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?

BASIC NUMBERS

THE OUTER PLANETS
Property	Jupiter	Saturn	Uranus	Neptune	Pluto
Semi-major axis (AU)	5.2	9.6	19.2	30.1	39.5
Orbital Period (years)	12	29	84	165	249
Orbital Eccentricity	0.048	0.053	0.043	0.010	0.250
Rotation Period (days)	0.41	0.44	0.72R	0.67	6.4R
Mass / Mearth	318	95	15	17	0.002
Diameter / Dearth	11.2	9.4	4.0	3.9	0.18
Surface Gravity / gearth	2.4	1.1	0.9	1.1	0.07

• 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)

JUPITER: BASICS

• 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 colored clouds.
  • 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.

JUPITER: STRUCTURE

• 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 spacecraft.
• Modeling shape implies dense, rocky core, roughly 10 M_earth.
  • 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 gravitational energy.

SATURN

• 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 (enough) water.
  • 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.

SATURN'S RINGS

• Discovered by Galileo (1610), recognized as rings by Huygens (1659).
• 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, radio waves.
• 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 and outside.
• 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.