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EARTH AS A PLANET
Chapter 7
SOME PROPERTIES of
EARTH
Semi major Axis
1.00 AU
Period
1.00 year
Mass
5.98 x 1024 kg
Diameter
12,756 km
Escape Velocity
112. Km/s
Rotation Period
23h 56m 4s
Surface Area
5.1 x 108 km2
Atmospheric Pressure 1.00 bar
EARTH as a PLANET
 Only
planet with water in liquid form.
 Composition: iron, silicates, oxygen.
 Density: highest in Solar System.
 Interior: 4 major layers - core, outer core,
mantle, crust.
EARTH as a PLANET
 Only
planet with water in liquid form.
 Composition: iron, silicates, oxygen.
 Density: highest in Solar System.
 Interior: 4 major layers - core, outer core,
mantle, crust.
 Magnetic Field and Magnetosphere: Earth
behaves as if it had a bar magnet inside it.
MAGNETOSPHERE and VAN
ALLEN BELTS
The MAGNETOSPHERE
SHAPE
EARTH’S STRUCTURE – HOW
DO WE KNOW IT? FROM
EARTHQUAKES
PLATE TECTONICS
 Earth’s
crust consists of about 12 plates.
PLATE TECTONICS
 Energy
escaping from the interior drives the
plates (few cm/year).
50 million years
from now
PLATE TECTONICS
 Where
plates interact, dramatic changes occur
in Earth’s crust.

Rift Zones: Plates pulling apart. Material
rises from mantle to fill spaces (volcanoes).
E.g.. Mid Atlantic Ridge.
PLATE TECTONICS
 Where
plates interact, dramatic changes occur
in Earth’s crust.

Subduction Zone: Two plates come together,
once forced down and melts. Earthquakes and
volcanoes common along subduction zones.
PLATE TECTONICS
 Coastal
BC is on the upper part of a
subduction zone.
SAN ANDREAS FAULT
ACTIVE SITES on EARTH’S
SURFACE
ACTIVE SITES on EARTH’S
SURFACE
CONTINENTAL DRIFT
FLASHCARD
AT 2 CM/YEAR, HOW LONG WOULD IT TAKE A
TYPICAL PLATE TO TRAVERSE THE PRESENT
WIDTH OF THE ATLANTIC OCEAN, ABOUT 6000 KM?
A) 3 X 106 YR
B) 1.2 X 107 YR
C) 3 X 108 YR
D) 1.2 X 109 YR
> 100 km
Ozone
Layer
Atmosphere very
thin, constant
leakage of H and
He
50 - 80 km, O3,
absorbs UV
radiation
10 - 80 km,
cold (-50ºC),
Stratosphere cloudless
< 10 km, most
atmosphere
Troposphere here. (N2 78%,
O2 21%, Ar 1%,
traces H2O, CO2)
GREENHOUSE EFFECT
 CO2
content of atmosphere is critical for its
role in retaining heat from Sun.
 Greenhouse Effect:
 Earth
absorbs sunlight
and re-emits it as
infrared (IR) radiation
(Earth acts like a
blackbody).
 CO2 transparent to
sunlight but absorbs IR
acting like a blanket.
 The more CO2, the
hotter Earth is.
GREENHOUSE EFFECT and
GLOBAL WARMING
 Greenhouse
effect has raised Earth’s average
temperature by 23ºC.
 Without
the greenhouse effect, Earth’s
average temperature would be below freezing
and Earth would be in a constant global ice
age.
GREENHOUSE EFFECT and
GLOBAL WARMING
 CO2 levels are increasing.
 Prediction: rising CO2 levels
will lead to
global warming with uncertain consequences.
THE MOON

1/4 the size of Earth
THE MOON

Earth’s Moon has no atmosphere.
THE MOON

The Moon is tidally locked to Earth.
Non rotating
Moon
Rotating Moon
THE MOON
Only extraterrestrial
object that humans
have visited.

APOLLO MISSIONS
MOON’S INFLUDENCE on
EARTH
Differential forces on Earth tend to stretch
Earth slightly.

TIDAL BULDGES in an
“IDEAL” OCEAN
SPRING and NEAP TIDES
SPRING and NEAP TIDES
MOON’S INFLUDENCE on
EARTH
Tidal friction slowly is increasing the length
of Earth’s day. (6 hours 4.5 billion years ago,
22 hours 620 million years ago)

LIFE on EARTH
 Earth
is the only planet known to be
harbouring life.
 Life arose early on. Fossils date back 3.5
billion years.
 Origin
of life is unsure.
 Miller experiment:
produced amino acids
and other pre-biological
molecules from
primitive Earth
atmosphere - mainly
CO2 (No O2).
LIFE on EARTH
 Development
Use
Free
of plants:
up CO2 and produce O2 - photosynthesis.
O2 in atmosphere 2 billion years ago.
 Formation
Protects
of the ozone (O3) layer
Earth from UV.
This allowed life to leave protective oceans
and colonize the land. Not possible earlier.
COSMIC INFLUENCES on
EARTH’S EVOLUTION
 Moon is heavily cratered
 Where are Earth’s
craters?
 Most lost by erosion
and geological
activity.
- caused by impacts.
COSMIC INFLUENCES on
EARTH’S EVOLUTION
 More
than 150 impact craters are still
recognized on Earth’s surface.
COSMIC INFLUENCES on
EARTH’S EVOLUTION
 Concern
is that a large object, ≥10 km, will
collide with Earth releasing a few billion times
as much energy as Hiroshima bomb.
COSMIC INFLUENCES on
EARTH’S EVOLUTION
 Concern
is that a large object, 10 km, will
collide with Earth releasing a few billion times
as much energy as Hiroshima bomb.
 One
such object crosses Earth’s orbit every
100 million years.
ORBITS of the 100 LARGEST
KNOWN NEAR-EARTH
ASTEROIDS
Mean Time to Impact Earth (Years)
IMPACT FREQUENCY
1.00E-12
Impactors on the surfce of the space shuttle
1m, 30sec
1.00E-09
Shooting stars
1 mm, 30 sec
1.00E-06
1.00E-03
Meteroites
1m, 1yr
1.00E+00
1.00E+03
Arizona Crater
100 m, 104 yr
1.00E+06
1.00E+09
1.00E-08
Sudbury, Ontario
10 km, 108 yr
1.00E-04
1.00E+00
Diameter (Metres)
1.00E+04
1.00E+08
COSMIC INFLUENCES on
EARTH’S EVOLUTION
 More
than 150 impact craters are still
recognized on Earth’s surface.
METEOR CRATER
0.1 km meteor
Crater size = 10 x
size of impacting object
1.2 km
TUNGUSKA EVENT
Siberia
June 30, 1908
Projectile
exploded before
impacting Earth
Projectile’s mass
estimated to be
100,000 tons
Caused
widespread
damage.
TUNGUSKA EVENT
SHOEMAKER-LEVI COMET
IMPACT
Earth’s
View
Voyager’s
View
SHOEMAKER-LEVI IMPACT
DEVELOPMENT of DUST CLOUD
Impact
IMPACT DUST CLOUDS
IR IMAGE of IMPACTS
Io
COSMIC INFLUENCES on
EARTH’S EVOLUTION con’t
EFFECTS of a MASSIVE
IMPACT
EFFECTS of a MASSIVE
IMPACT
Global earthquakes.
 Huge tidal waves.
 Dust high up in the atmosphere.
 Block out Sun for about one year, kills
photosynthesis.
 World-wide fires, forests and grasslands
destroyed.
 Highly acidic rains.
 Animals depending on plants die
(dinosaurs?), rise of mammals.

EXTINCTIONS, the SOLAR
SYSTEM and our GALAXY

Past 100 million years many large scale extinctions (~
10) identified on Earth.
In these, a significant fraction of existing
species were extinguished.

Best known is extinction
of the dinosaurs 65
million years ago after
flourishing for 100
million years.
EXTINCTIONS, the SOLAR
SYSTEM and our GALAXY

What caused the extinctions?
Impact of ~ 20 km
asteroid.
Evidence: crater in
Gulf of Mexico and
Iridium layer.
EXTINCTIONS, the SOLAR
SYSTEM and our GALAXY

What caused the extinctions?
Impact of ~ 20 km
asteroid.
Evidence: crater in
Gulf of Mexico and
Iridium layer.
Nearby supernova
(depletes O3,
forming NO2).
Crab Nebula
EXTINCTION of the
DINOSAURS
ALTERNATE DINOSAUR
EXTINCTION THEORY
EXTINCTIONS, the SOLAR
SYSTEM and our GALAXY

Supernovae and impacts more likely when Sun passes
through spiral arm in our Galaxy
Oort cloud comets get perturbed.
SPIRAL GALAXY
OORT CLOUD GETS
PERTURBED