Modeling Sar Backscattering of Bright Flows and Dark Spots on Titan

Introduction: The SAR imaging mode of the Cassini Radar instrument allows to map the surface of Titan with a high-resolution [1]. The first Cassini close flyby Ta was acquired on 26 October 2004 and revealed a complex surface, with areas of low relief and dome-like volcanic constructs, flows and sinuous channels [1]. In particular, channels and fan-like features with a strong radar backscattering were observed; the strong SAR signal was explained by a high component of volume scattering [2]. Such fan-like features, extending from tens of kilometers to more than 200 km in length, could be the result of cryovolcanism [3]. Also, a number of radar-dark spots up to 30 km across were observed: they could correspond to smooth hydrocarbon deposits [4]. We present here a first analysis of radar-bright and radar-dark features of the Ta flyby, based on the use of classical SAR backscattering mo dels. We considered two main materials which could constitute the surface of Titan, tholins and water-ammonia ice, and modeled both the single and two-layer cases, taking into account volume and sub-surface scattering. First results show that SAR-bright regions can be explained by both strong volume scattering in a water ice + ammonia layer or by the effect of a thin layer of such material covering a tholin substratum. Radar-dark spots can also be modeled with two scenarios: a rough tholin surface or a smooth one with some volume scattering. Radar-bright Regions: We studied the SAR image extract presented in Figure 1. We considered two regions, region #1 being characterized by a lower SAR backscattering typical of a “surrounding SAR-dark material”, while region #2 corresponds to SAR-bright fan-like features which could be related to flow features covering the SAR-dark material. SAR illumination is from the left, and the terrain is supposed to be rather flat. The studied region is located around coordinates 50.92N, 79.35W, and the SAR incidence angle there is close to 30. We shall consider in the following that the SAR-dark material could be representative of a tholincomposed surface, of dielectric constant estimated to ε = 2.2-0.01i [5], within the 2-3 range obtained from the radiometry mode [1]. If of cryovolcanic origin, SARbright flows could be composed of a mixture of water ice and ammonia, whose dielectric constant is estimated to ε = 4.5-0.04i [6]. We worked on SAR normalized cross-section values (σ), not corrected for incidenceangle effect, at a resolution of 175 m per pixel: region #1 presents an average backscattered power of -7.5 dB, while region #2 corresponds to a σ value around 0 dB.