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 because of 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 polarization ratios of icy satellites are correlated and vary together along a continuum, which is not satisfactorily explained by existing models. We modified a backscatter model that includes the coherent backscatter opposition effect (CBOE) and found that it fits successfully the observed continuum, indicative of CBOE's primary role in driving the circular polarization ratios of icy satellites. We predict that the linear polarization ratios and the incidence angle variation properties also vary along the observed continuum, and that, globally, 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. The unusual radar properties of icy satellites appear to be correlated and distributed along a spectrum of values. Only modelling including the coherent backscatter opposition effect can reproduce this behaviour.

Automated summary

The radar properties of icy satellites of Jupiter and Saturn differ significantly from those of rocky planets due to the lower absorptivity of water ice. However, the exact mechanisms behind these differences are uncertain. In this study, we modified a backscatter model and found that the coherent backscatter opposition effect (CBOE) plays a significant role in driving the circular polarization ratios of icy satellites. Our predictions suggest that the linear polarization ratios and the incidence angle variation properties also vary along a continuum. The modified CBOE model indicates that world beyond Saturn's orbit rich in non-water ices may have even more extreme radar properties.