An Introduction to Astronomy
Part VIII: Jovian Planets
Lambert E. Murray, Ph.D.
Professor of Physics
Size and General Composition
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Observations of the orbital period of Jupiter’s moons
indicates that Jupiter has more than twice the mass of all
the other planets, their moons, and the asteroids.
Jupiter’s diameter is about 10 times that of Earth, and its
volume is 1,300 times Earth’s.
Thus, Jupiter’s density is only 1.3 g/cc, only 1/4 of
Earth’s. This low density means Jupiter is composed of a
high percentage of light elements.
The Galileo probe showed that Jupiter is about:
– 86% hydrogen
– 14% helium
– small amounts of water (H2O), methane (CH3), and
ammonia (NH3).
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Thus, Jupiter is more like the Sun than the terrestrial
planets.
Orbital and Rotational Motion
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Jupiter is 5.2 AU from the Sun and takes 12 years to
complete one orbit of the Sun.
Jupiter spins on its axis very quickly, once every 9h50m.
Recall that all Jovian planets have much greater rotation
rates than do terrestrial planets.
On Jupiter cloud bands near the equator rotate slightly
faster (9h50m) than bands near the poles (9h56m).
Jupiter exhibits differential rotation—the rotation of an
object in which different parts have different periods of
rotation. This is possible only for gaseous objects.
Jupiter is very oblate, which is caused by its rapid rate of
rotation.
Jupiter’s equatorial diameter is 6% greater than its polar
diameter.
Jupiter’s Persistent Red Spot
Unlike terrestrial planets, surface features would
have little effect on Jupiter’s upper atmosphere,
allowing weather patterns to last for long
periods.
 Jupiter’s giant red spot, first seen by Galileo in
the mid-1600s, has persisted to this day, over
300 years.
 The red spot is approximately 40,000 km long
and 15,000 km across, large enough to swallow
the 13,000-km diameter Earth several times
over!
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Jupiter’s Giant Red Spot
Weather Patterns on Jupiter
The color patterns of Jupiter’s clouds are
striking.
 Colors seen in Jupiter’s upper atmosphere are
likely due to chemical reactions induced by
sunlight and/or lightning in its atmosphere.
 The bright bands are called zones, while the dark
bands are called belts.
 As we will show in the next diagram, the zones
are regions where the warmer gasses are rising,
while the belts are regions where the cooler
gasses are falling.
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Zones and Belts on Jupiter
The Winds of Jupiter
Just as on Earth, there are different atmospheric
cells generated by the convection currents in
Jupiter’s atmosphere.
 However, unlike the Earth, Jupiter appears to be
emitting more energy than is incident upon it from
the Sun – thus, it is being heated internally!
 The boundaries between the convection cells
exhibit high-speed winds, much like our jet
streams. These high-speed winds cause a great
deal of turbulence which can be seen in the upper
atmosphere of Jupiter.
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Close-up view of the Great Red Spot & associated turbulent
flows
rotates CCW
~ 6 day period
High pressure
region
Diagram of the possible internal structure of Jupiter
Jupiter’s Interior and Magnetic Field
The gaseous atmosphere on Jupiter is a few
thousand miles thick. As one goes deeper in
Jupiter’s atmosphere, gaseous hydrogen becomes
liquid hydrogen.
 At 15,000 km below the clouds, it is theorized that
pressure and temperature create a state of liquid
metal hydrogen.
 Jupiter’s core, if it exists, is very small,
contributing only 1% of the planet’s mass.
 Jupiter’s magnetic field is quite strong— nearly
20,000 times stronger than Earth’s.
 Jupiter’s magnetic field is believed to be generated
by its large mass of liquid metal hydrogen and its
rapid rotation rate.
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Jupiter’s Internal Energy Source
Jupiter emits about twice as much energy as it
receives from the Sun.
 It is thought that Jupiter’s excess energy is left
over from its formation; because of its great size,
Jupiter is cooling very slowly.
 The heat generated from the gravitational
collapse of a large gas cloud is sufficient to start
nuclear fusion in a star.
 However, Jupiter would have to be 100 times
more massive to release enough heat to support
nuclear fusion, so it cannot act like a miniature
star.
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Jupiter’s Moons
Jupiter has at least 63 moons. (Dedicated searches for
additional moons of both Saturn and Jupiter continue to
turn up new, small moons, most of which are too small
to be spherical. Saturn has at least for 60 moons.)
 The smallest of the four larger moons – the Galilean
moons – is 5000 times larger than the largest of the
smaller moons. The Galilean moons are spherical,
while the others are irregular in shape like asteroids.
 The four large Galilean moons; and the four small
inner moons (inside Io’s orbit), along with six of the
outer moons all revolve around Jupiter in a counterclockwise fashion (as observed from the North Pole).
The orbits of many of the remaining outer moons are
retrograde and often more eccentric.
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The Galilean Moons
Up Close and Personal
(in Order from Jupiter’s Surface)
The Earth’s Moon is about the same size as Europa.
Io’s Active Volcanoes
The Surface of Io
Io’s Volcanic Activity
Io, the Galilean moon closest to Jupiter, has
active volcanic geysers that spew hot sulfur onto
the surface.
 Io’s heat is produced by tidal forces caused by its
eccentric orbit around Jupiter, and the influence
of the gravitational pull of the other Galilean
moons (Ganymede, the larges moon in the solar
system, is outside Io’s orbit).
 The surface of Io can rise and fall as much as
100 meters.
 Io’s density is about 3.5, indicating that is it
composed mostly of rock. It’s surface features
indicate the erupted materials are sulfurous.
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This is a picture of Io’s plasma torus, made up of charged sulfur ions
trapped in Jupiter’s magnetic field. This demonstrates how volcanic
activity could supply the gases needed for an atmosphere.
Jupiter’s Torus
Quarter images of Io’s and Europa’s tori (also called plasma
tori because the gas particles in them are charged—plasmas).
Io is visible in its torus (green), while Europa is visible in its
torus (blue). Some of Jupiter’s magnetic field lines are also
drawn in. Plasma from tori flow inward along these field lines
toward Jupiter.
Europa
Close-up of Europa’s Surface
Close-up of icy
region of Europa
showing fractured
ice similar to our
polar ice caps.
What looks like an impact crater in
the ice pack.
Mosaic of Europa's Ridges, Craters
Credit: Jet Propulsion Lab, NASA
Surface Features of Europa I
Surface Features of Europa II
Europa’s Icy World
Europa’s surface appears to be covered with ice,
much like the polar ice pack at the North and
South Poles. The moon’s moderate density
indicates a rocky world covered by an ocean of
frozen water.
 Like Io, Europa also experiences some tidal
heating, which may mean the interior of this
moon is relatively warm; the seas below the ice
pack may be warm enough to sustain life.
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Ganymede and Callisto
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Like Europa, Ganymede and Callisto both appear to be
ice-covered moons. The ice may be thicker on these
moons which are farther from Jupiter (perhaps
thousands of miles thick).
Ganymede—larger than Mercury —is the largest moon
in the solar system, and appears to be less active than
Europa, with a darker surface.
However, Ganymede does appear to have some type of
crustal activity which appears to be rejuvenating the
surface.
Callisto, the outermost Galilean moon, shows more
cratering than Ganymede, as would be expected from a
less active surface, experiencing little tidal heating.
Callisto has the largest known impact crater—
Valhalla— in the solar system.
Grooved Terrain of Ganymede
Higher Resolution of Galileo
Surface Features on Europa
Lenticulae attributed to rising warmed ice and debris
travel up from the moon’s interior by convection,
arriving at and then leaking out at the surface. The
white domes are likely to be rising material that has not
yet reached the surface.
Callisto
Valhalla on Callisto
Irregularly Shaped Inner Moons
The four known inner
moons of Jupiter are
significantly different from
the Galilean satellites.
They are roughly ovalshaped bodies. Although
craters have not yet been
resolved on Adrastea and
Metis, their irregular
shapes strongly suggest
that they are cratered. All
four moons are named for
characters in mythology
relating to Jupiter (Zeus, in
Greek mythology).
Jupiter’s Ring
Only after the fly-by of Voyager I did we know
that Jupiter had a thin ring. This ring had not
been visible in telescopes from the Earth’s
surface.
 Most of the material in the ring is in a nearly flat
plain, but there is a small amount of material
scattered out around the ring as seen in the next
picture.
 The ring is relatively close to Jupiter, extending
only to about 1.8 planetary radii.
 We will discuss planetary rings more when we
look at Saturn.
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Jupiter’s Ring
Saturn
Saturn’s Vital Statistics
Saturn orbits the Sun at 9.5 AU; its distance from
the Earth varies from 8.5 AU to 10.5 AU.
 Saturn has an orbital period of 29.5 years.
 Saturn is tilted 27° with respect to its orbital plane,
so over time its rings appear in different
orientations when viewed from Earth.
 Like Jupiter, Saturn’s cloud belts rotate
differentially.
 Saturn’s rotation rate is 10h39m.
 Saturn is even more oblate (0.102) than Jupiter,
with its equatorial diameter 10% greater than its
polar diameter.
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Saturn’s Rings
Cassini Division
Enke Division
Description of Saturn’s Rings
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As seen from Earth, Saturn’s rings look almost solid,
except for two gaps.
– The largest gap is called the Cassini division, believed to be
caused largely by the gravity of Mimas acting synchronously
on the orbital path of nearby ring particles.
– The smaller gap is called the Enke division.
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Saturn’s rings are very thin, as can be seen by the
shadow they cast on the planet. In some cases they are
less than 100 meters across.
The rings are made up of small ice covered particles
ranging is size from centimeters to meters.
Each ring particle revolves around Saturn according to
Kepler’s laws.
Because the plane of the rings is tilted, we see them with
different orientations over time.
Numerous Thin Ringlets Constitute
Saturn’s Inner Rings
This Cassini image
shows that Saturn’s
rings contain numerous
ringlets. Inset: As
moons orbit near or
between rings, they
cause the ring ices to
develop ripples, often
like the grooves in an
old-fashioned record.
The F Ring and One of its Shepherds
Two tiny satellites, Prometheus and Pandora, each measuring
about 50 km across, orbit Saturn on either side of the F ring.
Sometimes the ringlets are braided, sometimes parallel to each
other. In any case, the passage of the shepherd moons causes
ripples in the rings. The gravitational effects of these two
shepherd satellites confine the particles in the F ring to a band
about 100 km wide.
Saturn’s Rings
Origin of Saturn’s Rings
Saturn’s rings are believed to result from the tidal
forces arising from the planet itself.
 Remember the double bulge of the Earth’s oceans
created by the gravitational pull of the Earth’s
moon. If the Moon were closer, this tidal bulge
would be bigger.
 In the same way, a moon close to Saturn would
feel a differential gravitational pull. When this
gravitational force is as great as the gravitational
forces holding the moon together, the moon will
come apart!
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The Roche Limit
This occurs when the moon is inside the Roche
limit of 2.44 planetary radii.
 All the ring particles of Saturn combined
together would only form a small moon about
the mass of Saturn’s moon Janus.
 The rings of all the Jovian planets are within the
Roche limit for that planet!
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Saturn’s Interior and Magnetic
Field
Much of our knowledge of Saturn comes from
the Pioneer 11 fly-by in 1979, followed by
Voyager 1 in 1980 and Voyager 2 in 1981.
 Saturn has a magnetic field as can be seen from
aurora present on the planet. However, Saturn’s
magnetic field is only 5% as strong as Jupiter’s.
We believe this is because Saturn’s liquid
metallic hydrogen only extends about half way
to its cloud tops.
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Saturn’s
Aurora
A Comparison of
the Interiors of
Jupiter and
Saturn
Saturn’s Clouds and Weather
Although the cloud tops reach a temperature of
only –180°C, Saturn, like Jupiter, radiates more
heat than it absorbs from the Sun. This radiated
heat helps to form weather patterns on Saturn
similar to those observed on Jupiter, but Saturn’s
wind speeds are 3 to 4 times faster.
 Saturn’s clouds are less colorful than Jupiter’s.
This may be because the colder temperatures at
Saturn’s distance from the Sun inhibit chemical
reactions that give Jupiter’s atmosphere its varied
colors. In addition, a layer of methane haze above
the cloud tops on Saturn blurs out color
differences.
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A Storm on Saturn
(recorded by Hubble)
Close-up
of Wind
Patterns
on Saturn
Saturn’s Moons
 Saturn
has at least 60 moons, most of which
consist of dirty ice. Major moons include
Titan, Mimas, Enceladus, Dione.
 Titan may be the most interesting moon in
the solar system because it has an
appreciable, permanent atmosphere,
composed mostly of nitrogen with 1%
methane and a trace of argon.
Saturn’s Moon Titan
 Titan
is the second largest moon in the solar
system with a diameter of 5,250 km.
 Titan’s atmosphere is denser than Earth’s
because its surface temperature of –220°C
is low enough to keep gas molecules of
nitrogen and methane from escaping.
Titan Surface
Features
by Cassini
Surface Features on Titan
The Huygens spacecraft took
this image at Titan’s surface
on January 14, 2005. What
appear like boulders here are
actually pebbles strewn
around the landscape. The
biggest ones are about 15
cm (6 in.) and 4 cm (1.5 in.)
across.
Uranus and its
Major Moons
Uranus Observation History
 Uranus,
though barely visible to the naked eye,
was unknown by the ancients.
 Plotted on star charts as early as 1690,
Uranus’s slow orbital motion caused it to go
unnoticed until Herschel discovered it in 1781.
 Galileo may have been the first to actually
observe Uranus – he drew a star on his pictures
of Jupiter’s moons, which is not present today,
but corresponds to where Uranus would have
been at the time!
Occultation Studies of Uranus
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An occultation is the passing of one astronomical object
in front of another (usually a star). From these
occultations, one can determine planetary diameters and
chemical compositions of the atmosphere.
Uranus has a diameter of 51,000 km (32,000 mi), 4
times that of Earth.
Uranus’s atmosphere is similar to Jupiter and Saturn:
mostly hydrogen and helium with some methane.
Uranus does not have cloud layers, so the methane in its
atmosphere, which absorbs red light, makes the planet
appear blue.
Uranus has a density of 1.2; it might have a very small
or no rocky core at all.
Discovery of Uranus’ Rings
Uranus’ rings cannot be seen from Earth. They
reflect only 5% of the sunlight incident upon them
(compared to 80% reflected from Saturn’s rings).
 Uranus has 13 known rings, the first nine
discovered in 1977 during the occultation studies
of Uranus.
 Thirty minutes before and after the planet occulted
the star, symmetrical dips in the star’s intensity
occurred – indicating the presence of the rings.
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Uranus’ Clouds and Weather Patterns
Uranus’s atmosphere is similar to Jupiter and
Saturn: mostly hydrogen and helium with some
methane.
 Uranus does not have cloud layers, so the methane
in its atmosphere, which absorbs red light, makes
the planet appear blue.
 However, it does have cloud bands that rotate
differentially—16 hours at the equator and 28
hours at the poles.
 Uranus orbital period is 84 years.
 The tilt of Uranus’s orbit is about 98°, exposing
one pole to the Sun for periods of about 42 years,
however, Uranus has a fairly uniform temperature
over its entire surface: –200°C.
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Uranus’s equatorial plane is tilted nearly 90° to
its plane of revolution.
Uranus and its Moons
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Five moons were known before Voyager; 10 more were
discovered by Voyager. All are low-density, icy worlds.
Still other, smaller moons have also been discovered.
Most orbit the planet in the plane of the equator.
The innermost major moon, Miranda, is perhaps the
strangest looking object in the solar system. It appears
as if it were torn apart by a great collision and then
reassembled. Cliffs on its surface are as high as 12
miles.
Two of Uranus’s moons are shepherd moons.
Uranus’ Major Moons in Color
 Miranda
is the smallest and innermost of
Uranus' five major moons.
 So far the only close-up images of Miranda
are from the Voyager 2 probe, which made
observations of the moon during its Uranus
flyby in January, 1986.
Miranda
Uranus’ Major Moons
Puck
Miranda
Ariel
Umbriel
Titania
Oberon
[Not to scale]
Neptune
Wispy white clouds
are thought to be
crystals of methane.
Neptune’s Discovery
Neptune is too far away to see with the naked eye,
but even if you could you might not detect its
motion, since Neptune has an orbital period of 165
years.
 Neptune was discovered because of unexplained
variations in Uranus’ orbit from that predicted by
Newton’s laws.
 It was theorized that another planet beyond
Uranus caused these variation. Based upon
precise calculations, observers discovered Neptune
just where it was predicted to be found.
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Weather and Cloud Patterns
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Neptune is similar to Uranus, in appearance and
composition. It is slightly smaller at 49,500 km in
diameter.
Unlike Uranus, however, weather patterns have been
observed on Neptune.
– Voyager sent back an image of a Great Dark Spot on
Neptune, similar in appearance to Jupiter’s Great Red
Spot.
– In 1994, astronomers using the Hubble Space
Telescope could find no trace of Neptune’s dark spot,
which may have disappeared.
Neptune radiates more internal heat than Uranus.
Neptune’s temperature is remarkably uniform at –216°C.
Wind speeds on Neptune appear to be the fastest in the
solar system.
Neptune’s Banded Structure
Several Hubble Space Telescope
images at different wavelengths
were combined to create this
enhanced-color view of Neptune.
The dark blue and light blue
areas are the belts and zones,
respectively. The dark belt
running across the middle of the
image lies just south of
Neptune’s equator. White areas
are high-altitude clouds,
presumably of methane ice. The
very highest clouds are shown in
yellow-red, as seen at the very
top of the image. The green belt
near the south pole is a region
where the atmosphere absorbs
blue light, probably indicating
some differences in
chemical composition.
Neptune’s Interior and Magnetic
Field
Neptune’s magnetic field rotates with a period of
16h3m, which is taken as the planet’s basic rotation
rate.
 Evidence for this magnetic field can be seen in the
aurora.
 Neptune’s axis is tilted less than 30° to its orbit.
 Neptune’s greater density is probably due to a
somewhat larger rocky core.
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Neptune’s Moons and Rings
Before Voyager Neptune was known to have 2
moons; we have now identified 13 moons.
 Triton, Neptune’s largest moon, is the only major
moon to revolve around a planet in a clockwise
(retrograde) direction.
 Nereid has the most eccentric orbit of any moon in
the solar system with a radius varying from ~ 1.5
to 5.5 million kilometers.
 Voyager also revealed a set or rings.
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Triton
Neptune’s Moon Tritan
Triton is the coldest world our space probes have
thus far visited.
 It has a light-colored surface composed of water
ice with some nitrogen and methane frost. Its
surface appears young, with few craters. Its
surface appears to have active geyser-type
volcanoes.
 Triton’s active volcanism is probably due to
internal heating from tidal forces caused by
Neptune’s gravity.
 Triton has a density of 1.76.
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The Magnetic Fields of Five Planets
This drawing shows how the magnetic fields of Earth, Jupiter, Saturn,
Uranus, and Neptune are tilted relative to their rotation axes. Note that the
magnetic fields of Uranus and Neptune are offset from the centers of the
planets and steeply inclined to their rotation axes. Jupiter, Saturn, and
Neptune have north magnetic poles on the hemisphere where Earth has its
south magnetic pole.
End of Part VII