Electromagnetic and geochemical characterization of volcanic rock samples in the framework of radar exploration of terrestrial planets

Introduction

The detection of anomalous bright reflections beneath the Martian SPLD by MARSIS radar has sparked a vast debate on the characterization of the basal material. The source of the reflections has been interpreted by someone as an evidence of basal liquid salty water [1, 2], while by others as clay sediments or volcanic rocks [3, 4]. Though it has been demonstrated that clays cannot be the responsible of the strong reflections [5], it has not yet been well understood if the dielectric permittivity of the volcanic rocks can be so high as to cause the reflectivity seen by MARSIS. Furthermore, although there are in literature several attempts to demonstrate a correlation between the dielectric properties of volcanic rocks and their whole-rock compositions [6], these data can hardly be compared with each other, since every work employs different experimental setup and procedures. Therefore, a definitive dataset of the permittivity of such samples and planetary simulants does not yet exist, especially at the operating frequencies of planetary radar sounders ( 1 MHz - 1 GHz). The present work aims to be a first step towards a reliable dataset of electromagnetic measurements of volcanic rocks, investigating the relation between their geochemical and their electric and magnetic properties.

Methodology

 

Sample characterization

 

We studied the granular samples of three different volcanic rocks that are plausibly representative of the overall surface compositions of the terrestrial planets [7]. In the following, the samples are named as: St. Augustine (2006), Etna (1991-1993) and Etna (Holocene). The St. Augustine (2006) sample is a lava rock coming from the 2006 eruption of the St. Augustine stratovolcano in Alaska [8]. Sample Etna (1991-1993) comes from the Piana del Trifoglietto lava field at SE of Etna summit craters generated by the eruption started on December 1991 and stopped on March 1993 [9]; the rock is characterized by a potassic geochemical signature. Sample Etna (Holocene) is a mugearite from Pizzi Deneri Formation and it comes from an ancient eruption (57000-15000 years ago), representing the more sodic volcanic activity [10]. In Fig. 1 and Tab. 1 are shown respectively the TAS diagram of the samples and their geochemical composition.

The samples exhibit different grain densities: ρ_{St. Augustine(2006)} = (2.736± 0.001) g/cm3, ρ_Etna(91-93)= (2.961 ± 0.001) g/cm³ and ρ_{Etna(Holocene)}= (2.787 ± 0.001) g/cm³.

Table 1. Whole-rock compositions of the samples.

Figure 1. Total Alkali vs. Silica diagram of the samples.

Electromagnetic measurements and mixing formulas

 

The electromagnetic measurements were carried out with a two port Vector Network Analyzer (VNA), employing a cage coaxial cell and using the experimental procedure and setup described in [11]. The complex permittivity ε and magnetic permeability µ were estimated by using the Nicholson-Ross-Weir algorithm and the equations slightly modified in [12]:

where F_g is a factor related to the geometry of the cell, Γ and Ψ are the reflection and transmission coefficients, l_e = 5 cm is the electrical length of the cell, c is the velocity of the light in a vacuum and ν is the frequency. The complex effective permittivity of the two phases granular rock-air was studied using Lichtenecker and Bruggeman mixing formulas:

where ε_i and ε_eare respectively the permittivity of the environment (the solid grains) and inclusions (air) and f = 1- Φ is the volume fraction of the grainsin the sample, with Φ the porosity.

Results and conclusions

 

Measurements with the VNA were performed on the granular samples at room temperature, for percentage porosities ranging from 31% to 55%.  The magnetic permeability µ is not reported because the three samples do not show a magnetic behavior (and then we can consider µ ≃ 1). Fig. 2 illustrates the complex dielectric permittivity as a function of frequency and at Φ = 0.36. Gray areas in the plots show values that are not reliable since at high frequencies the NRW algorithm diverges because of the cell resonances. Data have larger uncertainties at low frequencies due to VNA instrumental limits. The two Etna samples show a similar dielectric behavior, instead the St. Augustine sample has lower values of both real and imaginary part of permittivity. In the end, we fitted at 80 MHz the complex permittivity as a function of porosity using eqs. 3 and 4. In Tab. 1 we show the measurements at Φ = 0.36 and the values obtained with the mixing formulas at Φ = 0. The St. Augustine sample has the lower values of the solid complex permittivity, probably due to its lower iron abundances and higher abundances of SiO_2, while the two Etna samples show a similar dielectric behavior. Further measurements will be required in the future in order to identify the relation between the electromagnetic properties and the geochemical compositions of volcanic rocks that can be accounted as good simulants of the surface and subsurface of terrestrial planets, such as Mars and Venus.

Figure 2. Complex permittivity frequency spectra of the samples.

Table 2. Complex permittivity measured at Φ = 0.36 and fitted at Φ = 0 with eqs. 3 and 4.

References 

• Orosei R. et al. (2018) Science, 361(6401), 490-493
• Lauro S. E. et al. (2020) Nat. Astron., 5(1), 63-70
• Smith I. B. et al. (2021) Geophys. Res. Lett. 48, 15
• Grima C. et al. (2022) Geophys. Res. Lett. 49.2, e2021GL096518.
• Mattei, E. et al. (2022). Earth and Plan. Sci. Lett. 579, 117370.
• Rust A. C. (1999) Jour. of Volc. and Geoth. Res. 91.1: 79-96.
• McSween Jr et al (2009) Science, 324.5928: 736-739.
• Larsen J. F. et al. (2010). Rapp. tecn. US Geological Survey.
• Calvari S. et al. (1994) Acta Vulcan., 4: 1-14.
• Vona A. et al. (2017) Chem. Geol., 458: 48-67.
• Brin A. et al. (2021) Icarus 114800.
• Mattei E. et al. (2013). IEEE trans. on instr. and meas., 62(11), 2938–2942.