A Rover Permittivity Sensor for Lunar Water Ice Detection

A ROVER PERMITTIVITY SENSOR FOR LUNAR WATER-ICE DETECTION. L. Eckert1, C. Gscheidle1, R. Šeško1, R. Trautner2, P. Reiss1, 1Technical University of Munich, Ottobrunn, Germany, 2European Space Research and Technology Centre, European Space Agency (ESA), Noordwijk, The Netherlands. Introduction: In-situ resource utilization on the Moon has gained significant interest for future lunar missions. The presence of water, as detected by various missions, is of particular interest, as its utilization would facilitate long-term missions and an extended human presence on the lunar surface while also reducing costs. Therefore, increased efforts to detect, characterize, and map lunar water resources have been made in recent years. Indirect evidence for the existence of water, predominantly in the polar regions, was obtained by various remote sensing missions such as the Lunar Reconnaissance Orbiter [1, 2]. Earth-based investigation of lunar soil samples such as those returned by the Chang’E-5 mission provided direct evidence for water [3]. However, the lunar water abundance, spatial distribution, and temporal variation are still not fully understood. Permittivity Sensors: Permittivity sensors can perform in-situ water ice detection and quantification. These lightweight instruments measure a material’s complex electric permittivity, which is a combination of its polarizability (real part) – its ability to store energy in an electric field – and its conductivity (imaginary part) [4]. The permittivity is related to the bulk capacitance of the material mixture between the sensor’s electrodes. In the lunar subsurface, the bulk capacitance is a combination of the capacitances of vacuum, regolith, and – if present – water ice. Due to the significant difference between the relative permittivities of water/ice (typically εr ~ 80 [5]), lunar regolith (typically εr ~ 5 [6]), and vacuum (εr ~ 1), a measurement of this parameter can be used to determine a sample’s composition and therefore the water ice content in the lunar subsurface. The capabilities of permittivity sensors for space science have already been demonstrated successfully on missions such as Cassini-Huygens [7] and Rosetta [8]. Due to the extremely low conductivity of lunar surface materials, simplified instrument concepts can be employed. The Rover Permittivity Sensor (RPS): The Rover Permittivity Sensor (RPS) is developed as part of ESA’s contribution to an upcoming rover mission to the polar region of the Moon [9]. It is designed for in-situ characterization of the regolith in the shallow subsurface by measuring its electrical permittivity [10] and is an evolution of the permittivity sensor used in the PROSPECT instrument package [11]. Mounted on a rover wheel, it assesses the regolith’s density, porosity, and chemical composition at various locations, which allows the mapping of the water ice content along the rover track. The sensor comprises an electronics unit inside the rover body, wheel-mounted patch electrodes, and several temperature sensors. A newly developed coaxial pancake slip ring provides the interface between the rotating wheel and the stationary rover body. Electronics: The relaxation frequency of water ice strongly depends on its temperature and can be well below 1 Hz for temperatures of 40 K, representing the minimum temperature in some lunar polar craters [4]. Therefore, we chose the frequencies of the electrical field induced into the soil to lie in the range of a few Hz and lower. The square wave excitation frequencies are generated by a digital oscillator in the electronics, filtered by a low pass, and then applied in sequence via a current measurement resistor to the patch electrodes. The current, which is related to the capacitance of the electrodes – hence the permittivity of the materials the electrodes are in contact with – is measured via the voltage across the resistor. Afterwards, the voltage signal is amplified, filtered and digitized. The expected low current (on the order of nA) requires low parasitic capacitance in the instrument’s printed circuit board (PCB), slip ring, and harness. This low parasitic capacitance is ensured by using a guard signal to shield the measurement signal from the electrodes to the electronics in the rover body. Electrodes: The patch-shaped electrodes are mounted to one of the front rover wheels with appropriate angular spacing and come in contact with undisturbed regolith once every wheel revolution. Figure 1: A 3D-printed model of the rover wheel with one of the rover permittivity sensor’s patch electrodes. The depth to which the instrument can characterize the lunar regolith depends on the electrode size. Therefore, the sensor comprises multiple electrodes with varying dimensions. An isolating cap shields the sensor electrode to protect the electronics from electrostatic discharge and the electrode itself from mechanical damage. Slip Ring: The slip ring located inside the rover wheel ensures a shielded connection of the sensitive electrode measurement signal between the patch electrodes and the electronics. Its mechanical and electrical design is furthermore optimized for low parasitic capacitance. Additionally, the slip ring contains low-power signal lines for temperature sensors integrated into one of the electrodes and the rover wheel itself. To ensure that the slip ring does not affect the reliability of the rover wheel or cause its complete failure through increased friction between its rotary and stationary parts, we are currently investigating an open dust-tolerant design instead of a classic mechanically sealed design. The dust-tolerant design minimizes dust ingress and allows any collected dust particles to fall out during the wheel revolution. Dedicated surfaces for the electrostatic collection of regolith fines are added to the design in order to further mitigate the accumulation of fine dust inside the slip ring. Temperature Measurement Unit: As the permittivity of lunar regolith is temperature-dependent [4], it is essential to know the temperature at the measurement location to accurately evaluate and interpret the sensor data. To accomplish this, the instrument uses two distinct temperature detectors. A resistive temperature detector (RTD) integrated into one electrode comes into contact with the regolith directly at the electrode-soil interface to provide a direct measurement. Figure 2: Prototype of the temperature measurement unit. In addition, a contactless infrared (IR) temperature measurement unit determines the surface temperature of the undisturbed regolith a few centimeters ahead of the rover. As the IR measurement employs thermopile sensors, commonly used for hightemperature measurements, we performed a feasibility study to assess their compatibility with cryogenic temperatures and the linearity and sensitivity of the signal. The IR temperature measurement unit is mounted to the rover body at an angle to guarantee that the measurement is neither affected by IR radiation from the rover itself nor by direct sunlight. References: [1] Colaprete, A. et al. (2010) Science 330, 463– 468. [2] Hayne P. O. et al. (2015) Icarus 255, 58–69 (2015). [3] Liu, J. et al. (2022) Nature Communications 13, 3119. [4] Nurge, Mark A. (2012) Planetary and Space Science 65.1 76-82. [5] Uematsu, M. and Franck, E. U. (1980) Journal of Physical and Chemical Reference Data 9.4 1291-1306. [6] Chung, D. H. et al. (1972) Proceedings of the Lunar Science Conference, vol. 3, p. 3161. Vol. 3. [7] Fulchignoni, M. et al. (2002) Space Sci. Rev. 104, 395–431. [8] Seidensticker, K. J. et al. (2007) Space Sci. Rev. 128, 301–337. [9] Trautner, R. et al. (2024) European Lunar Symposium. [10] Gscheidle, C. et al. (2024) Frontiers in Space Technologies 4. [11] Trautner, R. et al. (2021) Measurement Science and Technology 32.12 125117.