1. Chapter 7 Earth and the Terrestrial Worlds

2. In this chapter we will see how did the terrestrial worlds come to be so different, when all were made of essentially the same ingredients.
In this class we will:
Explore the key processes that have shaped Earth and the other terrestrial worlds over time
In the next class:
The history of each world individually
We start with an overview of terrestrial planets…

3. Mercury airless and barren craters smooth plains, cliffs

4. Venus a searing hothouse thick atmosphere volcanoes few craters
Radar view of a twin-peaked volcano

5. Mars thin atmosphere, couldn’t keep water some craters the solar system largest volcanoes riverbeds?

6. Moon craters smooth plains

7. Earth atmosphere volcanoes craters mountains riverbeds

8. Why have the planets turned out so differently, when they formed at the same time from the same materials?

9. 7.1 Earth as a Planet Why is Earth geologically active?
Lets explore the key processes that have shaped Earth and the other terrestrial worlds over time. We will focus on earth, and then compare it with other worlds.
Our Goals for Learning:
Why is Earth geologically active?
What processes shape Earth’s surface?
How does Earth’s atmosphere affect the planet?

10. a. Why is Earth geologically active (continually reshaping)?

11. Short answer: the Earth is big enough to still have a hot interior.
So what do we know about the interior of the Earth and why is it hot?
So let’s see what that interior is and why it’s hot.

12. Internal Structure, by density
All the terrestrial worlds have layered interiors.
Mostly granite and basalt
Rocky material, mostly minerals with Si and O
Remove lithosphere label & arrow, and dotted circle
Mostly nickel and iron. On Earth it has two parts: a solid inner core and molten outer core. Molten outer core is the only layer of molten rock on earth.
Question: How does layering occur?

13. Why do water and oil separate?
Water molecules repel oil molecules electrically.
Water is denser than oil, so oil floats on water.
Oil is more slippery than water, so it slides to the surface of the water.
Oil molecules are bigger than the spaces between water molecules.

14. Why do water and oil separate?
Water molecules repel oil molecules electrically.
Water is denser than oil, so oil floats on water.
Oil is more slippery than water, so it slides to the surface of the water.
Oil molecules are bigger than the spaces between water molecules.

15. Differentiation
When they were just formed, planets were hot enough for their interior to melt. Gravity then separates materials by density, process called differentiation.

16. Internal structure, by rock strength
The “lithosphere” is the cool rigid rock that forms a planet’s outer layer: the crust and some of the mantle.
The lithosphere floats on the lower, warmer and softer layers. This layer can still deform.
Note: rocky material (while solid), can slowly deform and flow over the period of millions of years. Rock becomes softer and easier to deform when it is warm.

17. What happens to Silly Putty if you pull it VERY SLOWLY?
It stretches.
It breaks.
Nothing.

18. What happens to Silly Putty if you pull it VERY SLOWLY?
It stretches.
It breaks.
Nothing.

19. What happens to Silly Putty if you pull it SHARPLY?
It stretches.
It breaks.
Nothing.

20. What happens to Silly Putty if you pull it SHARPLY?
It stretches.
It breaks.
Nothing.

21. Do rocks s-t-r-e-t-c-h?
No - rock is rigid and cannot deform without breaking.
Yes - but only if it is molten rock.
Yes - rock under strain may slowly deform.

22. Do rocks s-t-r-e-t-c-h?
No - rock is rigid and cannot deform without breaking.
Yes - but only if it is molten rock.
Yes - rock under strain may slowly deform.

23. Sources of Internal Heat – since planets were young
Gravitational potential energy of accreting planetesimals
Question: describe what forms of energy are involved in this process.
Differentiation
Questions: how does differentiation provide heat?
Radioactivity: on Earth, it still supplies the heat

24. Why some terestrial planets are cooler than the others?
What cools off faster?
A grande-size cup of Starbucks coffee
A teaspoon of capuccino in the same cup.

25. What cools off faster? A grande-size cup of Starbucks coffee
A teaspoon of capuccino in the same cup.

26. What cools off faster? A big terrestrial planet.
A tiny terrestrial planet.

27. What cools off faster? Why? A big terrestrial planet.
A tiny terrestrial planet.
Why?

28. Heat Drives Geological Activity
Planets cool by convection: hot rock rises, cool rock falls.
1 cycle takes 100 million years on Earth.
Keep in mind that mantle convection primarily involves solid rock!

29. A large planet… Is still warm inside Has a convecting mantle
Has a thinner, weaker lithosphere
any molten rock it has is nearer the surface
which makes it more geologically active

30. Internal Structure of the Terrestrial Planets
The thickness of the lithosphere controls many geological processes.
Which planets have the most and least geological activity?

31. Comparing the Planets
Mars is probably still active, while mercury and moon are not.

32. Internal heat causes planetary magnetic fields
Moving charged particles create magnetic fields.
So can a planet’s interior, if the core it electrically conducting, convecting, and rotating.
Figure 7.5 goes here.

33. The charged particles can create aurorae (“Northern lights”)
Because of it strong magnetic field, the earth has Magnetosphere.
Earth’s magnetic fields protects us and our atmosphere from charged particles from the Sun
The charged particles can create aurorae (“Northern lights”)
Figure 7.6ab NEW should go here.

34. If the planet core is cold, do you expect it to have magnetic fields?
Yes, refrigerator magnets are cold, and they have magnetic field.
No, planetary magnetic fields are generated by moving charges around, and if the core is cold, nothing is moving.

35. If the planet core is cold, do you expect it to have magnetic fields?
Yes, refrigerator magnets are cold, and they have magnetic field.
No, planetary magnetic fields are generated by moving charges around, and if the core is cold, nothing is moving.

36. b. What processes shape Earth’s surface?

37. What processes shape Earth’s surface?
Impact cratering
Volcanism
Tectonics
Erosion

38. Impact Cratering
Asteroids or comets typically hit the ground with speeds 40,000 to 250,000 km/hr. The impact vaporizes solid rock and makes circular craters.
Figure 3e but of course it ain’t on the cd. (huh, and figure 10.7 isn’t either.)
This figure is Mars.

39. Impact Cratering Moon must be hit as often as Earth.
Where are Earth’s craters?
Erased by volcanic activity and erosion.
The more craters, the older the surface, because almost all of impact processes happened when planets were just formed. Very useful rule…

40. Volcanism
When underground molten rock (from just below the surface) finds its way to the surface.
Molten rock rises when it is:
Less dense than its surroundings, it has natural tendency to rise.
Squeezed by its surroundings.
Pushed by expanding trapped gas (water vapor, CO2, N2, H2S, SO2)

41. Volcanism Erases other geological features
In the process of outgassing provided gas for our atmosphere and water for our oceans
Question: Earth was created by rocky and metallic planetesimals. How did water and other ices (nowadays gases) arrive to earth? Why do we say that volcanoes provided earth with water and atmosphere?

42. Why doesn’t Mars have as much volcanic activity as Earth?
It’s too far from the Sun, so it cooled off faster.
It’s smaller than the Earth, so it cooled off faster.
It might, we just haven’t seen them erupt yet.

43. Why doesn’t Mars have as much volcanic activity as Earth?
It’s too far from the Sun, so it cooled off faster.
It’s smaller than the Earth, so it cooled off faster.
It might, we just haven’t seen them erupt yet.

44. Tectonics and Plate Tectonics
Tectonics: any surface reshaping from forces on the lithosphere (stretching, compression…)
Mostly result of mantle convection. On earth it fractured litosphere into more than a dozen of pieces, plates.
Plate tectonics: on earth litosphere consists of 12 plates which move around each other, appear to be unique to earth.
Like volcanism, depends on internal heat!

45. Erosion
Wearing down or building up of geological features by wind, water and ice.
Shaping of valleys by glaziers, carving of canyons by rivers, but also building things by sedimentary deposit (grand canyon is sedimentary rock that was built by erosion)
Question: erosion is important mostly on Earth - why?

46. Erosion
Wearing down or building up of geological features by wind, water and ice (weather)
Shaping of valleys by glaziers, carving of canyons by rivers, but also building things by sedimentary deposit (grand canyon is sedimentary rock that was built by erosion)
Question: erosion is important mostly on Earth - why?
It has water in solid and liquid form, and strong winds because of atmosphere and fast rotation.

47. c. How does Earth’s atmosphere affect the planet ?

48. Erosion (already mentioned) Protection from cosmic radiation
Changes the surface temperature: greenhouse effect
Makes the sky blue!
Question: On a standard globe, what do you think how thick would be layer of the atmosphere?

49. Erosion (already mentioned) Protection from solar/cosmic radiation
Changes the surface temperature: greenhouse effect
Makes the sky blue!
Question: On a standard globe, how thick would be layer of the atmosphere?
It would have a thickness of a dollar bill.

50. 2) Radiation Protection
Sun emits visible light but also dangerous ultraviolet and x rays.
All X-ray light absorbed very high in the atmosphere. They are very energetic and ionize molecules (which absorb it) high in the atmosphere.
Ultraviolet light is not so easily absorbed. All molecules, but ozone (O3) are transparent for UV. Without it the life on earth would be impossible.
Figure 7.13a would be good here.

51. 3) The Greenhouse Effect
Calculations show that visible light would heat up the atmosphere only up to –17C (1 F) if there would not reemission (“trapping”) of infrared due to greenhouse gasses.
The average temperature on Earth is 15C (59 F) instead.
Question: Do you recall the temperature of Venus?

52. How does the greenhouse effect alter surface temperature?

53. Greenhouse effect:
Certain molecules let sunlight through but trap escaping infrared photons
(H2O, CO2, CH4)
The greenhouse gasses tend to slow down th escape of infrared photons, while their molecular motions heat the surrounding air.

54. A Greenhouse Gas Any gas that absorbs infrared
Greenhouse gas: molecules with 2 different types of elements (CO2, H2O, CH4)
Not a greenhouse gas: molecules with single or 2 atoms of the same element (O2, N2)

55. Greenhouse Effect: Bad?
The Earth is much warmer because of the greenhouse effect than it would be without an atmosphere…but so is Venus.

56. 4) With atmosphere the sky is blue!
Most of the visible light passes through the atmosphere without being disturbed. But small portion of it, specially blue color, gets scattered randomly around the sky – we see the sky as bright and blue.
Question: without atmosphere how would our sky look like?

57. What have we learned? • Why is Earth geologically active?
Internal heat drives geological activity, and Earth retains plenty of internal heat because of its relatively large size for a terrestrial world. This heat causes mantle convection and keeps Earth’s lithosphere thin, ensuring active surface geology. It also keeps part of Earth’s core melted, and the circulation of this molten metal creates Earth’s magnetic field.

58. What have we learned? • What processes shape Earth’s surface?
The four major geological processes are impact cratering, volcanism, tectonics, and erosion. Earth has experienced many impacts, but most craters have been erased by other processes. We owe the existence of our atmosphere and oceans to volcanic outgassing. A special brand of tectonics—plate tectonics—shapes much of Earth’s surface. Ice, water, and wind drive rampant erosion on our planet.

59. What have we learned? • How does Earth’s atmosphere affect the planet?
Two crucial effects are (1) protecting the surface from dangerous solar radiation—ultraviolet is absorbed by ozone and X rays are absorbed high in the atmosphere— and (2) the greenhouse effect, without which the surface temperature would be below freezing.

60. 7.2 Mercury and the Moon: Geologically Dead
The simplest histories: very small – they cooled down fast and their gravity was to small to keep an atmosphere.
Our Goals for Learning
Was there ever geological activity on the Moon or Mercury?

61. Moon
Impact cratering was the most important geological activity. But,…
Some volcanic activity must have flooded lunar craters, creating lunar maria: smoother and darker regions.
During heavy bombardment period (when it was?) the moon was cratered. At that time moon had already cooled, so craters were everywhere.
Radioactivity in the mean time was building up the heat in its interior, and 3 to 4 billion years ago, when the material in mantle melted, molten rock welled up through the cracks in the lithosphere.
Figure 7.15, make a line pointing to the Mare Humorum.

62. The Moon is now geologically dead.
The only ongoing geological change is very slow “sand-blasting”: its surface is constantly bombarded by micrometeorites (sand size particles from space) which pulverize the surface layer of the Moon. They will even erase the footsteps left by astronauts in this powdery surface. After millions of years.
Question: Do micrometeorites bombard also the Earth? What happens to them when they hit the Earth?

63. Mercury
Figure 7.17a
It has plenty of craters, suggesting very old surface. But they are less crowded together than in the dense cratered parts of the Moon’s surface. What does this tell us?

64. Mercury
It has plenty of craters, suggesting very old surface. But they are less crowded together than in the dense cratered parts of the Moon’s surface. What does this tell us?
Some craters were later covered by lava. The heat came from radioactivity, much like on the Moon.

65. Mercury has a huge impact ‘basin’
Mercury has a huge impact ‘basin’. It spans more than half of the planets radius.
It also has many tremendous cliffs unlike anything we found on other planets. They are 3 or more km high, and run for hundreds of kilometers across the surface.
Cliff on Mercury, cutting through craters and other surface features. Some of these cliffs are more than a mile in height and hundreds of miles in length.

66. Could it be that the all planet simply shrank?
Early in its history, Mercury gained more internal heat than the Moon because of its larger size. This heat swelled the size of its iron core.
Figure 7.18
Steep long cliffs formed when the core cooled, shrinking the planet by ~20 km.
Mercury is now geologically dead.

67. What have we learned?
• Was there ever geological activity on the Moon or Mercury?
Both the Moon and Mercury had some volcanism and tectonics when they were young. However, because of their small sizes, their interiors long ago cooled too much for ongoing geological activity.

68. 7.3 Mars: A Victim of Planetary Freeze-drying
Our Goals for Learning
What geological features tell us that water once flowed on Mars?
Why did Mars change?

69. Mars vs. Earth 50% Earth’s radius, 10% Earth’s mass
1.5 A.U from the Sun
Similarities:
Axis tilt about the same as Earth.
Similar rotation period. But orbital period twice as long as the Earth’s.
Has polar caps (but made of frozen CO2)
Differences:
Thin CO2 atmosphere: little greenhouse effect. The average temperature on Mars is –53C(-63F).
Orbit is more elliptical than Earth’s: seasons more extreme in the south than the north.
Strong pole to pole winds, due to extreme seasons.

70. Main Difference seems to be:
Mars is SMALLER!

71. What geological features tell us water once flowed on Mars?
Today Mars has no liquid water (What would happen to a spoonful of it on today's mars?). But, it seems that Mars may once have been a much warmer and wetter place…
What geological features tell us water once flowed on Mars?

72. Lets start with general features of Martian geology…
Need Figure 7.20.
South hemisphere has relatively high elevation and is scarred by numerous impact craters.

73. Lets start with general features of Martian geology…
North hemisphere, in contrast, shows few impact craters and tends to be below the average Martian surface level. Which surface (N or S) is older?

74. Past volcanism and tectonics:
Meteoritic evidence, radiometric dating
Volcanoes…as recent as 180 million years ago… this is the largest volcano in the solar system, the base the size of Arizona, and it is three times higher than Mount Everest.

75. Past tectonic activity…The long deep system of valleys
Past tectonic activity…The long deep system of valleys. Long as the US is wide and four times as deep as Grand Canyon.

76. What do we know about ancient water flows?
Surface of Mars appears to have ancient river beds
They are covered by craters, probably 3 billion years old.

77. Eroded crater
Condition of craters indicates erosion by liquid water

78. Closeup of eroded bottom of a crater
Closeup of eroded bottom of a crater. It appears it must have held lakes.

79. Low-lying regions may once have had oceans

80. Low-lying regions may once have had oceans

81. Opportunity
Spirit
Inset shows hypothetical ancient ‘water line’ for Gusev Crater where Spirit landed

82. 2004 Opportunity Rover provided strong evidence for abundant liquid water on Mars in the distant past.
How could Mars have been warmer and wetter in the past?

83. Today, most water lies frozen underground (blue regions)…
Some scientists believe accumulated snowpack melts to carve gullies even today
The hydrogen content of the martian surface soil.
If volcanoes are so recent, and there is still geological activity on mars we might still see some of its ice melts to liquid water…

84. Why did Mars change?
Primarily because of it small size. Two main reasons for liquid water loss:
Enlarge labels? 1)
2) Also, uv radiation broke water molecules, and H2 easily escaped.

85. Would “terraforming” Mars work?
Yes No
Hint: How would releasing all of the frozen CO2 from polar caps help warm the atmosphere? Would that be enough heat for Mars to again gets its oceans?
Discussion question: release the frozen carbon dioxide…warm up the planet…would the oceans stay? Would the hydrogen stick around?

86. What have we learned?
• What geological features tell us that water once flowed on Mars?
Dry river channels, rock-strewn floodplains, and eroded craters all show that water once flowed on Mars, though any periods of rainfall seem to have ended at least 3 billion years ago. Mars today still has water ice underground and in its polar caps, and could possibly have pockets of underground liquid water.

87. What have we learned? • Why did Mars change?
Mars’s atmosphere must once have been much thicker with a much stronger greenhouse effect, so change must have occurred due to loss of atmospheric gas. Much of the lost gas probably was stripped away by the solar wind, which was able to reach the atmosphere as Mars cooled and lost its magnetic field and protective magnetosphere. Water was probably also lost because ultraviolet light could break apart water molecules in the atmosphere, and the lightweight hydrogen then escaped to space.

88. 7.4 Venus: A Hothouse World
Our Goals for Learning
Is Venus geologically active?
Why is Venus so hot?

89. Is Venus geologically active?
Venus is only about 5% smaller than Earth in radius. And very similar in composition.
Figure 7.29 bc and maybe d..
Twin volcanoes
Similarities: Radio images show lots of volcanic & tectonic features, as expected for a large terrestrial planet
Tectonic stress marks

90. Differences:
Almost no magnetic field: why? (hint: what is rotational period of Venus?)
No erosion: why? (hint:does it have a liquid water? How about wind?)
No plate tectonics: still a mystery

91. Why is Venus so hot? Greenhouse effect.
But why is it so prominent on Venus?

92. Venus has very thick atmosphere (its surface pressure is about 90 times that on Earth). And this atmosphere is about 96% CO2.
 altogether Venus has 200,000 times more CO2 in the atmosphere than the Earth.
Why its atmosphere is so much different than the Earths?

93. Where is Earth’s CO2?

94. (and some in plant life and in the ocean)
Where is Earth’s CO2?
It is still around, but in the solid form! CO2 dissolves in water and makes carbonate rocks - like limestone
(and some in plant life and in the ocean)
Why did this happen on Earth and not on Venus?
Venus lacks oceans to dissolve the carbon dioxide and lock it away in rock on the seafloor
And why is that?
‘strip tease’ of bullet-by-bullet unveiling is warranted on this slide

95. One reason why there is no water on Venus is the same one with Mars.
Remember: there is no ozone on mars, …
But, then, why Venus didn’t get oceans of liquid water, like the Earth, which would have prevented its water from being lost to space?

96. We can understand Venus’ history by thinking about what would happen if Earth were moved to Venus’ orbit.
How would the initial heating affect the oceans and the greenhouse effect?

97. Note: Venus is only 30% closer to the Sun than the Earth, but the difference in distance was apparently critical!
A runaway greenhouse effect would then occur.
Eventually, water molecules would break down & escape to space, just as apparently happened on Venus

98. What have we learned? • Is Venus geologically active?
Venus almost certainly remains geologically active today. Its surface shows evidence of major volcanic or tectonic activity in the past billion years, and it should retain nearly as much internal heat as Earth. However, geological activity on Venus differs from that on Earth in at least two key ways: lack of erosion and lack of plate tectonics.
• Why is Venus so hot?
Venus’s extreme surface heat is a result of its thick, carbon dioxide atmosphere, which creates a very strong greenhouse effect. The reason Venus has such a thick atmosphere is its distance from the Sun: It was too close to develop liquid oceans like those on Earth, where most of the outgassed carbon dioxide dissolved in water and became locked away in rock. Thus, the carbon dioxide remained in the atmosphere, creating the strong greenhouse effect.

99. 7.5 Earth as a Living Planet
Our Goals for Learning
What unique features on Earth are important for human life?
How might human activity change our planet?
What makes a planet habitable?

100. What unique features of Earth are important for life?
Surface liquid water
Atmospheric oxygen
Plate tectonics
Climate stability throughout its history

101. What unique features of Earth are important to human life?
Earth’s distance from the Sun and moderate greenhouse effect make liquid water possible
Surface liquid water
Atmospheric oxygen
Plate tectonics
Climate stability

102. What unique features of Earth are important to human life?
Surface liquid water
Atmospheric oxygen
Plate tectonics
Climate stability
PHOTOSYNTHESIS (plant life) is required to make high concentrations of O2, which produces the protective layer of O3.

103. What unique features of Earth are important to human life?
Surface liquid water
Atmospheric oxygen
Plate tectonics
Climate stability
Plate tectonics are an important step in the carbon dioxide cycle.

104. The Carbon Dioxide Cycle
Over tens of millions of years, any piece of seafloor crust makes its way across the ocean (with speed with which our nails grow) bottom, then finally gets recycled into the mantle – subduction, when it meets with continental plates.

105. Earth’s ice ages end as oceans freeze over and volcanoes release CO2 into the atmosphere

106. What unique features of Earth are important to human life?
Surface liquid water
Atmospheric oxygen
Plate tectonics
Climate stability
The CO2 cycle acts like a thermostat for the Earth’s temperature. Higher the temperature, more CO2 dissolves in the oceans.

107. These unique features are intertwined:
• plate tectonics creates climate stability • climate stability allows liquid water • liquid water is necessary for life • life is necessary for atmospheric oxygen
How many other connections between these can you think of?
For example:
atmospheric oxygen allows life on land
Stable climate good for life
Water may enable plate tectonics

108. How might human activity affect Earth’s climate?

109. Human activity is increasing the concentration of greenhouse gases in the atmosphere, which may strengthen the greenhouse effect and lead to global warming. There are many dangers of runaway effects or the responses of ecosystems we still do not understand.
Question: should we worry about human made global warming, since CO2 cycle should regulate our atmosphere? Why or why not?

110. What makes a planet habitable?
Located at an optimal distance from the Sun for liquid water to exist.

111. What makes a planet habitable?
Large enough for geological activity to release & retain water and atmosphere.

112. Planetary Destiny
Earth is habitable because it is large enough to remain geologically active and at the right distance from the Sun so oceans could form.

113. What have we learned?
• What unique features of Earth are important for life?
Unique features of Earth on which we depend for survival are
(1) surface liquid water, made possible by Earth’s moderate temperature;
(2) atmospheric oxygen, a product of photosynthetic life;
(3) plate tectonics, driven by internal heat; and
(4) climate stability, a result of the carbon dioxide cycle, which in turn requires plate tectonics.

114. What have we learned? • How might human activity change our planet?
Ozone depletion can leave surface life more vulnerable to dangerous solar ultraviolet radiation, and the high rate of extinctions could have unknown consequences. The human release of greenhouse gases into the atmosphere may already be causing global warming and certainly would affect the climate if it continues.

115. What have we learned? • What makes a planet habitable?
We can trace Earth’s habitability to its relatively large size and its distance from the Sun.
Its size keeps the internal heat that allowed volcanic outgassing to lead to our oceans and atmosphere, and also drives the plate tectonics that helps to regulate our climate through the carbon dioxide cycle.
Its distance from the Sun is neither too close nor too far, thereby allowing liquid water to exist on Earth’s surface.