1. The Earth’s Atmosphere
Evolution and Composition

2. Thickness of the Atmosphere
Earth’s atmosphere is a relatively thin envelope of gases and tiny, suspended particles that encircles the globe.
Compared to the planet’s diameter, the atmosphere is like the thin skin of an apple.

3. The atmosphere is essential for life on Earth.
The atmosphere shields organisms from exposure to hazardous levels of solar ultraviolet radiation.
It has the gases necessary for life-sustaining processes like photosynthesis and cellular respiration.
It supplies the water required by all forms of life.

4. 4.5 billion years ago
The solar system evolved from an immense rotating cloud of dust, ice and gasses, called a nebula, more than 4.5 billion years ago.
Earth’s atmosphere was probably mostly hydrogen, helium, methane, and ammonia.
Since these atoms and molecules are light, they probably escaped to space.

5. 4.4 billion years ago
Outgassing, the release of gases from rock through volcanic eruptions and the impact of meteorites, is probably the principal source of the Earth’s early atmosphere.
The primeval atmosphere was mostly carbon dioxide, with some nitrogen, and water vapor.
Argon was added from radioactive decay.
A small amount of oxygen was added by separating water vapor molecules.

6. 4 billion years ago
The planet started to cool and the atmosphere changed.
Cooling caused water vapor to condense into clouds and produce rain.
Precipitation and runoff gave rise to the ocean that eventually may have covered as much as 95% of the planet’s surface.
Some carbon dioxide dissolved in ocean water.

7. 2.5 billion years ago
Photosynthesis, the process whereby green plants use sunlight, water, and carbon dioxide to manufacture food, played an important role in the evolving atmosphere.
A byproduct of photosynthesis is oxygen.

8. 438 million years ago
From the oxygen in the atmosphere, the ozone (O3) shield formed.
The ozone shield protects the earth’s surface from harmful UV rays and makes it possible for organisms to live and evolve on land.

9. Nitrogen – most abundant
Nitrogen, a product of outgassing, became the most abundant atmospheric gas because it does not easily chemically combine and it does not easily escape from the atmosphere.

10. The modern atmosphere 78% Nitrogen 21% Oxygen 0.9% Argon
0.04% Carbon dioxide
Also:
Neon
Helium
Methane
and others

11. The bottom line…
The earth’s atmosphere was not always what it is today.
Processes of physics and chemistry made the atmosphere what it is today.
The atmosphere is still evolving.