Abstract
Laboratory measurements of the photoabsorption cross section of 15NH3 at wavelengths between 140 and 220 nm are presented for the first time. Incorporating the measured photoabsorption cross sections of 15NH3 and 14NH3 into a one-dimensional photochemical diffusive model, we find that at 400 mbar, the photolytic efficiency of 15NH3 is about 38% greater than that of 14NH3. In addition, it is known that ammonia can condense in the region between 200 and 700 mbar, and the condensation tends to deplete the abundance ratio of 15NH3 and 14NH3. By matching the observed ratio of 15NH3 and 14NH3 at 400 mbar, the combined effect of photolysis and microphysics produces the ratio of (2.42 ± 0.34) × 10-3 in the deep atmosphere, in excellent agreement with the Galileo spacecraft measurements. The usefulness of the isotopic composition of ammonia as a tracer of chemical and dynamical processes in the troposphere of Jupiter is discussed.
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