This proposal is a follow-up of the LWS graing spectrum obtained of Saturn as
part of the guranteed time GIANTS proposal. Three, weak, rotational methane
absorption features are observed but with a greater strength than
anticipated. The original guaranteed time proposal included a full Fabry-Perot
scan of Saturn, but in light of the reduced sensitivity of ISO, this has been
replaced by several miniscans targeting specific lines; methane was not
included. It is proposed to observe these methane lines at high spectral
resolution to determine the tropospheric concentration of methane. Methane is
an important source of opacity, so that further to obtaining the C/H ratio,
its abundance tells us of its contribution to the overall thermal balance of
the planet. The determination of the methane vertical concentration profile
using ISO LWS at far-infrared wavelengths does not suffer from the drawbacks
of previous near-infrared studies (saturation, scattering etc.) and should be
an important new measurement of the currently uncertain value for the
Satrunian C/H ratio.  Methane is important also as the starting point for a
network of photochemical reactions occurring in the the
stratosphere. Determination of the tropospheric vertical mixing ratio profile
(which is expected to be uniform) and comparison with the profiles of ethane
(C2H6), etylene (C2H2) and acetylene (C2H4) from the guaranteed time proposal
will provide a picture of the balance of both the carbon photochemistry and
vertical mixing as a function of altitude.