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The 12-14 micron region lies outside the terrestrial atmospheric
windows and can not be observed from the ground. The only information
available in this spectral range was provided by the Voyager infrared
data acquired in 1980 with a spectral resolution of 4.3 cm-1 (0.096
micron). At this resolution and due to the high noise level of the
infrared spectrometer (IRIS) aboard Voyager, only mean mole fractions
of the gases with emission signatures in the thermal infrared range
could be obtained, with significant error bars. No information was
obtained on the vertical distributions of these components, except for
a 2-altitude-levels profile from a particular observing sequence over
Titan's north pole. The profiles showed a general tendency for increase
with height, implying formation of the species in the upper atmosphere
(in accordance with photochemical models).
In order to determine with precision the vertical distributions
of C3H8 and C2H6, the ISO/SWS F-P capabilities are required in the 12-14
micron region where, within two short spectral intervals (around 748 and 821
cm-1), each molecule presents an emission band, with a strong Q-branch
and some multiplets.
This part of the spectrum is also covered in the grating mode by the
SWS in the Central Programme at 0.5 cm-1, which does not permit to resolve
the bands. The ISO/SWS F-P will allow us to determine with accuracy
the molecular abundances of the two species, and to constrain current
photochemical models, while providing information on the eddy mixing profile.
At the same time we will combine these FP observations with SWS grating
scans in the 2.38 - 2.6 mu region, unexplored up to now, at no additional
time cost.