37th DPS Meeting, 4-9 September 2005
Session 46 Titan's Surface and Magnetic Environment

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[46.15] The reflectance spectrum of Titan's surface as determined by Huygens

S.E. Schröder (MPI fur Sonnensystemforschung), M.G. Tomasko (LPL), H.U. Keller (MPI fur Sonnensystemforschung), DISR Team

The Huygens mission was designed to investigate the atmosphere and surface of Saturn's largest moon Titan. The Huygens images show that after its successful descent through Titan's atmosphere the probe landed in a dry lakebed. Determining the composition of the dark surface material at the landing site is of paramount importance, as it will certainly lead to further understanding of Titan's atmospheric methane cycle. For this purpose the Huygens probe carried spectrometers that recorded data all the way down to the surface. As at many wavelengths the atmosphere is completely opaque due to atmospheric methane absorption, a complete reflectance spectrum cannot be acquired from outside the atmosphere, and this makes the Huygens measurements unique.

The Descent Imager/Spectral Radiometer (DISR) aboard Huygens included visual and infrared spectrometers, as well as a lamp that illuminated the surface shortly before and after landing. We employ two methods to derive the reflectance spectrum of the surface at the landing site. The first divides up- and downward fluxes just before landing with the lamp off, and the second directly measures the spectrum of the illuminated surface after landing. The last method has the advantage that it also yields the surface reflectance in the methane absorption bands. Both methods pose serious challenges. Whereas the outcome of the first depends on the input of an atmosphere model, the second requires precise knowledge of the lamp flux at the measurement location, which in turn requires knowledge of the probe attitude after landing. We attempt to reconcile the results of the two methods to arrive at the true reflectance spectrum of the surface at the landing site. We describe the result and investigate how it compares to the reflectance of various compounds (tholins, water ice) theorized to be present on the surface of Titan.

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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.