1. Earth
The Model Planet

2. If you encountered another planet, what would you want to learn about it?
Basic physical parameters
How old is the planet? How was it formed?
How is it structured internally? Externally?

3. Learning more
Does it have a magnetic field? How strong is it? How is it structured? What does it tell us about the planet’s interior?
How is its atmosphere structured? Of what is it made? What are its weather patterns? How does the atmosphere help control the planet’s energy budget?

4. Learning more (2)
Has the atmosphere always been the same as it is now? How has the atmosphere interacted with the surface?
What kinds of physical processes have produced the planet’s landforms?
Is there life on the planet? How does it interact with the various ecosystems?

5. Physical Properties Diameter: 12,756 km at the equator
Why specify “at the
equator”?
Because the polar diameter is only 12,697 km

6. Physical Properties The bulging at the equator and flattening at
The poles is called OBLATENESS.
It’s due to the rotation of the planet.
Earth is the most spherical (least oblate)
of all the planets.

7. Physical Properties Volume: 1.1 trillion cubic kilometers (km3)
Mass: 5.97 x 1024 kilograms
Mt. Everest is about than 2400 km3
This is equal to about 81 moons!

8. Physical Properties Density: 5500 kg/m3 or 5.5 g/cm3
We compare the density of materials like rocks & metals to the density standard: WATER!
Water’s density is 1.0 g/cm3

9. Physical Properties
Most rocks have densities between 2.5 and 4.0 g/cm3
Most metals have densities greater than 6.0 g/cm3
Iron is 7.8 g/cm3
Nickel is 8.9 g/cm3

10. Physical Properties The density of an object reflects its composition.
What does Earth’s density tell you about its composition?
Earth must be made of a
combination of rocks
and metals.

11. How old is the Earth? About 4.6 billion years.
The oldest rocks are found on the north slope of Canada, the Canadian Shield. These rocks are 4.0 billion years old. They were dated from the radioactive decay of uranium into lead.
The other 0.6 billion years is an estimate of how long it took the earth to form.

12. How was the earth formed?
Accretion
Differentiation
Interior Structure
Evidence, or “How sure are we?”

13. A rotating cloud of gas & dust:
a nebula

14. Rotation causes the nebula to flatten

15. A star ignites in the center and a
temperature gradient forms.

16. Solid chunks, called planetissimals,
begin to condense close to the star.

17. Accretion
Planetissimals (dust-sized to small moon-sized solid bodies) begin to form near the sun.
Gravity attracts planetissimals into larger and larger bodies. Planets begin to grow.
Most of the “stuff” of planetissimals is rock (silicates: Na, Ca, Mg, O, Si) and heavy metals (Fe, Ni).

18. Differentiation As earth grows in size, its gravity grows too
It begins to pull in other planetissimals, which impact on its surface.
What happens when you repeatedly hit a piece of metal with a hammer?

20. Differentiation
All the heat generated by planetissimals impacting on the surface, plus the heat generated by radioactive decay in the young earth’s interior causes the entire earth to melt.

21. Differentiation
When the entire earth is molten, the heavy elements (iron, nickel) sink to the interior.
The lighter materials (granite-type rocks) rise to the surface.
The medium density rocks (basalt-type) ends up in the middle.
Layers form: core, mantle, crust.

22. Molten Earth, heated
by impacts & radioactive
decay.
Differentiated (layered)
Earth after cooling.

23. Interior Structure
Inner Core (kept solid by the immense pressure of all the material on top of it.)
Outer Core (less pressure allows it to be a liquid.)
Mantle
Asthenosphere
Lithosphere
Crust

24. Solid Inner Core
2400 km diameter
Iron & Nickel
Liquid Outer Core
2270 km thick
Iron & Nickel
Crust
km thick
Granitic rocks:
Feldspars
Mantle
2900 km thick
Basaltic rocks:
Olivine, Pyroxene
Interior section of mantle is a thick fluid called the
asthenosphere. The outer mantle + the crust are rigid
and are collectively called the lithosphere.

25. More about the layers
The difference between the mantle and the crust is based on chemical composition.
The difference between the asthenosphere and the lithosphere is based on viscosity (the ability to flow under pressure.)

26. How do we know?
What evidence do we really have that the interior of the earth is the way we think it is?
Deep mines are hot!
Heat and molten material escape from volcanos and geysers.
Earthquake waves

27. Earthquake Waves
When an earthquake occurs it produces 2 types of waves:
P or primary waves. These are waves of compression of the rock. They travel fastest.
S for secondary or shear waves. The rock moves up & down or sideways.

29. Earthquake waves
P waves are capable of traveling through both liquids and solids, so they travel through mantle and cores.
S waves can’t travel through liquids, so they stop when they hit the outer core.
Look closely at the next diagram.

30. Cornell Univ.

31. How big’s that core?
With the right placement of seismometers around the earth’s surface, we get a good estimate of the size of the outer core.
The size of the inner core is calculated from theory.

32. Earth’s Magnetic Field
Why does Earth have a global or world-wide magnetic field, while other similar planets either have no magnetic fields or very different kinds of fields?
Why should we care about Earth’s magnetic field? What does it do for us?

33. Earth’s Magnetic Field
Magnetic fields are made wherever there is an electric current, that is the movement of electrons.
In a regular bar magnet, the magnetic field comes from the electrons orbiting around the nuclei of the iron atoms. All the electrons orbit in the same direction.

35. Earth’s Magnetic Field
In the earth, electrical currents run through the molten iron core. Friction within the molten, flowing iron knocks electrons off iron atoms.
The molten iron flows at about 0.8 inches per second, but the electrical currents can flow faster.

36. Earth’s Magnetic Field
The electrical currents within the earth cause earth to act like a gigantic electromagnetic generator.
This is called the Dynamo Theory of magnetic field generation.

38. A little more info…
The earth’s magnetic field isn’t strong enough for us to feel, but many animals can sense it and even use it to navigate. It’s only about 0.4 Gauss, much weaker than a small magnet you can hold in your hand.
On average, the North & South poles “flip” every 390,000 years. There have been 9 flips in the past 3.5 million years.

39. The poles “flip” ?
No one knows how long the process takes, maybe a few years, maybe a few minutes.
Every so often, what was the North magnetic pole suddenly becomes the South magnetic pole.
Lava that cools quickly on the sea floor records these flips and lets us date them.

40. Stripes of different magnetic polarity form in the rocks as the lava from the mid-ocean ridge cools.

41. Strange Things Going On
Earth’s magnetic field is NOT aligned with its rotational axis.
The magnetic field is tilted 12o to the rotational axis, and doesn’t even pass directly through the center of the earth.
Does this mean that the electrical currents don’t flow evenly and uniformly inside the earth? Is there turbulence inside?

42. Magnetic Fields in Space
Earth’s magnetic field extends 7-10 times the earth’s diameter outward from the earth.
The earth’s magnetic field would be spherical, but the solar wind compresses it on the side closest to the sun, and stretches it out into a long tail on the side opposite the sun.
Overall, it’s kind of tadpole shaped.

43. Magnetic Field Structure
The whole magnetic field is called the magnetosphere.
On the side closest to the sun, where the solar wind compresses the field, there is a “bow shock”, just like a boat pushes some water out of the way at its bow as it sails forward.

46. Why do we care?
Earth’s magnetic field isn’t just “there” with no purpose. Without it, you and I and every living thing on this planet would be dead (including the cockroaches!)
The magnetic field channels away the solar wind.
It also prevents erosion of the atmosphere.

47. Solar Wind So what is the solar wind anyway?
It’s radiation: extremely hot, high-energy, fast-moving charged particles (ions) given off by the sun. Most of these particles are protons.
If you were exposed to it for just a few hours without protection, your skin and every organ in your body would be burned, and you’d have a fatal dose of radiation poisoning.

48. How does the magnetic field protect us?
The magnetic field captures the solar wind and channels much of it into a donut of radiation around the earth.
This donut (actually 2 layers – one inside the other) is called the Van Allen Radiation Belt (V.A.R.B.)

50. Van Allen Radiation Belts
Satellites must orbit either below or above the V.A.R.B., or their electronics would be fried.
The problem is even worse when we send manned missions into space. The ship must pass through the radiation belts as quickly as possible or the crew is toast !

51. Where does the radiation go?
Since the sun continually supplies new solar wind, where does the solar wind go that the earth has already captured?
The magnetic field channels some of it into our atmosphere at the north & south poles. Here it ionizes oxygen and nitrogen atoms, causing the beautiful northern and southern lights.

52. Northern Lights?
The northern lights are properly called the “aurora borealis.” They’re nothing more than a very large fluorescent light display (without the fluorescent tube!)
The northern lights are sometimes seen as far south as Florida, especially when the sun is very active.

54. This aurora was photographed in Tennessee in October, 2002.

55. Where does the rest of the radiation go?
Much of it flows through the magnetic field, around the earth, and “drips” off the tail of the magnetic field. The tail is called the “magnetotail.”
Without our “Teflon-coating” of magnetic field, the earth would have been cooked many billions of years ago.

56. Switch Gears…!
Let’s switch topics, from the magnetic field to the atmosphere.
Earth’s atmosphere is unlike any other planet’s in chemical composition, but it is like every other planet’s in the processes that go on within it.

57. Chemical Composition Our current atmosphere is: 78% nitrogen (N)
21% oxygen (O)
1% argon (Ar), helium (He), carbon dioxide (CO2), water vapor (H2O), and about 20 other rare gases.

58. Chemical Composition
The % of water in the atmosphere can vary from near 0% over deserts to 0.5% in the tropics.
The % of carbon dioxide has doubled in the past 300 years, from 150 parts per million (ppm) to about 340 ppm today.
This means that our atmosphere is evolving! Could it have evolved in the past?

59. Atmospheric Pressure
Pressure is the downward push of the column of air above you.
At earth’s surface, the air (barometric) pressure is 14.7 pounds / square inch.
Other units are inches of mercury in a barometer, and 1013 millibars.

61. The Atmosphere’s Structure
Earth’s atmosphere has both vertical and horizontal structure.
Vertically, the atmosphere is divided into 4 layers.
Horizontally, the atmosphere is divided into 6 circulation cells, 3 in the northern hemisphere & 3 in the southern.

62. 4 Layers
Troposphere, the weather layer. From the earth’s surface to 10 km up. It gets colder the higher up you go within this layer.
Stratosphere, the circulation layer. The jet stream and ozone layer that protects us from UV light are in this layer. Extends from 10 to 40 km up. Temperature rises as you go up within this layer.

63. 4 Layers continued
Mesosphere, a middle layer, up to 75 km. Here the air pressure is only 1/10,000th of the pressure at the earth’s surface. The temperature again falls as you go up within this layer. Thermosphere, the hot layer, up to 120 km. This is the outer edge of earth’s atmosphere. Here, the temperature equalizes with the temperature of the hot solar wind. This is where auroras form.

65. What sets off one layer from the next is the way the temperature varies within it.

66. How does the atmosphere affect the surface?
…in 4 ways:
35% of the sunlight that hits the atmosphere is reflected back into space by clouds.
The % of visible light reflected by a planet is called its albedo. Earth’s albedo is 0.35.
Clouds, ice, deserts all increase albedo.
A high albedo generally means that the planet has a cold surface (lots of ice.)

68. How does the atmosphere affect the surface?
33% of the sunlight is absorbed by gases and dust, but then is re-radiated as infrared (heat). Much of this infrared light goes back into space and is lost.
The absorption of light is called attentuation or extinction (just like the dinosaurs!)
Greenhouse gases (water vapor, carbon dioxide, methane or CH4) help to trap the heat and prevent it from going back into space.
Without the greenhouse gases, earth’s surface would be about -18oC.

70. How does the atmosphere affect the surface?
Dust in the atmosphere also causes reddening, a process where the blue light is scattered, but red light is allowed to pass straight through.
We see the scattered blue light as the blue of our sky. We also see the red of sunset as it passes straight through the atmosphere.
Reddening also happens in space when starlight passes through dusty nebulas.
By the way, only 32% of the sunlight makes it to the surface.

71. The reddening effect.

72. Atmospheric Circulation
Earth’s atmosphere has 3 circulation cells in each hemisphere (called Hadley cells on other planets).
The northern-most is the polar cell, from 90o to 60o north latitude
We live in the temperate cell, from 60o to 30o north latitude.
The southern-most is the tropical cell, from 30o north to the equator.

73. Atmospheric Circulation
How does the air circulate?
Warm air rises at the equator, cools off at high altitude, then falls back to the surface at 30o north latitude. It eventually circulates back to equator.
The polar cell operates by cold air falling at the north pole, flowing away from the pole, warming and rising at about 60o north latitude.
The temperate circulation cell is just caught in the middle between the tropical & polar cells.

75. Atmospheric Circulation
If the earth didn’t rotate, the air in the circulation cells would simply move north and south.
However, the earth’s rotation causes the Coriolis Effect. This causes moving wind to turn or deflect to the right in the northern hemisphere. The circulation cells turn into tubes, allowing the winds to move all the way around the globe.

76. Click here for an animation of the Coriolis Effect

77. Other Planets
How are other planet’s atmospheres different? Other planets rotate faster or slower, are hotter or cooler.
How would rotating faster affect the atmosphere? It might turn circulation cells into bands, where the winds simply move from west to east or east to west.
Hotter temperatures could be expected to make the wind speeds higher.

78. East – west bands of winds result from a very rapid rate of rotation.

79. Where did the atmosphere come from?
Some water and gases were contributed by comets, meteors, and other planetissimals impacting on earth’s surface.

81. Where did the atmosphere come from?
But most of the atmosphere came from volcanic outgassing. Volcanoes release over 100 billion kilograms of water vapor and gases into the atmosphere every year.
Over 4.5 billion years, 5 x 1020 kilograms of water and gases have been released. This is enough for the atmosphere and the oceans!

83. Did the atmosphere change?
Earth has had 3 atmospheres.
The first atmosphere was hydrogen (H) and helium (He) from the original solar nebula.
Since these gases are light and the earth was hot way back then, most of the H and He was eventually lost to space.

84. 2nd atmosphere
After the H and He escaped into space, only the gases that were too heavy to be lost were left behind: nitrogen (N2) and carbon dioxide (CO2).
We still have the N2 today, but where did the CO2 go?
Most of it dissolved in the oceans, combined with calcium (Ca) and was turned into limestone. Some was absorbed by photosynthetic bacteria.

85. The layer of limestone below formed by the chemical equation:
CO2 + CaO CaCO3

86. 3rd atmosphere
As the photosynthetic bacteria began to use the CO2, it began to produce oxygen (O2).
Some of the bacteria evolved into photosynthetic plants which increased the rate of O2 production.
Our present atmosphere is N2 and O2 in about a 4:1 ratio.

87. First came the photosynthetic bacteria, then the green plants
First came the photosynthetic bacteria, then the green plants. Both added
oxygen (O2) to our atmosphere.

88. Earth’s Geology
Earth’s surface changes by processes that are similar to some of the other planets:
Wind blows particles that cause erosion and build up structures like sand dunes and dust fields.
Flowing water cuts canyons and river beds, and transports material. Flood can erode huge channels.
Flowing ice or lava can act much like flowing water.
Subsurface movements cause hills, mountains, volcanoes, huge cracks and rift valleys.

89. Earth’s Geology
Earth also has larger-scale processes that other planets don’t have: plate tectonics.
Earth’s crust is divided up into about 20 large pieces or plates of rigid crust that float on top of flowing mantle.
Where these plates come together or pull apart is where we get mid-ocean ridges, chains of volcanoes and mountains, and earthquakes.

92. Earth’s Geology
The movement of plates is driven by hot currents of magma welling up from deep within the mantle of the earth.
Look at the following diagrams closely. Look especially for places where the plates are separating or crushing together, because we’ll be looking for similar features on other planets too!

93. A plume of hot magma is welling up from the deep mantle. Notice
how it pushes up the crust. A mid-ocean ridge may form here.

96. The next time you think of the
earth, remember all of the
parts that make it up.
We’ll use this
information
again, when
we start
looking at
the other
planets!

97. Photo & Audio Credits NASA U.S. Geological Survey
Donald E. Davis – NASA
Cornell University
B. Tissue