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Shortly after the earth formed, its atmosphere was very similar to that of Venus indeed. The table below explains its composition. Make up of Atmosphere Lots of Carbon dioxide Lots of steam Traces of methane and ammonia Reason This was released by the ton in the many volcanic eruptions early Earth experienced. The temperature of the earth was very high (above boiling point). This is due to the greenhouse effect caused by all the carbon dioxide. Also released in volcanic eruptions. Obviously, something happened to change our atmosphere (or we would not be here!). The table overleaf shows how the atmosphere of the Earth looks like today…. Then we will look at how this change might have happened. Gas Nitrogen Oxygen Carbon Dioxide. SEE PAGE 161 FOR A DIAGRAM OF THE CARBON CYCLE. Noble gases. THESE ARE INERT, UNREACTIVE GASES LIKE ARGON, AND HELIUM. Percentage of atmosphere 78% (often referred to on exams as around 80%) 21% Less than 1% Less than 1% The following is a step by step guide as to how the atmosphere of the Earth changed into what it is today STEP ONE: THE EARTH COOLED, EVENTUALLY DROPPING BELOW 100 DEGREES. THIS ALLOWED THE STEAM TO CONDENSE TO FORM THE OCEANS STEP TWO: MUCH OF THE CARBON DIOXIDE REACTED WITH THE WATER IN OCEANS AND WAS REMOVED FROM THE ATMOSPHERE. THE REACTION FORMED CARBONATE ROCKS E.G. LIMESTONE STEP THREE: THE APPEARANCE OF WATER KICKSTARTED THE EVOLUTION OF EARLY PLANT LIFE. THIS PRODUCED THE OXYGEN WE SEE TODAY (THROUGH PHOTOSYNTHESIS). PHOTOSYNTHESIS ALSO HELPED REMOVE SOME OF THE CARBON DIOXIDE THAT WAS LEFT. STEP FOUR: THE OXYGEN PRODUCED REACTED WITH THE METHANE AND AMMONIA. THESE REACTIONS REMOVED THEM FROM THE ATMOSPHERE AND THE REACTION WITH AMMONIA PRODUCED ALL THE NITROGEN THAT MAKES UP MOST OF TODAYS ATMOSPHERE.