We propose to observe the HD R(2) (37.702 micron) line
on Jupiter and Saturn with SWS. The goal is to complement the LWS
measurements of the HD R(0) and R(1) lines at 112 and 56 microns,
that will be performed in the central program, to improve the
accuracy of the determination of the D/H ratio in these two planets.
The D/H abundance in Jupiter and Saturn is thought to be representative
of the conditions in the primitive nebula from which the Solar System
was formed. As such, it is a key parameter constraining (i) models
of galactic evolution (ii) physical conditions prevailing in the solar nebula
(iii) the origin and history of the grains embedded in the nebula.
So far (and besides the very recent in situ measurement by the
Galileo Probe in Jupiter), the D/H ratio in Giant Planets has been measured
spectroscopically from visible lines of HD and infrared features
of CH3D. Important complications are associated with both methods,
however. ISO offers a unique opportunity to improve significantly
the D/H determination by giving access to the rotational lines of HD
in the far infrared range. Preliminary analyses of the HD lines observed by
ISO in technical time show however that cloud opacity and thermal
profiles effects in the planetary atmospheres must be carefully
considered when deriving the D/H ratio. It thus appear necessary
to observe so many HD lines as possible (i.e. three) to disentangle
betwen the various effects. The S(0) and S(1) quadrupolar line of H2
near 28.2 and 17.0 micron will also been observed to constrain the Jupiter's
and Saturn's thermal profile, facilitating the interpretation of the
HD measurements.