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New laboratory measurements of CH4 in Titan's conditions and a reanalysis of the DISR near-surface spectra at the Huygens landing site |
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| Authors: | D Jacquemart E Lellouch C de Bergh N Lacome M Tomasko |
| Institution: | a Université Pierre et Marie Curie-Paris 6, LADIR, CNRS, UMR 7075, Case Courrier 49, Bât F 74, 4, Place Jussieu, 75252 Paris Cedex 05, France b LESIA, Observatoire de Paris, CNRS, Université Pierre et Marie Curie-Paris 6, Université Paris-Diderot, 5 Place Jules Janssen, 92195 Meudon, France c LPG, Université Joseph Fourier, 38041 Grenoble, France d LPL, University of Arizona, Tucson, AZ 85721, USA |
| Abstract: | Laboratory spectra of methane-nitrogen mixtures have been recorded in the near-infrared range (1.0-1.65 μm) in conditions similar to Titan's near surface, to facilitate the interpretation of the DISR/DLIS (DISR—Descent Imager/Spectral Radiometer) spectra taken during the last phase of the descent of the Huygens Probe, when the surface was illuminated by a surface-science lamp. We used a 0.03 cm−1 spectral resolution, adequate to resolve the lines at high pressure (pN2∼1.5 bar). By comparing the laboratory spectra with synthetic calculations in the well-studied ν2+2ν3 band (7515-7620 cm−1), we determine a methane absorption column density of 178±20 cm atm and a temperature of 118±10 K in our experiment. From this, we derive the methane absorption coefficients over 1.0-1.65 μm with a 0.03 cm−1 sampling, allowing for the extrapolation of the results to any other methane column density under the relevant pressure and temperature conditions. We then revisit the calibration and analysis of the Titan “lamp-on” DLIS spectra. We infer a 5.1±0.8% methane-mixing ratio in the first 25 m of Titan's atmosphere. The CH4 mixing ratio measured 90 s after landing from a distance of 45 cm is found to be 0.92±0.25 times this value, thus showing no post-landing outgassing of methane in excess of ∼20%. Finally, we determine the surface reflectivity as seen between 25 m and 45 cm and find that the 1500 nm absorption band is deeper in the post-landing spectrum as compared to pre-landing. |
| Keywords: | Methane Titan Huygens DISR near-surface spectra |
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