A comparative study of the early terrestrial atmospheres

The CO2 abundance required for maintenance of the climatic conditions speculated to have existed 4 billion years ago on the earth, Mars, and Venus is presently sought by means of a one-dimensional radiative-convective atmospheric model. It is ascertained on this basis that the terrestrial and Martian atmospheres were highly stable, and unlikely to proceed to a moist or runaway greenhouse state even if CO2 partial pressure were equivalent to 100 bars on earth. The proximity of Venus to the sun rendered its atmosphere highly susceptible to rapid water vapor photodissociation and hydrogen escape.