The following document lists the file abstract/ACOUSTEN_TITAN16M.abs
from catalogue VI/111.
A plain copy of the file
(without headers/trailers) may be downloaded.
The 16 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. Furthermore, in the 16 micron region, two species (C3H4 and C4H2) appear with bands which overlap in such a way that the retrieval of their abundances was even more inaccurate than for the other gases. The C3H4 abundance was inferred by combining the nu_9 band with another isolated C3H4 band at 328 cm-1. 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 heigh, implying formation of the species in the upper atmosphere (in accordance with photochemical models), which however could be confirmed only in the case of C3H4 given the large uncertainties of the Voyager data. In order to determine with precision the vertical distributions of C4H2 and C3H4, the ISO/SWS F-P capabilities are required in the 16 micron region where, within a short spectral interval (615-645 cm-1) each molecule presents an emission band, the nu_8 of C4H2 at 627.9 cm-1 and the nu_9 of C3H4 at 634.7 cm-1. This part of the spectrum is also covered in the grating mode by the SWS in the Central Programme at a 0.5 cm-1, which does not permit to resolve the bands. The ISO/SWS F-P will allow us to separate the contributions of the two species, to obtain mixing ratio vertical profiles for the stratospheric C4H2 and C3H4 on a disk-average basis, and to constrain current photochemical models, while providing information on the eddy mixing profile.