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 tex2html_wrap_inline14 m (43-90 tex2html_wrap_inline14 m) and 0.6 tex2html_wrap_inline14 m (90-197 tex2html_wrap_inline14 m). Weak methane features due to forbidden rotational transitions were detected in absorption against the hydrogen continuum at 106, 120 and 137 tex2html_wrap_inline14 m.

The saturnian spectrum was modelled using a multilayer, line-by-line radiative transfer code. The feature at 106 tex2html_wrap_inline14 m is well-matched by a methane mole fraction of tex2html_wrap_inline26 , while at 120 and 137 tex2html_wrap_inline14 m a mole fraction of tex2html_wrap_inline30 gives a better fit. Assuming a solar C/H ratio of tex2html_wrap_inline32 , 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 tex2html_wrap_inline14 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.