Observed DISR Spectra at Different Altitudes
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Observed DISR Spectra at Different Altitudes

November 16, 2009
A graph showing three spectra obtained at different altitudes of 150 km, 500m and 20m during the descent of Huygens to the surface of Titan.

The regions of lowest intensity in the spectra correspond to wavelengths where light is strongly absorbed by methane. The intensity does not fall to zero (see arrows alongside the red line) in the centre of these bands due to the diffusion and scattering of light in the atmosphere.

Close to the surface, methane is a strong absorber of sunlight (as shown in the blue line).
The reflected spectrum from illumination by a 20m lamp on the spacecraft, shown by the green line, has a lower level of methane absorption and allows scientists to measure the methane abundance at the surface. It also yields the reflectivity of this surface at the landing site, along with precious information on the composition of the surface, an important goal for Huygens.

NOTE - the intensity is given in arbitrary units and the reflected spectrum (green line) has been corrected for residual sunlight and light colour from the lamp.

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