The following document lists the file abstract/BBEZARD_NEP_2.abs
from catalogue VI/111.
A plain copy of the file
(without headers/trailers) may be downloaded.
We propose to use the SWS in the grating mode to search for
spectral signatures from benzene (C6H6), methylacetylene (C3H4),
diacetylene (C4H2), and methyl radical (CH3) on Neptune. These
still undetected compounds are expected to be produced from the
photochemistry of methane in the high atmosphere. Detecting these
molecules and measuring their concentrations (or even getting
stringent upper limits) would provide crucial tests for some chemical
schemes proposed in photochemical models, and help determining some
important atmospheric parameters.
We will search for thermal emission from the C6H6 band at 14.8
micron, the C3H4 band at 15.8 micron, the C4H2 band at 15.9 micron,
and the CH3 band at 16.5 micron. These are by far the most favorable
rovibrational bands for these molecules in the thermal IR. They occur
in spectral regions inaccessible from the ground. Bands at shorter
wavelengths, in the 8-12 micron telluric window, are intrinsically
weaker and would produce much less intense emission features due to
the exponential decrease of the Planck function with frequency at the
cold temperatures prevailing in Neptune's stratosphere. ISO hence
offers the only opportunity in the near future to detect for the first
time these important compounds.
To facilitate the analysis of the observed emission features, we
also propose to record the two quadrupolar lines of hydrogen at 28.2
and 17.0 micron, using the F-P mode of the SWS. These observations
would provide information on the thermal structure on Neptune in the
atmospheric region from which the hydrocarbon emissions originate.
Mixing ratios could then be more reliably retrieved.
Synthetic calculations based on realistic abundances of C6H6,
C3H4, and C4H2 show that the relevant spectral signatures should be
detectable by SWS in the amount of time requested.