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T. Fouchet, E. Lellouch, P. Drossart, B. B\'ezard, Th. Encrenaz (DESPA), H. Feuchtgruber (MPI Garching)
Observations of Jupiter with the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO) show, from 12 to 16 \mum, stratospheric emissions due to ethane (C2H6) at 12.5 \mum, acetylene (C2H2) at 13.7 \mum, and methylacetylene (C3H4) at 15.8 \mum. This is the first detection of C3H4 in Jupiter outside the polar regions (Kim et al., 1985). At the ISO-SWS spectral resolution ({\rm R}=1500), C2H2 lines due to the \nu5+\nu4-\nu4 hot band and to the \nu5 band are clearly separated. These two bands probe the atmosphere respectively at 1 and 4 mbar, which allow us to determine the acetylene vertical distribution in this region. Some information can also be retrieved on the vertical distribution of ethane in the following way. Between C2H6 lines, the thermal flux mostly originates from the upper troposphere. However, this flux is screened by the wings of C2H6 lines in the lower stratosphere. Therefore, adjusting the observed flux level allows us to constrain the C2H6 vertical distribution in this region. On the contrary, since C3H4 lines are optically thin, no vertical information can be inferred for this constituent.
We have performed the analysis of these emissions using the stratospheric temperature profile determined from ISO-SWS observations of the \nu2 and \nu4 methane bands at 6.5 and 7.7 \mum along with the S(1) H2-quadrupolar line at 17.0 \mum. Our temperature profile is in good agreement with that measured by the Galileo Probe. We find a C2H2 mixing ratio increasing from 9.1\times 10-8 at 4 mbar to 2.4\times 10-7 at 1 mbar. For ethane, we find that its mixing ratio must decrease at least as p-1 at pressures higher than 1 mbar. Finally, we determine a C3H4 column density of 1.8\times1015 molecules\,cm-2.