The methane mole fraction in Titan's stratosphere from DISR measurements during the Huygens probe's descent

We present a determination of the methane mole fraction in Titan's stratosphere using spectral measurements by the Upward Looking Infrared Spectrometer (ULIS) of the Descent Imager/Spectral Radiometer (DISR) aboard the Huygens probe. We analyzed the 1.4-μm band of methane for which a complete linelist, down to very low intensities, was recently made available. The DISR/ULIS measurements were used to derive the methane transmittance along the path from the probe to the Sun during the descent from 135 to 29 km. Fitting the transmittance ratios to remove residual instrumental effects, we derived a stratospheric methane mole fraction of 1.44 (+0.27/−0.11)%, taking into account random errors and uncertainties in the spectroscopic parameters. This value is fully consistent with the simultaneous measurement of the methane profile by the Huygens Gas Chromatograph/Mass Spectrometer (GCMS). It disagrees with the ∼1% mixing ratio recently inferred from Cassini/CIRS spectra at low latitudes near 85 km. Possible reasons for the discrepancy and uncertainties in the methane spectroscopic parameters are discussed.