Status Report

Visible and IR Spectrum of Titan’s surface and atmosphere recorded by DISR onboard Huygens

By SpaceRef Editor
February 22, 2005
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During the Huygens descent to the surface of Titan the DISR instrument recorded visible and infrared spectra (the change in the amount of light with varying colour or wavelength) of the ambient sunlight which is absorbed, diffused, and reflected by both the surface and the atmosphere. The figure shows spectra obtained by the Downward Looking Infrared Spectrometer at three altitudes: 150 km, 500 m and 20 m above the surface.

Observed DISR Spectra at Different Altitudes (note – Intensity is given in arbitrary units) Copyright: ESA, NASA, LPL/University of Arizona/Observatoire de Paris-Meudon

Solar radiation is scattered and absorbed by the gas and aerosols in the atmosphere. 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.

Since the instrument is looking downward, this measurement is a sensitive probe of the abundance of methane and of haze/cloud particles below Huygens.

Close to the surface, methane is a strong absorber of sunlight (as shown in the blue line). As a result, a 20 W lamp was used to give uniform spectral illumination for optimum surface reflectivity (colour) measurements in order to infer the composition of the surface material.

The reflected spectrum, shown by the green line1, 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.

The plateau at around 1430 to 1580 nanometres is believed to be due to absorption by water ice.  Laboratory work is in progress to validate these assumptions.

1Shown here at a different scale after removal of the residual sunlight and correction for the colour of the light from the lamp.


Bruno Bézard
LESIA/Observatoire de Paris
Tel: + 33 1 45 07 77 17

Marty Tomasko
LPL/University of Arizona

Olivier Witasse
Huygens Project Scientist Team, ESTEC
+31 71 565 8015

SpaceRef staff editor.