Revolution 17 began with apoapsis at 41 planetary radii on 20 October 2005. The T8 fly-by on 28 October was at an altitude of 1,353 kilometres. This time, the radar imaged the dark Belet terrain to the west of Xanadu, which was revealed to contain a field of 100-metre-tall dunes. The individual dunes ran for hundreds of kilometres, and the field extended at least 1,000 kilometres along the equatorial zone.268,269 The 'cat scratches' of T3 were seen to be smaller dune fields. ''It's bizarre,'' said R.D.
Lorenz. "These images from a moon of Saturn look just like radar images of the deserts in Arabia and Namibia on Earth. Titan's atmosphere is thicker than Earth's, its gravity is lower, and its 'sand' is certainly different - everything's different except the physical process that forms the dunes and resulting landscapes.''
Calculations showed that the variable tidal wind caused by Saturn's gravitation was combining with the west-to-east zonal circulation to create surface winds that would produce a sinuous pattern of longitudinal dunes that were aligned with the direction of the prevailing wind flow, except near elevated terrain that controlled the local wind direction.270 This process could transfer fine material from the mid-latitudes into the equatorial zone. Furthermore, the presence of dunes implied the absence of persistent liquids on the surface in the equatorial zone that would serve as sand traps - a conclusion that seemed to conflict with the idea that Titan was a land of lakes.
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