined.
Divisions and Resonances
• Some moons act as “shepherds” that “herd”
material into rings with gravitational pull.
• Moons can also create divisions (gaps) when the orbital
period of a moon is a small ratio of the orbital period of
material in the disk (for example “2:3 resonance”).
Titan
video clip
video clip
• 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 is 50% greater
than air pressure on Earth
Surface temperature is -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, are heavily cratered and
appear geologically dead.
Tethys
Iapetus
Enceladus
Heavily cratered
and marked by
3 km deep, 1500
km long crack.
Leading (upper
right) side darker
than rest of
surface because
of dark deposits.
Possibly active with
regions of fewer craters,
containing parallel
grooves, possibly filled
with frozen water.
Saturn’s Smaller Moons (2)
Hyperion is too small to pull
itself into spherical shape.
video clip
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
(SLIDESHOW MODE ONLY)
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?