Oxygen in the stratospheres of giant planets and Titan

Helmut Feuchtgruber ¹, E. Lellouch ², Th. Encrenaz ², B. Bézard ², A. Coustenis ², P. Drossart ², Th. de Graauw ³, & G. R. Davis ⁴

¹ Max-Planck Institut für extraterrestrische Physik, Postfach 1603, D-85748 Garching, Germany

² DESPA, Observatoire de Paris, F-92105 Meudon, France

³ SRON, P.O. Box 800, NL-9700 AV Groningen, The Netherlands

⁴ Dept. of Physics, University of Saskatchewan, Saskatoon, S7N 5E2, Canada

Infrared spectra of the Short-Wavelength Spectrometer (SWS) of ISO at wavelengths between 25 - 45 micron have provided the first detection of stratospheric H2O on all four Giant Planets (Feuchtgruber et al. 1997, Lellouch et al. 1997) and Titan (Coustenis et al.1998). Together with SWS observations of CO₂ at 14.98 micron, leading to first detections on Neptune (Feuchtgruber et al. 1997), Saturn (de Graauw et al., 1997) and Jupiter (Lellouch et al. 1998) an external source of oxygen is required to explain the derived upper stratospheric mixing ratios of up to several ppb at mbar-microbar levels.

Two possible sources of this external input flux are discussed, the local source, sputtering material from rings and icy satellites, and the interplanetary source, which could be either micrometeoritic particles or larger objects like cometary nuclei. Similar measured input fluxes to within a factor of 10 on all Giant planets and Titan favour the interplanetary source. The short lifetime ( 10 years) of stratospheric H2O after SL9 like impacts also tends to exclude unfrequent massive impacts as a candidate origin.

Like H₂O, CO₂ is trapped by the cold tropopause on Neptune and therefore has to be also of external origin. Photochemical reaction of CO, which is relatively abundant in Neptune's stratosphere and absent on Uranus, with OH could produce CO2 in amounts compatible with the detection on Neptune and the non-detection on Uranus. This scenario may also hold on Jupiter and Saturn. Since CO₂ does not condense at the tropopause of these planets, CO₂ could also conceivably be transported by vertical upward transport from the deep troposphere although a preliminary assessment of this mechanism suggests that it does not provide the correct CO/CO₂ ratio. In the case of Jupiter CO₂ could also be produced by photochemical conversion of H₂O to CO₂ in the years after the SL9 impacts. This is supported by the fact that the CO₂ band shows latitudinal variability.

We provide an overview on the required amounts of external oxygen fluxes and a detailed discussion on the various scenarios for the origin of CO₂ in the stratospheres of the giant planets.