Earth's Atmosphere: Formation and Evolution

Earth's Unique Atmosphere

• Earth's atmosphere is primarily composed of 78% nitrogen and 21% oxygen, a unique composition supporting life.
• Unlike Venus (primarily CO₂) and Mars (primarily CO₂), Earth's atmospheric balance enables biological processes and moderates climate.

Formation of Earth's Atmosphere

• Two main theories explain Earth's atmospheric origins:
  • Volcanic Outgassing: Volcanic activity released gases (CO₂, methane, water vapor, nitrogen) from magma. Over time, CO₂ was absorbed, leaving nitrogen dominant.
  • Comet Impact: Comets, containing icy gases, may have contributed to the atmosphere. However, this theory is less conclusive due to comet unpredictability.

Early Atmosphere and Oxygenation

• Early Earth's atmosphere was anoxic, primarily CO₂, nitrogen, and methane.
• Photosynthesis by cyanobacteria (blue-green algae) introduced oxygen.
• The Great Oxidation Event (~2.3 billion years ago) led to a dramatic increase in oxygen levels.
• This caused a decrease in CO₂ and a subsequent ice age.

Tectonic and Biological Evolution

• The Archean Era saw the formation of landmasses and tectonic activity.
• Prokaryotes (anaerobic bacteria) were early life forms thriving without oxygen.
• Eukaryotic organisms developed after the GOE, leading to plants and animals.
• The ozone layer's development (~800-542 million years ago) enabled life transitions to land by shielding from harmful UV radiation.

Mars and Venus Atmospheres

• Mars lost its atmosphere due to cooling core and lack of a magnetic field, allowing solar wind to strip it.
• Venus has a thick, CO₂-rich atmosphere, causing extreme greenhouse effects (surface temperatures around 400°C).

Atmospheric Composition and Pressure

• Earth's atmosphere has layers:
  • Troposphere: Weather patterns; temperature decreases with altitude.
  • Stratosphere: Ozone layer; temperature increases with altitude (UV absorption).
  • Mesosphere: Meteors burn up; temperature decreases.
  • Thermosphere: Absorbs solar radiation; temperature increases.
• Atmospheric pressure and density decrease with altitude.

Weather Phenomena and Heat Transfer

• Weather results from uneven heating, creating pressure differences and driving wind.
• Heat transfer mechanisms are:
  • Conduction: Heat transfer between objects in contact.
  • Convection: Heat movement through fluid motion.
  • Radiation: Energy transfer through electromagnetic waves.
• Solar radiation is mostly absorbed by land/oceans (50%).

Humidity and Precipitation

• Humidity (water vapor) is key for cloud formation and precipitation.
• Condensation occurs as warm air rises and cools to the dew point.
• Collision-coalescence process: Water droplets collide and grow to form rain.
• Bergeron process: Ice crystals grow and melt to form precipitation (rain, hail, sleet).

Climate Change and Current Concerns

• Human activities (burning fossil fuels) increase CO₂ levels, intensifying the greenhouse effect and causing global warming.
• Melting ice caps and changing weather patterns are consequences.
• The Montreal Protocol (1987) highlights the importance of global cooperation in addressing atmospheric issues.