Cassini radar observation of Punga Mare and environs: Bathymetry and composition

In January 2015 (fly-by T108), the Cassini radar observed Punga Mare, Titan's northernmost and third-largest sea, in altimetry mode during closest approach. The ground track intercepted a section of the mare and a system of channels and flooded areas connecting Punga to Kraken Mare. We use a processing technique, successfully adopted for Ligeia Mare and Ontario Lacus, for detecting echoes from the sea floor and constraining the depth and composition of these liquid bodies. We find that, along the radar transect, Punga Mare has a maximum measured depth of 110 m. The relative reduction in backscatter of the seafloor, as a function of increasing depth, suggests a liquid loss tangent of 3 ± 1 ×10^-5. While this value is within the formal uncertainty of the loss tangent derived for Ligeia Mare, the best-fit solution is lower and is consistent with a nearly pure binary methane-nitrogen liquid with little to no ethane or higher order components. The indication of very low amounts of ethane toward the pole suggests that atmospheric processes are controlling the surface liquid composition of Titan's seas.