Evolution of a nitrogen atmosphere on titan.

Photochemical calculations indicate that if NH(3) outgassed from Titan it should have been converted to a dense N(2) atmosphere during the lifetime of the satellite. A crucial step in the process involves a gas phase reaction of N(2)H(4) with H. The most favorable conditions for this step would be the intermediate production of a CH(4)-H(2) greenhouse capable of raising the gas temperature to 150 degrees K. Subsequently about 20 bars of N(2) could have evolved. The pressure-induced opacity of 20 bars of N(2) should suffice to explain the recently measured 200 degrees K surface temperature. Unlike the situation on Jupiter, NH(3) is not recycled on Titan by reactions involving N(2) or N(2)H(4). This may explain the failure of recent attempts to detect NH(3) in the upper atmosphere of Titan.