Small-scale lava tube detection on the Moon using Ground Penetrating Radar

Abstract

Introduction: The exploration of the Moon has nowadays gathered a new interest for many scientific purposes [1]. In particular, the detection of lava tubes reveals important for the identification of favorable environments for human activities [2]. Recently, the potentialities of a Ground Penetrating Radar (GPR) for the detection of buried lava tubes on the Moon have been evaluated by different authors, either from ground-based observations (e.g., [3], [4], [5]) or from orbit [6].Here, an evaluation of the detection potentiality by means of a GPR on small-scale lava tubes is performed, taking into account different frequencies, dielectric properties and lava tube dimensions.Methodology: A semi-elliptic lava tube geometry is considered, having the horizontal semiaxis () longer than the vertical one (), and placed at a depth d. The dielectric properties of lunar subsoil have been taken from [7] and [8]. The two frequencies of the Lunar Penetrating Radar onboard the Chang’e-3 mission (60 and 500 MHz) have been studied. The parameter space considered in this analysis is reported in Table 1.  Parameter Values Geometric scenario (m) 1 2 a = 8, b = 5, d = 15 a = 13, b = 8, d = 18 Dielectric scenario 1 2 [7] εr = 3.00 + 0.03i εb = 5.74 + 0.0805i [8] εr = 3.52 + 0.0176i εb = 7.12 + 0.0486i Frequency 60 MHz 500 MHz Table 1. Parameter space considered for the simulations. The basaltic lava flow is covered by a 8 m thick regolith layer [7]. A 2 m thick blocky layer is also present above the lava flow, comprising blocks having dimensions up to 1 m. The complex permittivity of the basalt (εb) has been evaluated from the one of the regolith (εr), considering an inverse Bruggeman mixing rule and a porosity of 0.45 [7]. Figure 1 shows the geometry considered for the simulations. Note that, a rough surface has been considered for the lava flow top, and for lava tubes ceil and floor. Fig. 1. Sketch of the studied subsurface geology. GPR cross-sections simulations: To simulate a GPR signal propagation in a lunar subsoil, the finite-difference time-domain (FDTD) code developed by [9] is used. The code is capable to study the propagation of a synthetic radar wave considering the most relevant parameters, i.e., the dielectric properties of subsoil (expressed in terms of permittivity and conductivity), and the antenna parameters (central frequency and antenna separation). Regarding the antenna separation, the values of 0.8 m and 0.26 m have been considered for the 60 and 500 MHz antennae, respectively, taking into account the values of [10] for the Lunar Penetrating Radar onboard the Chang’e-3 mission. The mesh has been built in order to be adequate to the wavelength considered for each run.Results: Figures 2-4 show the radar cross-sections emerging from the simulations in the dielectric scenario 2 [8], and considering the two frequencies and the different lava tube geometries. In Fig. 2, the main reflectors are also highlighted to help the interpretation of radar cross-sections. In all the studied cases, the GPR is capable to clearly detect the reflection coming from the lava tube ceil. Regarding the reflection from the floor, this can be poorly identified in the 60 MHz cross-sections. Note that, even if not reported, results for dielectric scenario 1 are similar to the ones shown. Fig. 2. Radar cross-section for geometric scenario 1, dielectric scenario 2, and a 60 MHz antenna. Fig. 3. Radar cross-section for geometric scenario 1, dielectric scenario 2, and a 500 MHz antenna. Fig. 4. Radar cross-section for geometric scenario 2, dielectric scenario 2, and a 60 MHz antenna. Fig. 5. Radar cross-section for geometric scenario 2, dielectric scenario 2, and a 500 MHz antenna.    Acknowledgement: We acknowledge support from the research project: “Moon multisEnsor and LabOratory Data analYsis (MELODY)”, selected in November 2020 in the framework of the PrIN INAF (RIC) 2019 call.