Since the Jovian atmosphere is composed almost entirely of captured solar nebula gas, it is expected that the 15N/14N ratio of the Sun and Jupiter are the same. The upper value of this ratio has been measured in the solar wind to be < 2.8 x 10~3 (Hashizume et al. 2000). The 15N/14N ratio in ammonia on Jupiter was estimated from Infrared Space Observatory Short Wavelength Spectrometer (ISO/SWS) observations (Fouchet et al., 2000) to be (1.9^1.0) x 10and an analysis of the Galileo Galileo Probe Mass Spectrometer (GPMS) measurements (Owen et al., 2001) yielded an estimate of (2.3 ± 0.3) x 10~3. More recently, analysis of the Cassini CIRS observations of Jupiter have yielded an estimate of the 15N/14N ratio of (2.22 ± 0.52) x 10~3 (Fouchet et al., 2004b) and (2.23 ± 0.31) x 10~3 (Abbas et al., 2004). All these measurements are consistent with the solar wind estimate. In addition, both Abbas et al. (2004) and Fouchet et al. (2004b) found negligible variation of the 15N/14N ratio with latitude, suggesting that this ratio is not affected by the fractionation effects caused by the condensation of ammonia and thus that this ratio represents the true bulk ratio of Jupiter.
The 15N/14N ratio in HCN in the current local ISM is estimated to be (2.2 ± 0.5) x 10~3 (Dahmen et al., 1995). As mentioned earlier, the 15N/14N ratio is expected to reduce over time as the 15N produced by early Type II supernovae is diluted with 14N coming from the death of longer lived, intermediate-mass stars. Hence, we would expect that the 15N/14N ratio should have been higher in the presolar cloud than in the current ISM. The 15N/14N ratio in HCN for Comet Hale-Bopp was estimated to be roughly 3 x 10~3 (Jewitt et al., 1997), which, assuming the ratio for this comet is representative of the proto-solar value, is consistent with this expectation. We also expect that the 15N/14N ratio in molecules such as HCN and NH3 in the ISM was higher than in N2 due to the same ion-molecule reactions that increase the D/H ratio. Hence, the lower 15N/14N ratio found in ammonia in Jupiter's atmosphere and in the solar wind suggest that nitrogen was captured by the Sun and by Jupiter mainly in the form of N2 and not as NH3 or HCN (Owen et al., 2001). The fact that the terrestrial value of the 15N/14N ratio of 3.7 x 10~3 is substantially higher than that of Jupiter suggests that much of the Earth's nitrogen probably arrived later in the form of HCN and NH3 in comets.
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