If ammonia is transported to sufficiently high altitudes in an atmosphere (where pressures are of the order of 100mbar) then it may be photodissociated to form hydrazine (N2H4) via the reactions:
where M is any other molecule. Hence, we expect the abundance of ammonia to decrease above the 100mbar level. In fact, the ammonia decreases at lower altitudes also due to vertical eddy mixing and, as we saw in Section 4.2, the rate of decrease with height is thus determined by the strength of sunlight and the degree of eddy mixing.
Hydrazine itself may also be photolyzed into other products and the photoabsorption cross-sections of ammonia, hydrazine, and phosphine are shown in Figure 4.4. Hydrazine should condense at the temperatures found in Jupiter and Saturn's upper troposphere and the resultant ice particles may thus be a constituent of the hazes found in the upper tropospheres of these planets.
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