The Nitrogen Isotopic Ratio in Jupiter’s Atmosphere from Observations by the Composite Infrared Spectrometer on theCassiniSpacecraft

The Composite Infrared Spectrometer (CIRS) on the Cassini spacecraft made infrared observations of Jupiter's atmosphere during the flyby of 2000 December to 2001 January. The unique database in the 600-1400 cm -1 region with 0.53 and 2.8 cm -1 spectral resolutions obtained from the observations permits retrieval of global maps of the thermal structure and composition of Jupiter's atmosphere, including the distributions of 14 NH 3 and 15 NH 3 . Analysis of Jupiter's ammonia distributions from three isolated 15 NH 3 spectral lines in eight latitudes is presented for evaluation of the nitrogen isotopic ratio. The nitrogen isotopic ratio 14 N/ 15 N (or 15 N/ 14 N) in Jupiter's atmosphere in this analysis is calculated to be 448 ± 62 [or (2.23 ± 0.31) × 10 -3 ]. This value of the ratio determined from CIRS data is found to be in very close agreement with the value previously obtained from the measurements by the Galileo Probe Mass Spectrometer. Some possible mechanisms to account for the variation of Jupiter's observed isotopic ratio relative to those of various astrophysical environments are discussed.