Download What are the Jovian Planets? Characteristics of Jovian Planets

Key Concepts: Lecture 15: Jovian Planets (Gas Giants)
Space Missions to Jovian Planets
[You do not need to remember the details of these missions]
Background, Discovery, Basic properties: rotation, orbits, seasons
Internal Structure
Atmosphere: clouds, winds, storms
Magnetic field
Voyager
What are the Jovian Planets?
• 4 Giant Planets in the
outer solar system,
beyond Mars,
–Jupiter
–Saturn
–Uranus
–Neptune
Known to
the ancients
Discovered relatively
recently with
telescopes and
Newton’s Laws
Characteristics of Jovian Planets
• Much larger and
more massive than
terrestrial planets
• They are not solid gaseous
• All have rings
• All have many
satellites
Characteristics of Jovian Planets
Questions
ecliptic plane
• Which of the above planets experience
seasons and why?
• What would the seasons be like?
Rotation & Orbits
Planet
Rotation
Period
Rotate
Orbital
Period
Period
(hours) (years)
Jupiter
9.9
11.9 5.2 AU
Saturn
10.7
29.5 9.5 AU
Uranus
17.2
84.1 19.2 AU
Neptune
16.1
164.8 30.1 AU
Fast rotation (i.e. quite short days)
Orbital periods obey Kepler’s 3rd Law: P2=a3
• Jupiter
–axis of rotation tilted by 3o - no seasons
• Saturn
–axis tilted by 27o - has seasons
• Neptune
–axis tilted by 29o - has seasons
–seasonal changes slower than Saturn’s
• Uranus
–axis tilted by 98o - strange seasons
S
E
A
S
O
N
S
Seasons on Uranus
– poles see sun for 42
years then darkness for
next 42 years
• What caused this
extreme tilt?
–Perhaps a collision
with a large body
during formation
Composition & Structure of Jovians
• Mostly Hydrogen & Helium
• They have low densities
• At most they have only small rocky cores
• At high pressures, Hydrogen acts like a metal:
conducts electricity
• H and He are light and can
escape more easily than
heavier atoms
• The gas giants are massive
so they have high escape
velocities
• They are far from the Sun
so they are cold and the
atoms in their atmosphere
move quite slowly
gas particle at a given temperature.
Northern winter solstice
Southern summer solstice
Escape speed from planet or speed of
Why do they have H and He?
• Orbit takes 84 years
• Amount of sunlight
varies dramatically &
depends on location
Internal Heat Sources
• Primordial Heat from gravitational contraction
associated with formation
– Larger planets are heated more strongly during formation
by collapse of material and rapid initial differentiation.
Also it is more difficult for this heat to escape from a
larger planet: takes longer to cool down.
• Continued generation of heat by gradual
differentiation: e.g. condensation of He in Saturn
• Radioactive decay relatively unimportant because
heavy elements are smaller fraction of total mass
Effect of internal heat: raises the temperature of interior &
atmosphere to higher values than expected from only Sun’s
heating
Jupiter
Saturn
Uranus/Neptune
Clouds & Atmosphere
Atmospheres
• Atmosphere mostly Hydrogen &
Helium
• Clouds give Jovians their appearance
– Jupiter & Saturn :
• upper decks of frozen ammonia (NH3)
• lower decks - ammonium hydrosulfide
(NH4SH) - dark yellows & browns
Jupiter
– Uranus & Neptune:
• methane - pale blue appearance
• Uranus is featureless - stable atmosphere
Galileo probe
Winds & Circulation
• Features observed are due to circulation of clouds &
atmosphere
• Jovian planets are fast rotators
– horizontal circulation patterns parallel to equator
• No solid surface
– less friction for circulation patterns to lose energy
– longer lived storms
– can have different rotation rates at pole vs. equator
• Internal heat source causes vertical convection
currents
Winds & Circulation
• Alternating light & dark bands parallel to equator
stretch around planet
• Semi-permanent features
–shift in intensity and position from year to year
• East-west wind patterns
–do not appear to change at all, even over decades
Storms
• Storms - local disturbances in
the regular atmospheric
circulation patterns
• Seen on Jupiter, Neptune,
Saturn
–Great Red Spot - Jupiter - long
lived (>=300 years)
–Great Dark Spot - Neptune probably shorter lifetime
–Seasonal storms on Saturn
The Great Red Spot on Jupiter
• A giant cyclone that has been going for
centuries
• ~30,000 km long
• Changes in size over time
The Great Dark Spot on Neptune
• Dark Spot seen in Voyager images
1989
– similar to Jupiter Great Red Spot
• same shape
• same latitude
– 10,000 km long
• Dark Spot not seen in Hubble Space
Telescope images 1994
– faded or disappeared?
– storms may form & dissipate more
quickly on Neptune
Uranus/Neptune have strange
magnetic field orientation
Storms on Saturn
• Large storms are rarer
• Connected with seasons
• ~ every 30 years outbreak of
spots are seen in the equatorial
regions
• Storms resemble turbulent cloud
structures
• Very large - 10 times size of
Earth
Hubble Space Telescope
image - 1990
Magnetic Fields
• Jovians have strong magnetic fields
• Magnetic fields generated in the same manner as on Earth
–metallic liquid regions plus rotation of planet causes dynamo
–Jupiter & Saturn may have liquid metallic hydrogen cores
–Uranus & Neptune may have liquid metallic hydrogen
mantles