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R.D. Lorenz (Lunar and Planetary Lab, University of Arizona)
Various models for the anomalous radar reflectivity of the icy satellites have been proposed. Some invoke exotic subsurface structures (refractive lenses, hemispherical craters etc.) while others, notably the coherent backscatter theory, can achieve the observed high backscatter and polarization ratios with more-or-less random scatterers. Hapke's (Icarus vol.88, 407-417) succinct paper assumes the scatterers are voids or silicates in an ice surface. However, the mechanism for emplacing silicate rocks or voids in a 'solid ice' Europan surface is not clear.
I suggest here that a more likely scenario is ice boulders embedded in a porous ice matrix. Photometry indicates the upper surface is very porous, probably due to a steady resurfacing of 'snow' from eruptions along linea. This unconsolidated snow would not support impact-generated boulders - these would penetrate to some depth, larger ones deeper. Dust capture in aerogel is an appropriate analog. This model is qualitatively consistent with the low radar backscatter at 70cm wavelength - the large scatterers necessary are not abundant enough at shallow depths because there are fewer of them, and they penetrate deeper into the surface.
Tests of this model include nondetection in high-resolution optical imaging of surface boulders, even in rays from recent impact structures, and a paucity of scatterers at depth (where selfcompaction of the regolith removes the dielectric contrast between the boulder and the porous regolith). This latter implication should be apparent in, and favorable towards, subsurface sounding by an orbiting radar.