Download 09 Giant Planets

The Giant Planets
The Outer Planets
• Hydrogen-rich
atmospheres
• Belt-zone
circulation
• Shallow
atmospheres
• Interiors mostly
liquid H2
• Large satellite
systems
The Outer Planets
This figure shows the solar system from a vantage
point that emphasizes the relationship of the outer
planets to the rest of the system
Spacecraft Exploration of the
Outer Solar System
Pioneer 10 & 11 and Voyager 1 & 2 flew through the outer solar system.
This is Voyager. Galileo orbited Jupiter. Cassini orbited Saturn
Jupiter
Interior
Magnetosphere
Atmosphere
History
Rings
Roche Limit
Galilean
Satellites
Io
Europa
Ganymede
Callisto
Earth
Jupiter
Semi-major Axis
1 A.U.
5.203 A.U.
Inclination
0°
1° 18’
Orbital period
1.000 tropical year
11.86 tropical year
Orbital eccentricity
0.017
0.048
Rotational period
23 h 56 min 4.1 s
9 h 50.5 min
Tilt
23° 27’
3° 7’
Radius
6378 km
71,492 km
Mass
5.97 x1024 kg
1.90 x 1027 kg
Bulk density
5.52 g/cm3
1.33 g/cm3
Atmosphere
N2, O2
H2, He, CH4, NH3
Albedo
0.40
0.51
Surface temperature
250-300 K
110-150 K
Escape speed
11.2 km/s
59.6 km/s
Magnetic moment (equator)
8 x 1010 G.km3
1.3 x 1015 G.km3
Jupiter
Largest and most
massive planet in
the solar system:
Contains almost 3/4 of
all planetary matter in
the solar system.
Visual image
Infrared falsecolor image
Most striking
features visible
from Earth: Multicolored cloud
belts and the Great
Red Spot
Jupiter’s Interior
Jupiter’s composition consists mostly of hydrogen and helium.
Because of high
pressure, H2 is
compressed into
a liquid, and even
into a metallic
(quantum
degenerate) state.
He, which is
heavier than H2,
continues to sink
toward the center.
It is in a sense a
continuance of
gravitational infall
and it generates
heat.
T ~ 30,000 K
Electron energy
Degenerate Matter
 A point is reached below
the liquid H2 boundary
where the gravitational
pressure forces a phase
change from liquid to
degenerate matter. Gravity
forces electrons into the
lowest energy levels of the
atoms.
 Pressure in the degenerate
core comes from electrons
that cannot be packed
arbitrarily close together
(Pauli exclusion principle).
 Liquid metallic hydrogen is
an excellent conductor of
electricity and heat.
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
magnetosphere.
Extremely intense
radiation belts.
Very high energy
particles can be
trapped; radiation flux
corresponds to ~100
times a lethal dose for
humans!
Jupiter’s Magnetosphere
Jupiter is surrounded by belts of charged particles,
much like the Van Allen belts but vastly larger
A plasma torus is produced in the orbit of Io
Jupiter’s
Magnetosphere
The
magnetosphere
extends beyond
the orbit of Saturn
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.
Jupiter’s Atmosphere
Jupiter’s liquid hydrogen
ocean has no surface.
There is a gradual transition
from a gaseous to a liquid
phase as temperature and
pressure combine to exceed
the critical point.
Jupiter shows limb darkening
⇒ hydrogen atmosphere
extends above the cloud
layers.
There is only a very thin
atmosphere above the cloud
layers.
There is a transition to a liquid
hydrogen zone ~1000 km
below the clouds.
Clouds in Jupiter’s Atmosphere
There are 3
layers of
clouds:
1. Ammonia
(NH3)
crystals
2. Ammonia
hydrosulfide
(NH4SH)
3. Water
crystals
The Cloud Belts of Jupiter
 Dark belts or
bands alternate
with bright
zones.
 Zones are
higher and
cooler than the
belts; they are
regions of rising
warm gas.
 Belts are lower
and hotter than
the zones; they
are regions of
sinking cool
gas.
The Cloud Belts on Jupiter
Just like on Earth, high-and low-pressure zones are
bounded by high-pressure winds (jet streams).
• Hadley circulation
forms three wind zones
on Earth and five on
Jupiter in each
hemisphere.
• Jupiter’s rapid rotation
and size causes the
winds to be parallel to
the equator.
• This gross structure
has remained
unchanged for over
300 years.
The Atmosphere of Jupiter
However, real wind speeds are complicated.
This graph
shows wind
speed with
respect to
internal rotation
rate at a single
point in time.
There are
continual
changes to this
graph with time.
The Atmosphere of Jupiter
 The Great Red Spot is an anticyclonic storm that
has existed for at least 300 years and probably
much longer
 Rotating storms on Jupiter are anticyclones, i.e.
they circulate around high pressure not low.
The Atmosphere of Jupiter
Lightning-like flashes
have been seen; also
shorter-lived rotating
storms
One example: a Brown
Oval, is really a large gap
in clouds
The Atmosphere of Jupiter
Recently, three white storms were observed to merge
into a single storm, which then turned red. This may
provide some clues to the dynamics behind Jupiter’s
cloud movements.
Atmospheric Heating
Jupiter radiates more energy than it receives
from the Sun because the core is still radiating
heat caused by gravitational compression.
Internal heating feeds energy to storms from
below which causes them to be anticyclones.
Could Jupiter have been a star?
No; it is far too cool and too small for that. It
would need to be about 80 times more massive
to be even a very small star.
The History of Jupiter
• Formed from
cold gas in the
outer solar
nebula, where
ices were able to
condense.
• Rapid growth
• In the interior,
hydrogen became
metallic (very good
electrical conductor)
• Rapid rotation ⇒
strong magnetic
field
• Soon able to
trap gas directly
by gravity
• Rapid rotation and
large size ⇒ beltzone cloud pattern
• Heavy materials
sank to the center
• Dust from
meteorite impacts
onto inner satellites
were trapped to
form rings
The History of Jupiter
Jupiter’s energy output was larger in the past; it may
have been 100 times brighter than the Moon as seen
from Earth
A dwarf star in Jupiter’s place probably would have
made stable planetary orbits impossible
Jupiter played an invaluable role in sweeping the
solar system clear of debris before too much reached
Earth—otherwise life on Earth might not have been
possible
Jupiter’s Ring
Galileo spacecraft
image of Jupiter’s
ring, illuminated
from behind
Not only Saturn,
but all four gas
giants have rings.
Rings must be constantly resupplied with new dust.
Jupiter’s ring: dark
and reddish; it was
discovered by the
Voyager 1
spacecraft.
Composed of
microscopic particles of
rocky material
Location: Inside the Roche limit.
Ring material cannot be old
because radiation pressure and
Jupiter’s magnetic field force dust
particles to spiral down into the
planet.
The Roche Limit
The Roche limit is defined by the radius where the tidal force
between two particles = the gravitational force between them.
Particles cannot accrete inside this radius. All stable ring
systems are inside this limit.
Jupiter’s Family of Satellites
Over 60 natural satellites are now known; new
ones are still being discovered.
The four largest satellites were discovered by
Galileo and are called the Galilean satellites
Io
Europa
Ganymede
Callisto
They have interesting and diverse individual
geologies.
Jupiter’s Family of Satellites
Jupiter with Io and Europa. Note the relative sizes!
Comparison of Medium-size Bodies in
the Solar System
Mass (kg)
Mercury
Ganymede
Titan
Callisto
Io
Moon
Europa
Triton
Pluto
3.30 x 1023
1.47 x 1023
1.34 x 1023
1.06 x 1023
8.89 x 1022
7.35 x 1022
4.92 x 1022
2.16 x 1022
1.1 x 1022
7.8 x 10-5
2.36 x 10-4
5.7 x 10-4
4.7 x 10-5
1.2 x 10-2
2.5 x 10-5
2.1 x 10-4
Mass ratio
(ms/mp)
Radius
(km)
2439
2631
2575
2400
1815
1738
1569
1350
1140
Bulk
density
(g/cm3)
5.43
1.93
1.88
1.83
3.55
3.34
3.04
2.1
2.1?
15.1
20.25
26.60
5.95
60
9.47
14.04
Orbital
radius /
planetary
radius
(rs/Rp)
The Galilean Satellites of Jupiter
Interiors
Io
Io
 Most active of
all Galilean
satellites
 No impact
craters are
visible at all.
 Over 100 active
volcanoes!
 Activity
powered by
tidal
interactions
with Jupiter.
 Average density
~3.55 g/cm3 ⇒
Interior is
mostly rock.
Io
Yellow, orange and browns are probably from
sulfur and sulfur compounds in the ejecta.
White is probably from SO2 ice.
Io
Cause of
volcanism:
Gravity!
Io is very
close to
Jupiter, and it
also feels
gravitational
forces from
Europa. The
tidal forces
are huge, and
provide the
energy for the
volcanoes.
The tidal forces oscillate to act like a
pump that heats up the interior.
Io
Volcanic eruptions eject charged particles; these interact
with Jupiter’s magnetosphere and form a plasma torus.
Particles in the torus can travel to Jupiter along magnetic
field lines.
Europa
Average 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 are rapidly erased.
The Surface of Europa
Cracked surface and high albedo (reflectivity)
provide additional 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 is not molten
⇒ No magnetic
field.
Europa has a liquid
water ocean ~15 km
below the icy
surface. Some
speculate that life
could exist in this
submerged ocean.
Ganymede
Craters
(Palimpsests)
Bright terrain
Dark surface
Ganymede
Average density = ~1.9 g/cm3
Largest of the 4 Galilean satellites.
• 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 the surface broke
Ganymede
Ganymede is the largest satellite in the solar
system – it is larger than Pluto and larger in
diameter than Mercury
It has a history similar to Earth’s Moon, but
made of dirty water ice instead of rock
Ganymede
Areas once thought to be flat turn out to have
structure; they might result from a form of
plate tectonics
Callisto
Tidally locked to Jupiter, like all of Jupiter’s satellites.
 Average density:
~1.79 g/cm3
 ⇒ composition:
mixture of ice and
rocks
 Dark surface, heavily
pocked with craters
(palimpsests).
 No metallic core:
Callisto never
differentiated to form
core and mantle.
Layer of liquid water, ~10 km thick, ~100 km below
surface, probably heated by radioactive decay.
 ⇒ No magnetic field.
Saturn
Principal Features
Atmosphere
Rings
Satellites
Titan
Small Satellites
Jupiter
Saturn
Semi-major Axis
5.203 A.U.
9.522 A.U.
Inclination
1° 18’
2° 9’
Orbital period
11.86 tropical year
29.46 tropical year
Orbital eccentricity
0.048
0.054
Rotational period
9 h 50.5 min
10 h 14 min
Tilt
3° 7’
26° 44’
Radius
71,492 km
60,268 km
Mass
1.90 x 1027 kg
5.68 x 1026 kg
Bulk density
1.33 g/cm3
0.69 g/cm3
Atmosphere
H2, He, CH4, NH3
H2, He, CH4, NH3
Albedo
0.51
0.50
Surface temperature
110-150 K
95 K
Escape speed
59.6 km/s
35.6 km/s
Magnetic moment (equator)
1.3 x 1015 G.km3
4 x 1013 G.km3
Spacecraft Exploration of Saturn
Besides Voyager 1 & 2, the Cassini spacecraft
is orbiting among Saturn’s satellites; it used
many gravity assists to get there.
Saturn
 Mass: ~1/3 of mass of Jupiter
 Radius: ~16% smaller than Jupiter
 Average density ≈ 0.69
g/cm3: Less than water!
 Rotates about as fast as
Jupiter, but is twice as
oblate ⇒ No large core of
heavy elements.
 Mostly H2 and He; liquid
hydrogen and liquid metallic
hydrogen core.
 Saturn radiates ~1.8 times
the energy received from the
Sun. Probably heated by
liquid helium droplets falling towards center, i.e. gravitational
collapse
Saturn
Because the rotational
axis of Saturn is tilted
26° 44’ to the ecliptic
plane, the view of its rings
from Earth changes as
Saturn orbits the Sun.
When they are aligned
with the ecliptic plane,
they practically disappear
because they are so thin.
Saturn’s Atmosphere
Saturn’s atmosphere also shows zone and band structure like
Jupiter, but coloration is much more subdued than on Jupiter.
It is mostly H2, He, CH4, and NH3; the He fraction is much less than
on Jupiter. This true-color image shows the delicate coloration of
the cloud patterns on Saturn.
Saturn’s Atmosphere
 Three-layered
cloud
structure, just
like Jupiter
 Main
differences
from Jupiter
 More belts
and zones (7)
 Much stronger
winds: Winds
up to ~500 m/s
near the
equator!
Saturn’s Atmosphere
 Wind patterns on
Saturn are like
those on Jupiter,
with zonal flow.
 Seven on Saturn
compared to 5 on
Jupiter and 3 on
Earth.
Saturn’s Atmosphere
Anticyclones are rare on Saturn; they do not form often
and they quickly dissipate when they do. Colors are more
subtle compared to Jupiter probably because it is colder.
Saturn’s Atmosphere
This image shows what is
thought to be a vast
thunderstorm on Saturn,
as well as the polar
vortex at Saturn’s south
pole.
Saturn’s Magnetosphere
Saturn also has a strong magnetic field, but
only 5% as strong as Jupiter’s
Creates aurorae.
Saturn’s Rings
 Ring consists of 3
main segments
 A, B, and C Rings
 Separated by
divisions: visually
empty regions
 Rings could not have
been formed together
with Saturn because
material would have
been blown away by
particle streams from
a hot Saturn at the
time of formation.
Composition of Saturn’s Rings
 Rings are
composed of ice
particles ranging
from ~1 cm to ~1 m
in size
 They move at large
Keplerian velocities
around Saturn
 They have small
relative velocities
(all moving in the
same direction).
Shepherd Satellites
Some satellites in
orbits close to the
rings focus the
ring material,
keeping the rings
confined.
 Rings
 Inner
Satellites
 Shepherd
Satellites
Divisions and Resonances
Besides some satellites acting as “shepherds,” some
satellites have orbital periods that are in a small-number
fractional multiple (e.g., 3:1) of the orbital period of material
in the disk. This is an orbit-orbit resonance and disk
material is cleared out by tidal forces
⇒ 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: Saturn’s Largest Satellite
About the size of
Jupiter’s satellite
Ganymede.
Rocky core, but also
a large amount of
ice.
A thick atmosphere
of N2 and Ar, hides
the surface from
direct view.
Titan’s Atmosphere
 Because of the thick, hazy
atmosphere, surface features are
only visible in infrared images.
 Surface pressure: 50% greater
than air pressure on Earth
 Surface temperature: 94 K
(-290 oF)
 ⇒ Methane (CH4) and ethane
(C2H6) are liquid!
 CH4 is gradually converted to
C2H6 in the atmosphere
 ⇒ CH4 must be constantly
replenished, probably through
breakdown of ammonia (NH3).
North Polar Lakes
 Many of the organic compounds
in Titan’s atmosphere may have
been precursors of life on Earth.
Titan’s
Atmosphere
Trace chemicals
in Titan’s
atmosphere
make it
chemically
complex
Titan
Some surface features
on Titan are visible in
this Cassini infrared
image
Titan
The Huygens probe has landed on Titan and returned
images directly from the surface
Titan
Based on measurements made by Cassini and
Huygens, this is the current best estimate of the
interior structure of Titan
Saturn’s Mid-size Satellites
 Mimas, Enceladus, Tethys, Dione, and Rhea
all orbit between 3 and 9 planetary radii
from Saturn, and all are tidally locked—this
means they have “leading” and “trailing”
surfaces
 Iapetus orbits 59 radii away and is also
tidally locked
Saturn’s Mid-size Satellites
Saturn’s smaller satellites 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 Mid-size Satellites
Dione and Tethys are similar, having icy, heavily
cratered surfaces.
Saturn’s Mid-size Satellites
Rhea has a highly reflective, heavily
cratered surface. Wispy features are
on trailing side but not leading;
origin not yet understood.
Mimas is the closest satellite to
Saturn, and it has a crater covering
one-third of its surface, the result of
an impact that must have almost
destroyed the satellite.
Saturn’s Small Satellites
Masses of many small satellites are not well known
Janus and Epimetheus share a single orbit
Saturn’s Small Satellites
Telesto and Calypso
are located at the
L4 and L5
Lagrangian points of
Tethys, respectively
Uranus
Atmosphere
Interior
Rings
Satellites
Jupiter
Uranus
Semi-major Axis
5.203 A.U.
19.2 A.U.
Inclination
1° 18’
0° 46’
Orbital period
11.86 tropical year
84.01 tropical year
Orbital eccentricity
0.048
0.048
Rotational period
9 h 50.5 min
17 h 14 min – retro
Tilt
3° 7’
97° 52’
Radius
71,492 km
25,559 km
Mass
1.90 x 1027 kg
8.66 x 1025 kg
Bulk density
1.33 g/cm3
1.27 g/cm3
Atmosphere
H2, He, CH4, NH3
H2, He, CH4
Albedo
0.51
0.66
Surface temperature
110-150 K
58 K
Escape speed
59.6 km/s
21.3 km/s
Magnetic moment (equator)
1.3 x 1015 G.km3
3.8 x 1012 G.km3
The Discovery of Uranus
Discovered in 1781 by William Herschel; first
planet to be discovered in more than 2000 years
Little detail can be seen from Earth; arrows point
to three of Uranus’ satellites.
Rotation of Uranus
Uranus has the peculiarity of an axis of rotation which
lies almost in the plane of its orbit. Seasonal
variations are extreme.
The Atmosphere of Uranus
 Like other gas giants: No surface.
 Gradual transition from gas phase to liquid
interior.
 Mostly H2; 15% He, a few %
methane, ammonia and water
vapor.
 Optical view from Earth: Blue
color from methane, absorbing
longer optical wavelengths
 Cloud structures not visible even
after artificial computer
enhancement of optical images
taken from Voyager 2 spacecraft.
Cloud
Structures of
Uranus
A Hubble Space
Telescope image of
Uranus shows cloud
structures in lower,
warmer layers not
visible to Voyager 2 in
1986.
⇒ Possible seasonal
changes of the cloud
structures.
Interior of Uranus
 Deep hydrogen
& helium
atmosphere
 No liquid
metallic
hydrogen
 Ice/rock mantle
 Mantle may be
slushy so that it
can convect
The Rings of Uranus
Rings of Uranus and Neptune are similar to Jupiter’s rings.
Confined by shepherd satellites; consist of dark material.
Rings of Uranus were
discovered using
occultations of a
background star
Apparent motion
of star behind
Uranus and rings
The Satellites of Uranus
Umbriel
 5 largest satellites: Miranda,
Ariel, Umbriel, Titania, and
Oberon
 Visible from Earth.
 Tidally locked to Uranus.
 10 more discovered by Voyager
2; more are still being found.
Ariel
Miranda
 Dark surfaces, probably ice
darkened by dust from meteorite
impacts.
Miranda
Neptune
Atmosphere
Rings
Satellites
Triton
Magnetospheres
of the Giant
Planets
Jupiter
Neptune
Semi-major Axis
5.203 A.U.
30.07 A.U.
Inclination
1° 18’
1° 46’
Orbital period
11.86 tropical year
164.8 tropical year
Orbital eccentricity
0.048
0.007
Rotational period
9 h 50.5 min
16 h 3 min
Tilt
3° 7’
29° 34’
Radius
71,492 km
25,269 km
Mass
1.90 x 1027 kg
1.03 x 1026 kg
Bulk density
1.33 g/cm3
1.64 g/cm3
Atmosphere
H2, He, CH4, NH3
H2, He, CH4, C2H6
Albedo
0.51
0.62
Surface temperature
110-150 K
56 K
Escape speed
59.6 km/s
23.8 km/s
Magnetic moment (equator)
1.3 x 1015 G.km3
2 x 1012 G.km3
Neptune
 Discovered in
1846 by J. G. Galle
at a position
predicted from
gravitational
disturbances on
Uranus’ orbit by J.
C. Adams and U.
J. Leverrier.
 Blue-green color
from methane in
the atmosphere
 4 times Earth’s
diameter
 4% smaller than
Uranus
The Atmosphere of Neptune
 Cloud-belt structure with high-velocity winds; origin not well
understood.
 Darker cyclonic disturbances, similar to Great Red Spot on Jupiter,
but not long-lived.
 White cloud features of methane (CH4) ice crystals
The Rings
of Neptune
Interrupted between
denser segments (arcs)
 Made of dark
material, visible in
forward-scattered
light.
 Ring material must
be regularly resupplied by dust
from meteorite
impacts on the
satellites.
Focused by small
shepherd satellites
embedded in the ring
structure.
The Satellites of Neptune
Two satellites (Triton and Nereid) are visible
from Earth; 6 more discovered by Voyager 2
Unusual orbits:
Triton: Only
satellite in the
solar system
orbiting
clockwise, i.e.
retrograde.
Nereid: Highly
eccentric orbit;
very long orbital
period (359.4 d).
The Surface of Triton
 Very low temperature (34.5 K) ⇒
Triton can hold a tenuous
atmosphere of nitrogen (N2) and
some methane (CH4); 105 times
less dense than Earth’s
atmosphere.
 Surface composed of ices: N2,
CH4, carbon monoxide (CO),
carbon dioxide (CO2).
 Possibly cyclic N2 ice
deposition and re-vaporizing on
Triton’s south pole, similar to
CO2 ice polar cap cycles on
Mars.
 Dark smudges on the N2 ice
surface, probably from CH4
rising from below the surface,
forming carbon-rich deposits
when exposed to sunlight.
The Surface of Triton
 Ongoing surface
activity: Surface
features probably
not more than 100
million years old.
 Large basins might
have been flooded
multiple times by
liquids from the
interior.
 Ice equivalent of
greenhouse effect
may be one of the
heat sources for
Triton’s geological
activity.
Comparing the
Rings of the
Giant Planets
Magnetospheres and Internal Structure
of the Giant Planets
Uranus and Neptune both have substantial magnetic
fields, but at a large angle to their rotation axes.
The rectangle
within each
planet shows a
bar magnet that
would produce
a similar field.
Note that both
Uranus’ and
Neptune’s are
significantly off
center.
Magnetospheres and Internal Structure of the
Giant Planets
If magnetic fields of Uranus and Neptune are produced
by dynamos, then the “slush” must be conducting and
convecting.
Interior structure of Uranus and Neptune, compared to
that of Jupiter and Saturn.