LECTURE 18: JUPITER AND SATURN
- How do we learn about the outer planets?
- What do the densities of Jupiter and Saturn imply about their composition?
- What causes bands in Jupiter's atmosphere?
- What is Jupiter's internal structure, and how is it inferred?
- Why are Jupiter and Saturn oblate, instead of spherical?
- What are the structural differences between Jupiter and Saturn?
- What are Saturn's rings composed of? Are they solid?
- What determines the detailed structure of Saturn's rings?
|THE OUTER PLANETS
Semi-major axis (AU)
Orbital Period (years)
Rotation Period (days)
Mass / Mearth
Diameter / Dearth
Surface Gravity / gearth
- Jupiter, Saturn, Uranus, Neptune called the Jovian (Jupiter-like) planets.
- They all have many moons, some big, some tiny.
- Pluto has one moon, Charon, that is nearly as big as Pluto itself.
LEARNING ABOUT THE OUTER PLANETS
- Can see basic features through Earth-based telescopes.
- Hubble Space Telescope especially useful because of sharp imaging.
- Distances from geometry (Copernicus), diameters from angular
size and distance.
- Masses from orbits of moons, more recently (and accurately) from
gravitational effects on spacecraft.
- Space probes:
- Pioneer 10 to asteroid belt and Jupiter (launched 1972).
- Pioneer 11 to Jupiter and Saturn (launched 1973).
- Voyager 1 to Jupiter and Saturn (launched 1977).
- Voyager 2 to Jupiter, Saturn, Uranus, and Neptune (launched 1977).
- Galileo orbiter to Jupiter (launched 1989).
- Cassini orbiter to Saturn (launched 1997, reached Saturn last July)
- More mass than all other planets combined (by factor of 2.5).
- Density 1300 kg / m3, much less than terrestrial planets
(4000-5500 kg / m3).
- Implies different composition, structure.
- Mainly hydrogen and helium, most abundant elements in Sun and universe.
- Rapid rotation, 10-hour day.
- Equator rotates slightly faster than polar regions.
- Circled by dark "belts" and light "zones."
- Belts slightly hotter, denser. See further down, to different
- Winds in belt and zone regions blow in opposite directions.
- Get cyclone storms in between. Most prominent is Great Red Spot,
a hurricane the size of the Earth, persistent since at least 1610.
- Four large moons (discovered by Galileo), many smaller moons.
- Inferred by combination of observations and theoretical modeling.
- Fast rotation causes outward bulge at equator, oblate shape.
- Can learn about shape of interior from gravitational effect on
- Modeling shape implies dense, rocky core, roughly 10 Mearth.
- This inference is controversial; some claims that rocky core not needed.
- Existence of core important to understanding how Jupiter formed.
- Standard theory says core formed first by coagulation of solid material,
then collected gas by gravity.
- If there is no core, then this theory can't be right.
- Similar analyses of Saturn, Uranus, Neptune provide clear evidence
for solid cores in those planets.
- Core surrounded by layer of helium and liquid metallic hydrogen.
- Metallic means electrons can hop easily from atom to atom.
- Hydrogen becomes metallic at very high pressure.
- Conducts electricity, produces Jupiter's strong magnetic field.
- Surrounded by layer of liquid molecular hydrogen and helium.
- Atmosphere of gaseous hydrogen and helium, traces of other elements.
- Jupiter emits 2.5 times more radiation than it absorbs from the Sun.
- Implies planet is slowly contracting, generating heat from
- Similar composition to Jupiter, mostly hydrogen and helium.
- 1/3 mass of Jupiter, but nearly same radius.
- Lower density, 700 kg / m3. Saturn would float in
- Hydrogen/helium more compressible than rock, iron.
- Less massive planet, lower gravity, lower density.
- Surface gravity about 1/3 Jupiter, similar 11-hour rotation period.
- Less gravity, same rotation speed, so even more oblate (flattened).
- Atmospheric structure similar to Jupiter, but less visually dramatic.
- Interior structure similar to Jupiter, with less metallic hydrogen.
- Also generates heat by gravitational contraction.
- One large moon (Titan) and many smaller ones.
- Discovered by Galileo (1610), recognized as rings by
- Maxwell (1857) proves they are not solid, or tidal gravity
(stronger pull on inside than outside) would tear them apart.
- Composed of particles centimeters to meters in size.
- Each particle orbits Saturn like tiny moon, held by gravity.
- Occasional collisions: bouncing, sticking, breaking.
- Composed of (or at least coated by) water ice, highly reflective.
- Size inferred by transparency to optical light, infrared light,
- Total mass comparable to small moon, 300 km across.
- Not unique! Jupiter, Uranus, Neptune have thinner, darker rings,
not easily visible from Earth.
STRUCTURE OF SATURN'S RINGS
- Images from Voyager missions and, most recently, Cassini mission
reveal intricate structure in spectacular detail.
- Several major rings (identified with Earth-based telescopes),
composed of thousands of ringlets.
- Some rings ultra-thin, less than 100 m. Like sheet of paper
1 mm thick, 10 km wide.
- Gaps caused by gravitational perturbations of moons.
- Some thin rings maintained by gravity of "shepherd" moons inside
- Gravity of small moons creates ripples, waves on edges of some rings.
- If ring material collected into a moon, Saturn's tidal gravity
would tear it apart.
- Rings may be material that was too close to Saturn to make a moon,
or could be remains of moon that came too close and was torn apart.
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Updated: 2005 May 22 [dhw]