We have analysed the ISO-SWS spectrum of Jupiter in the 12-16 micron range, where several hydrocarbons exhibit rovibrational bands. Using temperature information from the methane and hydrogen emissions, we derive the mixing ratios (q) of acetylene and ethane at two independent pressure levels. For acetylene, we find $q=(8.9^{+1.1}_{-0.6})times10^{-7}$ at 0.3 mbar and $q=(1.1^{+0.2}_{-0.1})times10^{-7}$ at 4 mbar, giving a slope $-dln q / dln P=0.8pm0.1$, while for ethane $q=(1.0pm0.2)times10^{-5}$ at 1 mbar and $q=(2.6^{+0.5}_{-0.6})times10^{-6}$ at 10 mbar, giving $-dln q / dln P=0.6pm0.2$. The ethane slope is consistent with the predictions of Gladstone et al. (1996), but that predicted for acetylene is larger than we observe. This disagreement is best explained by an overestimation of the acetylene production rate compared to that of ethane in the Gladstone et al. (1996) model. At 15.8 micron, methylacetylene is detected for the first time at low jovian latitudes, and a stratospheric column density of $(1.5pm0.4)times10^{15}$ molecule.cm-2 is inferred. We also derive an upper limit for the diacetylene column density of $7times10^{13}$ molecule.cm-2.
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