Federico Di Paolo

Dielectric properties of Jovian satellite ice analogs for subsurface radar exploration: A review

The first European mission dedicated to the exploration of Jupiter and its icy moons (JUpiter ICy moons Explorer—JUICE) will be launched in 2022 and will reach its final destination in 2030. The main goals of this mission are to understand the internal structure of the icy crusts of three Galilean satellites (Europa, Ganymede, and Callisto) and, ultimately, to detect Europas subsurface ocean, which is believed to be the closest to the surface among those hypothesized to exist on these moons. JUICE will be equipped with the 9 MHz subsurface-penetrating radar RIME (Radar for Icy Moon Exploration), which is designed to image the ice down to a depth of 9 km. Moreover, a parallel mission to Europa, which will host onboard REASON (Radar for Europa Assessment and Sounding: Ocean to Near-surface) equipped with 9MHz and 60MHz antennas, has been recently approved by NASA. The success of these experiments strongly relies on the accurate prediction of the radar performance and on the optimal processing and interpretation of radar echoes that, in turn, depend on the dielectric properties of the materials composing the icy satellite crusts. In the present review we report a complete range of potential ice types that may occur on these icy satellites to understand how they may affect the results of the proposed missions. First, we discuss the experimental results on pure and doped water ice in the framework of the Jaccard theory, highlighting the critical aspects in terms of a lack of standard laboratory procedures and inconsistency in data interpretation. We then describe the dielectric behavior of extraterrestrial ice analogs like hydrates and icy mixtures, carbon dioxide ice and ammonia ice. Building on this review, we have selected the most suitable data to compute dielectric attenuation, velocity, vertical resolution, and reflection coefficients for such icy moon environments, with the final goal being to estimate the potential capabilities of the radar missions as a function of the frequency and temperature ranges of interest for the subsurface sounders. We present the different subsurface scenarios and associated radar signal attenuation models that have been proposed so far to simulate the structure of the crust of Europa and discuss the physical and geological nature of various dielectric targets potentially detectable with RIME. Finally, we briefly highlight several unresolved issues that should be addressed, in near future, to improve our capability to produce realistic electromagnetic models of icy moon crusts. The present review is of interest for the geophysical exploration of all solar system bodies, including the Earth, where ice can be present at the surface or at relatively shallow depths.

Radar Signal Penetration and Horizons Detection on Europa Through Numerical Simulations

We propose a strategy to evaluate the performance of a radar sounder for the subsurface exploration of the Europa icy crust and, in particular, the possibility to detect liquid water at the base of the ice shell. The approach integrates the information coming from experimental measurements of the dielectric properties of icy materials, thermal models related to different crustal scenarios, and numerical simulations of radar signal propagation. The radar response has been evaluated in terms of cumulative attenuation, signal-to-noise ratio (SNR), and reflectivity. Our simulations indicate that a subsurface radar operating at 9 MHz can identify shallow-buried targets and to detect the ice/water interface in various thermal scenarios. Under our assumptions the ice/water interface can be detected almost down to a depth of 15 km under a conductive ice shell, whereas for a convective ice shell, the maximum depth is about 12 km (in the cold downwelling plume). We also discuss the possibility to detect shallow targets associated with interfaces between pure water ice and MgSO4 · 11H2O ice mixtures at various salt contents, using the data of laboratory dielectric measurements.

Pitfalls in GPR Data Interpretation: False Reflectors Detected in Lunar Radar Cross Sections by Chang’e-3

Chang’e-3 (CE-3) has been the first spacecraft to soft land on the moon since the Soviet Union’s Luna 24 in 1976. The spacecraft arrived at Mare Imbrium on December 14, 2013, and the same day, Yutu lunar rover separated from lander to start its exploration of the surface and the subsurface around the landing site. The rover was equipped, among other instruments, with two lunar penetrating radar systems having a working frequency of 60 and 500 MHz. The radars acquired data for about two weeks while the rover was slowly moving along a path of about 114 m. At navigation point N0209, the rover got stacked into the lunar soil and after that only data at a fixed position could be collected. The low-frequency radar data have been analyzed by different authors and published in two different papers, which reported totally controversial interpretations of the radar cross sections. This paper is devoted to resolve such controversy by carefully analyzing and comparing the data collected on the moon by Yutu rover and on earth by a prototype of LPR mounted onboard a model of the CE-3 lunar rover. Such analysis demonstrates that the deep radar features previously ascribed to the lunar shallow stratigraphy are not real reflectors, rather they are signal artifacts probably generated by the system and its electromagnetic interaction with the metallic rover.

Young sea ice electric properties estimation under non-optimal conditions

Sea ice monitoring is important for both climate change studies and potential trans-Arctic shipping. Ground Penetrating Radar (GPR) has been demonstrated to be a powerful method to retrieve sea ice thickness and gain information about its internal structure. Nevertheless, its applicability can be strongly limited in the case of very low ice thickness and high salinity content. This paper presents results from a field experiment performed under such conditions which integrated GPR data and s-parameters measurements with Vector Network Analyzer (VNA) on artificial sea ice grown at the SERF research site in Winnipeg, Canada. The observed dielectric behavior has been used to monitor sea ice growth, relating the electrical conductivity to temperature evolution and brine content. Results demonstrate the capability of both GPR and VNA techniques in the investigation of sea ice properties under non-ideal conditions.