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[18.26] On the Isotopic Fractionation of Ammonia upon Condensation in the Jovian Atmosphere

K. S. Kalogerakis (SRI International), E. R. Wouters (Thermo Electron Corporation), R. T. Meharchand (Florida State University)

Infrared spectra of ammonia recorded by ground-based telescopes,the ISO, Galileo, and Cassini space missions and ammonia mass spectra recorded by the Galileo Probe indicate that the ¹⁵N/¹⁴N isotope ratio in the atmosphere of Jupiter is roughly half of that observed in the Earth's atmosphere (Fouchet et al. 2004, Wong et al. 2004, and references therein). These observations posed the question of whether the observed Jovian isotope ratio reflects the composition of the protosolar nebula or rather results from isotope-selective fractionation by photochemical and thermochemical processes that sequester ¹⁵N in clouds or other unobserved reservoirs in the lower Jovian atmosphere.

We report calculations of the isotopic fractionation coefficient of ammonia by condensation based on the best available vapor pressure laboratory measurements at low temperature (King et al. 1989), previously unused by the planetary community. In order to explain the accepted Jovian ¹⁵N/¹⁴N isotope ratio the condensation fractionation coefficient must be greater than 1.10 near 160 K. In contrast, the laboratory data indicate that this coefficient decreases monotonically from 1.016 at 165 K to 1.003 at 195 K, and thus cannot explain the observed ¹⁵N depletion.

When combined with the lack of spatial variation of the ¹⁵N/¹⁴N ratio observed by Cassini (Fouchet et al. 2004) and the persistence of ¹⁵N depletion between 0.8 and 2.8 bar observed by the Galileo Probe (Mahaffy et al. 2000, Owen et al. 2001), we conclude that all information currently available supports the inference that the observed ¹⁵N/¹⁴N ratio is representative of Jupiter's global atmospheric composition and thus is likely to reflect the nitrogen isotopic ratio of the protosolar nebula.

This work was supported by the NSF Research Experiences for Undergraduates and Planetary Astronomy Programs under grants PHY-0353745 and AST-0206270.

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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.