For the next few weeks: Terrestrial Planets, their Moons, and the Sun

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For the next few weeksTerrestrial Planets,
their Moons, and the Sun
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Announcements

• Reading Assignment
• Box 7-2 (pp. 168-169) , Sections 9-5 and 9-6 (pp
  223-226)
• 3rd Homework will be posted on Thursday (due
  Tuesday 3/10)
• Note that this is a slight change from the
  schedule (which is now updated)
• Second-project details will be posted early next
  week (due 4/21)
• Results from first exam will be briefly discussed
  today
• Solutions will be posted soon
• If you want to discuss the exam with me, please
  see me in my office (either during regular office
  hours, or another time)

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First Exam Results

• Average 72.3
• Median 74.5
• High 100
• Total tests taken 58
• 89 and above 10 (17.2)
• 78 88 15 (25.9)
• 67 77 12 (20.7)
• 56 66 9 (15.5)
• 55.5 and below 12 (20.7)

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First mid-term exam grade distribution
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Today Planetary Interiors and Surfaces
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Review Formation of terrestrial planets

• Small dust particles accreted to make
  planetesimals
• Planetesimals accreted (and collided with other
  planetesimals) to form protoplanets
• The protoplanets were at least partially molten
• denser iron-rich material fell to the center,
  bringing heavier metals with it, making an
  iron-rich core (differentiation)
• A terrestrial planet!

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Internal structure of a terrestrial planet
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All of the terrestrial planets, and Earths moon
have a similar internal structure, but the
relative sizes are different depending on how hot
the interior got, how rapidly the object cooled,
and how much mass it has The largest factor in
determining this structure is the objects size
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Sources of heat

• Accretion
• Conversion of gravitational energy of incoming
  material to kinetic energy, into thermal energy
• Chemical Differentiation
• Conversion of gravitational energy of falling
  denser materials within the interior to thermal
  energy
• Radioactive decay of elements within the bodys
  interior
• leads to a slight mass difference between initial
  and final elements which is converted to energy
  E?mc2

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Cooling processes in terrestrial planets

• Mantle convection in the interior
• Hot material rises, cold material falls (like
  boiling water on a stove)
• Thermal conduction in the lithosphere
• Like when a metal plate is heated at one end
  the other end will soon get hot too
• Radiative loss through the surface into space
• Volcanic eruptions

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Activity 1

• 1 (a) Which planet at the right would you expect
  to have the highest internal temperature shortly
  after formation?

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Activity 1

• 1 (b) Which of the two planets would you expect
  to cool the most rapidly? Why?

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Activity 1

• 1 (c) Assuming that the two planets have the
  same composition, which one will have the highest
  heating rate due to the radioactive decay of
  elements in the interior?

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Activity 1

• 1 (d) what is the most important factor in
  determining how active a planets interior is
  likely to be, and how long it is likely to stay
  active? (Circle one)
• (A) The objects distance from the Sun.
• (B) The objects composition.
• (C) The objects size

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Another source of internal heat is tidal
heating
Io

• This results from the tidal force caused by
  differences in the gravitational force from a
  nearby massive object
• causes tides on Earth
• Reason for the Moons 11 synchronous rotation
• Tidal heating is important for Io and Europa
  (moons of Jupiter) and possibly other moons
• Tidal heating is not an important internal heat
  source for the planets in our solar system

Europa
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Cassini images of a spray emanating from
Enceladus Evidence of liquid water beneath the
surface?
Do these particles populate Saturns E ring?
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Probing the interiors of planets

• The only reliable way to accurately probe the
  interior of a planet is by analyzing seismic
  activity like Earthquakes
• Another reasonable approach is to measure how the
  body rotates (by measuring its libration)
• The existence of a planetary magnetic field
  provides some basic information
• Theoretical and numerical models can be
  constructed, but these are not as definitive

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Seismic Waves

• P and S waves (Primary and Secondary)
• Move through the Earths interior
• Provide information about interiors structure
• Surface Waves
• The rolling waves that are felt on the surface
• Like water waves

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P- and S-waves
P-waves, or compression waves

• P-waves
• Compressional (or longitudinal) waves
• Can travel through solids or liquids
• S-waves
• Shear (or transverse) waves Cannot travel through
  liquids

S-waves, or transverse waves
Water waves
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• S-waves do not travel through the Earths core
  (creating a shadow zone as shown at below
  right)
• This proves that part of the core is liquid

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Planetary Magnetic Fields

• Another important tool for probing the interior
  of a planet
• Mercury has a global magnetic field
• this is somewhat of a puzzle!
• Venus does NOT have a global magnetic field
• Slow rotation rate
• Earth has a very strong global magnetic field
• Mars does not have a global magnetic field, but
  there is evidence that it had one in the past

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The Earths Dynamo

• Permanent magnets lose their field if raised to a
  temperature above about 500oC
• The Earth is hotter than this nearly everywhere
• Earths field is also known to change
  periodically
• Pole reversals
• It must be generating its own internal magnetic
  field
• Need a circulating electric current
• Circulation and convection of electrically
  conductive molten iron in the Earth's outer core
  produces the magnetic field

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Activity 2

• Why are scientists so certain that part of
  Earths core is liquid?