Earth and Space: Unit Three
Earth and Space: Unit Three
(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
(b) evaluate the role of volcanic outgassing and
impact of water-bearing comets in developing Earth's
atmosphere and hydrosphere;
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.
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.
The geosphere includes the solid Earth portion of the Earth Systems. Rocks
The atmosphere is seen here as the blue haze above the
and soil (regolith) at the surface, and all the deep interior portions of the
Earth. It differs from the Lithosphere, which only includes the planet’s crust.
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, but until differentiation
occurred, our atmosphere was likely negligable.
Second Atmosphere Origin and Composition:
Produced by volcanic out gassing. Gases
produced were probably similar to those created by
modern volcanoes Uniformitarianism (James Hutton)
• (H2O, CO2, SO2, CO, S2, Cl2, N2, H2, NH3 (ammonia) and CH4 (methane)
• No free O2 at this time (not found in volcanic gases).
1. How does the Earth’s atmosphere protect and nurture
2. The hydrosphere includes what areas on Earth?
3. How do the geosphere and the lithosphere differ?
4. What are the main gases in Earth’s atmosphere, and their
respective proportions today?
5. Describe the hypothesized first atmospheric gases on
6. Why was the Earth unable to hold onto the first
7. What was the Earth’s second atmosphere probably like,
and what was its origin?
Ocean Formation - As the Earth cooled, H2O
produced by out gassing was allowed to condense,
and exist as liquid in the Early Archean (4-3.8 bya),
allowing oceans to form.
Evidence - pillow basalts, deep marine beds in greenstone
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
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
to one school
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 (a long-period comet)
contained substantial amounts of heavy
While studies suggest that
water, rich in the hydrogen isotope
most of Earth's water probably
did not have a cometary origin,
there is contradictory data as
If the type of comets bombarding the
well. It is hotly debated to
Earth were like Hale-Bopp, it suggests
that Earth's ocean water should be rich this day!
in deuterium, which in fact it is not.
8. What is one scientific explanation for the
origin of our oceans, and what is the
9. Why couldn’t there have been oceans
during Earth’s earliest years?
10.What evidence is there that comets were
NOT the big contributors to the Earth’s
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:
Photochemical dissociation - breakup of water molecules by
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.
• Chemical Weathering - through oxidation of surface materials
(early consumer) Produces iron oxide
• Animal and Plant 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
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, and tend not to form now.
Banded Iron Formation (BIF) - Deep water deposits in which
layers of iron-rich minerals alternate with iron-poor layers. 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.
The primordial atmosphere had 1,000
times more CO2 than it does now.
Where did it all go?
• H2O condensed to form the
• CO2 dissolved into the oceans and
precipitated out as carbonates
Most of the present-day
CO2 (the largest carbon
sink) is locked up in crustal
rocks and dissolved in the
By contrast, N2 is chemically
inactive, and stayed a gas in
the atmosphere and become
its dominant constituent.
11. Describe two processes that produce oxygen on
12. What processes consume oxygen on our planet?
13. What evidence is there that the amount of O2
in Earth’s atmosphere has increased with time?
14. If Earth’s CO2 was 1,000 times higher in the
primordial atmosphere, where did it all go?
15. Why hasn’t N2 (atmospheric nitrogen) changed
like CO2 levels have?