In May 2013, the bottom of Ligeia Mare (LM), Titan, was detected in the active altimetry mode of the Cassini RADAR at a maximum depth of 160 m (Mastroguiseppe et al., 2014). This was the first and, so far, only detection of the floor of an extraterrestrial sea. The difference of amplitude of the surface and bottom echoes was also investigated in order to evaluate losses by absorption in the liquid layer. In this paper, we analyze the passive radiometry data that were acquired concurrently with the active data, in order to provide an independent estimate of the liquid loss tangent and to determine the dielectric constant of both the liquid and the seafloor. We then used these results to convert the radiometry mosaic of LM into a low-resolution bathymetry map. For the last 10 years, the passive radiometer incorporated in the Cassini RADAR has been observing the 2.2-cm wavelength thermal microwave emission from Titan. Its calibration has been recently refined to an unprecedented accuracy of <1% (Janssen et al., this meeting). To date, all LM has been mapped in high-spatial resolution. The 2.2-cm emissivity measured over it is directly related to the depth of the seafloor, the most emissive areas being the deepest and vice-versa. Comparing the radiometry data acquired in May 2013 to a two-layer model and using as an input the altimetry-derived depth profile, we find that the loss tangent value that best fits data is very low and only slightly smaller than that found by Mastroguiseppe et al. (2014) (3.0±1.0 10-5). This strongly suggests that the sea is composed of pure hydrocarbons with no or few suspended particles. A dielectric constant of 2.9 is inferred for the sea bottom pointing to water ice as its most likely composition rather than organic sediments. Lastly, the dielectric constant of the liquid is found to be <1.7, which, together with the low loss tangent, supports the idea of a methane-dominated composition (rather than ethane, Mitchell et al., submitted).