g01.    What was the second atmosphere like?

Outgassing began when interior of the planet began to heat up. The gases were brought to the surface through volcanic and other related exhalations. All but the lightest gases are held to the surface by gravity. The atmosphere became suitable for a chain reaction to produce life because it lacked abundant free oxygen producing a reducing, non-oxygenic environment. The dissociation of hydrogen and oxygen in the upper atmosphere caused by ultraviolet radiation and the dissociation of CO₂ into carbon and free oxygen through photosynthesis produced an oxygen rich atmosphere. Supporting evidence is present in the rock record. Chert is found in rocks 3.5 billion years old.  It only forms in an atmosphere free of ammonia. Carbonate rocks, which need an alkaline environment, are also absent during this time period. Iron rocks are not oxidized until 3 billion years ago; they then show striations of oxidations until 1.8 billion years ago. After that, iron formations are fully oxidized.  This shows that oxygen was gradually being added to the atmosphere. Carbonate rocks become abundant at about 1.8 billion years ago.  If CO₂ were abundant, these rocks would not have been deposited. At the end of the Precambrian, the atmospheric conditions allowed for cooling resulting in an ice age.  

e16.    What do computer models of greenhouse changes show?

m35.    How can global warming affect biodiversity?

m38.     Why is ozone being depleted?

m41.    How can matter affect the environment?

m52.    What causes air pollution?