A. The past atmosphere:

1. At the early formation of the Earth, as it emerged from the Solar Nebula as a Proto-planet, it is theorized that the Atm. Was made mostly of Hydrogen and Helium. This early atmosphere was "blown off" by collision with high energy particles of the "solar wind" and the heating of the atmosphere by energy from the sun. This left the early Earth rocky with a thin atmosphere. The outer gas planets are too far away from the sun to be affected in this way.

2. 4 Billion years ago atmosphere made mainly of:
   
   • a. Methane (CH₄) (each molecule made of 1 Carbon atom and 4 Hydrogen atoms)
     
   • b. Ammonia (NH₄) (each molecule made of 1 Nitrogen atom and 4 Hydrogen atoms)
     
   • c. a little Water (H₂O) (each molecule made of 2 Hydrogen atoms and 1 Oxygen atom
     

   This was TRULY NASTY! Very Poisonous! Obviously the air is not poisonous now, What happened?

3. 3.8 Billion years ago Sunlight triggered chemical reaction with the (CH₄), (NH₄) , and (H₂O) which broke the molecules apart into a mix of chemicals (Carbon, Hydrogen, Oxygen, Nitrogen) as well as the original substances.
   
4. Hydrogen is VERY low mass, so much of it could not be held by Earth's gravity and drifted off into space
   
5. Reactions occurred between the other chemicals in the atmosphere forming other molecules like:
   
   a. Carbon Dioxide (CO₂)
   b. Ozone (O₃)
   

   Formed a thin layer in the upper atmosphere ESSENTIAL for life: OZONE protects the surface from Ultra violet Radiation (UV) from the Sun by absorbing it and changing it from Ultra violet wavelengths to Infrared wavelengths (like heat)

   Before the ozone layer formed, the only things that could live on the Earth were microscopic organisms deep in the water. The water protected them from UV. They probably got their energy from chemicals in the water NOT from the sun (called Chemotrophs meaning "Chemical fed ")

   After OZONE layer formed organisms could survive closer to the surface of the water because the ozone protected them.

   MAJOR BREAKTHROUGH organisms start to use sunlight for energy source (called autotroughs ("fed by self"): PHOTOSYNTHESIS BEGINS!!

   With the beginning of photosynthesis the atmosphere begins to change "rapidly"
   The reaction for photosynthesis is:

   6CO₂ + 6H₂0 ---> C₆H₁₂O₆ + 6O₂

   It is like a recipe: Take 6 carbon dioxide molecules, and 6 water molecules and Use the energy in sunlight to run the reaction (not written in the equation). When photosynthesis is done 1 Glucose molecule and 6 Oxygen molecules have been made

   This is a Key development in the changing atmosphere: the production of OXYGEN (O₂)

   Leads to a build up of oxygen in the atmosphere (kind of nice for breathing!)

   Before the development of photosynthetic organisms (that make molecular oxygen (O₂)) the atmosphere had about 3% oxygen (definitely NOT enough to breath :( )

   AFTER they developed the atmosphere changed! NOW the atmosphere has 21% molecular oxygen! (thank goodness! )