No oceans on Titan from the absence of a near-infrared specular reflection

Abstract

With its substantial atmosphere of nitrogen, hydrocarbons and nitriles, Saturn's moon Titan is a unique planetary satellite. Photochemical processing of the gaseous constituents produces an extended haze that obscures the surface. Soon after the Voyager fly-bys in 1980 and 1981 photochemical models1,2,3 led to the conclusion that there should be enough liquid methane/ethane/nitrogen to cover the surface to a depth of several hundred metres. Recent Earth-based radar echoes imply that surface liquid may be present at a significant fraction of the locations sampled4. Here we present ground-based observations (at near-infrared wavelengths) and calculations showing that there is no evidence thus far for surface liquid5. Combined with the specular signatures from radar observations, we infer mechanisms that produce very flat solid surfaces, involving a substance that was liquid in the past but is not in liquid form at the locations we studied.

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Figure 1: A cylindrical projection map of Titan at 2-µm wavelength11.
Figure 2: The 2.1-µm brightness of four locations on the surface of Titan measured over four days as a function of solar zenith angle.
Figure 3: Synthesized images and plots of reflectivity.

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Acknowledgements

We benefited from radiative transfer benchmark calculations provided by H. Gordon and from discussions with D. Campbell, R. Lorenz and S. Ostro. Part of this work was performed by the Jet Propulsion Laboratory, California Institute of Technology under contract with the National Aeronautics and Space Administration. Author Contributions M.E.B., A.H.B. and H.G.R. supplied the Keck observations and data analysis. S.V.S. provided the light-scattering model calculations.

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Correspondence to R. A. West.

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Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.

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West, R., Brown, M., Salinas, S. et al. No oceans on Titan from the absence of a near-infrared specular reflection. Nature 436, 670–672 (2005). https://doi.org/10.1038/nature03824

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