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The Giant Planets as seen by ISO
Th. Encrenaz 1 - E. Lellouch 1 - H. Feuchtgruber 2 - B. Altieri 2 - B. Bézard 1 - Th. de Graauw 3 - P. Drossart 1 - M. J. Griffin 4 - M. F. Kessler 2 - P. G. Oldham 4
1 DESPA, Observatoire de Paris, Meudon, France -2 ISO Science Operation Center, Villafranca, Spain -
3 SRON, Groningen, the Netherlands -
4 Queen Mary and Westfield College, London, UK
The study of the giant planets's atmospheres -thermal structure, chemical composition, abundance ratios - has greatly benefited from spectroscopic observations with ISO. New discoveries have been achieved in the following areas: (1) determination of the D/H ratio; (2) discovery of new molecules; (3) retrieval of the stratospheric thermal structure; (4) determination of vertical distributions of several species.
The determination of D/H, which has important cosmological and cosmogonical implications, has been achieved through the first detection of HD rotational lines: R(2) on Jupiter with SWS-FP (Encrenaz et al., 1996; Lellouch et al., 1996), R(1) on Saturn with LWS-FP (Griffin et al., 1996); R(2) on Neptune with SWS-grating (Feuchtgruber et al., 1997a). The results are presented by Lellouch et al. at this workshop.
The detection of new species includes: H2O in the stratospheres of all giant planets; CO2 in the stratospheres of Jupiter, Saturn and Neptune; CH3C2H in the stratospheres of Jupiter and Saturn; C4H2 in the stratosphere of Saturn; H2O in the deep troposphere of Saturn. The Saturn detections (apart from stratospheric H2O) have been announced by de Graauw et al. (1997). Other results concerning the stratospheric H2O and CO2 have been announced by Feuchtgruber et al. (1997b) and Lellouch et al. (1997); they will be presented by Lellouch et al. at this workshop. All these detections have been achieved with SWS in the grating mode.
Information upon the thermal structure of the giant planets is retrieved
from the emission bands of methane at 3.3
m (v3 band)
and 7.7 m (v4 band) (both observed with SWS-grating)
which probe pressure levels around a bar and a mbar respectively.
In particular, in the case of Jupiter, a temperature of about 800 K,
higher than previously expected from Voyager, has been inferred
at a pressure level of 0.16 bar (Encrenaz et al., 1996); this
new result is in agreement with the recent measurements from Galileo.
The 7.7 m CH4 band has been used to constrain the temperature
distributions in the lower stratospheres of Jupiter (Encrenaz et al.,
1996) and Saturn (de Graauw et al., 1997).
In the case of Saturn, the vertical distributions of NH3 and PH3
(Davis et al., 1996) and CH4 (Davis et al., 1997) have been retrieved
from the LWS in the grating mode. On Neptune, the stratospheric
abundances of CH4, C2H2, C2H6 and CH3D have been
retrieved by Bézard et al. (1997) in the 7-14 m range,
using SWS-grating; these result lead to a new determination of D/H,
in agreement with the HD measurement. On Uranus, only the C2H2
band is visible in this spectral range; from its analysis, an eddy
mixing coefficient of about 2 104 cm2s-1 is inferred
at the homopause (Encrenaz et al., 1997); this confirms the very
low diffusion in Uranus' stratosphere as compared to the other
giant planets.
Other spectroscopic data have also been obtained using PHT-S on Uranus and Neptune, and using CAM-CVF on Jupiter. In particular, the CAM images of Jupiter, coupled with SWS-grating observations at various points of the Jovian disk, will be used for studying the spatial variations of the Jovian atmospheric structure, in view of a better understanding of the planet's global dynamics (see the poster presentation by Encrenaz et al. at this workshop).
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Postscript version