Feasibility of a Rover-wheel-based Permittivity Sensor for Prospecting Lunar Water

Introduction: Quantifying and mapping the precise amount of water ice in the subsurface of the Moon has proven challenging using only remote sensing data. However, this knowledge is crucial for planning long-term exploration missions that rely on the utilization of potential water reserves. Higher-resolution data and ground truth can be obtained by in-situ measurements. Instruments on stationary landers, such as ESA’s upcoming PROSPECT instrument package [1], will analyze the vertical distribution of water up to 1 m depth. In addition, rover missions promise to be highly valuable assets for lunar exploration as their mobility allows them to analyze the lateral distribution of water ice around its landing site at much higher resolution than presently possible [2].

Studies have shown that water ice in the lunar regolith can be detected using electric permittivity sensors [3,4,5]. By measuring the temperature- and frequency-dependent electric permittivity in the extremely low-frequency band, the soil’s porosity and water ice content can be deduced. Permittivity sensors of varying complexity have been part of the Rosetta/Philae [6] and Cassini/Huygens payloads [7], and a drill-based sensor will be employed in the upcoming Intuitive Machine’s IM-4 mission as part of ESA’s PROSPECT instrument package.

Rover Permittivity Sensor: For an ESA contribution to an upcoming lunar rover mission by the MBRSC (UAE), we are currently investigating the feasibility of a rover-wheel-based permittivity sensor. A prototype of this Rover Permittivity Sensor (RPS) is currently being tested at the Technical University of Munich [8]. The sensor architecture is based on the PROSPECT P-Sensor design with added sensor capabilities and circuitry adaptations. Advantages of the instrument are its simplicity, operational robustness, and low mass, power, and data requirements.

The RPS design features four distinct elements: Two electrodes mounted on one of the rover wheels, one of which is equipped with a thermometer, a coaxial pancake slip ring to transfer the signal from the rotating wheel to the static rover body, a backend electronics board performing sensor signal conditioning, acquisition, and analog processing, and a thermopile sensor to measure the surface temperature in the close vicinity of the rover.

RPS utilizes two electrodes, which allows to periodically investigate the soil’s porosity and its water-ice content, taking advantage of the mobility offered by the rover to map resources along its traverse. As the sensing depth correlates with the electrode’s size, the two electrodes have different dimensions and allow for determining the depth-dependent stratification and stability of water ice in the shallow subsurface.

The back-end electronics board includes circuitry to cycle four different excitation frequencies. For this mission, read-out of the sensor will be performed by the rover’s digital electronics, reducing the complexity and power demand of the sensor electronics.

In between the electrode and the backend electronics, a novel coaxial pancake slip ring is used to transfer the excitation and signal from the static body to the rotating rover wheel. The slip ring was specifically developed to cope with the harsh lunar environment and dust contamination while providing effective shielding for the sensitive measurement signal of the permittivity sensor.

The permittivity measurements are accompanied by measurements of the regolith surface temperature, with both, a contact temperature sensor integrated into one electrode and a thermopile sensor attached to the rover’s body. The sample temperature is a crucial parameter for the soil permittivity; therefore these additional sensors provide important contextual information for data interpretation.

Figure 1: RPS wheel architecture (second electrode obstructe).

Figure 2: Assembled electrode prototype.

Results: In the feasibility study, a fully functional prototype of RPS has been developed and evaluated. Figure 1 conceptually shows the wheel section of the instrument and Figure 2 depicts one of the wheel-mounted sensor electrodes. The results of the feasibility study confirm the measurement concept and the predicted magnitude of geometric capacitance of the novel electrodes. In addition, experiments with the thermopile temperature sensor showed that this concept is also viable for samples at cryogenic temperatures.

Conclusion and Outlook: The recent study has proven the technical feasibility of the RPS concept for lunar exploration. With a newly developed slip ring and two electrodes, mapping of water ice on the lunar surface and investigation of its stratification in the shallow subsurface is possible. A special development effort was put into selecting materials and components suitable for an ongoing development to facilitate a fast progression towards a flight instrument.

At TUM, a specialized dusty thermal vacuum system is being assembled to assess electrode robustness, slip ring dust tolerance, and combined system performance in a representative environment and with icy regolith simulant. Refined and extensive testing of the thermopile sensor is also foreseen as a next step.

In addition to their application on lunar rovers, similar permittivity sensors could also be integrated into other exploration systems, such as penetrators or landers, to provide valuable context information about the soil properties and potential water-ice content.

Acknowledgement: The RPS Feasibility Study was funded by ESA.

References: [1] Trautner, R et al. (2025), Frontiers in Space Technologies, 5 [2] Gscheidle, C. et al. (2022), PSS, 212, 105426 [3] Nurge, M. A. (2012) PSS., 65, 76-82 [4] Trautner, R. et al. (2021) Meas. Sci. Technol, 32, 125117 [5] Gscheidle, C. et al. (2024) Frontiers in Space Technologies, 4 [6] Seidensticker, K. J. et al. (2007) Space Sci. Rev., 128, 301-337 [7] Grard, R. et al. (2006) PSS, 54, 1124-1136 [8] Trautner, R. et al. (2024) Euro. Lunar Symposium, Dumfries, UK