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  • Huygens Articles
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Rain, winds and haze during the Huygens probe's descent to Titan's surface

Abstract

The irreversible conversion of methane into higher hydrocarbons in Titan's stratosphere implies a surface or subsurface methane reservoir. Recent measurements from the cameras aboard the Cassini orbiter fail to see a global reservoir, but the methane and smog in Titan's atmosphere impedes the search for hydrocarbons on the surface. Here we report spectra and high-resolution images obtained by the Huygens Probe Descent Imager/Spectral Radiometer instrument in Titan's atmosphere. Although these images do not show liquid hydrocarbon pools on the surface, they do reveal the traces of once flowing liquid. Surprisingly like Earth, the brighter highland regions show complex systems draining into flat, dark lowlands. Images taken after landing are of a dry riverbed. The infrared reflectance spectrum measured for the surface is unlike any other in the Solar System; there is a red slope in the optical range that is consistent with an organic material such as tholins, and absorption from water ice is seen. However, a blue slope in the near-infrared suggests another, unknown constituent. The number density of haze particles increases by a factor of just a few from an altitude of 150 km to the surface, with no clear space below the tropopause. The methane relative humidity near the surface is 50 per cent.

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Figure 1: View of Titan from 34 km above its surface.
Figure 2: View from 8 km.
Figure 3: View from 1.2 km.
Figure 4: The view from Titan's surface.
Figure 5: View of ‘shoreline’ and channels.
Figure 6: Topographic model of highland region 5 km north of the Huygens landing site.
Figure 7: Titan's surface.
Figure 8: Distribution of rock on the surface.
Figure 9: Probe ground track.
Figure 10: Observed winds.
Figure 11: The surface of Titan displayed in true colour.
Figure 12: Reflectivity samples of Titan's surface.
Figure 13: Spectral comparison of bright highlands and dark lowlands.
Figure 14: Derivation of methane mole fraction.
Figure 15: Reflectance of Titan's surface.
Figure 16: Haze properties.
Figure 17: Atmospheric spectra.
Figure 18: Haze models versus observations.
Figure 19: Vanishing sunlight.
Figure 20: Determination of total haze optical depth.
Figure 21: Haze vertical structure and total optical depth.
Figure 22: Thin cloud layer observation at 21 km.

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Acknowledgements

We thank the people from the following organizations whose dedication and effort have made this project successful: AETA (Fontenay-aux-Roses, France), Alcatel Space (Cannes, France), Collimated Holes Inc., EADS Deutschland GmbH (formerly Deutsche Aerospace AG, Munich, Germany), ETEL Motion Technology (Mortiers, Switzerland), The European Space Agency's (ESA) European Space and Technology Centre (ESTEC), The European Space Operations Centre (ESOC), The Jet Propulsion Laboratory (JPL), Laboratoire de Planétologie de Grenoble (CNRS-UJF), Loral Fairchild (Tustin, California, USA), Martin Marietta Corporation (Denver, Colorado, USA), Max-Planck-Institut für Sonnensystemforschung (Katlenburg-Lindau, Germany), Observatoire de Paris (Meudon, France), Technische Universität Braunschweig (TUB), Thomson-CSF (Grenoble, France), University of Arizona's Kuiper Lunar and Planetary Laboratory (LPL), and the US Geological Survey (Flagstaff, Arizona, USA).

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Tomasko, M., Archinal, B., Becker, T. et al. Rain, winds and haze during the Huygens probe's descent to Titan's surface. Nature 438, 765–778 (2005). https://doi.org/10.1038/nature04126

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