• Other planets and moons in our solar system have atmospheres, but none of them could support life as we know it.
• They are either too dense (as on Venus) or not dense enough (as on Mars), and none of them have much oxygen.
• The Earth’s atmosphere is special and able to support life as we know it.
• Some scientists describe three stages in the evolution of Earth’s atmosphere as it is today.
• The three stages are not clearly separated but are used to help explain the changes in the atmosphere over time.
• The three stages are:
• The first stage involved outgassing from volcanoes and the release of gases from inside the Earth.
• The second stage was the rise of photosynthetic organisms which produced oxygen as a byproduct.
• The third stage was the development of the ozone layer which allowed life to move onto land and protected organisms from harmful solar radiation.

Origin of Atmosphere

• Earth formed approximately 5 billion years ago
• A dense atmosphere emerged in the first 500 million years from expelled vapor and gases
• Cooling of the earth led to degassing, releasing hydrogen (H2), water vapor, methane (CH4), and carbon oxides
• Prior to 3.5 billion years ago, the atmosphere likely consisted of CO2, CO, H2O, N2, and H2
• The hydrosphere formed 4 billion years ago, resulting in oceans of water in which sedimentation occurred
• The ancient environment lacked free oxygen, evidenced by early rock formations containing many reduced elements
• Blue-green algae began photosynthesis 1 billion years ago, leading to the accumulation of oxygen in the atmosphere and a decrease in CO2
• Ozone molecules formed from oxygen atoms, shielding Earth from biologically lethal UV radiation
• The presence of ozone enabled organisms to develop and live on land, playing a significant role in the evolution of life on Earth.

3 Phases of Atmosphere Formation

• Just Formed Earth:
  • Just formed Earth had a very warm atmosphere of hydrogen (H2) and helium (He).
  • These gases moved very fast and eventually escaped Earth’s gravity.
  • The original atmosphere of Earth was likely composed of hydrogen and helium because they were the main gases in the dusty, gassy disk around the Sun from which the planets formed.
  • The Earth and its atmosphere were very hot during this time.
  • 
• Young Earth:
  • Volcanoes released gases including H2O (water) as steam, CO2 (carbon dioxide), and NH3 (ammonia).
  • CO2 dissolved in seawater, and simple bacteria thrived on sunlight and CO2.
  • The by-product of the bacteria’s metabolic process was oxygen (O2).
  • Earth’s “second atmosphere” came from the gases released by volcanoes.
  • There were many more volcanoes on early Earth because the planet’s crust was still forming.
  • 
• Current Earth:
  • Plants take in carbon dioxide (CO2) and give off oxygen (O2), while animals take in oxygen (O2) and give off CO2
  • Burning stuff also releases CO2
  • CO2 dissolves into oceans
  • Simple bacteria evolved that could use energy from the sun and carbon dioxide in water to produce oxygen as waste
  • Oxygen started to build up in the atmosphere, while carbon dioxide levels decreased
  • Ammonia molecules in the atmosphere were broken apart by sunlight, leaving nitrogen and hydrogen
  • Hydrogen, being the lightest element, rose to the top of the atmosphere and eventually drifted off into space.
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• Earth’s third atmosphere is the one we have now, containing enough oxygen for animals, including humans, to evolve.
• Plants and some bacteria use carbon dioxide and give off oxygen, while animals use oxygen and give off carbon dioxide.
• The atmosphere upon which life depends was created by life itself.

Formation of the layered structure in the lithosphere:

• The earth was in a volatile state during its primordial stage.
• Increase in density caused a rise in temperature inside.
• Material inside the earth started getting separated by density, which is called differentiation.
• Heavier materials like iron sank towards the center of the earth, while lighter ones moved towards the surface.
• This division led to the formation of the crust (outermost), mantle, outer core, and inner core (innermost).
• Density of the material increased from the crust to the core.