Models of the Jovian upper atmosphere

Recent developments in photochemical models of the upper atmosphere of Jupiter and Titan are reviewed. Measurements of the Jovian Lyman alpha airglow favor a homopause value of approximately 10 7 cm² s −1 for the eddy diffusion coefficient. A relatively hot exosphere as implied by the electron density measurements results in an extended atmosphere of hydrogen. Larger than solar C/H and N/H ratios are obtained on reinterpretation of the sunlight reflected from Jupiter; and substantially lower ultraviolet photoabsorption cross sections measured for methane and ethane influence the distribution of CH 4 , NH 3 , and their photochemical products. Possible coupling between PH 3 and NH 3 may inhibit ultraviolet destruction of NH 3 on Jupiter. The red color of the Great Red Spot and other minor spots may be due to the presence of red phosphorus crystals formed after photolysis of PH 3 . The detection of such gases as PH 3 , GeH 4 , and CO in the stratosphere of Jupiter gives an indication of the presence of thermochemical nonequilibrium processes, and a strong vertical mixing may be required to explain their large observed upper atmospheric abundances. This argument is further strengthened by a laboratory finding that C 2 H 2 can efficiently scavenge photolysis products of PH 3 . Other alternate mechanisms to account for the presence of CO in the stratosphere are the oxygen atom transfer from the Galilean satellites to Jupiter either by radial diffusion, or along magnetic flux tubes, or on ablation of meteoroidal material. Subsequent reactions of the thermal oxygen atoms with photolysis products of methane lead to the production of CO. Finally, the discussion of reducing atmospheres is extended to include Titan. It is found that if NH 3 has outgassed from Titan and if the surface temperature during the initial phases of its evolution was 150°K or greater, due perhaps to a CH 4 ‐H 2 greenhouse, it is inevitable that Titan would have accumulated a thick atmosphere of nitrogen over geologic time. The pressure‐induced thermal opacity of 20 bars of nitrogen can also explain the recently measured 200°K surface temperature of Titan. On the other hand, the composition of Titan's atmosphere would be far less certain if the surface temperature were as low as 100°K.