Isotopic Fractionation of Nitrogen in Ammonia in the Troposphere of Jupiter

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