Session 27P. Solar System Astronomy with ISO and Prospects for SIRTF I, II
Invited Poster Session, Wednesday, October 14, 1998, 5:10-6:10pm, Hall of Ideas

[27P.05] Jupiter Troposphere as seen with ISO/SWS

Th. Fouchet, E. Lellouch, P. Drossart, Th. Encrenaz, B. B\'ezard (Obs. Paris), H. Feuchtgruber (Max Planck Inst.)

Observations of Jupiter with ISO/SWS span from 2.5 to 12.5 \mum at a mean resolving power of 1500. First interpretations of these observations were published by Encrenaz et al., {\it Astron. Astrophys.}, 1996. A complete NH3 vertical distribution from 6 to 0.2 bar is obtained combining 5 \mum and 10 \mum spectral regions. The 5 \mum-window is well known to probe atmospheric levels down to 6-7 bars where, from weak lines of NH3 between 2000 and 1800 cm-1, we retrieved a deep N/H = 1.2 \times solar. This result is in agreement with findings from the Galileo Probe Net Flux Radiometer (Sromovski et al., {\it Science}, 1996) and from centimetric observations (de Pater et al., 1986). The ammonia vertical distribution in the upper troposphere is infered from the 10 \mum region. It is found to be subsaturated by a factor of 3 above 700 mbar and then rapidly removed by photo-dissociation above 300 mbar. Our resolution and high signal to noise ratio also allows us to detect several 15NH3 lines from which we inferred a 14N/15N = 500 - 1000, thus significantly higher than the terrestrial ratio (14N/15N = 270). This discrepancy may rather reveal an isotopic frationation taking place during the condensation or photo-dissociation of ammonia in the atmosphere of Jupiter than a fractionation which occured during the formation of the solar system.

Other tropospheric species of interest are observed in both the 5\mum and 10 \mum regions : CH3D mixing ratio is derived from \nu6 (1155 cm-1) and \nu2 (2200 cm-1) bands, and is in agreement with the D/H ratio (2.1 10-5) derived from ISO observations of HD lines (Lellouch et al., this conference). We derived a complete PH3 distribution between 3-4 bar and 0.3 bar from \nu2 (992 cm-1), \nu4 (1118 cm-1) and \nu1 (2323 cm-1) bands, which shows an enhancement of photo-dissociation above 0.3 bar. ISO/SWS has provided the first detection of NH3 ice in the 3 \mum region. Brooke et al, {\it Icarus}, 1998 have obtained from this part of Jupiter spectrum a particle radius of 10 \mum. NH3 ice also exhibits an absorption at 9.5 \mum, which is not detected in ISO/SWS observations. It allows us to infer a radius equal or larger than 10 \mum for the ice particles.

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