Methane Absorption Measured inside the Atmosphere of Titan

Details Methane Absorption Measured inside the Atmosphere of Titan Methane Absorption Measured inside the Atmosphere of Titan

Sep 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006dps....38.2205k&link_type=abstract

American Astronomical Society, DPS meeting #38, #22.05; Bulletin of the American Astronomical Society, Vol. 38, p.520

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Scientific paper

The Descent Imager / Spectral Radiometer recorded spectra in the spectral ranges 480 - 970 nm and 830 - 1700 nm. In both cases, several hundred spectra were taken between 140 and 0 km altitude looking up and looking down. We used these data to constrain properties and vertical distribution of the aerosols and to measure the transmission spectrum of methane as function of methane abundance.
Assuming a methane mixing ratio of 1.7 percent above 30 km altitude, increasing to 5 percent toward the surface, the observed depths and shapes of some methane bands agree well with calculations based on laboratory methane absorption coefficients.
For example, below 830 nm wavelength, the observed methane absorption is very consistent with the expected absorption at the temperature of Titan's atmosphere. On the other hand, in the 890 nm methane band, absorptions are weaker than expected, indicating problems in previous extrapolations of the methane absorption coefficient from laboratory room temperature to Titan's temperature.
Further in the infrared, various estimates of methane absorption coefficients at Titan's temperature existed before the descent of Huygens. Our data can exclude some of these estimates. For example, the strong methane band at 1400 nm wavelength appears three times weaker than expected in high-altitude observations of the sun, but as strong as expected in spectra of the surface illuminated by our lamp from 20 m altitude. This indicates that the necessary correction is not in the methane absorption coefficient, but in its pressure coefficient.
This work is supported by JPL contract 961160.

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