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(6) Earth in space and time. The student knows the
evidence for how Earth's atmospheres,
hydrosphere, and geosphere formed and changed
through time. The student is expected to:
(a) analyze the changes of Earth's atmosphere that
could have occurred through time from the original
hydrogen-helium atmosphere, the carbon dioxidewater vapor-methane atmosphere, and the current
nitrogen-oxygen atmosphere;
(b) evaluate the role of volcanic outgassing and
impact of water-bearing comets in developing Earth's
atmosphere and hydrosphere;
Atmosphere
The atmosphere is a layer of gases surrounding the planet that is held close to
us by our gravitational field. It protects organisms by absorbing the relatively
dangerous part of the EM spectrum known as ultraviolet radiation. The
atmosphere helps keep the Earth’s surface warm through retention of thermal
energy, which helps reduce temperature extremes between day and night.
Hydrosphere
The hydrosphere describes the combined mass of water found on, under, and
over the surface of a planet. It includes liquid water in the oceans, rivers,
lakes, clouds, soil, and groundwater; solid water in snow and ice found in cold
regions and in ice caps; gaseous water found in the atmosphere.
Geosphere
The geosphere includes the solid Earth portion of the Earth Systems. Rocks
and soil (regolith) at the surface,
and all the
deep
interior
portions
of the
The atmosphere
is seen
here
as the blue
haze above
the planet
Earth. It differs from the Lithosphere, which only includes the planet’s crust
and upper mantle.
Chemical Composition Today:
• Nitrogen (N2)- 78%,
• Oxygen (O2)- 21%,
• Trace Gases-Argon, CO2, H2O and others…
First Atmosphere’s composition:
- Probably H2, He
These gases are relatively rare on Earth compared to other places in
the universe and were probably lost to space early in Earth's history
because
• Earth's gravity is not strong enough to hold lighter gases
• Earth still did not have a differentiated core (solid inner/liquid outer core) which creates Earth's
magnetic field (magnetosphere = Van Allen Belt) which deflects solar winds.
Once the core differentiated the lighter gases could be retained
Second Atmosphere Origin and Composition:
Produced by volcanic out gassing. Gases
produced were probably similar to those created by
modern volcanoes Uniformitarianism
• (H2O, CO2, SO2, CO, S2, Cl2, N2, H2, NH3 (ammonia) and CH4 (methane)
• No free O2 at this time (not found in volcanic gases).
How are the geosphere and the lithosphere different?
Why do scientists think the first atmospheric gases were H2 and He?
How would Earth’s first atmosphere get “lost” in space?
How was Earth’s 2nd atmosphere likely produced?
What gas was likely not present in Earth’s 2nd atmosphere?
Ocean Formation - As the Earth cooled, H2O produced by out
gassing could exist as liquid in the Early Archean, allowing oceans to
form.
Evidence - pillow basalts, deep marine beds in greenstone belts.
The term Archean means “ancient” and was
originally used to refer to the oldest known rocks.
Rocks of Archean age contain the earliest fossils of
life on Earth. Because these rocks were formed
very long ago—between 2.5 to 4.0 billion years
ago—most have long since been covered by
younger sediments, eroded, or subducted into the
Earth's mantle. Nevertheless, some Archean strata
survived in the central parts of continents. These
Archean “shields” lie in the heart of Canada,
Australia, Africa, India, and Siberia.
The Earth in its earliest years was a horribly hot and violent place. Asteroids,
comets, and other chunks of space debris left over from the solar system's
formation continually bombarded the young planet, releasing huge amounts of
heat.
The decay of radioactive elements
inside the Earth also generated
great quantities of heat. At the
same time, frequent volcanic
eruptions may have covered much
of the planet's surface in red-hot
flows of lava. The early Earth's
surface was hot enough to turn any
liquid water instantly into steam.
Nonetheless, the planet eventually
cooled enough and obtained
enough water to fill a vast ocean.
Some of the water in the Earth's oceans came from
condensation following the outgassing of water vapor from
volcanoes on the surface of the planet, while some was
delivered by impacting comets. An important question in
recent years has been the relative importance of these two
sources.
According to one school of thought,
comets may have supplied the bulk of
oceanic water during the heavy
bombardment phase of the solar system,
between about 4.5 and 3.8 billion years ago.
If this is true, it increases the
chances that the delivery of
organic matter, (also found in
comets) played an important part
in the origin of life on Earth.
However, cosmochemists found that comet
Hale-Bopp contains substantial amounts of
heavy water, which is rich in the hydrogen
isotope deuterium.
If Hale-Bopp is typical in this respect and if
cometary collisions were a major source of
terrestrial oceans, it suggests that Earth's ocean
water should be similarly rich in deuterium,
whereas in fact it is not.
Shoemaker-Levy
While studies suggest that most
of Earth's water probably did
not have a cometary origin,
there is contradictory data as
well. It is hotly debated to this
day!
1. What evidence is there that there was water on
the Earth during the early Archaen?
2. What evidence is there that water was delivered
by comets?
3. What evidence is there that water was formed
from volcanic outgassing?
4. Which theory has more evidence today?
Today, the atmosphere is 21% free oxygen. How did
oxygen reach these levels in the atmosphere? Let’s
look at processes that contribute to the cycling of
O2 on our planet:
Oxygen Producers:
•
Photochemical dissociation - breakup of water molecules by
ultraviolet radiation
Produced O2 levels approx. 1-2% current levels
At these levels O3 (Ozone) can form to shield Earth surface from UV
• Photosynthesis - CO2 + H2O Sunlight C6H12O6+ O2
produced by cyanobacteria, and eventually higher plants –
probably supplied the rest of O2 to atmosphere.
Oxygen Consumers
• Chemical Weathering - through oxidation of surface materials
(early consumer)
• Animal Respiration (much later)
• Burning of Fossil Fuels (much, much later)
Today, the atmosphere is 21% free oxygen. How did
oxygen reach these levels in the atmosphere? Let’s
look at processes that contribute to the cycling of
O2 on our planet:
Oxygen Producers:
•
Photochemical dissociation - breakup of water molecules by
ultraviolet radiation
Produced O2 levels approx. 1-2% current levels
At these levels O3 (Ozone) can form to shield Earth surface from UV
• Photosynthesis - CO2 + H2O Sunlight C6H12O6+ O2
produced by cyanobacteria, and eventually higher plants –
probably supplied the rest of O2 to atmosphere.
Oxygen Consumers
• Chemical Weathering - through oxidation of surface materials
(early consumer)
• Animal Respiration (much later)
• Burning of Fossil Fuels (much, much later)
Evidence from the Rock Record includes
Iron (Fe), which is extremely reactive with oxygen. If we look at the
oxidation state of Fe in the rock record, we can infer a great deal about
atmospheric evolution.
Archean – minerals that only form in non-oxidizing environments in these sediments: Pyrite
(Fools gold; FeS2), Uraninite (UO2). These minerals are easily dissolved out of rocks under
present atmospheric conditions.
Banded Iron Formation (BIF) - Deep water deposits in which layers of iron-rich minerals
alternate with iron-poor layers, primarily chert. These are common in rocks 2.0 - 2.8 B.y. old,
but do not form today.
Red beds are never found in rocks older than 2.3 B. y., but are common during later times. Red
beds are red because of the highly oxidized mineral hematite (Fe2O3)
Conclusion – the amount of O2 in the atmosphere has increased with
time.
no oxygen, iron is dark
Lots of oxygen, iron is red
Banded Iron
formations
The primordial atmosphere had 1,000 times more CO2
than it does now. Where did it all go?
• H2O condensed to form the oceans.
• CO2 dissolved into the oceans and precipitated out
as carbonates (e.g., limestone).
Most of the present-day CO2 (the largest carbon
sink) is locked up in crustal rocks and dissolved in
the oceans.
By contrast, N2 is chemically inactive, and stayed a
gas in the atmosphere and become its dominant
constituent.
The primordial atmosphere had 1,000
times more CO2 than it does now.
Where did it all go?
• H2O condensed to form the
oceans.
• CO2 dissolved into the oceans and
precipitated out as carbonates
(e.g., limestone).
Most of the present-day
CO2 (the largest carbon
sink) is locked up in crustal
rocks and dissolved in the
oceans.
By contrast, N2 is chemically
inactive, and stayed a gas in
the atmosphere and become
its dominant constituent.
1. How are BIFs and Red beds evidence of Earth’s
oxygen atmosphere?
2. What are two oxygen producers?
3. What are two oxygen consumers?
4. Why is ozone important for the origin of life?
5. Where is the Earth’s largest carbon sink today?
6. Why is N2 the largest part of Earth’s atmosphere
today?
Just a reminder…
The geosphere
refers to
everything from
the core of the
Earth to its
surface.
The biosphere is the layer of life
on Earth. It exists beneath,
upon, and above the surface in
the atmosphere as well
Axolotl
Soil Nematodes
Airborne Bacteria
Harvestman
One thing most geologists
agree on is that the Earth’s
first atmosphere contained
no free oxygen. There were
trace amounts of Oxygen
bound in water molecules,
and Carbon dioxide…but
none of it was “free”, or
molecular oxygen. (O2)
Photochemical Dissociation Hypothesis states that the sun’s energy helped the
atmosphere evolve through the following processes:
 The ultraviolet light combined with the water vapor to set the hydrogen off
into space and free the oxygen.
2H2O + UV light energy ----> 2H2 (freed into space) + O2
The newly freed oxygen reacted with methane, forming carbon dioxide and
additional water vapor.
CH4 + 2O2 ----> CO2 + 2H2O
The oxygen also reacted with ammonia, producing nitrogen and water.
4NH3 + 3O2 ----> 2N2 + 6H2O
After converting the ammonia and methane to carbon dioxide and nitrogen,
free oxygen began to accumulate as further dissociation of water vapor
continued.
3. What is meant by “free oxygen”?
4. Besides water, what other molecules play a
huge role in photochemical dissociation?
5. What happens when freed oxygen combines
with methane?
6. What happens when freed oxygen combines
with ammonia?
This theory states that our atmosphere was
delivered to us from the Earth’s interior through
volcanic eruptions. In contrast to the
Photochemical Dissociation Hypothesis, the
Outgassing Hypothesis argues that the free
oxygen came from the photosynthesis of primitive
organisms which existed 1.5 - 3.5 billion years ago.
The oxygen took approximately 2 billion years to
become free, but when it did, it formed the ozone
layer, eliminating the dangerous radiation and
setting up the foundation for a habitable planet.
This theory states that our atmosphere was
delivered to us from the Earth’s interior through
volcanic eruptions. In contrast to the
Photochemical Dissociation Hypothesis, the
Outgassing Hypothesis argues that the free
oxygen came from the photosynthesis of primitive
organisms which existed 1.5 - 3.5 billion years ago.
The oxygen took approximately 2 billion years to
become free, but when it did, it formed the ozone
layer, eliminating the dangerous radiation and
setting up the foundation for a habitable planet.
It is obvious that Earth
contains O2 now, and without
it, aerobic life would not be
possible.
What life could have evolved
all those billions of years ago,
before there was significant
O2 in our atmosphere?
Anaerobic life forms
http://www.teachersdomain.
org/resource/tdc02.sci.life.ce
ll.stetteroxygen/
Although the early Earth was mostly
devoid of molecular oxygen, high
volcanic activity released significant
amounts of molecular hydrogen.
With little oxygen available to
convert that hydrogen into water,
hydrogen gas probably accumulated
in the atmosphere and oceans in
concentrations as high as hundreds
to thousands of parts per million.
Thus, the early Earth was likely a
paradise for methanogens
(methane-producing bacteria) that
fed directly on hydrogen and carbon
dioxide, at least until the
atmospheric hydrogen was depleted.
Many anaerobic microbes including methanogens are easily
poisoned by oxygen, and the recent discovery of banded
sediments with rusted iron suggests that oxygenproducing, photosynthetic microbes called cyanobacteria
were able to gather sunlight for photosynthesis. These
BIFs would not have formed without O2 present in the
atmosphere.
The evolution of O2 in our
atmosphere spelled doom
for the proliferate
methanogens, and other
types of extremophiles that
had evolved during this early
period in Earth’s past.
Despite their small stature, one of the
first aerobic organisms (require the
presence of O2) set in motion a process
that would change everything.
These cyanobacteria which evolved 3.5-1.5
billion years ago (also known as blue-green
algae), were remarkably self-sufficient
creatures that could use the sun’s energy
to make their own food, and fix nitrogen,
a process where nitrogen gas is converted
into ammonia or nitrate. (NH3; NO3)
While this may not seem significant, the cycling of nitrogen on Earth is
essential for life. It is found in amino acids, proteins, and genetic material.
Nitrogen is the most abundant element in the atmosphere (~78%).
However, gaseous nitrogen must be 'fixed' into another form so that it can
be used by living organisms.
7. What primitive organism uses photosynthesis to
combine CO2 and water in the presence of sunlight
to make sugar and O2?
8. What are methanogens, and why was early Earth a
paradise for them?
9. Where did these methanogens retreat to, when
oxygen started evolving in our atmosphere?
10.Where did the molecular hydrogen of the Earth’s
first atmosphere likely go?
11.How do are banded iron formations created…and
why can’t they form anymore?
And then...nothing else happened. At least,
not for another two billion years.
It wouldn't be until about 600 million years
ago, that the first multicellular organisms
finally emerged.
So what happened during that immense,
multi-billion year gap? Why did it take so
long for more complex life to arrive on the
scene?
For that matter, why did oxygen suddenly spike 2.5 billion years ago?
The simple, uncomfortable answer is that
we don't really know.
???
We already know that over time, the Earth’s
crust cooled. The crust is thin, relatively,
varying from a few tens of kilometers thick
beneath the continents to less than 10 km thick
beneath the oceans.
The crust and upper mantle together constitute
the lithosphere, which is typically 50-100 km
thick and is broken into large plates. These
plates sit on the asthenosphere.
The asthenosphere is kept plastic largely
through heat generated by radioactive decay.
This heat source is relatively small, but
nevertheless, because of the insulating
properties of the Earth's rocks at the surface,
this is sufficient to keep the asthenosphere
plastic in consistency.
13. Why was there a huge 2 billion year gap between
the first origins of life and oxygen in the atmosphere, and
the appearance of more complex life forms?
14. What is the lithosphere composed of, and what does
it sit on top of?
15. What keeps the asthenosphere plastic?
Energy can be transferred in three ways…
 Radiation
Energy transfer across the vacuum of space
 Conduction
Energy transfer directly from molecule to
molecule (solids)
 Convection
Energy transfer through fluids (liquids and
gases)
Very slow convection currents flow in the asthenosphere,
(upper portion of the mantle) and these currents provide
horizontal forces on the plates of the lithosphere much as
convection in a pan of boiling water causes a piece of cork
on the surface of the water to be pushed sideways
http://www.youtube.
com/watch?v=p0d
WF_3PYh4
16. Give an example of how Earth experiences the
transferal of thermal energy in the form of radiation.
17. Give an example of conduction of thermal energy.
18.Give an example of convection of thermal energy.
19. In what media does each type of thermal energy
transfer?
Of course, the timescale for convection in
the pan is seconds and for plate tectonics is Differentiation within
10-100 million years, but the principles are the Earth is crucial to
similar.
plate tectonics,
because it is
responsible for
producing an interior
that can support
tectonic motion.
The heat generated by the lower mantle, drives the convection currents upward against the
lithospheric plates. As the currents cool, they move laterally, pushing and pulling the
lithosphere apart. Then, the currents move downward again, where they begin to heat up once
more due to proximity to lower mantle heat.
• While seemingly static, the geosphere is in fact a very
active player in the Earth systems, affecting the
atmosphere and the hydrosphere, as well as critical
processes such as the hydrologic cycle and other
biogeochemical cycles.
 The types of minerals contained in soils--a factor of
geologic processes--help to determine the vegetative
cover and ecosystems at the surface.
 Carbon – an essential element of life – is bound in organic
matter and is carried to the ocean via wind and water
erosion where eventually it becomes part of the ocean
floor.
20. How does the geosphere influence the biosphere?
21. How does the geosphere influence the hydrosphere?
22. Why has the differentiation of the Earth been so
important a factor in tectonic movement?
Tectonic movement carries the ocean deposits into the
Earth's interior. On geologic timescales, volcanic activity can
vent the carbon to the Earth's atmosphere as carbon dioxide.
The carbon cycle is one of the
key cycles linking the Earth’s
subsystems: geosphere,
atmosphere, hydrosphere, and
biosphere.
The outer core of the Earth contains liquid iron. Its motion is
thought to drive the Earth's magnetic field – the
magnetosphere - which extends far beyond the atmosphere
protecting Earth and its biosphere from solar wind and cosmic
radiation.
Being dynamic, the Earth is still changing. 150 million years in the
future, the continents should look something like this.
In 250 million years, we will have another Pangea supercontinent.