Development of Permittivity Sensors for Lunar Water Prospecting

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Professorship of Lunar and Planetary Exploration Technologies
TUM School of Engineering and Design
Technical University of Munich
Figure 1: Lunar Volatiles Scout instrument: (left) Schematic with important
elements highlighted; (right) Analog model for testing.
1Technical University of Munich (Lise-Meitner-Str. 9, 85521 Ottobrunn, Germany, c.gscheidle@tum.de)
Christian Gscheidle1and Philipp Reiss1
Development of Permittivity Sensors for Lunar Water Prospecting
Figure 3: Comparison of measured and calculated Fourier coefficients for
different insertion depths for the LVS geometry.
Introduction
Precise knowledge of the distribution, abundance and physical state of lunar volatiles is crucial for planning future missions. The data from numerous remote sensing missions around the Moon
has provided a strong basis for such analyses, however ground truth data for verification and model correlation is still lacking especially from the lunar poles.Although upcoming exploration
missions, such as ESA’s PROSPECT [1] or NASA’s VIPER [2], will improve the ground truth data availability,broader coverage and finer spatial resolution is still necessary.
Measuring the regolith’s electrical permittivity is a useful technique to quickly determine the state and abundance of water in lunar regolith.Multiple missions to the Moon and other celestial
bodies have used or intend to use permittivity sensors [1,3,4] and the Technical University of Munich (TUM) is currently developing this measurement technique for several applications [5].
Electrical Permittivity Sensor for Lunar Exploration
The electrical permittivity (or dielectric constant), describes a material’s ability to store energy in an electric
near field.Any mixture of materials between two electrodes and their respective (relative) effective
permittivities influence the system’s electric capacitance.In the context of lunar and planetary
exploration,this phenomenon can be exploited as the relative permittivity of vacuum (=1), dry regolith (~3-
10), and water (liquid and ice, ~80-100)differ significantly in both magnitude and behavior in the frequency
domain and over temperature [6]. Measuring the capacitance of a calibrated system thus allows for
deduction of the material’s permittivity and subsequently the water content.
Permittivity sensors exhibit several advantages for use on spacecraft: They are lightweight,require very
low power and can cope with harsh environments while providing scientifically valuable data. Based on
aRC-circuit, the measurement principle is focused on applying a square wave excitation potential with
given low frequency (< 50 Hz) and measuring the transient charging process. This characteristic time series
yields frequency domain information when being Fourier-transformed. Besides the (relative) electrical
permittivity, such sensors can also provide information on the material’s electric conductivity, magnetic
properties over frequency or even geotechnical properties, such as density and porosity.
References
[1] Trautner R., Reiss P. and Kargl, G. (2021) Meas. Sci. Technol. 32, 125117. [2] Andrews D.(2022) IAC 2022, IAC-22,A3,2A,x67570 [3] Seidensticker K.J. et al. (2007) Sp Sci Rev 182, 301-
337. [4] Hamlin, M. et al. (2016) Icarus 270, 272-290. [5] Gscheidle C. et al. (2020) Euro. Lunar Symp. [6] Nurge M. A. (2012) Plan Sp Sci 65, 76-82. [7] Biswas J. et al. (2020) Plan. Sp. Sci.
181, 104826. [8] Reiss P. et al. (2021) JGR Planets 126. [9] Smolka A. (2020) Semester Thesis, RT-SA-2020/11.
Currently, TUM is involved in multiple permittivity sensor development activities:
•The Lunar Volatiles Scout (LVS) is adrilling instrument designed for exploring the lunar poles by characterizing thermally extracted volatiles with an integrated mass spectrometer and
pressure sensors [7]. Its drill shell geometry forms a cylindrical capacitive system with the central heating element, which can be employed to perform permittivity measurements to extend the
LVS capabilities. An analog LVS setup for testing with representative dimensions is shown in Figure 1.
•The instrument package PROSPECT, developed by ESA, includes a permittivity sensor integrated into its drill to measure the subsurface regolith properties and detect water ice, as well as
determine the geometry of the borehole [1].
•Drawing from the above involvements, we are currently investigating different forms of patch electrodes for permittivity sensors. These flat and light electrodes could be attached to otherwise
unused surfaces of exploration systems, such as wheels of rovers or pads of lander foots.Valuable data on the subsurface can be acquired herby using only minimal resources of the host
system.
Multi-Physics Simulations
Simulations of the physical system are crucial both for developing an optimized sensor configuration and for
interpretation and correlation of the sensor data. However, as permittivity is dependent on material
mixtures, temperatures, and frequencies, sophisticated multi-physics simulations are required.
Exemplary results from multi-physics simulations (heat and mass transport, electric field) for our LVS system
are presented in Figure 2 [8,9].
The simulations predict that the system can detect different quantities of water in the regolith. A permittivity
measurement before and after heating thus allows estimating the initial hydrated and final dry
concentration of water in the sampled volume, providing an important reference for the correlation with
other instrument data on gas pressure and composition, as well as temperature.
Experimental Verification
The developed measurement electronics with low frequency excitation has been
calibrated with known capacitances to estimate its own internal parasitic
capacitance (𝐶!= 9.6 pF, r"= 0.99).
Subsequently, the analog model shown in Figure 1b was used to determine the
LVS’ capacitance over the insertion depth into dry simulant (quartz sand). Figure
3 shows the resulting correlation of calculated and measured Fourier coefficients
with aminimum 𝑟"= 0.96,highlighting the suitability to use this method for
insertion depth determination.
Furthermore, comparing the absolute values of the measurements with the
Maxwell capacitance values of the multi-physics simulation, a good correlation
can be seen.
Lunar Polar Volatiles Conference 2022
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b
Figure 2: Result fields from preliminary multi-physics simulations for the LVS: (a) Temperature, (b) Adsorbed
concentration of water, (c) Relative permittivity. The domain is rotationally symmetric around the LVS’ heater.