Observations of hydrocarbons in the giant planets

B. Bézard ¹, H. Feuchtgruber ², & T. Encrenaz ¹

¹ Observatoire de Paris, Section de Meudon, F-92195 Meudon, France

² Max-Planck Institut für Extraterrestrische Physik, D-85748 Garching, Germany

The Short Wavelength Spectrometer (SWS) of the Infrared Space Observatory (ISO) recorded spectra of the four giant planets at a resolving power of 1500. On Jupiter, Saturn, and Neptune, the 7-16.5 m range exhibits prominent emission bands from CH₄ (and its deuterated isotope CH₃D), C₂H₆, and C₂H₂. On Uranus, only C₂H₂ emission is detected in this spectral range. Information on the stratospheric temperature profile can be retrieved from the analysis of the 7.7-m CH₄ band combined with the S(0) and S(1) lines of H₂ at 28.2 and 17.0 m respectively. The inferred (disk-averaged) temperature profiles show significant differences with respect to those derived from Voyager measurements about a decade ago.

Analysis of the C₂H₆ and C₂H₂emission bands yield constraints on the abundance profiles of these compounds, and indirectly on the eddy mixing rate of the atmosphere in the 0.1-3 mbar region, a poorly-known parameter. In addition, on Saturn, methyl-acetylene (CH₃C₂H), diacetylene (C₄H₂), and benzene (C₆H₆) have been detected for the first time. ISO also found evidence for benzene on the Jovian disk, while the Voyager spacecraft could see it only in the northern auroral region.

Photochemical models are currently being developed with reference to these ISO measurements to better understand the quantitative details of hydrocarbon photochemistry. On Saturn, the overall agreement between model and observations is excellent except for C₂H₄ and C₃H₈ which were not seen by ISO and are overpredicted by the model. On the other hand, the C₆H₆ column density is underestimated, a problem probably linked to an incomplete chemical scheme. On Uranus, analysis of the C₂H₂emission combined with the lack of emission in the 7.7-m band provided an estimate of the eddy mixing coefficient in the high atmosphere and confirmed the sluggishness of the atmospheric mixing rate.

Finally, the methyl radical (CH₃), a direct product of methane photolysis, was detected on both Saturn and Neptune through emission in its band at 16.5 m. The CH₃ abundance is mostly sensitive to the poorly-known methyl recombination rates and to the eddy mixing profile. The Saturn measurement implies either a CH₃ self recombination rate higher than assumed in current models or an eddy mixing coefficient smaller than inferred from Voyager ultraviolet measurements.