Constraining the physical properties of Titan’s empty lake basins using nadir and off-nadir Cassini RADAR backscatter

Abstract

We use repeat synthetic aperture radar (SAR) observations and complementary altimetry passes acquired by the Cassini spacecraft to study the scattering properties of Titan's empty lake basins. The best-fit coefficients from fitting SAR data to a quasi-specular plus diffuse backscatter model suggest that the bright basin floors have a higher dielectric constant, but similar facet-scale rms surface facet slopes, to surrounding terrain. Waveform analysis of altimetry returns reveals that nadir backscatter returns from basin floors are greater than nadir backscatter returns from basin surroundings and have narrower pulse widths. This suggests that floor deposits are structurally distinct from their surroundings, consistent with the interpretation that some of these basins may be filled with evaporitic and/or sedimentary deposits. Basin floor deposits also express a larger diffuse component to their backscatter, which is likely due to variations in subsurface structure or an increase in roughness at the wavelength scale [5]. We generate a high-resolution altimetry radargram of the T30 altimetry pass over an empty lake basin, with which we place geometric constraints on the basin's slopes, rim heights, and depth. Finally, the importance of these backscatter observations and geometric measurements for basin formation mechanisms is briefly discussed.

Raised Rims Around Titan's Sharp‐Edged Depressions

“…As the delay‐Doppler processed altimetry show local surface slopes of rims that are comparable to the incidence angle of SAR images (ranging from 23° to 45°; Michaelides et al, ), the local incidence angle is thus near specular, and so we suspect that this explains the brightness of the rims.…”

“…With this spacecraft vector imposed, we calculate the angle between the surface normal of each pixel and this vector, which provides us with the local incidence angle ( θ c ). We then apply a composite scattering law that has been applied to Titan previously (Michaelides et al, ; Wye et al, ), which takes the following form: where the total backscatter ( σ 0 ) is a mixture of both a quasi‐specular ( σ 0 − q ) and diffuse ( σ 0 − d ) component. The quasi‐specular component is defined using a Hagfors () model, given by the following: where ρ is the Fresnel reflection coefficient, related to the dielectric constant by .…”

“…At low incidence angles σ 0 − q will dominate, while at high incidence angles, scattering will be primarily diffuse. We then assume values for A d i f f , n , B , and ε for both the interior and exterior of the SED separately that have been published previously (Michaelides et al, ; Wye et al, ). For the interior, we assume a flat ( σ h = 0.1°), pure methane liquid surface with ε = 1.7 as measured by Mitchell et al ().…”

“…With this spacecraft vector imposed, we calculate the angle between the surface normal of each pixel and this vector, which provides us with the local incidence angle ( c ). We then apply a composite scattering law that has been applied to Titan previously (Michaelides et al, 2016;Wye et al, 2007), which takes the following form:…”

Liquid‐filled canyons on Titan

“…In an attempt to confirm canyon widths obtained by means of the SAR images analysis, we applied the advanced altimetry data processing technique described in detail by Michaelides et al [2016], who used it to enhance along-track resolution for May 2007 (T30) altimetry observations and measure the profile of empty lake basins on Titan. Iso-Doppler regions, extending within the area illuminated by the altimetric pulse, are limited across track by the antenna beam width (~8.5 km) but span along track for only~1 km.…”