Preliminary tests to combine X-ray microtomography and dielectric measurements to assess the radar properties of pure water ice

Two large space missions, JUICE and EUROPA Clipper are on their way to reach the icy satellites of Jupiter in the early 2030s. One of the main scope of these missions is to find liquid water below/inside the icy crusts and to assess the habitability conditions of such ocean worlds. Radar sounders, on board these missions, will play a fundamental role in detecting position, depth and composition of the water. However, presently our understanding of the composition and thermal state of such icy crusts is poorly constrained, which makes the detection of liquid water using radio waves very difficult. Therefore, it is of paramount importance to perform systematic measurements of the dielectric properties of a large set of icy materials having different salt composition and temperature, to define the range of penetration of the radar signals in different scenarios and to assess the detectability limit of the water.

To reach this goal, as a first step, it is important to determine the dielectric properties of pure water ice in the frequency range typical of planetary radar sounders (1-100 MHz). The aim of this work was to optimize the laboratory procedure to assess such properties, combining X-ray micro-computed tomography images with low/high frequency dielectric measurements. The experimental activity was first focused on defining a procedure to produce polycrystalline Ih ice samples, minimizing the presence of defects like air bubbles and cracks - which are known to affect the results of the dielectric measurements. To achieve this purpose, different samples were prepared using different sample holders and cooling rates and then analysed qualitatively and quantitatively using microtomography. Once the most reliable procedure to minimize ice defects was assessed, samples of pure ice were produced in a climatic chamber simultaneously using the microtomography and the dielectric cells, to test the possibility to perform structural analysis and dielectric measurements on the same type of ice. Dielectric measurements were performed using both a capacitive cell connected to an LCR-meter instrument and a coaxial line connected to a VNA. The results of this work confirm that this procedure can be successfully applied to control the integrity of the sample and to assess, at the same time, the dielectric properties of pure Ih ice.