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 r^ant spectral signatures should be detectable by sws in the amount of time requested.
Instrument
SWS02 , SWS07
Temporal Coverage
1996-10-26T19:47:20Z/1996-10-27T00:14:53Z
Version
1.0
Mission Description
The Infrared Space Observatory (ISO) was the worlds first true orbiting infrared observatory. Equipped with four highly-sophisticated and versatile scientific instruments, it was launched by Ariane in November 1995 and provided astronomers world-wide with a facility of unprecedented sensitivity and capabilities for a detailed exploration of the Universe at infrared wavelengths.
European Space Agency, BEZARD et al., 1998, 'SEARCH FOR UNDETECTED HYDROCARBONS ON NEPTUNE', 1.0, European Space Agency, https://doi.org/10.5270/esa-tp5209s
