Terrestrial Planetary Geology: Basic Processes

1
Terrestrial Planetary GeologyBasic Processes
Earth
2
What are terrestrial planets like on the inside?
Planets (inc. Moon) to scale, with typical
surface features
3
Earths Interior

• Core Highest density nickel and iron
• Mantle Moderate density silicon, oxygen, etc.
• Crust Lowest density granite, basalt, etc.
• Other terrestrial planets have similar layers

4
Terrestrial Planet Interiors

• Applying what we have learned about Earths
  interior to other planets tells us what their
  interiors are probably like

5
Differentiation

• Gravity pulls high-density material to center
• Lower-density material rises to surface
• Material ends up separated by density
• This generates heat inside planet

6
Lithosphere

• A planets outer layer of cool, rigid rock is
  called the lithosphere crust outer mantle
• It floats on the warmer, softer rock that lies
  beneath most of the mantle is plastic-- the
  rock slowly deforms

7
Strength of Rock

• Rock stretches when pulled slowly but breaks when
  pulled rapidly--like Silly Putty but MUCH slower
• The gravity of a large world pulls slowly on its
  rocky content, shaping the world into a sphere if
  bigger than about 300 km in diameter
• Rapid shear, like an earthquake or impact breaks
  rock

8
Seismic Waves Let UsKnow Whats Inside a Planet

• P (primary) waves push matter back and forth
  (longitudinal like sound waves)
• S (secondary) waves shake matter side to side
  (transverse, like water waves)

9
SEISMIC WAVES REVEAL INTERIOR

• SEISMOGRAPHS detect EARTHQUAKES, VOLCANOS, and
  EXPLOSIONS at varied distances.
• Long surface (L) waves travel fastest, but are
  not very useful as they don't probe the interior.
• P-waves, PRIMARY, (push-pull waves) are
  COMPRESSIONAL, LONGITUDINAL waves. Propagate
  through liquids as well as solids. VP
  function of (compressibility composition, T,
  P)
• S-waves, SECONDARY, (side-slip) are SHEAR,
  TRANSVERSE waves. CANNOT propagate through
  liquids (OUTER core). VS (a new) function of
  (rigidity composition, T, P)
• We have some seismic measurements for the Moon
  too seismometers left by Apollo astronauts

10
Seismic Wave Paths

• Both S and P waves can be detected from
  earthquakes on the same side of the earth,
• ONLY P waves are detected on the opposite side of
  the earth
• shadow zone region where no S (absorbed)
  only weak P (refracted) waves are detected.
• Tells us the outer core of the earth is LIQUID

11
Seismographic measurements have found for Earth

• Boundaries between inner/outer
  core outer core/mantle composition
  changes in mantle (600 km) oil and natural gas
  deposits in crust.
• Seismic measurements can even detect excess
  rotation of earths inner core, upwellings and
  sideways motions in the mantle.
• MODERN SEISMOLOGY CAN GIVE A 3-D PICTURE,
  CHANGING IN TIME, OF THE EARTH'S INTERIOR! ---
  Tomography
• There is a good analogy with 3-D images of
  people's interiors from MRI or CAT scans.

12
Thought Question

• What is necessary for differentiation to occur
  in a planet?
• a) It must have metal and rock in it
• b) It must be a mix of materials of different
  density
• c) Material inside must be able to flow
• d) All of the above
• e) b and c

13
Thought Question

• What is necessary for differentiation to occur
  in a planet?
• a) It must have metal and rock in it
• b) It must be a mix of materials of different
  density
• c) Material inside must be able to flow
• d) All of the above
• e) b and c

14
Causes of Geologic Activity

• Heating of Interior
• Accretion and differentiation when planets were
  young
• Radioactive decay is most important heat source
  today

15
Cooling of Interior

• Convection transports heat as hot material rises
  and cool material falls (outer core inner
  mantle)
• Conduction transfers heat from hot material to
  cool material (lithosphere)
• Radiation sends energy into space (surface
  atmosphere)

16
Role of Size is Dominant

• Smaller worlds cool off faster and harden earlier
• Moon and Mercury are now geologically dead
• Mars lasted longer Venus may still be active
• Earth is VERY active

17
Surface Area to Volume Ratio Gives Cooling Time

• Heat content depends on volume
• Loss of heat through radiation depends on surface
  area
• Time to cool depends on surface area divided by
  volume

• Larger objects have smaller ratio and cool more
  slowly

18
Why do some planetary interiors create magnetic
fields?
Iron filings follow the magnetic field lines of a
bar magnet
19
Sources of Magnetic Fields

• Motions of charged particles create magnetic
  fields
• Electromagnets via currents in coil of wire
  (usually amplified by magnetic material, like
  iron)
• Permanent magnets electron spins act as currents
  in iron or nickel

20
Sources of Magnetic Fields in Planets

• A world can have a magnetic field if charged
  particles are moving inside
• 3 requirements
• Molten interior
• Convection
• Moderately rapid rotation

Earth has molten iron outer core Outer planets
have metallic hydrogen Stars have ionized H
21
Practice Question Answers

• True a CCD is more linear and preferred over
  film.
• False Jupiter is 11.2 Earth radii but 318 Earth
  masses, not about 100.
• True oldest rocks on Earth 4 Gyr, oldest on
  moon from Apollo 4.4 Gyr
• False While most large telescopes are
  reflectors, they spend most of their time taking
  spectra, not pictures.
• False The earths magnetic field is generated in
  its liquid outer core. (Mantle is rocky and
  plastic.)
• False Twice the wavelength means 1/2 the energy
  E hf hc/?
• True liquids, solids dense gases give
  continuum thermal spectrum

22
More practice answers

• True this is stimulated emission
• B 293 K is earth or body temp. and thermal
  emission peaks in IR
• D 500 atoms after 1 half-life of 30 yrs, 250
  after 2, 125 after three half-lives, or 90 years
• E angular momentum conservation means
  flattening, gravity means condensation,
  collisions meant extra flattening as vertical
  energy is lost.
• A getting above atmosphere means less turbulence
  and less absorption by water vapor
• D LA/LB(RA/RB)2(TA/TB)4 22(1/2)44/161/4

23
What processes shape planetary surfaces?
Their surfaces are amazingly different, yet same
forces act!
24
Key Processes that Shape Surfaces

• Impact cratering
• Impacts by asteroids or comets
• Volcanism
• Eruption of molten rock onto surface
• Tectonics
• Disruption of a planets surface by internal
  stresses
• Erosion
• Surface changes made by wind, water, or ice

25
Impact Cratering

• Most cratering happened soon after solar system
  formed the heavy bombardment era
• Craters are about 10 times wider than object that
  made them
• Small craters greatly outnumber large ones
• Areas with many craters are old those with few
  were repaved

26
Impact Craters Classical
Tycho (Moon)
Barringer Meteor Crater (Arizona)
Rim at edge of shock rebound makes peak in center
27
Impact Craters on Mars Evidence for
Liquid Water in the Past
standard crater
impact into icy ground
eroded crater
Cratering History Movie
28
Volcanism

• Volcanism happens when molten rock (magma) finds
  a path through lithosphere to the surface
• Molten rock is called lava after it reaches the
  surface
• It solidifies to create volcanoes

29
Lava Properties Volcano Types
Runny lava makes flat lava plains
Slightly thicker lava makes broad shield volcanoes
Thickest lava makes steep stratovolcanoes
30
Outgassing

• Volcanism also releases gases from Earths
  interior into atmosphere in the past for other
  terrestrial planets
• Well talk more about this later

31
Tectonics

• Convection of the mantle creates stresses in the
  crust called tectonic forces
• Compression forces make mountain ranges
  (Appalachian Mts on Earth on left)
• Valley can form where crust is pulled apart
  (Ceraunius Valleys on Mars on right)

32
Plate Tectonics on Earth

• Earths continents slide around on separate
  plates of crust
• Plate Tectonics Applet
• Tectonics Mantle Convection

33
Erosion

• Erosion is a general term for weather-driven
  processes that break down or transport rock
• Processes that cause erosion include
• Glaciers
• Rivers
• Wind (which demands an atmosphere!)

34
Erosion by Water

• Colorado River continues to carve Grand Canyon
• Land has been uplifted, but river manages to wear
  it down

35
Erosion by Ice

• Glaciers carved the Yosemite Valley
• They covered most of the Northern US and Europe
  in recent ice ages

36
Erosion by Wind

• Wind wears away rock and builds up sand dunes
• Frequent on Earth, also seen on Mars

37
Erosional Debris

• Erosion can create new features by depositing
  debris
• Again, frequent on Earth, but remnants seen on
  Mars too

38
The Earth as a Planet

• You gotta love it, it's our pretty home!
• You gotta know something about it too!
• (At least to pass this class.)

39
The Earth is (nearly) a Sphere and it Rotates

• Proofs (other than Greek) Masts of ships
  visible before their decks. Cicumnavigation in
  1522! Distances corresponding to degree of
  latitude change bulges near equator.
• Newton realized that a rotating object has extra
  support perpendicular to its axis
• Re 6.378136 x 106 m
• Rp 6.356753 x 106 m
• ELLIPTICITY (Re -Rp )/Re 1/298.3
  0.0033528. If the earth were mushier, the same
  rotation speed would yield a higher ellipticity.

40
More Proofs of Earths Rotation

• FOUCAULT PENDULUM --- fixed plane of
  oscillation, with the Earth rotating underneath
  it.
• Satellites in POLAR ORBIT see different sections
  every 90 minutes as the Earth rotates under the
  satellite's FIXED path.