Barbara Cosciotti

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.

Thermal and electromagnetic models for radar sounding of the galilean satellite icy crusts

The icy satellites of Jupiter are known to host a water ocean beneath an ice shell. The ice penetrating Radar for Icy Moon Exploration (RIME) housed on board the JUpiter ICy moons Explorer (JUICE) mission is expected to probe the crust of Europa, Ganymede and Callisto up to a depth of 9 km. The main objectives of RIME are the detection of the relic Brittle-Ductile Transition in the ice shell of Ganymede and Callisto, and the identification of melt materials on Europa. The penetration of the radar signal is strictly connected to the electromagnetic properties of the ice, that in turn depends on the presence of contaminants and temperature profile inside the crust. Laboratory measurements carried out in the temperature range of (100-273)K provided the dielectric properties of pure, salty and dusty ices, whereas temperature profiles are obtained taking into account the heat conduction and thermal convection models for the Galilean satellites. The combination of electromagnetic and thermal properties of the icy crusts allowed us to generate simulated radar data at the operation frequency of RIME (9 MHz). Such simulations are important to determine the radar performance, estimating the signal penetration and the capability to resolve buried layers.

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.

Dielectric Measurements of Saline Ices: Implication for Jovian Satellites Radar Exploration

of Jupiter’s moon Ganimede. Recently, Marsis and Sharad missions on Mars showed the capability of orbiting radar sounder, to detect the subsurface features of the planet .Therefore, the use of such techniques could prove the presence of a subsurface Ganimede ocean. The achievement of such an objective depends on the electromagnetic properties of Ganimede ice shell, which in turn will depend on the ice’s purity and temperature gradient, as well as on the radar wavelength. In this scenario, we have performed measurements of dielectric properties of pure and saline ice (mixing MgSO4 or Na2SO4 with bidistilled water) both in time and frequency domain. Both methods set show the capability of TDR and LCR techniques to estimate the electromagnetic properties of salty ice.

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.

Electromagnetic signal penetration in a planetary soil simulant: Estimated attenuation rates using GPR and TDR in volcanic deposits on Mount Etna: EM Signal Penetration in Planetary Soil

Ground-penetrating radar (GPR) is a well-established geophysical terrestrial exploration method and has recently become one of the most promising for planetary subsurface exploration. Several future landing vehicles like EXOMARS, 2020 NASA ROVER, and Chang’e-4, to mention a few, will host GPR. A GPR survey has been conducted on volcanic deposits on Mount Etna (Italy), considered a good analogue for Martian and Lunar volcanic terrains, to test a novel methodology for subsoil dielectric properties estimation. The stratigraphy of the volcanic deposits was investigated using 500 MHz and 1 GHz antennas in two different configurations: transverse electric and transverse magnetic. Sloping discontinuities have been used to estimate the loss tangents of the upper layer of such deposits by applying the amplitude-decay and frequency shift methods and approximating the GPR transmitted signal by Gaussian and Ricker wavelets. The loss tangent values, estimated using these two methodologies, were compared and validated with those retrieved from time domain reflectometry measurements acquired along the radar profiles. The results show that the proposed analysis, together with typical GPR methods for the estimation of the real part of permittivity, can be successfully used to characterize the electrical properties of planetary subsurface and to define some constraints on its lithology of the subsurface.

Combined GPR and TDR measurements for snow thickness and density estimation

The Ground Penetrating Radar (GPR) is a technique capable to perform a fast monitoring of snowpack and glaciers, providing an estimation of some snow parameters like thickness, density and Snow Water Equivalent (SWE). The most important quantity to know to understand GPR data is the wave velocity in the snow, in order to transform the traveltime in depth. Independent measurements of wave velocity could be performed using the Time Domain Reflectometry (TDR) or estimating the electrical permittivity from density measurements. Here an evaluation of the accuracy of TDR measurements to estimate the wave velocity is proposed and the results are corroborated by independent measurements of snow height and density.

Electromagnetic parameters measurements of clay soils for Mars radar sounding

The martian shallow crust has been studied through two subsurface sounding radars, MARSIS and SHARAD which operate at 3-5 and 20 MHz, respectively. The capability of radar to resolve the subsurface structures and the stratigraphy relates both on the radar features and the electromagnetic parameters of the shallow crust. For small grain size sediments, like clay minerals, the dielectric properties strongly affect the penetration depth of the radar signal. In this study, we measured several clayey samples, collected from various geological settings, with different mineralogy and water content using a Vector Network Analyzer (VNA) in the frequency range 1MHz-6GHz. The electrical properties were estimated as a function of temperature (200-298K) via the Nicolson-Ross-Weir algorithm using a coaxial-cage probe connected to the VNA. These measurements are used to interpret radar data in terms of depth penetration and surface echoes strength which could be affected by the temperature diurnal variation of martian surface.

Electromagnetic characterization of saline mixture for shallow radar exploration

The first recognition of the Jupiter system, performed by the Voyager spacecraft, suggested the possibility of an ocean of liquid water beneath the ice shell two of Jupiters moons, Ganymede and Europa. In addition, the Galileo mission detected in the icy shell the presence of impurities like magnesium and sodium sulfates, and sulfuric and hydrochloric acids. JUICE - JUpiter ICy moons Explorer will investigate the potentially habitable zones in the Ganymede, Europa and Callisto moons. In particular a radar sounder (RIME) operating at 9 MHz, optimized for the penetration of the icy shell, will provide the subsurface survey up to a depth of about 9 km. The performance of RIME depends on the electromagnetic properties of the icy shells, which in turn are related to the impurity contents and temperature. In this scenario, we have performed measurements of dielectric properties of ice doped with MgSO4, Na2SO4. The tests were performed as a function of frequency from 20 Hz to 1 MHz and temperature down to 100 K. The electromagnetic behaviour of saline-ices have shown that the attenuation coefficient becomes very small when the temperature decreases.