A continuum of icy satellites’ radar properties explained by the coherent backscatter effect

The radar properties of icy satellites of Jupiter and Saturn commonly differ by more than an order of magnitude from those of rocky planets, due to the lower absorptivity of water ice than that of rock. However, the specific mechanisms behind these differences are not confidently known yet. Here we show that the global radar albedos and the circular polarisation ratios of icy satellites are correlated and vary together along a continuum, which is not satisfactorily explained by existing models. We modify a backscatter model that includes the coherent backscatter opposition effect (CBOE) and find that it fits successfully the observed continuum, indicative of CBOE’s primary role in driving the circular polarisation ratios of icy satellites. We predict that also the linear polarisation ratios and the incidence angle variation properties vary along the observed continuum, and that Europa and Enceladus have very heterogeneous surfaces/subsurfaces with very little microwave absorption. The modified CBOE model suggests that some worlds beyond Saturn’s orbit that are rich in non-water ices may have even more extreme radar properties.