Chapters 23 and 24:
The Jovian Planets (“Gas Giants”):
Jupiter, Saturn, Uranus, Neptune
Galileo’s view of Saturn. He could not see that the “lobes” were rings,
and did not understand why they sometimes disappeared.
Images of Saturn taken by students at Seattle University
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Ming-Hua Chang
Matthew Day
Saturn -- Hubble Space Telescope
Saturn: Voyager 2
Saturn’s most striking feature is its ring system
Cassini Spacecraft View of Saturn
Saturn’s Ring system, by Cassini
Close-up of clouds on Saturn
Gas giants (Jovian planets)
• Are mostly gases (H, He, H2O, CH4, NH3); the rest = ice + rock
• Have no solid surface: gases --> solid at high pressure
• Have ring systems and many moons
• Form faster, and in a different way, compared to terrestrial planets:
large enough to accumulate gas directly from the solar nebula
• They are far from the Sun (in the case of the solar system)
• So far, all known extrasolar planets are gas giants, but they are all
close to their parent star (why?)
Jupiter, imaged with the
Hubble Space Telescope
Colorful cloud bands are
the most striking
characteristic of the
planet, and the Great Red
Spot
Uranus
Neptune: Its existence was
predicted because Uranus did
not seem to obey Kepler’s Law!
Images taken by the Voyager 2 spacecraft
The giant planets are much more massive and less dense than Earth
They rotate much faster than Earth
318 MEarth
15 MEarth
95 MEarth
17 MEarth
Gas giants rotate differentially: rotation is faster at their equator
than their poles (like which other object in the solar system??
They all have magnetic fields (molten interiors)
Belts and zones on Jupiter
Differential rotation
Jupiter’s most distinguishing
features:
• The Great Red Spot
• Cloud bands
• the “Galilean moons”
Storms on gas giants last
so long because there are
no continents over which
they can lose their
energy and dissipate
What causes Jupiter’s cloud bands?
• Strong winds and Jupiter’s differential rotation produce
bands parallel to equator
• hydrogen, helium, water (H2O),
ammonia (NH3) and methane
(CH4)
• These gases are colorless, and
their ices are white
• Colors due to sulfur compounds
and organic hazes particles (?)
• Sunlight and lightning affect the
chemistry of Jupiter’s atmosphere
Clouds and convection in Jupiter’s atmosphere
Zones: high pressure, lighter
Belts: Lower pressure,
darker color
Rotation channels the winds
into east-west pattern that
wraps around entire planet
Zonal flow: stable eastward
and westward winds deeper
in the atmosphere
300 km/hour easterly, at
equator
0 at poles - bands disappear
Entry site of Galileo Probe: the first man-made object to
enter the atmosphere of a giant planet
Saturn is not as colorful as Jupiter, but does have bands and
storms
Saturn has less hydrogen and helium than Jupiter, and the ratio
of helium to hydrogen is much lower: why?
Helium seems to have
liquified and sank to
Saturn’s center
Helium precipitation
seems to be the cause
of Saturn’s energy
output
color
enhanced
Like Jupiter, Saturn has:
• bands
• oval storm systems
• turbulent flow patterns
• stable east–west zonal flow
• wind speed is > Jupiter with fewer east–west alternations.
• The equatorial eastward jet stream = 1500 km/h
Storms on Saturn are more rare than on Jupiter
In 1990, a storm developed in Saturn’s equatorial region
and soon completely encircled the planet.
The storm dissipated in a few weeks.
Uranus
Neptune
Both worlds are composed mostly of hydrogen and helium
Ammonia is in the form of ice crystals (snow), not as a gas
Methane gives these planets their blue color : absorbs red and
yellow photons from sunlight, reflects blue
Neptune has more methane than Uranus
Uranus
•
•
•
•
•
Does not have distinct atmospheric
features (storms, bands, etc.) the way
Jupiter does
No high-altitude clouds; deeper
features are obscured by haze layer
Clouds move around planet in same
direction as rotation (200-500
km/hour)
colder than Jupiter & Saturn (53 K)
has a thin ring
computer enhanced
Uranus’ axial tilt is 98o -- compared to Earth’s 23o tilt
New hypothesis of Uranus and Neptune formation: they originally
formed between Jupiter and Saturn (5-9 AU) and migrated
outward!
Neptune
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•
•
•
•
•
•
Slightly warmer than Uranus
even though further from Sun
temperature = 59 K
Haze layer not as thick
Has more obvious features than
Uranus: storms, spots, bands
slightly more massive than
Uranus
2000 km/hour winds -- much
faster than Uranus
The Great Dark Spot lasted only
a few years
Using a telescope with a spectrometer, we can observe the
wavelength at which Neptune radiates most of its photons. From
Wien’s law,
max =
0.29
T
cm
Neptune, max = 0.005 cm, so Neptune’s temperature = 59 K
if it were only heated by the Sun, Neptune should be 50 K.
Neptune has an internal source of energy: unknown at this time!
Neptune’s Great dark
Spot, discovered by
Voyager 2 in 1989
Neptune, imaged by the
Hubble Space Telescope
Great Dark Spot is gone
by 1994
Atmospheric structure
changes over just a few
days’ time
Computer-enhanced images of Neptune from Hubble Space Telescope
Seasons are changing!
Laws of physics and chemistry + observations = model for giant planets
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•
•
•
All 4 giant planets have about the
same mass rocky core (10 Mearth)
J & S have much more hydrogen &
helium than U & N
J & S have metallic hydrogen
J & S & N radiate away more energy
than they receive from Sun
Aurorae on earth: Interaction of solar wind with Earth’s magnetic field
The Earth has the
properties of a big
magnet
Solar wind (charged
particles) respond to this
magnetic field
The giant planets
all have
magnetic fields,
& atmospheres,
so we expect
them to have
aurorae too!
Jupiter’s magnetosphere
is affected by the solar
wind
We can measure the
strength of J’s magnetic
field : much stronger
than Earth’s
Formation of the gas giant planets:
Gas giants form via core accretion followed by accumulating H
and He gases from the solar nebula.
In the solar nebula, ice was an abundant core-building material > 5
AU from the Sun, because temperatures drops as you go further
from the Sun
Gas giants must
form before the
solar nebula
dissipates (< 10
million years)
Disks are seen around many young stars
Earth
Jupiter
Jupiter and Saturn’s
atmospheres to scale
cooler than Jupiter, less
dense atmosphere, thicker
cloud layers