Two Types of Planets: Reminders and Additions

• Terrestrial planets
  • low mass (£ 1 MÅ )
  • high density (rocky, metallic)
  • slow rotators (P ³ 24 hours)
  • few satellites
  • close to Sun (a £ 1.6 AU)
  • Thin atmospheres
  • Weak or no mag field
• Jovian planets
  • high mass (³ 15 MÅ )
  • low density (gaseous, but some gas is solid)
  • rapid rotators (P £ 18 hours)
  • many satellites
  • far from Sun (a ³ 5 AU)
  • Thick atmospheres
  • Strong magnetic field

The Jovian Planets

• Jupiter, Saturn, Uranus, and Neptune have all been visited by the Voyager space probes.
  • Galileo is now studying Jupiter
  • Cassini is on its way to Saturn
• Jupiter, Saturn, Uranus, and Neptune are all massive bodies
  • formed in outer part of pre-solar nebula where ices condense
  • growth by accretion and coalescence
  • Massive enough that gravity probably played an important role as well
• Jovian planets are gaseous/fluid bodies
  • supported by balance between pressure and gravity: Hydrostatic equilibrium - important concept

Hydrostatic Equilibrium

• Atmosphere is stable:
  • Pressure and gravity in equilibrium
  • Pressure must increase downwards because of increasing weight of gas above
  • Equation of state for gas (pressure proportional to density times temperature):

Pµ r T

• Since J,S,U,N are much more massive than Earth, expect much higher pressures.

Interiors of Jovian Planets

• Very different structure and composition from terrestrial planets - mostly gas, not rock

• Jupiter and Saturn are massive enough to have metallic hydrogen zones
  • large mass causes high internal pressure
  • liquid metallic hydrogen zones are interior to liquid molecular hydrogen zones
  • electrons not bound to individual atoms

• Jovian planets have internal heat sources
  • Jovian planets radiate more energy than they absorb from the Sun - energy conservation?.
  • Heat generated by contraction is slowly radiated away.
  • Recall how you measure "heat" or radiated energy:

E=4p R² s T⁴

Atmospheres of Jovian Planets

• Appearance of atmosphere depends on temperatures at highest levels.
  • 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 appearance
• 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 Jovian planets stretches clouds into bands.
• The different colored bands in Jupiter’s atmosphere show convective motion.
  • Zones: Higher, cooler, lighter colors
  • Belts: Lower, warmer, darker colors

Jupiter’s Zones and Belts Some Peculiarities

• Jupiter’s "Great Red Spot"
  • Large cyclonic storm, first discovered by Cassini (1665). Movie below:
• Uranus: Rotation axis inclined by 98°
  • Consequence: Solar day at poles is ½P_rev = 48 years.
  • Implication: angular momentum considerations most likely imply that a violent event disturbed Uranus early in its history

Atmospheric Circulation

• In spite of tilt of Uranus’ axis, dominant wind direction is still East-West, i.e. parallel to planet's equator.
• This means Coriolis force dominates solar heating

The Satellites of the Jovian Planets

Jovian Satellites

• The 4 Galilean satellites probably formed like miniature Solar System.
• Smaller Jovian satellites are probably captured asteroids.
• Remember: "Joy Is Eating Good Chocolate: Jupiter, Io, Europa, Ganymede, Callisto

Satellite            Distance (in                 Density
                        Jupiter  radii)                (g/cm³) 
Io                        5.9                                3.6
Europa                9.4                                3.0
Ganymede           15.0                              1.9
Callisto               26.4                              1.9

• Io - the pizza satellite
  • Most geologically active object in the Solar System.
  • 100 m of new surface every million years.
  • No impact craters.
  • Too small for much radioactivity. Energy derived from tidal forces.
  • Volcanic Eruption on Io
• Ganymede and Calllisto: icy surfaces with embedded hydrocarbons.
• Europa: Similar, but with fracture patterns and flooding.

Titan

• Saturn’s moon Titan is the only satellite in the Solar System with an atmosphere. Why?
• Probably has liquid N₂ surface
• Atmospheric composition:
  • 99% N₂
  • 1% CH₄

Magnetospheres of Jovian Planets: Basic Facts

• Jovian planets have large magnetospheres.
• Appear to be 3 requirements for magnetic fields:
  • Liquid metallic interior (iron [earth] or hydrogen)
  • Convection currents
  • Relatively rapid rotation (Venus has none!)
• Magnetosphere filled with charged particles (aurora)
• Solar wind distorts magnetosphere
• Orientation of fields differ from planet to planet
• Important difference: size of magnetosphere (how much stored energy) and strength at any one point (analogy: flowing water)
• Jupiter: Magnetic field traps charged particles from volcanic eruptions on Io.
• Strong magnetic field causes strong bow shock in the solar wind
• Magnetosphere and Bow Shock

Magnetic Field Orientations

• Uranus: orientation and location of magnetic field is strange: highly inclined relative to rotation axis; non-central
• Uranus’ field is tilted and offset from axis
• The bar magnet is only a model for true field
• Conclude: Not much understanding of planetary magnetic fields.

Planetary Rings - the Roche limit

• All Jovian planets have equatorial rings inside the Roche limit.

Roche limit = 2.5 x R x (r /m)

• R is radius of planet
• r density of planet
• m is density of orbiting object

• Solid objects get torn apart by tides because of differential forces.
• Reflected sunlight spectrum shows that the rings are made of tiny particles (not a solid ring) in Keplerian orbits:  remember that according to Kepler, V_orbital is proportional to r ^-1/2
• Rings are very thin.
  • Saturn’s rings are a few 10s of meters thick, but 280,000 km diameter.
  • Disappear once every 14.5 years when Earth crosses the orbital plane of rings.

Origin of the Rings

• Rings might represent:
  • unformed bodies, or
  • broken-up (tidally disrupted) bodies
• Current evidence favors second explanation because rings are not expected to be long-lived phenomenon.
• Do they get replenished?

• Three stages in formation

1 Orbiting cloud of particles ® thin disk (remember conservation of angular momentum)

2 Keplerean shear ® spreading of rings (speed in orbit depends on distance; collisions are soft) .

3 Collisions and evolution stop (never actually achieved - perturbations by external objects dominate and collisions continue.)

Ring Destruction

• Large particles will undergo collisions that grind them down.
• Small particles are removed by solar wind and radiation pressure.
• Particles slowly diffuse away from their initial orbits.
  • This process can be controlled and avoided by shepherd satellites.

Shepherd Satellites

• Shepherd satellites are small satellites within the ring system that tend to move ring particles only into the orbits that are least affected by perturbations from the satellites.

Ring Structure

• Rings can have a great deal of fine structure.
• Structure is due to perturbations by satellites.
  • Particles in Cassini division in 2:1 resonance with Mimas, the largest inner satellites of Saturn.

Sizes of Ring Particles

• Sizes are determined by scattering characteristics
• Large particles (compared to wavelengths of light) are seen in backscattering (essentially reflection).
• Small particles (compared to wavelengths of light) are seen in forward scattering.

Forward Scattering

• Particles comparable to wavelength of light, e.g. fog.
• Photons hit particles and most are scattered in the forward direction.

Back Scattering of Light

• Particles doing the scattering are much larger than the wavelength of light - e.g. baseballs.

• Photons hit particles; most are scattered in the backward direction.

Sizes of Particles in Saturn’s Rings

• Sizes of particles in Saturn’s ring range from mm to several meters (house-size).
• Composition seems to be mostly water ice (highly reflective).

Other Jovian Planets:

• Except for Saturn, rings of the other Jovian planets - Jupiter, Uranus, Neptune - are best seen in forward scattering, ie. with the sun behind them (Question - how can that be arranged?)
• Forward scattering shows that much of the ring material is microscopic dust.
• Except for Saturn, rings are dark.
  • Mostly rocky or ice with embedded hydrocarbons.
• Uranus also has large particles (centimeter to meter size), but they are hard to see because they are so dark.
  • Reflect only about 5% of visible light, similar to coal!

Spokes in Saturn’s Rings

• Persistent radial features called spokes are sometimes seen in Saturn’s rings.
• These do not rotate in a Keplerian fashion.
• These are probably due to microscopic dust particles that are electrically charged and trapped by Saturn’s magnetic field.

Origin of Spoke Material

• Origin of dust in spokes is unclear, but thought to be due to meteoroids
• Dust in spokes suggests that Saturn’s rings are young; prolonged accumulation of dust would make them dark.

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