EVIDENCE FOR SURFACE HETEROGENEITY ON TITAN

UNLIKE all other planetary satellites, Saturn's moon Titan has a massi ve atmosphere1-8. At visible wavelengths, a thick stratospheric haze h ides 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, a nd narrow windows exist in which the atmosphere absorbs only weakly9-1 4, and through which we might therefore catch a glimpse of the surface . Within two of these windows, Lemmon et al.15,16 recently observed a difference in Titan's albedo when the satellite was at eastern and wes tern elongation with respect to Saturn. Although these observations co uld be taken to imply that Titan's surface is heterogeneous (and there fore is not covered by a global methane-ethane ocean as predicted prev iously7), they could also be explained by transient clouds. Here I pre sent observations from two more rotational periods which record the sa me albedo difference, indicating that the heterogeneity is most unlike ly to be associated with transient features and must be intrinsic to t he surface. These results also imply that Titan is locked in a synchro nous orbit about Saturn.