m (43-90 m) and 0.6 m (90-197 m).
Weak methane features due to forbidden rotational transitions were
detected in absorption against the hydrogen continuum at 106, 120 and
137 m.
First detection of far-infrared methane features in Saturn
G.R. Davis (Saskatchewan), P.G. Oldham, M.J. Griffin (QMW), D.A. Naylor (Lethbridge), B.M. Swinyard (RAL), P.A.R. Ade (QMW), M. Burgdorf (VILSPA), G.S. Orton (JPL), S.B. Calcutt, P.G.J. Irwin (Oxford), D. Gautier, Th. Encrenaz, E. Lellouch (Meudon), Th. de Graauw (SRON)
The Long Wavelength Spectrometer (LWS) is one of four instruments on
the ESA Infrared Space Observatory (ISO) satellite. The far-infrared
spectrum of Saturn was measured on 16 July 1996, with spectral
resolutions of 0.29 m (43-90 m) and 0.6 m (90-197 m).
Weak methane features due to forbidden rotational transitions were
detected in absorption against the hydrogen continuum at 106, 120 and
137 m.
The saturnian spectrum was modelled using a multilayer, line-by-line
radiative transfer code. The feature at 106 m is well-matched by a
methane mole fraction of 
, while at 120 and 137 m
a mole fraction of 
gives a better fit. Assuming a
solar C/H ratio of 
, the above values imply a
saturnian C/H ratio of 1-2.1 times the solar value, which is less than
the C/H ratio determined from studies of the 1.1 m methane band
(2-8 times solar).
The discrepancy between the C/H ratios at the three different
wavelengths is under investigation. We plan to repeat these
observations with lower bias and longer integration time later in the
mission. We will also observe these features with higher spectral
resolution, from which we hope to determine the vertical distribution
of tropospheric methane on Saturn.