Laboratory investigations of the electromagnetic properties of a powdered L-chondrite

Introduction

Radio Echo Sounding (RES) has been employed in planetary missions to survey the Moon, Mars, and Comet 67P/Churyumov-Gerasimenko (Porcello et al., 1974; Plaut et al., 2007; Kofman et al., 2015) and it will be applied, among others, in future investigations of the Galilean moons (JUICE mission - Grasset et al., 2013; EUROPA CLIPPER mission – Blankenship et al., 2018), and asteroids (e.g. AIDA mission - Herique et al., 2019). The ability of a radar signal to penetrate and image the subsurface layers depends on both the dielectric permittivity and magnetic permeability of the materials composing the crust. Thus, laboratory experiments are essential to improve our capability to estimate the radar performance and interpret the radar data collected in different scenarios. We present here the results of the electromagnetic measurements conducted for different porosities of a powdered L5 chondrite sample (a good simulant of the surface of Ganymede and S type asteroids), in the frequency range of interest for planetary radar sounders (1 MHz-1 GHz).

Methodology

The analyzed meteorite was classified as an ordinary L5 chondrite, with shock grade S2 and weathering W2 (Cosciotti et al.,2021). A fragment of the solid meteorite was pulverized in a jaw crusher and then sieved, to obtain five different granulometric samples, from <125 μm to 800 μm. The grain density of the powdered sample is ρ_g=(3.331±0.001)g/cm³, value that agrees with those found in literature for similar samples (Li et al., 2019). The electromagnetic measurements were carried out with a two port Vector Network Analyzer (VNA), employing a cage coaxial cell (Mattei et al., 2013). Sample porosity was progressively decreased by vibrating the sample inside the cell using a vibrating plate. The complex permittivity and permeability were estimated by the scattering parameter measurements and the Nicolson-Ross-Weir (NRW) algorithm (Nicolson and Ross, 1970). The real part of the effective permittivity of the two phases granular meteorite-air was studied using four different mixing formulas (table 1): Maxwell-Garnett, Bruggeman, Looyenga-Landau-Lifshitz (LLL) and Lichtenecker formula (Sihvola, 1999).

Table 1. ε_i and ε_e are respectively the permittivity of the environment and inclusions (i.e. air). f=1-Φ represents the volume fraction of the sample, where Φ is its porosity.

Results

Measurements with the VNA were performed on the granular chondrite sample at room temperature, for different porosities (27%-40%). For example, figure 1 and 2 illustrate the frequency spectra of the electromagnetic parameters for a porosity Φ=0.3. The plots also report the values of the air permittivity obtained with the empty cell, as they represent the instrument lower limit for an accurate measurement. Gray areas in the plots show values that cannot be considered reliable due to the NRW algorithm divergence (above 600 MHz) caused by the cell resonance or associated to the instrumental limits.

Figure 1. Complex permittivity frequency spectrum.

Note that, for the whole dataset, the real part of permittivity is fairly constant with frequency, whereas for frequencies lower than 10 MHz the imaginary part is below the limit of the instrument (figure 1). This result agrees with literature data as chondrites usually have low dielectric losses.

Moreover, our measurements show that the meteorite sample is slightly magnetic, with a Debye-like relaxation between 10 and 100 MHz (figure 2).

Figure 2. Complex permeability frequency spectrum.

Finally, the real part of permittivity at 100 MHz for all porosities was fitted with Lichtenecker formula (figure 3), to retrieve the value of solid permittivity (the value at zero porosity). We obtained  ε_sol=9.2±0.2 , which is slightly lower than the value of the solid meteorite measured in Cosciotti et al., 2021.

Figure 3. of meteorite powder as a function of porosity fitted with Lichtenecker formula.

 

Conclusions

In this work we performed the electromagnetic characterization of crushed samples coming from an ordinary chondrite solid sample as a function of frequency and porosity. We found, in good agreement with literature data (e.g., Olhoeft and Strangway, 1974), that complex permittivity is strongly dependent on porosity. The presence of a Debye-like magnetic relaxation suggests that, not only dielectric but also magnetic loss should be accounted for when radar signal attenuation is estimated. In future laboratory work we will evaluate the effect of temperature, to better characterize the electromagnetic behavior of the samples.

Acknowledgements

This work has been supported by the Italian Space Agency through Contract ASI/INAF 2018-25-HH.0.

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