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The Jovian Planets
© Sierra College Astronomy Department
The Jovian Planets
Composition, Structure, and Dynamics
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Unlike terrestrial planets, Jovian planets are
made of gas and liquid.
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Jupiter and Saturn: mostly hydrogen and helium, with a
few percent hydrogen compounds and a small fraction
of rock and metal.
Uranus and Neptune: Less than half the mass is H and
He, with most of the composition made of hydrogen
compounds such as water, ammonia, methane.
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The Jovian Planets
Composition, Structure, and Dynamics
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The Jovian planets formed beyond the frost line and
were thought to have grown from planetesimals of about
the same mass – 10 Earth masses.
At greater distances, it took longer for small particles to
accrete into large, icy planetesimals with gravity strong
enough to pull in more material from the solar nebula.
Jupiter was first to form and was able to pull in the most
material followed by Saturn and Uranus-Neptune.
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Neptune is slightly more massive and denser than Uranus
which suggests it formed from a slightly more massive icerich planetesimal.
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The Jovian Planets
Composition, Structure, and Dynamics
Density Differences
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Saturn is considerably less dense than the other planets.
This makes sense if you compare what each planet is
made of: Uranus and Neptune have less % of H and He
than Saturn.
But then Jupiter should be the least dense of all! But it is
not because its large gravity compresses the
atmosphere.
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This also, explains why Jupiter is only slightly larger in radius
than Saturn
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Lecture 11: The Jovian Planets
Composition, Structure, and Dynamics
Shape of the Planets
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The Jovian planets are non-spherical and have
larger circumferences around their equators
than around the great circles through the poles.
Saturn is the most oblate as it is about 10%
wider than it is tall.
The strong gravity should make these large
worlds spherical but rapid rotation makes the
equatorial region bulge out.
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The Jovian Planets
Composition, Structure, and Dynamics
Inside Jupiter
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The Galileo probe penetrated to a depth of 200 km (or 0.3% of
Jupiter radius) before contact was lost.
At a depth of 80–100 km, predictions indicate the temperature is
Earth-like and the pressure is 10 times greater than that at the
Earth’s surface.
As one goes deeper in Jupiter’s atmosphere, gaseous hydrogen
becomes liquid hydrogen (~7,000 km). The pressure here is
500,000 times that of the Earth surface.
At ~15,000 km below the clouds, it is theorized that pressure and
temperature create a state of liquid metallic hydrogen (exists only
in Jupiter and Saturn).
The core which contains roughly 10 Earth masses is only about the
radius of the Earth (~6500 km).
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The Jovian Planets
Composition, Structure, and Dynamics
Comparing Jovian interiors
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Saturn is the most similar to Jupiter and has
liquid metallic hydrogen too, but much deeper
beneath the visible clouds.
The pressures are not high enough to form
liquid metallic hydrogen in Uranus and
Neptune; however, hydrogen compounds
reside in a layer above the central core of rock
and metal.
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The Jovian Planets
Composition, Structure, and Dynamics
Magnetic Fields
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Jupiter’s magnetic field is quite strong - nearly 20,000 times stronger
than Earth’s.
Jupiter’s low density and distance from Sun implies that iron is not the
source of the strong magnetic field.
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Since hydrogen dominates and high pressures exist inside Jupiter,
theorists predict that liquid metallic hydrogen is best possible
generator of the magnetic field.
Fast rotation period increases field strength.
Jupiter collects far more charged particles than the Earth does.
The other Jovian planets have smaller magnetic fields (though larger
than Earth’s if compared side-by-side).
 Uranus and Neptune’s magnetic field is generated by the core
“oceans” of hydrogen compounds, rock, and metal.
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The Jovian Planets
Composition, Structure, and Dynamics
The Atmosphere (Weather)
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Weather is driven on the Jovian planets by
energy from the Sun and from within (plus the
rotation of the planet).
Internal energy (present in all but Uranus) is
likely coming from the conversion of potential
energy to kinetic energy as gasses are slowly
falling or condensing inside these planets.
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Jupiter from Cassini: 4 October 2000
Courtesy of NASA/JPL-Caltech
Jupiter from New Horizions: 10 February 2007
Red Spot
Red Jr.
Courtesy of NASA/JPL-Caltech
The Jovian Planets
Composition, Structure, and Dynamics
Clouds and Colors
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The Jovian planets have clouds which condense from
a gas when the temperature becomes cold enough.
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For the Earth only one gas – water vapor – can condense.
For Jupiter, from high to low altitude: ammonia, ammonium
hydrosulfide, and water condense to form cloud layers, so
most of the time we see ammonia clouds. This happens at
about 30 to 100 km below the upper cloud tops.
For Saturn the same layers form but deeper in the
atmosphere (200 km below) and farther apart .
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Why? Saturn is colder and has weaker gravity.
For Uranus and Neptune, methane clouds dominate the
atmosphere. These absorb red light very well and make the
planets blue.
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The Jovian Planets
Composition, Structure, and Dynamics
Storms on Jupiter
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Jupiter’s Giant Red Spot, first seen in the mid-1600s, has lasted for over
300 years (or at least 150 years).
The Giant Red Spot is a high-pressure storm system that rotates
counterclockwise every 6 days.
The red spot is 40,000 km long and 15,000 km across, larger than the
13,000-km diameter Earth.
Cause of red color is still debated.
Several (12) zones and belts can be seen too. This banded structure is
due to the Coriolis effect and rapid rotation.
A New Red Spot?
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Oval BA formed in 2000 when three smaller spots collided and merged.
White in November 2005, brown in December 2005, and then red in February
2006.
As of March 2006, Red Jr. is about half the size of the Great Red Spot.
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The Jovian Planets
Composition, Structure, and Dynamics
Weather on other Jovian planets
 Saturn has zone and belts which are harder to
see since they are deeper in the atmosphere
 Uranus had nearly no clouds when Voyager
passed in 1986, but Earth observations has
shown more weather when northern “spring”
comes to Uranus.
 Neptune had a Great Dark Spot seen by
Voyager (1989) but it has since disappeared.
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Picture from Cassini: 9 February 2004
Cassini arrived
1 July 2004
http://saturn.jpl.nasa.gov
Courtesy of NASA/JPL-Caltech
Cassini Close-up of Saturnian Atmosphere
http://saturn.jpl.nasa.gov
Courtesy: NASA
l=727 nm
contrast
enhanced
Voyager’s Uranus (1986)
Enhanced
Normal
Courtesy: NASA
Keck’s Uranus (2004)
Rings and planet
taken with separate
exposures
Courtesy: Keck Telescope
Atmospheric Notes
 Neptune’s winds driven by an internal
heat source - reach
speeds of 2200
km/hr (1300 mph).
Courtesy: NASA
The Jovian Planets
Jupiter’s Moons
Jupiter’s Moons
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Jupiter’s family of 67 moons can be divided into
3 groups:
1. Outer moons, eccentric orbits, many
retrograde, dark surfaces, captured asteroids.
2. 4 inner moons orbit very close to Jupiter and
are probably fragmented moonlets (form and
shape Jupiter’s ring).
3. 4 Galilean moons, nearly circular orbits,
smallest is 5,000 times more massive than
the largest of the other moons.
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Closer to Jupiter
Further to Jupiter
Dense
Less Dense
Younger
Io
Older
Europa
Ganymede
Callisto
Courtesy: NASA
Size of Earth’s Moon
Surface age determined
by crater counts
The Jovian Planets
Jupiter’s Moons
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Io, the Galilean moon closest to Jupiter, has
active volcanic sulfuric geysers.
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Creates many surface layers
But does not build high volcanoes (lava too fluid)
Io’s heat is produced by tidal forces caused
by Europa and its eccentric orbit around
Jupiter.
Io is surrounded by a halo of sodium atoms,
which itself is embedded in a sodium torus
that surrounds Jupiter.
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The Jovian Planets
Courtesy: NASA
Io during eclipse
Jupiter’s Moons
Volcanoes on Io
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How? Should have cooler interior than
Mercury and Mars (smaller object).
Io’s elliptical orbit forced by resonance with Europa
and Ganymede causes differential tidal heating.
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Io is tidally distorted more when closer to Jupiter than farther
away.
This constant flexing heats the interior.
Spewed material from volcanoes forms torus of
sodium(?) around Jupiter (Io Torus).
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The Jovian Planets
Jupiter’s Moons
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Europa’s surface is ice; its moderate density
indicates a rocky world covered by an ocean
of frozen water.
Europa also experiences some tidal heating
which has resurfaced it.
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Ice rafts and lenticulae (100-m ice mounds)
This tidal heating of Europa also suggests
that an interior liquid ocean of water may
exist.
Europa is the smallest of the Galilean moons
(and is smaller than Earth’s Moon).
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The Jovian Planets
Jupiter’s Moons
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Ganymede has a surface that appears similar
to our moon.
The surface is composed mostly of ice.
With fewer craters than Callisto, some
resurfacing has occurred.
Ganymede is the largest moon in the solar
system.
It also generates its own magnetic field
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How? There may be a layer of salty-water buried 150
km beneath the surface
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The Jovian Planets
Jupiter’s Moons
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Callisto also has a surface that appears
similar to our moon.
The surface is composed mostly of ice.
There may be a water ocean below the
surface.
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Callisto is very heavily cratered implying that
it is tectonically inactive.
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Radioactive heating may contribute
There is a detectable magnetic field
May be the oldest surface in the solar system
Callisto’s interior is appears to be
undifferentiated.
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The Jovian Planets
Jupiter’s Moons
Resonances
 Previous examples: spin-orbit
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New examples: spin-orbit
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Moon (1:1) around Earth, Mercury (3:2) around Sun
All other major satellites to parent planet (1:1)
New examples: orbit-orbit
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Io-Europa (2:1)
Europa-Ganymede (2:1)
Later, in Saturn’s rings: Mimas-Cassini Division (2:1)
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The Jovian Planets
Saturn’s Moons
Moons of Saturn
 Saturn has 62 moons, second only to
Jupiter in number.
 Major moons include (from largest to
smallest): Titan (second largest moon in
the Solar System), Rhea, Iapetus, Dione,
Tethys, Enceladus and Mimas.
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Some Moons of Saturn
Rhea
Mimas
Courtesy: NASA
Enceladus
Close-up of surface
Enceladus
Enceladus has a very shiny
surface (albedo = 0.9) and
has just been discovered to
have a “significant”
atmosphere (which must be
Courtesy: NASA
replenished)
False-color image of
anti-Saturn hemisphere
Dione
Tethys
Courtesy: NASA
Iapetus (the two toned moon)
Phoebe
enlarged
Courtesy: NASA
Phoebe
Hyperion (next page too)
Hyperion
Courtesy: NASA
The Jovian Planets
Saturn’s Moons
Titan
 Titan may be the most interesting moon in
the solar system because it has an
atmosphere (How?).
 It is composed mostly of nitrogen with 1%
methane and a trace of argon.
 When sunlight strikes methane, it can
cause the formation of organic molecules,
which are a known precursor to life.
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Titan
Courtesy: NASA
Rhea
The Jovian Planets
Uranus’s Moons
Uranus’s Moons
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Five moons were known before Voyager (Miranda, Ariel,
Umbriel, Titania, Oberon); now 22 more are known (total
= 27).
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Many moons named for Shakesperian characters.
All the moons appear to be low-density, icy worlds (but
they appear to have had been more active than the
Saturnian satellites of a similar size).
The innermost, 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.
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The Jovian Planets
Neptune’s Moons
Neptune’s Moons
 Before Voyager 2, Neptune was known to have 2
moons; 13 moons are now known.
 Triton, Neptune’s largest moon, is the only major moon
to revolve around a planet in a clockwise (retrograde)
direction.
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Causes significant enough tides on Triton.
Triton is also tilted 23 deg relative to Neptune’s equator
Triton has a very thin atmosphere of N2 and CH4.
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The Jovian Planets
Neptune’s Moons
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Triton has a light-colored surface composed
of water ice with some nitrogen and
methane frost.
Its surface appears young, with few craters
and active geyser-type volcanoes observed
(nitrogen ice and carbon compounds).
Triton’s active volcanism is probably due to
internal heating from tides, heating from the
Sun or internal residual heat.
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The Jovian Planets
Planetary Rings - Saturn
Planetary Rings
 Saturn’s rings are very thin, in some cases
less than 100 meters thick.
 The rings are not solid sheets but are made
up of small particles of water ice or water-ice
mixed with dust.
 Three distinct rings are visible from Earth,
and were named (outer to inner) A, B, and C.
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Saturn from Earth
Voyager approaching Saturn
Courtesy: NASA
Voyager leaving Saturn
Courtesy: NASA
Mimas
The Jovian Planets
Planetary Rings - Saturn
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The largest division between rings is known as
the Cassini division.
This space is caused largely by the gravity of
Mimas acting synchronously (2:1 resonance)
on the orbital path of nearby ring particles.
Some other ring features are explained by the
presence of small shepherd moons.
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Close-up of Main Rings
Courtesy: NASA
A Ring
True
C Ring
Cassini
Division
B Ring
The Jovian Planets
Planetary Rings - Saturn
The Origin of Rings
 Saturn’s rings are probably about 100 million years old.
 The origin of Saturn’s rings is not well understood, but is thought
to be the result of:
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A close-orbiting, icy moon that shattered in a collision with an
asteroid .
A large comet which got too close to Saturn (much like
Shoemaker-Levy 9 did at Jupiter in 1994).
Rings around the Jovian planets are not billions of years old and
must be replaced or renewed on a much smaller time scale.
Tidal forces are greater on a moon in orbit close to a planet than
they are on a moon in an orbit farther out.
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The Jovian Planets
Saturn’s Rings
Roche limit is the minimum radius at
which a satellite (held together by
gravitational forces) may orbit without
being broken apart by tidal forces.
 Saturn’s rings are inside Saturn’s Roche
limit, so no moons can form from the
particles.
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The Jovian Planets
Planetary Rings - Jupiter
Voyager
from
“behind”
Jupiter
Courtesy: NASA
Jupiter’s Ring
Voyager I discovered a thin ring (system)
around Jupiter.
 The ring is close to Jupiter, extending to
only about 1.8 planetary radii.
 The ring is thought to be replenished from
the small moonlets within or near it.
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The Jovian Planets
Planetary Rings - Uranus and Neptune
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The rings of Uranus and Neptune and are
made of particles which are darker and
smaller than that of Saturn.
The Uranian rings are narrow, a few of which
are clearly confined by shepherding moons.
The Neptunian rings vary in width and are
confined by resonances of some of the
moons.
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The End
© Sierra College Astronomy Department
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