Constraining Depths and Wave Heights for Titan's lakes with Cassini RADAR Data
Abstract
Hayes et al. (JGR 2010) observed that the Cassini synthetic aperture radar (SAR) imaging magnitudes collected in Titan flybys T57 (22-June-2009) and T58 (8-July-2009) over Titan’s Ontario Lacus vary exponentially with distance from the lake shore, as expected if there is a deepening liquid layer that is attenuating the reflection from a roughened bottom. They deduced near-shore slopes at 13 locations around the lake perimeter. Previously, at the 2010 DPS meeting, we applied a similar model to the real-aperture radar data collected on Titan flyby T65 (12-January-2010) to obtain complete depth profiles across the entire width and length of Ontario Lacus, thereby constraining its maximum depths. The real-aperture reduction provides longer integration times, thus reducing the noise in the data. Consequently, we can detect bottom reflections from greater depths farther out into the lake. This improvement in signal-to-noise comes at the expense of resolution. We assume that the variation in backscatter across the lake is due largely to changes in depth, although changes in the lake bottom roughness or changes in the concentration of suspended scatterers would also affect the perceived backscatter. In this paper, we add the T57 and T58 data to our T65 Ontario Lacus analysis to more completely evaluate the subsurface topography across the lake. The shape of the lakebed may have implications for the lake geology. We also extend the bathymetry model to the northern lakes. While we map the depths of Ontario Lacus to be not much greater than several times the two-way absorption length (1.2 m for the assumed dielectric properties), the subsurface reflection in the northern lakes quickly extinguishes as the radar beam moves away from the shore, suggesting the lake floor to be at much greater depths. Only in Jingpo Lacus and Ligeia Mare do we find detectable subsurface structure away from the shore. Assuming dielectric properties similar to those deduced by Hayes et al. for Ontario Lacus' near shore regions, we evaluate partial depth profiles for these two northern lakes. We assume the dielectric properties to apply uniformly across the lake volumes, but we explore the effects that differing dielectric properties has on the depth interpretations. In our analysis, we allow for scatter from small-scale waves on the surface of these lakes. We find that the wave activity, assumed uniform across the lake, has to be less than 1 mm in rms height according to small perturbation theory. These results are consistent with the analysis of altimetry echoes from Ontario Lacus (Wye et al., GRL 2009) and suggest either that the wind is not strong enough at the time of the observations to initiate significant wave activity or that the liquid properties are quite different from what has been assumed (Lorenz et al., Icarus 2010). The outputs of this analysis will also be compared with those of another double layer model applied to SAR data. This model uses a Bayesian inversion algorithm to provide the mean values and the PDFs of the single parameter estimates, including wind speeds and optical thickness (Notarnicola et. al, IEEE TGRS 2009).
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.P31C1552W
- Keywords:
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- 0659 ELECTROMAGNETICS / Random media and rough surfaces;
- 5419 PLANETARY SCIENCES: SOLID SURFACE PLANETS / Hydrology and fluvial processes;
- 6281 PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS / Titan;
- 6964 RADIO SCIENCE / Radio wave propagation