We propose to search for PH3 and to determine the C/H ratio in
Neptune's troposphere, by an analysis of its infrared spectrum
in the regions 4.0-4.3 microns and 6.9-7.5 microns respectively.
Phosphine has never been observed in Neptune's atmosphere, but
has been detected in Jupiter and Saturn, and its presence
in Neptune in observable amounts is expected from disequilibrium
thermochemical models.The detection of PH3 will permit us to test
the efficiency of the convection within the planet as well as
theories of its formation.A PH3 enhancement, with respect to
solar abundance, would support, as in the case of CH4, the nucleation
model for Neptune's formation scenario. The C/H ratio is known
to be enriched in Neptune's troposphere, with respect to the solar
abundance, but with a factor 2 uncertainty. Because of CH4 saturation,
one has to probe very deep atmospheric levels, at pressures above 1.5b.
The best way to search for PH3 is to record the spectrum of Neptune
between 4.0 and 4.3 microns. Scattered solar radiation is observed
at these wavelengths. The nu1 and nu3 bands of PH3 are both
centered at 4.3 microns. A complete scan including the band center
and the far blue wing will allow to have maximum sensitivity for the
PH3 detection, and, if present, to retrieve its vertical distribution.
The best region to measure the abundance of tropospheric methane is
the 6.9-7.5 microns, in the far blue wing of the nu4 CH4 band. Thermal
emission is expected to be dominant. Both the PH3 and the C/H
observations require the use of ISO. The PH3 bands are centered inside
the terrestrial CO2 absorption at 4.3 microns. The 6.9-7.5 microns
range is not accessible from the Earth, due to terrestrial water vapor
absorption.Both observations require the high resolving power of
the SWS. In the case of Uranus, these observations will be performed
in the Central Programme (Griffin et al). The present proposal will
allow a comparative study of both Uranus and Neptune.