1. Chapter 9 Planetary Geology Earth and the Other Terrestrial Worlds

2. 9.1 Connecting Planetary Interiors and Surfaces
Our goals for learning:
What are terrestrial planets like on the inside?
What causes geological activity?
Why do some planetary interiors create magnetic fields?

3. What are terrestrial planets like on the inside?

4. Earth’s Interior Core: Highest density; nickel and iron
Mantle: Moderate density; silicon, oxygen, etc.
Crust: Lowest density; granite, basalt, etc.

5. Terrestrial Planet Interiors
Applying what we know about Earth’s interior to other planets helps us determine what their interiors are probably like.

6. Why are there layers? Differentiation
Gravity pulls high-density material to center
Lower-density material rises to surface
Material ends up separated by density

7. Lithosphere
A planet’s outer layer of cool, rigid rock is called the lithosphere.
Contains the crust and upper part of the mantle.
It “floats” on the warmer, softer rock that lies beneath.

8. Strength of Rock and Shape
Rock stretches when pulled slowly but breaks when pulled rapidly.
The gravity of a large object (>500 km) will slowly shape it into a sphere within about 1 billion years.
Larger objects will become spherical more quickly.
Molten objects will become spherical more quickly.
3476 km

9. 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

10. What causes geological activity?

11. Heating of Interior Heat of Accretion: Heat from differentiation:
Conservation of Energy
Gravitational Potential Energy
Kinetic Energy
Thermal Energy at Impact
Heat from differentiation:
when planets were young.
Denser Materials
Sink
Less Dense Materials
Rise
Friction Increases Temperature
Heat from Radioactive decay:
Most significant source of heat today.

12. Heating of Interior

13. Cooling of Interior
Convection transports heat as hot material rises and cool material falls.
Conduction transfers heat from hot material to cool material.
Radiation sends energy into space.
Note: Convection and Conduction only transport heat to the surface. The planet only loses heat through Radiation.

14. Role of Size Smaller worlds cool off faster and harden earlier
Moon and Mercury are now geologically “dead”
Why?

15. Surface Area to Volume Ratio
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

16. Why do some planetary interiors create magnetic fields?

17. Not … because there is a big bar magnet in the center of the Earth
The core far exceeds the Curie temperature for iron
Curie temperature: Temperature at which a permanent magnet loses its magnetism
The highest known value for this is 1400K (Cobalt)
The core temperature is estimated at 6000K

18. Magnetic Domains

19. Magnetic Domains = groups of atoms with aligned poles
Magnets can be temporary (like the needle used in the compass).
This nail has its atoms aligned, but the effect is only temporary.  You can get this affect by rubbing the nail on a magnet.
Neat fact:  Hitting the nail can demagnetize it, you are basically scrambling the atoms.

20. Sources of Magnetic Fields
Motions of charged particles are what create magnetic fields

21. Maxwell’s Equations of Electromagnetism
Gauss’ Law for Electrostatics
Gauss’ Law for Magnetism
Faraday’s Law of Induction
Ampere’s Law 21

22. Sources of Magnetic Fields
A world can have a magnetic field if charged particles are moving inside
3 requirements:
Molten interior
Convection
Moderately rapid rotation

23. Requirements for a strong magnetic field
Convecting charged particles
The charge particles need to be moving
Convection requires a heat source – there needs to be an energy source to drive the motion and produce a net electric current
Moderately rapid rotation
Without rotation, there is no clear direction for the current to be flowing in, and no net field.

24. When a planet cools If there was a magnetic field
There may be a remnant of the field locked into the rocks, minerals, and possibly the core after it cools
Much weaker than the dynamic field of a planet with a molten interior driving the field

25. Minor difficulties To have a magnetic field we need a current
To have a current we need a potential difference to drive the electrons and protons in different directions
What can be the source of such a potential difference that can keep the electrons and protons in a conducting liquid core separate?
It is not clear (at least to me, and lots of others), how this really works

26. Table of planetary valuesg
vesc
distance
temperature
atm. press.
rotation
mag. field
(* gE)
(km/s)
(A.U.)
(K)
(* Earth's)
Mercury
0.378
4.3
0.387
100 night, ~0
58.81 d
0.006
day
Venus
0.907
10.36
0.723
737 92 d
Earth 1
11.186
day h
Luna
0.16
2.4
170 night, 370 day
28 d
Mars
0.377
5.03
1.524
day h
Jupiter
2.364
59.5
5.203
165
> > 100
9.925 h
19,519
Saturn
0.916
35.5
9.539
134
10.50 h
578
Uranus
0.889
21.3
19.182 76
17.24 h
47.9
Neptune
1.125
23.5
30.06 72
16.11 h 27
Pluto
0.0675
1.1
39.53 50
0.003
6.405 d

27. Why do we care about the Earth’s magnetic field?
Are we in for trouble?
What about Mars?

28. How do we learn about the Earth’s core, mantle, and crust?
Deep drilling
Seismic waves
We know earth’s density from its size and applying Kepler’s 3rd law on the moon
All of the above
A and B

29. The inside of the earth is filled with molten lava
True
False
It is only molten in a very narrow region underneath the lithosphere.

30. How do we know that the Earth is geologically active?
Volcanoes
Seismology
Measuring the temperature inside
All of the above
A and B

31. What is the lithosphere?
Another name for a planet’s crust
The crust plus the mantle
A relatively rigid outer layer of rock that floats on more molten rock below
The boundary between the core and mantle

32. Where did the Earth’s (interior) heat come from?
Volcanoes
Impacts as the Earth was accreting
Radioactivity
All of the above
B and C

33. What is necessary for a differentiation to occur in a planet?
It must have metal and rock in it
It must be a mix of materials of different density
Material inside must be able to flow
All of the above
B and C

34. What is the source of the Earth’s magnetic field?
Magnetic rocks
Magnetized iron in the Earth’s crust
Magnetized iron in the Earth’s core
Molten metal circulating inside the Earth, moving electrons like in a wire

35. Why are smaller terrestrial bodies such as Mercury or the Moon “geologically dead”?
They don’t have volcanoes
They cooled off faster than the Earth did
They don’t have erosion
They were hit by fewer meteorites
They are made of different materials than the Earth

36. What is convection? Heat radiates from a planet into space
Heat travels from atom to atom from inside a planet to the outside
Hot material inside a planet rises, and cool material sinks towards the center
Metal conducts energy throughout the earth’s core

37. What have we learned? What are terrestrial planets like on the inside?
Core, mantle, crust structure
Denser material is found deeper inside
What causes geological activity?
Interior heat drives geological activity
Radioactive decay is currently main heat source
Why do some planetary interiors create magnetic fields?
Requires motion of charged particles inside planet

38. 9.2 Shaping Planetary Surfaces
Our goals for learning:
What processes shape planetary surfaces?
Why do the terrestrial planets have different geological histories?
How do impact craters reveal a surface’s geological age?

39. What processes shape planetary surfaces?

40. Processes that Shape Surfaces
Impact cratering
Impacts by asteroids or comets
Volcanism
Eruption of molten rock onto surface
Tectonics
Disruption of a planet’s surface by internal stresses
Erosion
Surface changes made by wind, water, or ice

41. Impact Cratering
Most cratering happened soon after solar system formed (First mil. years).
Asteroids and comets impact the surface at speeds between 40,000 and 250,000 km/hr (10-70 km/s)
Craters are about 10 times wider than object that made them.
Craters are about 10-20% as deep as they are wide.
Impact angle not a significant factor on shape of crater.
Small craters greatly outnumber large ones.

42. Impact Craters
Meteor Crater (Arizona)
Tycho (Moon)

43. Impact Craters on Mars “standard” crater impact into icy ground
eroded crater

44. 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.

45. Lava and Volcanoes Runny lava makes flat lava plains.
Slightly thicker lava makes broad shield volcanoes like in Hawaii (Not very steep).
Thickest lava makes steep stratovolcanoes (tall and steep).

46. Outgassing
Volcanism also releases gases from Earth’s interior into atmosphere (H2O, CO2, N2, H2S and SO2).

47. Tectonics
Convection of the mantle creates stresses in the crust called tectonic forces.
Compression forces make mountain ranges.
Valley can form where crust is pulled apart (mid-oceanic ridge).

48. Plate Tectonics on Earth
200 million years ago, the Earth was covered by one super-continent called Pangaea.
Pangaea broke up into Earth’s continents which slide around on separate plates.

50. Erosion
Erosion is a blanket term for weather-driven processes that break down or transport rock through the action of ice, liquid or gas.
Processes that cause erosion include
Glaciers (ice)
Rivers (liquid)
Wind (gas)

51. Erosion by Water
Colorado River continues to carve Grand Canyon

52. Erosion by Ice
Glaciers carved the Yosemite Valley

53. Erosion by Wind
Wind wears away rock and builds up sand dunes

54. Erosional Debris
Erosion can create new features by depositing debris

55. Why do the terrestrial planets have different geological histories?

56. Smaller worlds cool off faster and harden earlier.
Larger worlds remain warm inside, promoting volcanism and tectonics.
Larger worlds also have more erosion because their gravity retains an atmosphere.
Larger worlds may have a magnetic field if the core is molten and the spin is fast enough.

57. Planets close to Sun are too hot for rain, snow, ice and so have less erosion.
More difficult for hot planet to retain atmosphere.
Planets far from Sun are too cold for rain, limiting erosion.
Planets with liquid water have most erosion.

58. Planets with slower rotation have less weather and less erosion and a weak magnetic field
Planets with faster rotation have more weather and more erosion and a stronger magnetic field

59. How do impact craters reveal a surface’s geological age?

60. History of Cratering Most cratering happened in first billion years
A surface with many craters has not changed much in 3 billion years

61. Thought Question
How does the cooling of planets and potatoes vary with size?
a) Larger makes it harder for heat from inside to escape
b) Larger has a bigger ratio of volume (which needs to cool) to surface area (the surface is where cooling happens)
c) Larger takes longer to cool
d) All of the above

62. What is true of convection that stresses the surface crust in a planet?
Mountains may form where the crust is pushed together
Cracks and valleys may form where it is pulled apart
This happened when planets formed and were hot; convection has mostly ceased today
All of the above
A and B

63. What are the 4 basic processes that shape planetary surfaces?
Magnetic fields, impacts, volcanoes, erosion
Magnetic fields, earthquakes, volcanoes, erosion
Tectonics, impacts, volcanoes, erosion
Tectonics, impacts, erosion, magnetic fields

64. The lunar crater Tycho is about 80 km (50 miles) across.
It was probably made by:
The eruption of the large volcano in its center
An impactor nearly 80 km across
An impactor about 8 km across
Plate tectonics

65. Why do the lunar highlands have many more craters than the lunar maria?
They are on the side of the moon away from earth so they were hit by more impacts
Lava flooded the maria, hiding many craters
The less cratered surfaces are younger than those with more craters
All of the above
B and C

66. What evidence of water is seen on Mars?
Channels that look like dry riverbeds
Something has eroded rims of craters and erased small ones; that could have been rain
The Spirit and Opportunity rovers have found mineral evidence of water
There is ice in Mar’s polar caps
All of the above

67. The interior of the Earth consists of
A metallic core and solid rock outer shell.
A rocky core and metallic outer shell.
A metallic core and liquid rock outer shell.
A liquid rocky inner core and solid rock outer shell.
A mixture of rock and metals throughout.

68. Suppose Venus rotated on its axis as fast as Earth.
How would this change its relative levels of volcanism, tectonics, and erosion?
All would remain the same - independent of rotation.
Higher levels of all three.
Lower levels of all three.
The same levels of volcanism and tectonics, and a higher level of erosion.
Higher levels of volcanism and tectonics, and the same level of erosion.

69. What have we learned? What processes shape planetary surfaces?
Cratering, volcanism, tectonics, erosion
Why do the terrestrial planets have different geological histories?
Differences arise because of planetary size, distance from Sun, and rotation rate
How does a planet’s surface reveal its geological age?
Amount of cratering tells us how long ago a surface formed

70. 9.3 Geology of the Moon and Mercury
Our goals for learning:
What geological processes shaped our Moon?
What geological processes shaped Mercury?

71. What geological processes shaped our Moon?

72. Cratering of Moon
Some areas of Moon are more heavily cratered than others
Younger regions were flooded by lava after most cratering

73. Lunar Maria
Smooth, dark lunar maria are less heavily cratered than lunar highlands
Maria were made by flood of runny lava

74. Formation of Lunar Maria
Early surface covered with craters
Large impact crater weakens crust
Heat build-up allows lava to well up to surface
Cooled lava is smoother and darker than surroundings

75. Tectonics on the Moon
Wrinkles arise from cooling and contraction of lava flood

76. What geological processes shaped Mercury?

77. Cratering of Mercury
A mixture of heavily cratered and smooth regions like the Moon
Smooth regions are likely ancient lava flows

78. Cratering of Mercury Caloris basin is largest impact crater on Mercury
Region opposite Caloris Basin is jumbled from seismic energy of impact

79. Tectonics on Mercury
Long cliffs indicate that Mercury shrank early in its history.
Heavy cratering on Mercury suggest that it is not volcanically or tectonically active.

80. Geologically Dead
The Moon and Mercury are considered geologically “dead” because geological processes have virtually stopped

81. What have we learned? What geological processes shaped our Moon?
Early cratering still present
Maria resulted from volcanism
What geological processes shaped Mercury?
Cratering and volcanism similar to Moon
Tectonic features indicate early shrinkage

82. 9.4 Geology of Mars Our goals for learning:
How did Martians invade popular culture?
What are the major geological features of Mars?
What geological evidence tells us that water once flowed on Mars?

83. 1877-1888 observations by Giovanni Schiaperelli:
Martian Canali (Channels)

85. (Halloween broadcast of 1938)
HG Wells, “The War of the Worlds”
1897
(Halloween broadcast of 1938)

86. Mars in the popular imagination
Mars polar caps grow and shrink
We can see the surface (similar to Earth)
Imagined channels became canals
Smaller size  lower gravity  atmosphere leaking into space
Mars is dying (dead)  older  more advanced alien races
The Martians are coming! They must want our air and water!
And our gravity at three times theirs?

87. HG Wells, “The War of the Worlds”
1897

88. What are the major geological features of Mars?

89. Cratering on Mars Amount of cratering differs greatly across surface.
Few on the northern plains, more in the southern highlands.
Many early craters have been erased due to erosion and other geological processes.

90. Volcanism on Mars Mars has many large shield volcanoes.
Olympus Mons is largest volcano in solar system.
3x as high as Mt. Everest at 26 km above Martian surface.
Is Mars still volcanically active?
Scientists do not think so.
Its smaller size has allowed it to cool.
No known activity for at least tens of millions of years.

91. Tectonics on Mars
System of valleys known as Valles Marineris (a rift valley) is thought to originate from tectonics.
Valles Marineris is as long as the US is wide and 4x as deep as the Grand Canyon.

92. What geological evidence tells us that water once flowed on Mars?

93. Dry Riverbeds?
Close-up photos of Mars show what appear to be dried-up riverbeds.
Note how smaller riverbeds merge to form larger river in lower right-hand portion of image.

94. Erosion of Craters
Details of some craters suggest they were once filled with water

95. Martian Rocks
Mars rovers have found rocks that appear to have formed in water

96. Hydrogen Content
Map of hydrogen content (blue) shows that low-lying areas contain more water ice.

97. Crater Walls
Gullies on crater walls suggest occasional liquid water flows have happened less than a million years ago

98. What have we learned? How did Martians invade popular culture?
Surface features of Mars in early telescopic photos were misinterpreted as “canals”
What are the major geological features of Mars?
Differences in cratering across surface
Giant shield volcanoes
Evidence of tectonic activity

99. What have we learned?
What geological evidence tells us that water once flowed on Mars?
Features that look like dry riverbeds
Some craters appear to be eroded
Rovers have found rocks that appear to have formed in water
Gullies in crater walls may indicate recent water flows

100. 9.5 Geology of Venus Our goals for learning:
What are the major geological features of Venus?
Does Venus have plate tectonics?

101. What are the major geological features of Venus?

102. Radar Mapping
Thick atmosphere forces us to explore Venus’ surface through radar mapping

103. Cratering on Venus Impact craters, but fewer than Moon, Mercury, Mars.
No small craters since small impactors would have burned up in Venus’ atmosphere.

104. Volcanoes on Venus
Many volcanoes, including both shield volcanoes and stratovolcanoes.
Presence of sulfuric acid in clouds suggest that volcanic outgassing is still occurring.

105. Tectonics on Venus
Fractured and contorted surface indicates tectonic stresses.
Evidence is minimal compared to Earth.
Nothing during the last 750 million years.

106. Erosion on Venus Photos of rocks taken by lander show little erosion
Why?
Too hot and dry.
Very little wind due to slow rotation.

107. What have we learned? What are the major geological features of Venus?
Venus has cratering, volcanism, and tectonics but not much erosion
Does Venus have plate tectonics?
The lack of plate tectonics on Venus is a mystery

108. 9.6 The Unique Geology of Earth
Our goals for learning:
How do we know Earth’s surface is in motion?
How is Earth’s surface shaped by plate tectonics?
Was Earth’s geology destined from birth?

109. How do we know Earth’s surface is in motion?

110. Continental Motion
Motion of continents can be measured with GPS

111. Continental Motion
Idea of continental drift was inspired by puzzle-like fit of continents in 1912 by Alfred Wegener.
Similar types of rocks and rare fossils found in Eastern S.A. and West Africa.
Mantle material erupts where seafloor spreads – Mid-ocean ridges.

112. Seafloor Crust Compared to continental crust, seafloor crust is:
Thinner (5 – 10 km)
Denser
Younger
Seafloor crust is primarily made of basalt.

113. How is Earth’s surface shaped by plate tectonics?

114. Seafloor Recycling
Seafloor is recycled through a process known as subduction.
Denser seafloor crust pushes under the less dense continental crust.
Deep ocean trenches may be found here (~5 miles deep).
Descending seafloor plate heats up due to friction.
Stratovolcanoes often found here consisting of low density, thick lava.

115. Plate Motions
Measurements of plate motions tell us past and future layout of continents

116. Which two bodies show evidence of heavy cratering?
Mercury and Venus
Mercury and Earth
Mercury and Earth’s Moon
Earth and Venus
Earth and Mars

117. What have we learned?
How do we know that Earth’s surface is in motion?
Measurements of plate motion confirm idea of continental drift
How is Earth’s surface shaped by plate tectonics?
Plate tectonics responsible for subduction, seafloor spreading, mountains, rifts, and earthquakes

118. Hot Spots
Hawaiian islands have formed where plate is moving over volcanic hot spot

119. Was Earth’s geology destined from birth?

120. Earth’s Destiny
Many of Earth’s features determined by size, rotation, and distance from Sun
Reason for plate tectonics not yet clear

121. What have we learned? Was Earth’s geology destined from birth?
Many of Earth’s features determined by size, distance from Sun, and rotation rate
Reason for plate tectonics still a mystery

122. Surface Features
Major geological features of North America record history of plate tectonics

123. Surface Features
Himalayas are forming from a collision between plates

124. Surface Features
Red Sea is forming where plates are pulling apart

125. Rifts, Faults, Earthquakes
San Andreas fault in California is a plate boundary
Motion of plates causes earthquakes

126. Special Topic: How do we know what’s inside a planet?
P waves push matter back and forth
S waves shake matter side to side

127. Special Topic: How do we know what’s inside a planet?
P waves go through Earth’s core but S waves do not
We conclude that Earth’s core must have a liquid outer layer