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

Abstract

We have used the spectra obtained by the Composite Infrared Spectrometer (CIRS) onboard the Cassini spacecraft to search for latitudinal variation in the ¹⁵N/ ¹⁴N ratio on Jupiter. We found no variations statistically significant given the observational and model uncertainties. The absence of latitudinal variations demonstrates that ¹⁵NH ₃ is not fractionated in Jupiter's atmosphere, and that the measured ¹⁵N/ ¹⁴N represents Jupiter's global value. Our mean value for the global jovian ¹⁵N/ ¹⁴N ratio of (2.22±0.52)×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 ¹⁵N/ ¹⁴N ratio must represent the solar nitrogen isotopic composition. The solar ¹⁵N/ ¹⁴N ratio hence significantly differs from the terrestrial value: ( ¹⁵N/ ¹⁴N) _⊕=3.68×10 ^-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.