Announcements
• 25 people have still not joined the class
on Astronomy Place. You can not get credit
until you “join the class”. Once you join, all
your previous submissions will show up in
my gradebook.
• Planet Assignment 4, due Monday March 1,
– Part 1: Astronomy Place tutorial “Shaping
Planetary Surfaces”
– Part 2: Angel Planet Assignment 4
Example of a proto-Planetary Disk
Objectives:
• Describe the significance of bombardment in the
early solar system.
• Compare and contrast the structure & evolution of
the terrestrial planets.
• Describe the differentiation into core, mantle &
crust.
• Describe the heating and cooling processes in
planetary interiors. Describe the importance of
planetary size on its evolution.
Two Types of Planets
Terrestrial:
small
rocky, iron
inner
Jovian:
large
H, He, ices
outer
When Planets Form Bombardment & Differentiation
• Bombardment of planet as it forms, HEATS
and melts its surface
• Molten iron and other heavy elements sink
toward the center
• Falling iron converts gravitational potential
energy to kinetic energy -> HEAT
Inside the Terrestrial Worlds
• After they have formed, the molten planets
differentiate into three zones:
• core - made of metals
• mantle - made of dense rock
• crust - made of less dense rock
• Lithosphere - the rigid, outer layer of crust & part
of the mantle which does not deform easily
Inside the Terrestrial Worlds
Heating the Planets
• Planetary interiors heat up through:
• accretion
• differentiation
Supplies all the heat
at the beginning
• radioactivity
Supplies heat throughout
the planet’s life
Cooling the Planets
• Planets Cool by Radiating Heat to
Space from their Surface
• Heat is Transported from the Interior to the
Surface by:
• conduction - atoms & molecules moving &
hitting other atoms & molecules
• convection - macroscopic flows: hot material
rising & cool material sinking
Convection
Cooling Terrestrial Planets
Role of Planetary SIZE
• Larger planets heat more
– More iron to flow to center
– More radioactive elements to split
• Larger planets cool slower
– Extra layers provide extra insulation
Role of Planetary Size
•
•
•
•
Heating proportional to Volume ~ R3
Cooling proportional to Surface Area ~ R2
Heating/Cooling ~ Volume/Area ~ R = Size
Smaller Planets Cool Faster & Heat Less
(by impacts and splitting of radioactive nuclei)
-> Thicker rigid Lithosphere -> Less volcanism
& Tectonic Activity
Inside the Terrestrial Worlds
active geology
inactive geology
How do we Know
about the Interior of Earth
• Earthquakes make
waves
• Waves travel through
Earth, the higher the
density the faster the
waves travel
• Two types of waves
– Back & forth
– Side to side
Shaping Planetary Surfaces
• Major geological processes that shape planetary
surfaces:
• impact cratering: excavation of surface by asteroids or
comets striking the planet
• volcanism: eruption of lava from interior
• tectonics: disruption of lithosphere by internal stresses
• erosion: wearing down by wind, water, ice
Counting Craters to find Surface Age
• Cratering rate decreased as Solar Systems aged.
• The older the surface, the more craters are present.
Impact Craters - Mars
Volcanism - Mars Olympus Mons
Olympus Mons
Volcanism - Venus
Plate Tectonics
History of Plate Tectonics
• South America and Africa fit together like a
jig-saw puzzle, with matching geological
formations and fossils
• New Idea (Wegner 1912)
Continental Drift
• Rejected by Geologists
– Counter to established ideas
– Lack of overwhelming evidence
– No mechanism to drive motions
Evidence
Evidence
Mid-Atlantic Rift
in Iceland
Division between
North American
& European
Plates
Mechanism - Mantle Convection
Earth’s Plates
Earth’s Plate Motion
Valles Marinares - Mars
Erosion
Mars
Evidence for
water in past
Erosion
Wind
Summary