We have analysed the ISO-SWS spectrum of Jupiter in the 12–16 \mu { m } 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 } ) \times 10 ^ { -7 } at 0.3 mbar and q = ( 1.1 ^ { +0.2 } _ { -0.1 } ) \times 10 ^ { -7 } at 4 mbar , giving a slope - d \ln q / d \ln P = 0.8 \pm 0.1 , while for ethane q = ( 1.0 \pm 0.2 ) \times 10 ^ { -5 } at 1 mbar and q = ( 2.6 ^ { +0.5 } _ { -0.6 } ) \times 10 ^ { -6 } at 10 mbar , giving - d \ln q / d \ln P = 0.6 \pm 0.2 .
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 \mu { m } , methylacetylene is detected for the first time at low jovian latitudes , and a stratospheric column density of ( 1.5 \pm 0.4 ) \times 10 ^ { 15 } molecule cm ^ { -2 } is inferred .
We also derive an upper limit for the diacetylene column density of 7 \times 10 ^ { 13 } molecule cm ^ { -2 } .