The radar signal propagation through the icy crust of Jovian moons

The search for liquid water in the Solar System is one of the main goals of planetary exploration since it represents the fundamental ingredient for life. Radar instruments have already been successfully employed to detect subglacial lakes in Antarctica and Greenland [1][2] and below the Martian South Pole [3][4]. Radio Echo Sounding technique (RES) consists in sending pulses towards the surface of the investigated body and collecting echoes: parts of the signals come back from the surface, and those that do penetrate the surface are reflected by the electrical discontinuities of the subsurface layers. The intensity of these echoes and their time delay are linked to the material and the position of the reflective interface. RES technique allows a deep study of the interior of celestial bodies to understand how they formed, their characteristics and if they could host any living organisms.

Introduction

In 2030 and 2031 Europa Clipper and JUICE with two radar instruments, REASON and RIME [5][6] will probe Europa, Callisto, and Ganymede to study their characteristics and to search for possible shallow liquid water bodies. These moons are the ideal targets for this type of investigation. From Galileo mission data, we know that they host a liquid water ocean beneath the crust.  Their cold icy shell represents the perfect environment for RES surveys because in the frequency band of RIME and REASON (9MHz), cold ice is known to generate a low attenuation of radio waves allowing a deep penetration of the signals. Radars could detect liquid water coming from their inner oceans or caused by the local melting [7], and obtain information about their depth and composition.

The intensity of the reflections collected by the radar and the attenuation of radio signals are influenced by the complex value of dielectric permittivity of the ice and the other materials present in the shell.  In the case of an interface between two materials i, j the intensity of the echoes, I, caused by the discontinuity in electrical properties is linked to the values of the real part of their permittivity ε_i ,ε_j  (Equation1). Signals attenuation is linked to the value of the imaginary part. Liquid water has a permittivity higher than the other materials in the crust, then it causes strong reflections of radio waves. Besides liquid water, the presence of salts in Europa [8] and that of dust in Ganymede could originate features that would be detected by RES, moreover these impurities significantly affect the attenuation of radio pulses. Because of the large number of possible complex scenarios, the stratigraphy obtained by radars is usually difficult to interpret and it would be useful to consider what we could expect in advance making simulations.

 

Methods

 In the Roma Tre laboratory, we collected measurements of dielectric properties of different materials following the methods described in [9]. The trend of permittivity values with temperature of pure ice and ice doped with salts was studied to reproduce the condition of Europa. We also analysed the ice with inclusion of chondrites present in Ganymede spectra [10]. We examined different compositions and temperature profiles given by the literature [7][11] generating possible configurations of Ganymede and Europa subsurface environments. We performed several simulations assuming different scenarios and computed the intensity of the echoes caused by the presence of shallow liquid water bodies and other types of discontinuities.

Results and Conclusions

We can predict, through simulations, signals attenuation and at which depth the radar instruments could detect liquid water bodies depending on the shell temperature and thickness, and on which salt or dust is present. We made hypothesis of different scenarios (Figure1) and performed simulations to see if and how the radar would detect them.

The measurements of permittivity of doped ice in function of the temperature show different regions (Figure2). Until 250K the permittivity of the doped ice is approximately constant, then it rises to higher values and remains constant in the phase of liquid brine (a mixture of salty liquid water and ice) until 270K. After 270K the ice is completely melted, and the real and imaginary part of the permittivity jump to the values of salty liquid water.

The results of the simulation of the waves propagation through doped ice show that the gap in permittivity observed around 250 K causes reflections of the signal (Figure3). These echoes could be detected by the antenna of the radar, only if the signal is slightly attenuated.

This study of the features that electrical discontinuities in the icy crust of Europa and Ganymede would cause in a radar analysis are crucial for the interpretation of real data.

 

Figure1: Possible scenarios of dielectric discontinuities in the shell of the icy moons. On the right the echo could be caused by the gap in permittivity between ice and liquid brine. On the left it could be cause by the difference in permittivity between two parts of the crust with different salinities.

Figure2: Values of the permittivity of 50mM NaCl doped ice at the frequency of 9MHz in function of the temperature. The upper panel is the real part, the lower panel is the imaginary part. The three zones with different dielectric properties (ice, brine, and liquid) are marked by vertical grey lines.

Figure3: Simulation of the signal propagation through a NaCl doped crust. The main echo is caused by the discontinuity corresponding to the surface (100K), the weaker one (250K) is due to the presence of the brine condition.

 

References

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[2] Oswald, G. K. A. et al.(2008) In: J. Glaciol. 54, 94-106. 

[3] Orosei, R. et al.(2018) Sci., 361, 490-493

[4] Lauro, S. E. et al.(2020) Nature Astronomy, 5, 63-70

[5] Bruzzone L. et al.(2013) In: IEEE international geoscience and remote sensing symposium-IGARSS. 

[6] Blankenship D. et al.(2018)  In: 42nd cospar scientific assembly 42: B5-3 

[7] Chivers C. J. et al.(2023) In: The Planetary Science Journal

[8] Trumbo,S. et al.(2022) In: The Planetary Science Journal

[9] Brin,A. et al.(2022) In: Icarus

[10] Hibbits, C.A. (2023) In: Icarus

[11] Buffo, J. J. et al.(2021)  In: JGR Planets