The Origin and Structure of Earth’s Atmosphere Notes

The Origin and Structure of Earth's Atmosphere

Earth's Lost Atmosphere

1. During its early formation, Earth was surrounded by a cloud of silicate vapor.
2. As the planet cooled, this vapor condensed into molten and solid rock.
3. Any remaining gases from the solar nebula, such as hydrogen, were swept away by the solar wind, leaving Earth without an atmosphere.

Formation of Earth's Present Atmosphere

Outgassing

1. Outgassing refers to the release of gases trapped within Earth's interior.
2. As Earth cooled and formed a solid crust, volcanic eruptions continued to release gases such as:

Water vapor (\$\text{H}_2\text{O}\$)

Carbon dioxide (\$\text{CO}_2\$)

Hydrogen sulfide (\$\text{H}_2\text{S}\$)

Nitrogen (\$\text{N}_2\$)

Cometary Impacts

1. Comets and other icy bodies from the outer solar system also contributed to Earth's atmosphere.
2. These impacts released volatile materials such as:

Water

Carbon dioxide

Methane (\$\text{CH}_4\$)

Ammonia (\$\text{NH}_3\$)

Development of Earth's Atmosphere

A Reducing Atmosphere

Earth's early atmosphere was reducing, meaning it lacked oxygen.

Key processes included:

1. Dissolution of carbon dioxide: \$\text{CO}_2\$ dissolved in oceans and formed limestone.
2. Photochemical reactions: Ultraviolet radiation split water molecules into hydrogen and oxygen.
3. Oxygen removal: Oxygen reacted with minerals and volcanic gases, preventing accumulation.

An Oxidizing Atmosphere

The evolution of photosynthetic organisms (e.g., cyanobacteria) marked a turning point.

Photosynthesis released oxygen as a byproduct:

1. \$\$6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C} 6\text{H}{12}\text{O}_6 + 6\text{O}_2\$\$
2. Oxygen levels increased, forming an oxidizing atmosphere.

An Aerobic Atmosphere

Over time, oxygen levels rose to support aerobic organisms.

A balance emerged between:

1. Photosynthesis: Producing oxygen.
2. Respiration: Consuming oxygen.

The Structure of Earth's Atmosphere

The Composition of Air

Dry air consists mainly of:

1. Nitrogen (\$\text{N}_2\$): 78%
2. Oxygen (\$\text{O}_2\$): 21%
3. Argon (\$\text{Ar}\$): ~1%
4. Trace gases: Carbon dioxide, methane, neon, helium, and others.

Layers of the Atmosphere

The atmosphere is divided into four main layers based on temperature changes:

1. Troposphere

1. Altitude: 0–11 km
2. Temperature: Decreases with altitude.
3. Features: Contains most of the atmosphere's mass and all weather phenomena.

2. Stratosphere

1. Altitude: 11–50 km
2. Temperature: Increases with altitude due to ozone absorption of UV radiation.
3. Features: Contains the ozone layer, which protects life from UV rays.

3. Mesosphere

1. Altitude: 50–85 km
2. Temperature: Decreases with altitude.
3. Features: Coldest layer; meteors burn up here.

4. Thermosphere

1. Altitude: 85–600 km
2. Temperature: Increases with altitude due to absorption of solar radiation.
3. Features: Contains the ionosphere, which reflects radio waves and enables communication.

1. Tropopause: Between the troposphere and stratosphere.
2. Stratopause: Between the stratosphere and mesosphere.
3. Mesopause: Between the mesosphere and thermosphere.