Isotopic fractionation of nitrogen in ammonia in the troposphere of Jupiter

Abstract

Laboratory measurements of the photoabsorption cross section of (NH3)-N-15 at wavelengths between 140 and 220 nm are presented for the first time. Incorporating the measured photoabsorption cross sections of (NH3)-N-15 and (NH3)-N-14 into a one-dimensional photochemical diffusive model, we find that at 400 mbar, the photolytic efficiency of (NH3)-N-15 is about 38% greater than that of (NH3)-N-14. 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 (NH3)-N-15 and (NH3)-N-14. By matching the observed ratio of (NH3)-N-15 and (NH3)-N-14 at 400 mbar, the combined effect of photolysis and microphysics produces the ratio of (2.42 +/- 0.34) x 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.