Significance of Near-Surface Ice Fracture for Radio Sounding of Europa’s Ice

Abstract:

Ice-penetrating radar sounding is a powerful geophysical technique for directly imaging the near-subsurface structure of icy satellites. Scattering losses from subsurface features such as regolith, faults, or crevasses constrains the performance of radar sounders and can limit their ability to detect radar returns from subsurface features such as a global ocean. We present a systematic assessment of potential scattering loss mechanisms for radar sounding observations of Europa’s ice shell. We use a volume scattering model to evaluate losses from the Europan regolith, which is expected to be either impact-generated or tidal. While impact regolith does not extend deep enough to affect the radar signal, tidal regolith (if it exists) may be quite deep, based on thermal conductivity models. However, tidal regolith would only significantly attenuate the radar signal if it contained abundant pores at the decameter to meter scale. We also use a 1-D full-wave multi-layer scattering model to show that faults – regardless of their abundance and orientation– are unlikely to provide sufficient dielectric contrast to attenuate the radar signal. Finally, we use a rough-surface propagation model to show that vacuum-filled crevasses are not expected to have sufficient abundance to introduce significant losses. We conclude that, for the range of physical processes and material properties observed or hypothesized for Europa, scattering losses are unlikely to play a significant role in determining radar signal penetration and performance.