1. Planet Earth
1 February 2005
AST 2010: Chapter 7

2. Basic Facts
The Earth is a medium-sized planet with a diameter of 13,000 km
It is one of the inner or terrestrial planets
It is composed primarily of heavy elements, such as iron, silicon, and oxygen
It has much less light elements, such as hydrogen and helium, than the outer planets
Earth's orbit around the Sun is nearly circular
The Earth is the only planet in our solar system that is neither too hot nor too cold
It is warm enough to support liquid water on its surface
It is “just right” to sustain life — at least life as we know it
1 February 2005
AST 2010: Chapter 7

3. Basic Properties Some properties of the Earth Semi-major axis 1.00 AU
Orbital period
1.00 year
Mass
5.98 x 1024 kg
Diameter
12,756 km
Escape velocity
11.2 km/s
Rotation period
23 h 56 m 4 s
Surface area
5.1 x 108 km2
Atmospheric pressure
1.00 bar
1 February 2005
AST 2010: Chapter 7

4. Earth's Interior (1)
The interior of the Earth is difficult to study even with today's amazing technology
Its composition and structure must be determined indirectly from observation made near or at the surface only
Earth’s skin or crust is a layer only a few kilometers deep
The Earth is composed largely of metals and silicate rock
Most of this material is in a solid state, but some of it is hot enough to be molten
1 February 2005
AST 2010: Chapter 7

5. Earth's Interior (2)
The structure of the interior of the Earth has been probed in great detail by measuring the transmission of seismic waves through it
Seismic waves are waves that spread through the interior of the Earth from earthquakes or explosions
Seismic waves travel through Earth rather like sound waves through a struck bell
In a bell, the sound frequencies depend on what material the bell is made of and how it was constructed
Similarly, the way seismic vibrations behave depends on the composition and structure of the planet
1 February 2005
AST 2010: Chapter 7

6. Earth’s Internal Layers (1)
The Earth is divided into four main layers: crust, mantle, core, and inner core
The crust is the top layer, the part we know best
The crust under the oceans, which covers 55% of the surface, is typically about 6 km thick and is composed of volcanic rocks called basalt
Basalts are produced by cooling volcanic lava and made primarily of silicon, oxygen, iron, aluminum, and magnesium
The continental crust, which covers % of the surface, is 20 to 70 km thick and is mainly composed of another class of volcanic rocks called granite
The crust makes up only about % of the Earth’s total mass
1 February 2005
AST 2010: Chapter 7

7. Earth Internal Layers (2)
The mantle is the largest part of the solid Earth, stretching from the base of the crust down to a depth of 2,900 km
The mantle is more or less solid, but may deform and flow slowly due to its high pressures and temperatures
Below the mantle is Earth’s dense metallic core
In addition to iron, it contains nickel and sulfur, all compressed to a very high density
The core is 7,000 km in diameter
Its outer part is liquid
The inner core is 2,400 km in diameter and is probably solid
1 February 2005
AST 2010: Chapter 7

8. Rocks (1)
Basalt & granites are two examples of a class of rocks called igneous rocks
They are rocks that have cooled from a molten state
All volcanically produced rocks are igneous
There are two other kinds of rocks
Sedimentary rocks are made of fragments of igneous rocks or the shells of living organisms deposited by wind or water and cemented without melting
Metamorphic rocks are produced when high temperature or pressure alters igneous rocks physically or chemically
These are commonly found on Earth, but not on other planets
1 February 2005
AST 2010: Chapter 7

9. Rocks (2) A fourth group of rocks are called primitive rocks
Their formation dates back to formation of the planet
They have largely escaped chemical modification by heating
Thus, they represent the original material out of which the planetary system was made
No primitive rock is left on the Earth because it was heated early in its history
Primitive rocks may be found in comets, asteroids, or small planetary satellites
1 February 2005
AST 2010: Chapter 7

10. Differentiation
The separation of the earth interior into layers is an example of differentiation
Differentiation observed on Earth is evidence that it was once warm enough for the mantle rocks to melt
It allows the heavier metals to sink to the center and form a very dense core
1 February 2005
AST 2010: Chapter 7

11. Earth’s Magnetic Field
Much about Earth's interior can be learned from the Earth's magnetic field
Earth behaves in some ways as if a giant bar magnet were inside it
The magnet is roughly aligned with the rotational axis of the planet
Earth’s magnetic field is generated by moving material in Earth’s liquid metallic core
The circulating liquid metal sets up an electric current, which in turn produces a magnetic field
1 February 2005
AST 2010: Chapter 7

12. Earth’s Magnetosphere (1)
The Earth's magnetic field extends into surrounding space and traps small quantities of electric charges, such as electrons, that roam about the solar system
Within this region, called the magnetosphere, Earth’s field dominates over the weak interplanetary magnetic field extending outward from the Sun
Cross-sectional view of Earth’s magnetosphere as revealed by spacecraft missions
1 February 2005
AST 2010: Chapter 7

13. Earth’s Magnetosphere (2)
It was discovered in 1958 by instruments on the first U.S. Earth satellite, Explorer 1
This satellite recorded the ions (charged particles) trapped in the inner part of the magnetosphere
This region has a fairly complex structure
It is composed of more than one layer or part
The layer discovered in 1958 is called Van Allen Belts after the physicist who built the instrumentation for Explorer 1 and correctly interpreted its measurements
1 February 2005
AST 2010: Chapter 7

14. Solar Wind
Charges trapped in the magnetosphere flow outward from the Sun
Phenomenon is called solar wind
Flow of charged particles from the Sun is large
Trapped by the Earth’s magnetosphere
Their flow produces a deformation of magnetic field lines
Elongation far beyond Earth pointing away from the Sun
Magnetosphere typically extends  to km - 10 earth radii - from the earth - towards the Sun
Away from the sun, sizeable magnetic fields are measurable at a distance as large as the Moon's
1 February 2005
AST 2010: Chapter 7

15. Geology Study of processes that shape the crust
Although a fairly mature science, it is not until very recently that geologists were successful in understanding how landforms are created
1 February 2005
AST 2010: Chapter 7

16. Plate Tectonics (1)
A theory that explains how slow motions of the earth mantle move large segments of the crust
Resulting in slow drifting of the continents
Formation of mountains and other large scale geological features
Earth's crust and mantle divided into ~12 major plates fit together like the pieces of a puzzle
Plates observed to move slowly relative to one another
In some places, such as the Atlantic ocean, the plates are moving apart, elsewhere they are forced together

17. Plate Tectonics (2)
Driving power of the plates motion is provided by slow convection of the mantle
Convection: a process by which heat escapes from the interior of the mantle and produces and upward of warmer materials while cooler materials found above slowly sink down
1 February 2005
AST 2010: Chapter 7

18. Plate Motion
The plates’ motion brings them to collide into one another and brings about dramatic changes on the surface of the Earth
Basically four types of interactions are observed between the crustal plates:
They can pull apart
One plate can burrow under another
The can slide alongside each other
They can jam into each other
1 February 2005
AST 2010: Chapter 7

19. Rift Zones Plates pull apart from each other along rift zone
An important rift zone is found in the Mid-Atlantic ridge
Few rift zones are also found on land
E.g. central African rift - these rifts shall eventually break apart the African continent
Most of the rift zones are however found in the oceans
1 February 2005
AST 2010: Chapter 7

20. Subduction Zones
The point of contact where two plates come together is called subduction zone
Continental masses cannot be subducted but the thinner oceanic plates can be “easily” pushed down into the upper mantle
Subduction zones often marked by an ocean trench
Subducted plates forced down into regions of high temperature+pressure, eventually melts several hundred kilometers below the surface
1 February 2005
AST 2010: Chapter 7

21. Crust Regeneration
Calculations of the rate at which the sea floor is spreading reveal the approximate age of oceanic crust
60000 km of active rifts identified
Average separation of 4 cm per year
Correspond to an added area of 2 km2 per year
Enough to renew the entire oceanic crust in about 100 million years
Less than 3% the age of the planet
Oceans are a fairly recent feature of the planet
1 February 2005
AST 2010: Chapter 7

22. Fault Zones
Crustal plates slide parallel to each another  along much of their lengths
Boundaries so formed lead to the formation of cracks or faults
Along active fault zones, the motion of one plate relative to the other may amount to several centimeters per year - basically the same as the spreading along the rifts
1 February 2005
AST 2010: Chapter 7

23. San Andreas Fault
On the boundary between the Pacific and North American plates
Runs from the Gulf of California to the Pacific Ocean northwest of San Francisco
Pacific plate (west side) moves north carrying along Los Angeles, San Diego, and parts of Southern California
In a few million years, LA will be an island off the coast of San Francisco
1 February 2005
AST 2010: Chapter 7

24. Beware of Faults! Plates slide roughly alongside each other
The creeping motions of the plates builds up stresses in the crust
The stresses are eventually released in sudden, violent slippages, a.k.a. earthquakes
Average motion of the plates is constant
The longer the interval between earthquakes
the greater the stress and the larger the energy released when the surface finally moves
1 February 2005
AST 2010: Chapter 7

25. More about San Andreas
The San Andreas Fault, near Parkfield, has slipped every 22 years during the past century
moving an average of about 1 m each time
In contrast, the average interval between major Earthquakes in the Los Angeles region is about 140 years
the average motion is about 7 m
1 February 2005
AST 2010: Chapter 7

26. Mountain Building
When two continental masses are brought together by the motion of the crustal plates, they are forced against each other  under great pressure
The surface buckles and folds forcing  some of the rock deep below the surface and others to raise to large heights (sometimes many kilometers!)
This is how mountain ranges are formed on Earth 
The Alps result from the interaction of the African Plate with the European plate
We will see, however, that other mechanisms lead to formation of mountains on other planets
1 February 2005
AST 2010: Chapter 7

27. Volcanoes
Volcanoes mark the location where molten rock, called magma, rises from the upper mantle through the crust 
Volcanoes are formed numerously along oceanic rift zones where rising hot material pushes plates away from one another
Volcanic activity is also observed in subduction zones
In both cases, the volcanic activity brings to the surface large amount of materials from the upper mantle
1 February 2005
AST 2010: Chapter 7

28. More about Volcanoes
Volcanic activity also found near mantle "hot spots" areas
far from plate boundaries
but where heat rises from the interior of the planet. 
Best known hot spot lies under Hawaii
Supplies in magma three active volcanoes - two of which are on land, and the third in the ocean. 
It is estimated that the Hawaiian hot spot has been active for at least 100 million years.
Shaping the Pacific plate, the hot spot has generated a 3500-km long chain of volcanic islands
1 February 2005
AST 2010: Chapter 7

29. Earth’s Atmosphere Provides the air we breathe
The air of the atmosphere exerts a constant pressure (on the ground)
The atmosphere pressure at sea level is used to define the pressure unit called bar
Humans have existed mostly at sea level and are thus accustomed to such a pressure
The total mass of the atmosphere is ~ 5x1018 kg
Although this sounds like a lot, it constitutes only one millionth of the total mass of the Earth
Yet it composition is quite vital to us humans and other living creatures on the surface of this Earth
1 February 2005
AST 2010: Chapter 7

30. Structure of Earth’s Atmosphere
1 February 2005
AST 2010: Chapter 7

31. Troposphere Altitude range: Sea level - 9 miles
Densest area of the atmosphere
Most weather occurs and almost all aircraft fly in this region
Temperatures drop as elevation increases
Warm air, heated on the surface, rises and is replaced by descending currents of cooler air
The circulation generates clouds and other manifestations of weather
As one rises through the troposphere, one finds the temperature drops rapidly with increasing elevation
The temperature is near 50oC below freezing at the top of the troposphere
1 February 2005
AST 2010: Chapter 7

32. Stratosphere Altitude range: Dry and less dense
miles
Dry and less dense
The air in this layer moves horizontally and does not move up and down within it
Temperatures here increase with elevation
Near the top of the stratosphere, one finds a layer of ozone (O3)
Ozone is a good absorber of ultraviolet light
It thus protects the surface from the sun's ultraviolet radiation and makes it possible for life to exist on the planet
1 February 2005
AST 2010: Chapter 7

33. Mesosphere Altitude range:
miles ( km)
Temperatures fall as low as -93 degrees Celsius in this region
Chemicals are in an excited state, as they absorb energy from the sun
1 February 2005
AST 2010: Chapter 7

34. Ionosphere Altitude range: 62 - 124 miles.
This region is characterized by the presence of plasma.
Its boundaries vary according to solar activity.
1 February 2005
AST 2010: Chapter 7

35. Thermosphere Altitude range:
miles ( km)
Temperatures increase with altitude due to the sun's energy, reaching as high as 1,727 degrees Celsius
Auroras, caused by the sun's particles striking the earth's atmosphere, occur at this level
1 February 2005
AST 2010: Chapter 7

36. Exosphere Altitude range:
miles ( km)
The region begins at the top to the thermosphere and continues until it merges with interplanetary gases, or space
The prime components, hydrogen and helium, are present at extremely low densities
1 February 2005
AST 2010: Chapter 7

37. About Ozone Increasing evidence atmospheric ozone is being destroyed
Agents of destruction are industrial compounds called CFCs (chlorofluorocarbons)
Each year, a large ozone forms above the Antarctic continent
By now, the ozone loss has progressed into the temperate zone
The production of CFCs has been banned by international agreement
These chemicals are however destroyed slowly and are still  often released in the atmosphere
One can thus expect further reduction of the ozone layer in the next century
1 February 2005
AST 2010: Chapter 7

38. Weather and Climate All planets with atmospheres have weather
Weather is simply the name given to the circulation of air through the atmosphere
Climate is a term used to describe the evolution of weather through long periods of time: decades or centuries
Changes in climate are typically difficult to detect over short periods of time. However, their accumulating effects can be sizeable and sometimes quite dramatic
1 February 2005
AST 2010: Chapter 7

39. About the Weather
The energy that power this motion is derived primarily from the sunlight that heats the Earth's surface 
As the planet rotates, and orbits the Sun, the slower seasonal changes cause variations in the amount of heat of sunlight striking the different parts of the planet
The heat then proceeds to redistribute itself from warmer to cooler areas giving rise to various weather patterns
1 February 2005
AST 2010: Chapter 7

40. Hurricane Elena In the Gulf of Mexico on Sept 1, 1985
Wind speeds were in excess of 110 miles per hour
Eventually made landfall near Gulfport, Mississippi

41. Origin of Life
Early Earth atmosphere is believed to contain abundant carbon dioxide but no oxygen gas 
In the absence of oxygen, many complex chemical reactions are possible that lead to the production of amino acids, proteins, and many other chemical building blocks of life
Genetic studies of the many million species that now live on Earth suggest that they are related to one another
This led to the idea that all terrestrial life descends from a single common microbial ancestor
1 February 2005
AST 2010: Chapter 7

42. Evolution of Life
Blue-algae consume carbon dioxide and produce oxygen as a waste product 
They use the energy from sunlight, in a process called photosynthesis to develop and grow
They are thought to have proliferated and eventually evolved into what we know today as plants
1 February 2005
AST 2010: Chapter 7

43. Appearance of Oxygen in the Atmosphere
Studies suggest that oxygen started to accumulate in the atmosphere some 2 billion years ago
Led to formation of the Earth's ozone layer
Layer produced a shield under which more complex life could evolve and develop
Life is believed to arise from the vast oceans and venture into solid grounds
In this scenario, as animals evolved in environment increasingly rich in oxygen, they were able to develop techniques for breathing oxygen directly from the atmosphere (primitive lungs appeared)
1 February 2005
AST 2010: Chapter 7

44. Role of Carbon Dioxide (CO2)
Sunlight striking the surface is absorbed
heats the surface layers
re-emitted as infrared/heat radiation
Atmospheric CO2 transparent to visible light
does not impede sunlight to reach the surface
CO2 opaque to infrared energy
behaves as a blanket, trapping the heat in the atmosphere and impeding the flow back of energy back to space
This is called the greenhouse effect
1 February 2005
AST 2010: Chapter 7

45. Greenhouse Effect
On average, as much heat reaches Earth’s surface from the atmospheric greenhouse effect as from direct sunlight
This explains why
night-time temperatures are only slightly lower than daytime temperatures
life is actually possible on this planet
1 February 2005
AST 2010: Chapter 7

46. Global Warming (1)
Estimated that greenhouse effect elevates the surface temperature by about 23°C on the average
Without this effect, the average temperature would be below freezing
Earth would be covered with ice
Global ice age
An increase of atmospheric CO2  implies that the atmospheric temperature would rise to much higher average values
and then endanger life on our planet
1 February 2005
AST 2010: Chapter 7

47. Global Warming (2) Modern society increasingly depends on energy
Energy production is accomplished by burning fossil fuels which when burned release carbon dioxide
The problem is exacerbated by ongoing destruction of tropical forests in Asia, Africa, and South America
Atmospheric CO2 has increased by about 25% in the last 100 years
It is rising at a frightening pace of 0.5% per year
CO2 level will soon reach twice the value it had before the industrial revolution
Consequences are complex, not completely known
Sophisticated and elaborate computer models are used
Conclusions are not firm at this point
1 February 2005
AST 2010: Chapter 7

48. Why is there no clear evidence of craters on Earth?

49. Suggested Answer Geological activity!

50. Earth Craters
Evidence of fairly recent impacts can be found on our planet's surface
The best studied case took place on June 30, 1908, near the Tunguska River in Siberia, Russia 
8 km above the ground
Flattened more than a thousand square kilometers of forest
Blast wave spread around the world and was recorded by instruments designed to record changed in atmospheric pressure
1 February 2005
AST 2010: Chapter 7

51. 1 February 2005
AST 2010: Chapter 7

52. Arizona Meteor Crater Impact thought to have occured 50,000 years ago
Iron-nickel meteorite
Hurtling at about 40, miles per hour
Northern Arizona
Explosive force greater than 20 million tons of TNT
Estimated size of meteor about 150 feet across
Weigh several hundred thousand tons
Crater 700 feet deep, 4000 feet across
Today: crater is 550 feet deep, and 2.4 miles in circumference
1 February 2005
AST 2010: Chapter 7