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Week 1- Lecture Sept. 4, 2009.
The Origin of our Solar System and Earth
1) In the Beginning- The Big Bang
Today the most generally accepted scientific explanation of the origin of the universe is the Big
Bang theory.
According to this theory, our universe began between 12-14 billion years ago with a huge
explosion. Before that time, all of the matter and energy in the universe were compacted into a
single, inconceivably dense point. This is a concept that is difficult to grasp! The universe
continues to expand and to cool. During the past 12-14 billion years, galaxies stars and planets
have formed from the gas clouds.
The nebular hypothesis states that diffuse, slowly rotating clouds of has (hydrogen and helium)
and other particles, contract under the force of gravity. Contraction accelerates the rotation of
the particles and gas. Matter drifts to the center of the cloud and a proto-Sun may form.
Compressed under its own weight, the proto-Sun becomes hotter and hotter until a temperature
is reached where nuclear reactions occur and hydrogen fuses to form helium and energy is
released.
As the nebula collapses further, local regions begin to contract
gravitationally on their own because of instabilities in the
collapsing, rotating cloud
Our Solar System
-Our solar system formed from an immense rotating cloud of gas and dust called the
solar nebula., in one of the arms of our milky way galaxy. (it is estimated that our solar
nebula started contracting about 5 B.Y. ago).
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-Colud starts to contract under the influence of gravity--rotation speeds up;cloud
flattens
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-Matter in the center of the cloud accumulates to form the "protosun"
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-Disk around the sun, cools and condenses--planetisimals (km-scale) form and
start to colide and grow.
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-Discrete planets (and moons) form
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-Lighter elements are "blown away" from the hot regions close to the sun,
forming the light gaseous and icy outer planets.
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-Heavier material is concentrated closest to the sun, forming the "rocky"
terrestrial planets
The whole planetary assembly process probably took about 150 million years.
Followed by a 1 Billion year period during which the planets were subjected to heavy
bombardment by the remaining rocky & icy pieces leftover from planet formation.
Planetary compositions reflect the different conditions in which they formed.
Terrestrial planets (Earth-like planets or rocky planets) are rock & metal: They formed in
the hot inner regions of the Solar Nebula. Outer Planets (Jovian planets or Gaseous
planets) contain ice, ammonia, H & He: They formed in the cool outer regions of the
Solar Nebula.
By studying meteorites, which are thought to be left over material from the early phase
of the solar system, scientists have found that our earth is about 4.6 billion years old!
Earth's Early History
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-Earth forms around 4.6 B.Y ago. Homogeneous in composition.
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-About 20 million years later, a giant impact of Mars-sized body is thought to
have formed the Moon. Energy from the impact and radioactive heat cause partial
melting of the earth.
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-Partial melting caused differentiation about 4.4-4.2 B.Y. ago. Heavy elements
(Fe, Ni) sank to form the core, lighter elements (O, Si, Mg, Al ) floated toward surface,
forms mantle/'crust'. Degassing of interior (by volcanism; N, O, C, H locked in minerals)
and comets produced early atmosphere and hydrosphere. Early atmosphere was very
different from the present atmosphere, there was more CO2, CH4, and not much O2.
Only after photosynthetic (blue-green) algae (~3.5 B.Y.) evolved, did the O2 build up.
This created our second atmosphere which evolved into our present atmosphere.
Evolution of Earth's Atmosphere and Origin of Life
Effect of O2 in the ocean
Banded Iron Formations (BIF)
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- banded iron formations look like just what their name suggests, alternating
layers that are iron rich and iron poor
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- BIF's may represent seasonal blooms of blue-green algae with the iron
precipitating when photosynthesis gives off a lot of oxygen
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- BIF is a self-terminating process, because a point will be reached where more
O2 is being produced than can be used to oxidize the ferrous iron in solution (in the
ocean) and oxygen will escape to the atmosphere
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Effect of O2 in the atmosphere
Red Beds
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- as O2 becomes common in the atmosphere iron can no longer be transported
in solution and instead is oxidized at its source area on land
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- therefore, evidence for oxygen accumulating in the atmosphere comes from
continental sedimentary deposits, red beds
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- prior to 2.0 B.Y no continental deposits contain iron (or other easily oxidized
material like uranium)
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- after 2.0 B.Y red beds become increasingly common indicating the presence of
oxygen in the atmosphere
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- red beds are most common from 1.8-1.2 B.Y.
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-they are still forming today.
Importance of Oxygen in the Atmosphere
1. ozone layer
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- as O2 builds up in the atmosphere an ozone layer begins to accumulate in the
upper atmosphere
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- the ozone layer helps to shield the surface of the planet from radiation,
especially UV
2. respiration
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- all early organisms depend on fermentative (anaerobic) respiration
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- in fact, oxygen may have been toxic to early organisms
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- as oxygen increased in the atmosphere, organisms had to develop defences
against it
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- one way to do that was to turn a liability into an asset by using oxidative
(aerobic) respiration
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- one major advantage of oxidative respiration is that it is much more efficient,
about 20 times more efficient, than anaerobic respiration
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Oxygen content goes up in atmosphere as a result of photosynthesis by green
plants: CO2 (atmosphere)+ sunlight --> C-H sugar + O2 (atmosphere)
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O2 content rises from <1% to ~20%
Atmospheric Oxygen Concentration through Time
Origin of Life
Stromatolites
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- A special type of rock exists throughout the geologic record, called
stromatolites, which record the very first visible evidence of life, as early as 3.465 billion
years ago.
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- These rocks are actually complex colonies of different types of bacteria, each
type occupying a special niche in the colony. The most important are the photosynthetic
cyanobacteria (blue green algae), common pond scum.
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- These amazing life forms are highly adaptable and form the base of the first
food chain. Oh yes, they also are responsible for all the oxygen in the air. O2 is a waste
product of their photosynthesis.
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-Plants later likely simply incorporated a version of cyanobateria to carry out their
photosynthesis. Nature rarely reinvents a wheel
Mineralized Stromatolites from Ontario
present day stromatolites forming in Western Australia
Banded Iron Formation (B.I.F.), mainly 2 - 2.5 b.y. old, often found in greenstone belts
alternating bands of silica (e.g., chert) and Fe-oxide (hematite)
cyanobacteria fossils from the 3.5 B.Y.
Red Bed deposits by ~ 2 b.y. ago; continue to form today red beds = reddish-coloured
sedimentary rocks that formed in oxygen rich air.
Red Beds
A Differentiated Earth
The Internal structure Of the Earth
The Earth is divided into three chemical layers: the Core [Inner Core and Outer Core],
the Mantle [Upper mantle and Lower Mantle] and the Crust [Continental and Oceanic].
The Earth's outer layer, the crust, is 4 to 50 kilometers thick. The crust is rich in the
elements oxygen and silica with lesser amounts of aluminum, iron, calcium, potassium,
sodium, and magnesium. On the Earth, one finds two types of crust. Oceanic Crust is
made of relatively dense rock, called basalt. Continental Crust is made of lower density
rocks, such as granite.
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The mantle is approximately 2885 kilometers thick and is the middle layer of the
Earth. The Mantle is made up of minerals rich in the elements iron, magnesium, silica,
and oxygen. The Mantle makes up approximately ~70% of the Earth's volume. It is
separated from the crust by a sharp change to higher density and seismic velocity, and
more iron and magnesium rich composition.
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The core is predominantly composed of iron and nickel. Even after 4.6 billion
years of cooling, the Earth's core remains very hot. The Earth's core is divided into two
layers, a solid inner core, and a liquid outer core. The Inner Core is solid nickel-iron
alloy. The Outer Core, is a liquid, molten iron-nickel alloy.
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All of the above is known from the way seismic (earthquake) waves pass through
the Earth as we will discuss later in the course. The Earth has a radius of about 6370
km.