Dielectric Characterization of Salty-Ice Analogues and Simulations of Radar Signal Propagation Through the Icy Crust of Jovian Moons

 

The Jovian icy moons – Ganymede, Europa, and Callisto – are of great astrobiological and geophysical interest due to the potential presence of liquid water inside/beneath their icy shells. Among all geophysical methods, Radio Echo Sounding (RES) appears to be the most suitable technique to detect such hidden water, especially as it can operate from an orbiting platform. Starting early 2030s, RIME and REASON, the radar sounders aboard JUICE and Europa Clipper missions, will extensively explore the internal structure of the Galilean moons to search for any evidence of liquid water and to help assessing the habitability conditions of such icy bodies. In order to properly interpret the radar data, the dielectric behaviour of the material composing the crust must be known. Data regarding the dielectric behaviour of salty ices are sparse, especially in the frequency range of such radar sounders, and poorly understood.  

Given the ambiguity in the composition of the icy crusts, a large set of icy analogues should be explored, although laboratory measurements are time consuming and difficult to be properly performed. In this work we start addressing this problem, combining dielectric properties measured in laboratory with radar signal propagation simulations. 

Because the capability of radio waves to investigate deep in the crust depends on signal attenuation that, in turn, is controlled by temperature, type of salt and salt concentration, we performed dielectric measurements at various temperatures and salt concentrations.  We started by considering the most problematic salt, NaCl, as it is known to be able to enter the ice lattice and affect the conductivity of the icy mixture (and thus signal attenuation). We measured the complex dielectric permittivity of NaCl-doped ice samples over a radar frequency range of 1-100 MHz for the salt concentration range 10-1000 mM and the range of temperature 198-292 K, using a two-port Vector Network Analyzer (VNA) coupled with a coaxial cage cell inserted in a climatic chamber. Then, we used the results of such measurements to generate different subsurface scenarios and to run radar simulations at 9 MHz (one of the operational frequencies of RIME and REASON), to assess the detectability of various targets inside the icy crusts and to validate the performance of the radars. 

Our results provide a first hint on the detectability of the water inside/below an NaCl-icy crust and on the penetration depth of the radar signals in different thermal and salt concentration profiles.