15.Giant Planets

Giant Planets
Jupiter, Saturn, Uranus, Neptune
Size Comparison
Jupiter: 318 Earth-masses, Saturn: 95, Uranus: 14.5, Neptune: 17.2
Two subclasses: Jupiter-Saturn and Uranus-Neptune
•Terrestrial planets
•low mass
•high density
•slow rotators ( 24 hours)
•few satellites
•close to Sun ( 1.6 AU)
•Thin atmospheres
•Weak or no magnetic field
•Giant planets
•high mass
•low density
•rapid rotators (18 hours)
•many satellites
•far from Sun ( 5 AU)
•Thick atmospheres
•Strong magnetic field
Jupiter, Saturn, Uranus, and Neptune have all been
visited by the Voyager space probes.
•Galileo was crashed into Jupiter
•Cassini is now at Saturn
Jupiter, Saturn, Uranus, and Neptune are all
massive bodies
•formed in outer part of pre-solar nebula where ices
•growth by accretion and coalescence
Giant planets are gaseous/fluid bodies
•supported by balance between pressure and
gravity: Hydrostatic equilibrium
Solar Nebula Composition
98% of the nebula was in the form of gaseous H2 and He.
2% consisted of H2O, CH4, and NH3, (ices), and even smaller
amounts of rocks and metals (olivines, pyroxenes, iron, nickel, etc.).
Jupiter is closest to “solar” composition
Saturn less H2 and He
Uranus and Neptune mostly ices
The physical size of a planet depends on both its mass
and its chemical composition.
Chemistry of a Giant Planet
Atmospheres of Jovian Planets
Jupiter and Saturn
•For both planets, methane and frozen ammonia (NH3)
crystals are common.
•For Saturn, the NH3 extends over a greater depth and is
harder to see through, giving the planet a uniformly hazy
Uranus and Neptune
•For both planets only methane (CH4)
in atmospheres; NH3 completely frozen out.
•Uranus and Neptune have a greenish-blue
appearance because methane absorbs red light.
Rapid differential rotation of giant planets stretches clouds into bands.
•Jupiter -- reds and browns (ammonia, sulfur compounds, methane)
•Saturn -- reds and yellows (ammonia, sulfur compounds, methane)
•Uranus -- blues and greens (mostly from methane gas)
•Neptune -- blue (from methane gas)
Giant Planet Magnetic Fields
•At very high pressures inside Jupiter and Saturn hydrogen
begins to act like a liquid metal This provides an electrically
conducting fluid in which a magnetic field is generated.
•In Uranus and Neptune the magnetic fields are generated by
convection of water, ammonia and methane.
•Jupiter and Saturn have very strong magnetic fields which are
closely aligned with the planet's spin axis
•The magnetic fields of Uranus and Neptune are weaker,
irregular and highly tilted with respect to the planet's rotation
Magnetospheres of Giant Planets
Jupiter’s Great Red Spot
The Great Red Spot is
a huge storm
measuring 12,000 by
25,000 km (7,500 by
15,500 miles), which is
big enough to hold two
Earths side by side.
While Jupiter's cloud
patterns can change
within hours or days
like on Earth, the Spot
has lasted for over 300
Jupiter’s Interior
Jupiter’s Internal Sources of Energy
Jupiter radiates 1.6 times a much energy as falls on it
from the Sun. Thus, Jupiter has an internal heat source.
It is thought that much of this heat is residual heat left
over from the original collapse of the primordial nebula to
form the Solar System, but some may come from slow
This internal heat source is presumably responsible for
driving the complex weather pattern in its atmosphere,
unlike the Earth where the primary heat source driving
the weather is the Sun.
Saturn Facts
Spacecraft Encounters:
Pioneer 11 (1979);
Voyager 1 & 2 (1980, 1981)
Cassini - Huygens (2004)
Mean Distance from the Sun:
9.539 AU
Length of Year:
29.46 Earth years
Rotation Period:
10.66 hours
Mean Orbital Velocity:
9.64 km/s (6 mi/s)
Inclination of Axis:
26.73 degrees
Number of Observed
120,536 km (74,901 mi)
Now 34
Comparisons With Earth:
Average Distance from the
9.4 X Earth's
0.13 X Earth's
9.5 X Earth's
95 X Earth's
Saturn’s High-Velocity Winds
There are extremely high velocity winds in the atmosphere of Saturn.
Unlike the case for Jupiter, the variations in wind speeds are not
strongly correlated with the positions of the belts and bands. The wind
speeds in the atmosphere of Saturn have been measured to be as
high as 1800 km/hr, which is about 4 times the highest speeds in the
atmosphere of Jupiter.
Saturn’s Surface
The surface of Saturn bears many similarities with the surface of Jupiter, but
the color contrast is generally less. This is thought to be due to Saturn being
colder than Jupiter (further from the Sun, but also smaller with less internal
heat), so it has different chemical reactions in its atmosphere, leading to
different coloration.
There are large anticyclonic cells on the surface, apparently driven by the
planet's internal heat source, but none are as large as the Great Red Spot on
Jupiter, and they are not as abundant as on Jupiter.
1997 launch – 2004 Saturn arrival
The Cassini Orbiter's mission consists of delivering a probe (called Huygens,
provided by ESA) to Titan, and then remaining in orbit around Saturn for detailed
studies of the planet and its rings and satellites.
JPL technicians reposition
and level the Cassini orbiter
in the Payload Hazardous
Servicing Facility at the
Kennedy Space Center in
July 1997, after stacking the
craft's upper equipment
module on the propulsion
at Saturn
The international CassiniHuygens mission
successfully entered orbit
around Saturn at 9:12 p.m.
PDT on June 30, 2004. This
begins a four-year study of
the giant planet, its majestic
rings and 34 known moons.
Cassini will make 74 unique orbits around the planet, using close
flybys of Saturn's largest moon Titan for gravity assists and science
data acquistion. Because of the size of Titan, the flybys will allow for
major changes in orbital paths, allowing engineers to minimize fuel use
while maximizing science data collection.
Highlights of the Saturn Tour
74 Orbits of Saturn
44 Close flybys of Titan
8 close "targeted" flybys of other satellites:
3 close flybys of Enceladus
30 additional satellite flybys at distances less than 100,000 kilometers
(about 62,100 miles)
Many Saturn and Ring occultation opportunities
One "Titan 180 degree" transfer
One high inclination sequence
This false-colour composite was created with images taken during the
NASA/ESA/ASI Cassini spacecraft's closest fly-by of Titan on 16
April 2005.
This mosaic of three frames provides unprecedented detail of the high ridge area
including the flow down into a major river channel from different sources.
Saturn’s Rings
Nine days before it entered orbit,
Cassini spacecraft captured this
exquisite natural color view of
Saturn's rings.
The brightest part of the rings,
curving from the upper right to
the lower left in the image, is the
B ring.
Saturn's rings are made
primarily of water ice. Since
pure water ice is white, it is
believed that different colors
in the rings reflect different
amounts of contamination by
other materials such as rock
or carbon compounds.
Saturn’s South Pole
These images of Saturn's south pole, taken by two different instruments on Cassini,
show the hurricane-like storm swirling there and features in the clouds at various
depths surrounding the pole. Different wavelengths reveal the height of the clouds,
which span tens of kilometers in altitude.
Dione, Tethys and Pandora
A 330-kilometer-wide (205 mile)
impact basin can be seen near the
bottom right on Dione (at left).
Ithaca Chasma and the region
imaged during the Cassini
spacecraft's Sept. 24, 2005, flyby
can be seen on Tethys (middle).
Tethys is on the far side of the rings in this view; Dione and Pandora are much
nearer to the Cassini spacecraft. Dione is 1,126 kilometers (700 miles) across.
Tethys is 1,071 kilometers (665 miles) across and Pandora is 84 kilometers (52
miles) across.
Tethys Close-up
Saturn's moon Rhea is an alien ice world, but its cratered surface looks in some ways similar to our
own Moon, or the planet Mercury. Rhea's icy exterior would quickly melt if this moon were brought
as close to the Sun as Mercury. Rhea is 1,528 kilometers (949 miles) across.
Saturn's moon Enceladus is only 505 kilometers (314 miles) across, small
enough to fit within the length of the United Kingdom, as illustrated here.
The intriguing icy moon also could fit comfortably within the states of
Arizona or Colorado.
Uranus is the 3rd of the Gas Giant planets, and the first
planet discovered in "modern" times (1781).
It is barely visible from the Earth without a telescope, which
explains why it was not known as a planet to the ancients,
and why it had been observed various times after the
telescope had been invented without the observers realizing
that it was a planet and not a star.
Documented sightings go back to at least 1690 when
Flamsteed catalogued it as a star.
Uranus Facts
Sir William Hershel (1781)
Spacecraft Encounter(s):
Voyager 2 (1986)
Mean distance from Sun:
19.19 AU (2.871 billion km/1.784 billion mi)
Length of year:
84.01 Earth years
Rotation period:
17.24 hours
Mean orbital velocity:
6.81 km/s (4.2 m/s)
Inclination of axis:
Number of Observed
51,118 km
4.0 x Earth's
Mean Distance from Sun:
19.2 x Earth's
14.5 x Earth's
0.22 x Earth's
The picture on the right uses false colors and
contrast enhancement to bring out subtle details
in the polar region of Uranus.
Uranus has a relatively featureless
appearance at visible wavelengths.
Even from Voyager 2 at a distance of
80,000 km there were few
distinguishable features.
This is believed to be due to Uranus
being further from the Sun than Jupiter
and Saturn, which means its
temperature is lower (only 58 degrees
Kelvin in the upper atmosphere).
This decreases the likelihood of
chemical reactions making the colorful
compounds that give the surface
features on Jupiter and Saturn.
In addition, the upper atmosphere is
thought to have a high-level
petrochemical haze that obscures
features lower in the atmosphere.
•Neptune is the outermost of the four
gas giants
•Because of its distance from the Sun,
Neptune's atmosphere is a frigid -225°
C (-373° F)
•Neptune, like Jupiter and Saturn but
unlike Uranus, has an internal heat
source and produces 2.7 times more
heat than it absorbs.
•The blue-green color of the planet is
due to the presence of methane in the
atmosphere. The atmosphere consists
mostly of hydrogen, helium and
•Until the Voyager encounter in 1989,
the rings surrounding Neptune were
thought to be arcs. We now know that
the rings completely circle the planet,
but the thickness of each ring varies
along its length.
Neptune Facts
Neptune Facts
Galle, Challis, Adams, & Le
Verrier (1846)
Voyager 2
Mean Distance from
the Sun:
30.06 AU (4.497 billion
km/2.794 billion mi)
Length of year:
165 years
Rotation period:
16.11 hours
Mean orbital
5.43 km/s (3.3 mi/s)
49,528 km/30,775 mi
Inclination of axis:
Number of observed
Comparisons with Earth:
3.883 x Earth's
Average distance
from Sun:
30.06 x Earth's
17.14 x Earth's
0.31 x Earth's
Neptune has a great storm
in the southern hemisphere
called the “Great Dark Spot"
that is about half the size of
Jupiter's Great Red Spot and
so is roughly the same
diameter as the Earth, and
at least one other smaller
storm spot has been
detected as well.
Like the other gas giants,
there are rapid winds
confined to bands of latitude,
including one band that is
moving the Great Dark Spot
westward at over 1,100 km
(or 700 miles) per hour.
Indeed, Neptune has the
fastest planetary winds in
the Solar System, reaching
as fast as 2,000 km (over
1,200 miles) per hour.
Neptune’s Great Dark Spot