Different appearance of Titan's dunes
Résumé
In this paper we analyze Cassini's Titan Radar Mapper recent flybys and yet more evidence of dark linear dunes, in the latitude between 30° S and 30° N, whose orientations are somewhat comparable to previous dune observations but at closer study show morphological differences. The appearance of Titan's dunes depends on the projected look direction of the Cassini Radar antenna, incidence angle and resolution. Dune fields are generally oriented East/West on Titan, and for many radar observations the flyby is in the equatorial plane. At closest approach the imaging direction is most nearly normal to the dune direction such as in the central portion of the T8 swath. Away from that configuration, and especially past the -/+10 minutes from close approach, the relative azimuth angle that the projected look direction of the Cassini Radar antenna has with respect to the surface changes rapidly along with incidence angle and resolution resulting in signal attenuation of the imaged features. Observational biases in the SAR images are key for dunes comparison across Titan's equatorial belt. The results show that in some regions the projected look direction could be on the order of 60° and parallel to the long axis of the radar dark features direction (i.e. T16, T25, T28), therefore suggesting that the variation in backscatter must be a combination of compositional dunes dark material and bright interdune material, varying roughness and topography when present. This suggests that we cannot assume that all the dune fields currently imaged can be characterized simply on the bases of their orientation and therefore we suggest that the characterization of the imaged surface features should be divided into at least two categories: -1) topography driven (in which Radar-clinometry can be applied); -2) compositional or due to varying roughness.