Unlike all other planetary satellites, Saturn's moon Titan has a massive atmosphere1-8. At visible wavelengths, a thick stratospheric haze hides the surface from view. The emission from Titan in the infrared is largely from methane, nitrogen and hydrogen also in the stratosphere4. In the near-infrared, however, the extinction from haze decreases, and narrow windows exist in which the atmosphere absorbs only weakly9-14, and through which we might therefore catch a glimpse of the surface. Within two of these windows, Lemmon et a1.15,16 recently observed a difference in Titan's albedo when the satellite was at eastern and western elongation with respect to Saturn. Although these observations could be taken to imply that Titan's surface is heterogeneous (and therefore is not covered by a global methane-ethane ocean as predicted previously7), they could also be explained by transient clouds. Here I present observations from two more rotational periods which record the same albedo difference, indicating that the heterogeneity is most unlikely to be associated with transient features and must be intrinsic to the surface. These results also imply that Titan is locked in a synchronous orbit about Saturn.
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