Earth's Atmospheric and Hydrospheric Evolution

Fluid Layers of the Earth

The fluid layers of the Earth consist of the atmosphere and the hydrosphere.

Origin of the Fluid Layers

Approximately 4,570 million years ago, following the planet's formation, a proto-atmosphere emerged, composed of helium and hydrogen. Due to their low molecular weight, Earth's gravitational pull could not retain these gases. A second atmosphere subsequently formed through volcanic degassing, consisting of water vapor, CO2, nitrogen, and sulfur compounds. It is widely believed that most of the planet's water originated from comets. As the Earth cooled, the first oceans formed approximately 3,800 million years ago, with salinity resulting from volcanic emanations and rock erosion.

Evolution of the Atmosphere

Around 3,500 million years ago, cyanobacteria began producing abundant oxygen (O2) via photosynthesis, a process evidenced by fossilized stromatolites. While the exact composition of the early atmosphere remains uncertain, it differed significantly from today's. Following the condensation of the hydrosphere, molecular oxygen began to accumulate. Unlike other planets, Earth's high oxygen abundance is a direct result of biological photosynthesis, indicating that life originated very early in Earth's history.

Composition of the Atmosphere

The atmosphere is divided into distinct layers:

• Homosphere: Extending up to 80 km, this layer contains 99% of atmospheric gases with a homogeneous composition. Air consists of 78% nitrogen (N2), 20.95% oxygen (O2), 0.93% argon, and trace amounts of water vapor, ozone, and CO2. It also contains suspended particles known as aerosols, such as dust, ice crystals, and carbonic acid.
• Heterosphere: Located above the homosphere and extending to the upper limit of the atmosphere (approx. 10,000 km). This region is divided into several layers based on the molecular mass of constituent gases, including molecular hydrogen, oxygen, helium, and hydrogen atoms.
• Atmosphere: The gaseous layer surrounding Earth, held by gravitational attraction. Density and pressure decrease with altitude. While the functional atmosphere is considered to be within the first 100 km, the gaseous envelope extends to approximately 10,000 km.

Atmospheric Pressure and Barometry

Due to the weight of the air column, atmospheric pressure decreases exponentially with altitude because gases are highly compressible. These pressure gradients drive air movement. Common pressure units include:

• 1 mb = 100 Pascals
• 1 atm = 1013.2 mb = 76 mmHg