the atmosphere:

The atmosphere is a thin layer of gas surrounding the Earth, extending to ~100 km above the surface.

• Density & Pressure: Density decreases with altitude; at 5 km, air density is less than half of sea level.
• Layers of the Atmosphere:
  1. Troposphere: 0–~18 km; contains ~90% of atmospheric gases; where weather occurs; temperature decreases with height.
  2. Stratosphere: ~18–48 km; contains the ozone layer; temperature increases with height due to absorption of UV by O₃.
  3. Mesosphere: ~48–80 km; temperature decreases again due to low air density; ozone still present.
  4. Thermosphere: Above 80 km; temperature increases due to solar radiation breaking O₂ and N₂ into ions.
• Air Movement: Warm gases rise, cooler gases sink; winds disperse pollutants throughout the troposphere.

atmospheric composition:

• Air is a mixture of gases.

• Current composition of air (approx.):

  • 78% nitrogen (N₂)
  • 21% oxygen (O₂)
  • 0.93% argon (Ar)
  • 0.04% carbon dioxide (CO₂)
  • Traces of other gases (neon, helium, methane, water vapour).

Changes in air composition over time:

• Less carbon dioxide
  • Oceans formed → CO₂ dissolved into water, forming carbonates.
  • Plants absorbed CO₂ during photosynthesis.
  • Today: CO₂ is increasing again due to pollution and greenhouse gas emissions.
• Less water vapour
  • As Earth cooled, water vapour condensed → formed liquid water and oceans.
• More oxygen
  • Growth of plants and cyanobacteria → photosynthesis released O₂.
  • Led to the Great Oxygenation Event.
• Historically: changes in CO₂, H₂O vapor, and O₂ occurred slowly over millions of years.
• Modern era: human activity is causing rapid increases in CO₂, much faster than natural processes.
• Implication: the atmosphere is stable but vulnerable, and rapid changes threaten climate balance and life on Earth.

miller-urey experiment:

The Miller-Urey experiment, conducted in 1953 by Stanley Miller and Harold Urey, was a groundbreaking scientific experiment that aimed to simulate the conditions of early Earth and test the hypothesis about the chemical origins of life.

Setup: The experiment used a closed system that contained water, methane, ammonia, and hydrogen, which were believed to be present in Earth's early atmosphere.

Simulating Conditions: The apparatus included a flask of water to represent the ocean and electrodes to create electrical sparks, simulating lightning, which was thought to provide energy for chemical reactions.

Results: After a week of continuous operation, the experiment produced several amino acids, which are the building blocks of proteins. This finding suggested that organic compounds could form from inorganic precursors under conditions thought to resemble those of early Earth.

Significance: The experiment provided crucial evidence supporting the idea that life could arise from simple chemical processes. It sparked further research into abiogenesis, the origin of life from non-living matter.