Search for spatial variation in the jovian 15N/14N ratio from Cassini/CIRS observations

We have used the spectra obtained by the Composite Infrared Spectrometer (CIRS) onboard the Cassini spacecraft to search for latitudinal variation in the N-15/N-14 ratio on Jupiter. We found no variations statistically significant given the observational and model uncertainties. The absence of latitudinal variations demonstrates that (NH3)-N-15 is not fractionated in Jupiter's atmosphere, and that the measured N-15/N-14 represents Jupiter's global value. Our mean value for the global jovian N-15/N-14 ratio of (2.22 +/- 0.52) x 10(-3) agrees with previous measurements made by Fouchet et al. (2000, Icarus 143, 223-243) and Owen et al. (2001, Astrophys. J. 553, L77-L79). We argue that the jovian isotopic N-15/N-14 ratio must represent the solar nitrogen isotopic composition. The solar N-15/N-14 ratio hence significantly differs from the terrestrial value: (N-15/N-14)circle plus = 3.68 x 10(-3). This supports the proposition that terrestrial nitrogen originates from a nitrogen reservoir isolated from the main nitrogen reservoir in the proto-solar nebula. The origin and carrier of this isolated reservoir are still unknown. (C) 2003 Elsevier Inc. All rights reserved.