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Comins Chap 8 The Outer Planets

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					    Neil F. Comins • William J. Kaufmann III



Discovering the Universe
             Eighth Edition


          CHAPTER 8
    The Outer Planets
Key Questions….
• How do we know what we do?
• What value is there to ask these questions, and
build probes to answer them?
    Essay Questions for the Final
   Describe Jupiter's atmosphere. What
    are the similarities and differences
    between Jupiter and Saturn?

   Describe Jupiter's four largest
    satellites. Why are they similar, and why
    are they different?
    Essay Questions for the Final
   Describe Saturn and its Rings. How did
    they probably arise? How do we know?

   What do we now know about Titan? What
    were the scientific questions we wanted to
    answer by exploring Titan as we did?

   What are the properties shared by Uranus
    and Neptune? How are they unique?
    In this chapter you will discover…
   Jupiter, an active, vibrant, multicolored world
    more massive than all of the other planets
    combined

   Jupiter’s diverse system of moons

   Saturn, with its spectacular system of thin, flat
    rings and numerous moons, including bizarre
    Enceladus and Titan

   What Uranus and Neptune have in common and
    how they differ from Jupiter and Saturn
      8.1 A Different Kind of Planet

Our goals for learning:
 What are jovian planets made of?
 What are jovian planets like on the inside?
 What is the weather like on jovian planets?
What are jovian planets made
             of?
    Jovian Planet Composition
   Jupiter and Saturn
    —   Mostly H and He gas

   Uranus and Neptune
    —   Mostly hydrogen compounds: water (H2O),
        methane (CH4), ammonia (NH3)
    —   Some H, He, and rock
     Jovian Planet Formation
 Beyond the frost line, planetesimals could
  accumulate ICE.
 Hydrogen compounds are more abundant
  than rock/metal so jovian planets got
  bigger and acquired H/He atmospheres.
     Jovian Planet Formation
 The jovian cores are very similar:
  ~ mass of 10 Earths
 The jovian differences are in the amount of
  H/He gas accumulated.

Why did that amount differ?
    Differences in Jovian Planet
             Formation
 TIMING: The planet that forms earliest
  captures the most hydrogen and helium
  gas. Capture ceases after the first solar
  wind blew the leftover gas away.
 LOCATION: The planet that forms in a
  denser part of the nebula forms its core
  first.
             Density Differences
Density (g/cc)    2
                                Uranus and
                 1.5             Neptune are
                  1              denser than
                 0.5             Saturn because
                                 they have less
                  0              H/He,
                                 proportionately.
                  ep s
                  Sa r



                        ne
                  U rn
                       te


                 N nu
                     tu


                     tu
                    pi


                   ra
                  Ju
Density Differences
                  2
Density (g/cc)   1.5
                                But that
                                 explanation
                  1              doesn’t
                 0.5             work for
                                 Jupiter.
                  0
                  ep s
                  Sa r



                        ne
                  U rn
                       te


                 N nu
                     tu


                     tu
                    pi


                   ra
                  Ju
Sizes of Jovian Planets
                 Adding mass to
                  a jovian planet
                  compresses the
                  underlying gas
                  layers.
Sizes of Jovian Planets
                 Greater
                  compression is
                  why Jupiter is not
                  much larger than
                  Saturn even
                  though it is three
                  times more
                  massive.

                 Jovian planets with
                  even more mass
                  can be smaller
                  than Jupiter.
What are jovian planets like on
         the inside?
    Interiors of Jovian Planets
 No solid surface
 Layers under high pressure and
  temperatures
 Cores (~10 Earth masses) made of
  hydrogen compounds, metals, and rock
 The layers are different for the different
  planets — WHY?
Inside Jupiter
             High pressure
              inside of Jupiter
              causes the phase
              of hydrogen to
              change with
              depth.

             Hydrogen acts like
              a metal at great
              depths because its
              electrons move
              freely.
Inside Jupiter
             The core is
              thought to be
              made of rock,
              metals, and
              hydrogen
              compounds.

             The core is about
              the same size as
              Earth but 10 times
              as massive.
      Comparing Jovian Interiors




   Models suggest that cores of jovian planets have
    similar composition.
   Lower pressures inside Uranus and Neptune
    mean no metallic hydrogen.
         Jupiter’s Magnetosphere




                                           Aurora on Jupiter


   Jupiter’s strong magnetic field gives it an enormous
    magnetosphere.
   Gases escaping Io feed the donut-shaped Io torus.
What is the weather like on
     jovian planets?
Jupiter’s Atmosphere
               Hydrogen compounds
                in Jupiter form clouds.

               Different cloud layers
                correspond to freezing
                points of different
                hydrogen compounds.

               Other jovian planets
                have similar cloud
                layers.
                                          Jupiter’s
                                           Colors




   Ammonium sulfide clouds (NH4SH) reflect
    red/brown.
   Ammonia, the highest, coldest layer, reflects
                                        Saturn’s
                                         Colors




   Saturn’s layers are similar but are deeper in
    and farther from the Sun — more subdued.
Methane on Uranus and
      Neptune
                Methane gas on
                 Neptune and
                 Uranus absorbs
                 red light but
                 transmits blue
                 light.

                Blue light reflects
                 off methane
                 clouds, making
                 those planets look
                 blue.
Jupiter’
s Great
  Red
 Spot



   A storm twice as wide as Earth
   Has existed for at least 3 centuries
      Weather on Jovian Planets




   All the jovian planets have strong winds
    and storms.
          Thought Question

Jupiter does not have a large metal core
like the Earth. How can it have a magnetic
field?

 A.   The magnetic field is left over from when
      Jupiter accreted.
 B.   Its magnetic field comes from the Sun.
 C.   It has metallic hydrogen inside, which
      circulates and makes a magnetic field.
 D.   That’s why its magnetic field is weak.
          Thought Question

Jupiter does not have a large metal core
like the Earth. How can it have a magnetic
field?

 A.   The magnetic field is left over from when Jupiter
      accreted.
 B.   Its magnetic field comes from the Sun.
 C.   It has metallic hydrogen inside, which circulates
      and makes a magnetic field.
 D.   That’s why its magnetic field is weak.
        What have we learned?
   What are jovian planets made of?
    —   Jupiter and Saturn are mostly made of H and
        He gas.
    —   Uranus and Neptune are mostly made of H
        compounds.
   What are jovian planets like on the inside?
    —   They have layered interiors with very high
        pressure and cores made of rock, metals, and
        hydrogen compounds.
    —   Very high pressure in Jupiter and Saturn can
        produce metallic hydrogen.
        What have we learned?
   What is the weather like on jovian
    planets?
    —   Multiple cloud layers determine the
        colors of jovian planets.
    —   All have strong storms and winds.
8.2 A Wealth of Worlds: Satellites of Ice
              and Rock
Our goals for learning:
 What kinds of moons orbit the jovian planets?
 Why are Jupiter’s Galilean moons geologically active?
 What geological activity do we see on Titan and other
  moons?
 Why are jovian planet moons more geologically active
  than small rocky planets?
What kinds of moons orbit the
       jovian planets?
                Sizes of Moons
   Small moons (< 300 km)
    —   No geological activity
   Medium-sized moons (300–1,500 km)
    —   Geological activity in past
   Large moons (> 1,500 km)
    —   Ongoing geological activity
    Medium and
    Large Moons
   Enough self-gravity
    to be spherical
   Have substantial
    amounts of ice
   Formed in orbit
    around jovian
    planets
   Circular orbits in
    same direction as
    planet rotation
Small Moons
             Far more
              numerous than
              the medium
              and large
              moons
             Not enough
              gravity to be
              spherical:
              ―potato-shaped‖
Why are Jupiter’s Galilean
moons geologically active?
           Io’s Volcanic Activity




   Io is the most volcanically active body in
    the solar system, but why?
                Io’s Volcanoes




   Volcanic eruptions continue to change Io’s
    surface.
    Io Volcanoes IR
  Tidal Heating




Io is
squished
and
stretched is its
But why
orbit so
as it orbits
elliptical?
Jupiter.
   Orbital       The tugs add up over
 Resonances      time, making all three
                 orbits elliptical.

Every 7 days,
these three
moons line up.
Europa’s Ocean: Waterworld?
Tidal Stresses Crack Europa’s
          Surface Ice
Tidal stresses crack Europa’s
          surface ice

                     Tidal flexing
                     closes crack,
                     grinds up ice




                     Tidal flexing
                     opens crack,
                     leaving two
                     ridges
Europa’s Interior Also Warmed by Tidal
                Heating
Ganymede
     Largest moon in
      the solar system
     Clear evidence of
      geological activity
     Tidal heating plus
      heat from radio-
      active decay?
Callisto
            ―Classic‖
             cratered iceball
            No tidal heating,
             no orbital
             resonances
            But it has
             magnetic field !?
         Thought Question

How does Io get heated by Jupiter?

A.   Auroras
B.   Infrared light
C.   Jupiter pulls harder on one side than the
     other
D.   Volcanoes
        Thought Question

How does Io get heated by Jupiter?

A.   Auroras
B.   Infrared light
C.   Jupiter pulls harder on one side than the
     other
D.   Volcanoes
What geological activity do we
see on Titan and other moons?
Titan’s Atmosphere
               Titan is the only
                moon in the solar
                system which has
                a thick
                atmosphere.

               It consists mostly
                of nitrogen with
                some argon,
                methane, and
                ethane.
                  Titan’s Surface




   The Huygens probe provided a first look at Titan’s surface
    in early 2005.
   It had liquid methane, ―rocks‖ made of ice.
              Titan’s ―Lakes‖




   Radar imaging of Titan’s surface has
    revealed dark, smooth regions that may be
    lakes of liquid methane.
       Medium Moons of Saturn




   Almost all show evidence of past volcanism
    and/or tectonics.
Ongoing Activity on Enceladus

                    Fountains of ice
                     particles and
                     water vapor from
                     the surface of
                     Enceladus
                     indicate that
                     geological activity
                     is ongoing.
Medium Moons of Uranus
                Varying amounts
                 of geological
                 activity occur.

                Moon Miranda has
                 large tectonic
                 features and few
                 craters (episode of
                 tidal heating in
                 past?).
Neptune’s Moon Triton
                Similar to Pluto,
                 but larger

                Evidence for past
                 geological activity
Why are jovian planet moons
more geologically active than
   small rocky planets?
    Rocky Planets vs. Icy Moons




   Rock melts at         Ice melts at lower
    higher                 temperatures.
    temperatures.         Tidal heating can
   Only large rocky       melt internal ice,
    planets have           driving activity.
        What have we learned?
   What kinds of moons orbit jovian
    planets?
    —   Moons of many sizes
    —   Level of geological activity depends on
        size
   Why are Jupiter’s Galilean moons
    geologically active?
    —   Tidal heating drives activity, leading to
        Io’s volcanoes and ice geology on other
        moons.
        What have we learned?
   What geological activity do we see on Titan
    and other moons?
    —   Titan is the only moon with a thick atmosphere.
    —   Many other icy moons show signs of geological
        activity.
   Why are jovian planet moons more
    geologically active than small rocky planets?
    —   Ice melts and deforms at lower temperatures
        enabling tidal heating to drive activity.
        8.3 Jovian Planet Rings

Our goals for learning:
 What are Saturn’s rings like?
 Why do the jovian planets have rings?
What are Saturn’s rings like?
    What are Saturn’s rings like?
 They are made up of numerous, tiny
  individual particles.
 They orbit over Saturn’s equator.
 They are very thin.
Earth-Based View
Spacecraft View of Ring Gaps
Artist’s Conception of Close-Up
Gap Moons
          Some small
           moons create
           gaps within rings.
Why do the jovian planets have
            rings?
           Jovian Ring Systems




   All four jovian planets have ring systems.
   Others have smaller, darker ring particles than does
    Saturn.
Why do the jovian planets have
            rings?
   They formed from dust created in impacts
    on moons orbiting those planets.




            How do we know this?
          How do we know?
 Rings aren’t leftover from planet formation
  because the particles are too small to
  have survived this long.
 There must be a continuous replacement
  of tiny particles.
 The most likely source is impacts with the
  jovian moons.
           Ring Formation




 Jovian planets all have rings because they
  possess many small moons close-in.
 Impacts on these moons are random.
 Saturn’s incredible rings may be an
  ―accident‖ of our time.
        What have we learned?
   What are Saturn’s rings like?
    —   They are made up of countless individual ice
        particles.
    —   They are extremely thin with many gaps.


   Why do the jovian planets have rings?
    —   Ring systems of other jovian planets are much
        fainter with smaller, darker, less numerous
        particles.
    —   Ring particles are probably debris from moons.
Summary of Key Ideas
              Jupiter and Saturn
   Jupiter is by far the largest and most massive planet in
    the solar system.
   Jupiter and Saturn probably have rocky cores
    surrounded by a thick layer of liquid metallic hydrogen
    and an outer layer of ordinary liquid hydrogen. Both
    planets have an overall chemical composition very
    similar to that of the Sun.
   The visible features of Jupiter exist in the outermost 100
    km of its atmosphere. Saturn has similar features, but
    they are much fainter. Three cloud layers exist in the
    upper atmospheres of both Jupiter and Saturn. Because
    Saturn’s cloud layers extend through a greater range of
    altitudes, the colors of the Saturnian atmosphere appear
    muted.
            Jupiter and Saturn
   The colored ovals visible in the Jovian
    atmosphere are gigantic storms, some of which
    (such as the Great Red Spot) are stable and
    persist for years or even centuries.
   Jupiter and Saturn have strong magnetic fields
    created by electric currents in their metallic
    hydrogen layers.
              Jupiter and Saturn
   Four large satellites orbit Jupiter. The two inner Galilean
    moons, Io and Europa, are roughly the same size as our
    Moon. The two outer moons, Ganymede and Callisto,
    are approximately the size of Mercury.
   Io is covered with a colorful layer of sulfur compounds
    deposited by frequent explosive eruptions from volcanic
    vents. Europa is covered with a smooth layer of frozen
    water crisscrossed by an intricate pattern of long cracks.
   The heavily cratered surface of Ganymede is composed
    of frozen water with large polygons of dark, ancient crust
    separated by regions of heavily grooved, lighter-colored,
    younger terrain. Callisto has a heavily cratered ancient
    crust of frozen water.
              Jupiter and Saturn
   Saturn is circled by a system of thin, broad rings lying in
    the plane of the planet’s equator. Each major ring is
    composed of a great many narrow ringlets that consist of
    numerous fragments of ice and ice-coated rock. Jupiter
    has a much less substantial ring system.
   Titan has a thick atmosphere of nitrogen, methane, and
    other gases, as well as lakes of methane and ethane.
   Enceladus has areas with very different surface features:
    an older, heavily cratered region and a newer, nearly
    crater-free surface created by tectonic activity.
             Uranus and Neptune
   Uranus and Neptune are quite similar in appearance, mass,
    size, and chemical composition. Each has a rocky core
    surrounded by a thick, watery mantle; the axes of their
    magnetic fields are steeply inclined to their axes of rotation;
    and both planets are surrounded by systems of thin, dark
    rings.
   Uranus is unique in that its axis of rotation lies near the plane
    of its orbit, producing greatly exaggerated seasons on the
    planet.
   Uranus has five moderate-sized satellites, the most bizarre of
    which is Miranda.
   Triton, the largest satellite of Neptune, is an icy world with a
    tenuous nitrogen atmosphere. Triton moves in a retrograde
    orbit that suggests it was captured into orbit by Neptune’s
    gravity. It is spiraling down toward Neptune and will eventually
    break up and form a ring system.
              Key Terms
A ring                      liquid metallic hydrogen
B ring                      occultation
belt                        polymer
Cassini division            prograde orbit
C ring                      resonance
differential rotation       retrograde orbit
Encke division              ringlet
F ring                      Roche limit
Galilean moon (satellite)   shepherd satellite (moon)
Great Dark Spot             spoke
Great Red Spot              zone
hydrocarbon
     WHAT DID YOU THINK?
 Is Jupiter a “failed star”? Why or why not?
 No. Jupiter has 75 times too little mass to
  shine as a star.
     WHAT DID YOU THINK?
 What is Jupiter’s Great Red Spot?
 The Great Red Spot is a long-lived, oval
  cloud circulation, similar to a hurricane on
  Earth.
    WHAT DID YOU THINK?
 Does Jupiter have continents and oceans?
 No. Jupiter is surrounded by a thick
  atmosphere primarily of hydrogen and
  helium that gradually becomes liquid as
  you move inward. The only solid matter in
  Jupiter is its core.
     WHAT DID YOU THINK?
 Is Saturn the only planet with rings?
 No. All four outer planets (Jupiter, Saturn,
  Uranus, and Neptune) have rings.
     WHAT DID YOU THINK?
 Are the rings of Saturn solid ribbons?
 No. Saturn’s rings are all composed of
  thin, closely spaced ringlets consisting of
  particles of ice and ice-coated rocks. If
  they were solid ribbons, Saturn’s
  gravitational tidal force would tear them
  apart.

				
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