The extensive Medusae Fossae Formation (MFF) along the dichotomy boundary on Mars has geologic features indicative of wind erosion of low-density material. There is evidence suggesting a water ice component, but with considerable uncertainty linked to the unknown MFF porosity and compaction behavior. We use SHARAD radar sounder data to estimate the real permittivity and loss tangent of MFF deposits, and compare these to a model for sediment compaction and to the properties of ice in mid-latitude glaciers. In areas along the margins of Eumenides Dorsum, between Gordii Dorsum and Amazonis Mensa, and in northwest Zephyria Planum, the loss tangent is about 0.001 at 170 m and plateaus at about 0.003 for 310-550 m thickness. The real dielectric constant across the study areas ranges from 2 to 3. We propose that the MFF is a two-layer deposit, with 300-600 m of fine-grained, self-compacting material above up to 2 km of minimally compacting, low-loss material. The lower unit could be ice-free and very coarse-grained, but we see no evidence of extensive sand exposed by erosion. The lower layer might instead be ice-rich and protected from sublimation by the dry cover. The volume of cover relative to a high ice content in the lower layer implies hybrid MFF formation as glacial or polar layered deposits capped by a dry, perhaps pyroclastic ash, component.
