Radar - Bright Channels on Titan

In June 2008, during the T44 swath, the Cassini SAR (Synthetic Aperture Radar) observed sinuous channels in the South-west of the Xanadu region (Fig. 1 and 2). Some parts of these channels exhibit very large radar cross-sections, up to 5 dB whereas the angle of incidence was ~20°. This is larger than allowed by the coherent backscatter model considered to explain the unusual reflective and polarization properties of the icy satellites [1] and only a few radar scattering mechanisms can be responsible for such high radar returns. We propose the presence of (transparent) rounded, icy rocks with size larger than the radar wavelength (2.18 cm) in the channels to explain the large radar cross-sections measured in these units, and discuss the geological implications. This paper is intended to contribute to understanding the anomalously high radar backscatter measured in several regions of Titan and discussed in a companion paper [2]. Fig. 1: Xanadu region observed by the Cassini SAR superimposed on a combination of VIMS and ISS observations. Radar-bright channels revealed during the T44 radar swath are outlined in black. Radar-bright channels in South-west Xanadu: The channels revealed during the T44 swath are among the brightest units observed by the Cassini radar anywhere on Titan. Fig. 2 depicts their measured backscatter values. The highest radar cross-sections are close to 5 dB. Since parts of these channels probably have widths smaller than the pixelization of the radar reflectivity, their radar cross-sections may be underestimated. The flows appear to originate from rugged terrains consisting of overlapping mountain ranges. Their origin is likely to be fluvial, formed by rainfall of methane. The meandering morphology suggests a low regional southward gradient [3]. Fig. 2: Radar-bright rivers observed during the T44 radar swath with corresponding normalized radar cross-sections in dB. Backscattering interpretation: On Titan, the effective averaged dielectric constant is low (~1.7) [4]; surface materials are hydrocarbons, tholins and, in some places, water ice, possibly with ammonia. In such conditions, it is difficult to explain the measurements of radar brightness higher than 3 dB at incidence angles of ~20°. We might think of Titan’s surface as similar to the surfaces of highly reflecting icy satellites such as Europa and Enceladus [5] which have been modeled with some success as low-loss inhomogeneneous media that lead to multiple scattering and depolarization of incident radar signals. Coherent backscattering has been argued to explain the high reflectivities seen [1]. However, coherent backscattering cannot account for radar cross-sections in excess of 3 dB. Double bounce effect on the wall of the rivers will be a more efficient scattering mechanism but it is unlikely to occur in many places. We propose another hypothesis: channels are filled with rounded rock mainly composed of water-ice similar to those observed by the Huygens probe at its landing site. Water-ice is a low loss medium; its microwave dielectric constant ε’ is 3.13 and its loss tangent (ε’’/ε’) is less than 10 [6]. It has long been known that transparent (or low-loss) spheres with diameter larger than the wavelength backscatter significantly more (of about an order of magnitude) than metal spheres of the same size [7][8][9]. This results from internal reflection on the rear surface of the sphere (Fig. 3). Scattering by transparent spheres has been well described by the Mie theory. Fig. 4 shows the theoretical Mie radar cross-section for a wavelength of 2.18 cm as a function of a, the radius of the spheres, for values ranging from 0.1 to 150 cm. The radar cross-section σ of the ice spheres is normalized with respect to the correspond1533.pdf 40th Lunar and Planetary Science Conference (2009)