HISTORY OF EARTH’S ATMOSPHERE
             N
              2
             CO2
                                                   O
                                                    2      O reaches 
             HO                                              2 
              2                                            current levels; 
               oceans  CO
               form          2                             life invades 
                         dissolves                         continents
   Outgassing        Life forms  Onset of
                     in oceans    photosynthesis
   4.5 Gy           4 Gy             3.5 Gy                0.4 Gy                present
   B.P              B.P.             B.P.                  B.P.
      Evolution of oxygen and ozone over Earth’s history
                           Life appears on land
       RUNAWAY GREENHOUSE EFFECT ON VENUS
  due to accumulation of water vapor from volcanic outgassing early in its history  
   …did not happen on Earth because farther from Sun; as water accumulated it 
   reached saturation and precipitated, forming the oceans
                       EARTH   VENUS
        Comparing the atmospheres of Earth and Venus
                                           Venus       Earth 
               Radius (km)                  6100       6400
               Surface pressure (atm)        91          1
                                                           -4
               CO (mol/mol)                  0.96      3x10
                  2
                                                 -2
               N (mol/mol)                 3.4x10       0.78
                2
                                                 -5
               O (mol/mol)                 6.9x10       0.21
                2
                                                -3         -2
               HO (mol/mol)                 3x10       1x10
                2
        BIOGEOCHEMICAL CYCLING OF ELEMENTS:
               examples of major processes
      Physical exchange, redox chemistry, biochemistry are involved
                                Surface
                                reservoirs