Bathymetry and absorptivity of Titan's Ontario Lacus

Abstract

[1] Ontario Lacus is the largest and best characterized lake in Titan's south polar region. In June and July 2009, the Cassini RADAR acquired its first Synthetic Aperture Radar (SAR) images of the area. Together with closest approach altimetry acquired in December 2008, these observations provide a unique opportunity to study the lake's nearshore bathymetry and complex refractive properties. Average radar backscatter is observed to decrease exponentially with distance from the local shoreline. This behavior is consistent with attenuation through a deepening layer of liquid and, if local topography is known, can be used to derive absorptive dielectric properties. Accordingly, we estimate nearshore topography from a radar altimetry profile that intersects the shoreline on the East and West sides of the lake. We then analyze SAR backscatter in these regions to determine the imaginary component of the liquid's complex index of refraction (). The derived value, = (6.1 −1.3 +1.7 ) × 10 −4 , corresponds to a loss tangent of tan D = (9.2 −2.0 +2.5 ) × 10 −4 and is consistent with a composition dominated by liquid hydrocarbons. This value can be used to test compositional models once the microwave optical properties of candidate materials have been measured. In areas that do not intersect altimetry profiles, relative slopes can be calculated assuming the index of refraction is constant throughout the liquid. Accordingly, we construct a coarse bathymetry map for the nearshore region by measuring bathymetric slopes for eleven additional areas around the lake. These slopes vary by a factor of ∼5 and correlate well with observed shoreline morphologies.

Transient features in a Titan sea

“…These profiles, which are both above their respective noise floors, are correlated and the lesser T92 NRCS, relative to that of T91, is due to its greater incidence angle. This behaviour is consistent, for example, with the radar transmitting through the liquid and scattering off the seabed as claimed by other analyses 3,19,22,23 . In the region of the anomalies, however, the T92 profile exhibits a large spike in NRCS but no similar anomalous spike is observed in the T91 profile.…”

“…Including the additional physics of refraction and loss due to reflection at the atmosphere-sea interface 23 , we followed the same prescription as above to test models for submerged seamounts. We used the recently measured index of refraction for Ligeia Mare to calculate the refracted incidence angles of the radar 19 and considered a perfectly flat surface for Ligeia Mare, consistent with recent measurements 8 , when calculating the Fresnel transmission coefficients.…”

Geomorphological significance of Ontario Lacus on Titan: Integrated interpretation of Cassini VIMS, ISS and RADAR data and comparison with the Etosha Pan (Namibia)

“…These data have been mentioned previously by Hayes et al (2010) and Hayes et al (2011), but never presented as images in a published work. Hayes et al (2010) showed that the normalized radar backscatter cross-section (σ 0 ) observed over Ontario Lacus decreases exponentially with the distance to the border. They interpreted this behavior as a radio wave attenuation through a deepening liquid medium, thus in agreement with the altimetry interpretation of Wye et al (2009).…”

“…Since speckle noise is an effect of the observed surface roughness and subsurface volume scattering, not an instrumental effect , the lack of speckle noise on Unit A leads us to conclude that the salt-and-pepper texture observed on Unit B is characteristic of a specific geomorphological unit different from Unit A. Roughness on the order of the wavelength or significant volume scattering compared to that of Unit A could explain this difference between Units A and B. According to our interpretation, the exponential decrease in average radar backscatter observed by Hayes et al (2010) across the outer border of Unit B (Fig. 11) cannot be attributed to microwave attenuation through a deepening liquid medium.…”

“…According to our interpretation, Unit A (corresponding to the darkest parts of Ontario Lacus in RADAR images, R1d), is covered by liquids because the radar backscatter recorded over this unit is below the noiselevel, and likely attributable to a combination of specular reflection and absorption in these liquids. This hypothesis is inconsistent with a composition dominated by simple liquid light hydrocarbons according to the penetration depth of microwaves in these materials Hayes et al, 2010) or requires that the liquid layer is deep enough so that microwaves do not probe the floor. However, it might be consistent with more complex liquid compositions, involving dissolved or suspended materials.…”