1. Primordial Atmosphere
Our Atmosphere today is a secondary atmosphere.
When the Earth and other planets form they would have been surrounded by a primordial atmosphere (mainly H2, He) the abundant gassesin a solar nebula.
The primordial atmospheres of the inner planets were probably wiped out completely during a stage in sun evolution that caused substantial mass from its surface to be ejected in form of violent solar winds. These winds were effective in eroding the primordial atmospheres of the terrestrial planets.
This atmosphere erosion may have been enhanced by the lack of a strong magnetic field in the early Earth.
Also, Earth’s gravity (compared to Jovian planets) is not strong enough to prevent escape of H2 & He.

2. 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 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.
OUTGASSING

3. Produced by volcanic outgassing
Secondary Atmosphere
Produced by volcanic outgassing
Gases produced were probably similar to those created by modern volcanoes
(H2O, CO2, SO2, CO, N2, H2) and NH3 (ammonia) and CH4 (methane).
No free O2 at this time (not found in volcanic gases).
Chemical Composition Today:
• Nitrogen (N2)- 78%,
• Oxygen (O2)- 21%,
• Trace Gases-Argon, CO2, H2O and others…

4. The Earth in its earliest years was a horribly hot and violent place
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.

5. Ocean Formation
As the Earth cooled, H2O produced by out-gassing could exist as liquid in the Early Archean, allowing oceans to form.
Comet and meteorite delivery are potential donors of water to Earth’s oceans as well.

6. Ocean Formation
As the Earth cooled, H2O produced by out-gassing could exist as liquid in the Early Archean, allowing oceans to form.
Comet and meteorite delivery are potential donors of water to Earth’s oceans as well.

7. Today, the atmosphere is 21% free oxygen
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

8. Addition of O2 to Atmosphere
Photosynthesis - CO2 + H2O C6H12O6+ O2
produced by cyanobacteria, and eventually higher plants – probably supplied the rest of O2 to atmosphere.

9. Removal of CO2 from Atmosphere
Once the water vapor in the atmosphere condensed to form an ocean, it became a “sink” for dissolved CO2.
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.

10. Atmosphere: The gaseous area surrounding the planet
Divided into several concentric spherical strata separated by narrow transition zones. It’s in layers like clothing for the earth.
The upper boundary at which gases disperse into space lies at an altitude of approximately 100 km above sea level.
More than 99% of the total atmospheric mass is concentrated in the first 40 km from Earth's surface.
Atmospheric layers are characterized by differences in chemical composition that produce variations in temperature

11. Troposphere The troposphere starts at the Earth's surface and extends 8 to 14.5 kilometers high (5 to 9 miles). This part of the atmosphere is the most dense. Almost all weather is in this region. Stratosphere The stratosphere starts just above the troposphere and extends to 50 kilometers (31 miles) high. The ozone layer, which absorbs and scatters the solar ultraviolet radiation, is in this layer. Mesosphere The mesosphere starts just above the stratosphere and extends to 85 kilometers (53 miles) high. Meteors burn up in this layer Thermosphere The thermosphere starts just above the mesosphere and extends to 600 kilometers (372 miles) high. Aurora and satellites occur in this layer. Ionosphere The ionosphere is an abundant layer of electrons and ionized atoms and molecules that stretches from about 48 kilometers (30 miles) above the surface to the edge of space at about 965 km (600 mi), overlapping into the mesosphere and thermosphere. This dynamic region grows and shrinks based on solar conditions and divides further into the sub-regions: D, E and F; based on what wavelength of solar radiation is absorbed. The ionosphere is a critical link in the chain of Sun-Earth interactions. This region is what makes radio communications possible. Exosphere This is the upper limit of our atmosphere. It extends from the top of the thermosphere up to 10,000 km (6,200 mi). Credit: NASA/Goddard

12. Formation of Earth’s Early Spheres
Earth’s Atmosphere
Formation of Earth’s
Early Spheres