An Infrared Temperature Measurement Unit for the Rover Permittivity Sensor

Introduction: In-situ resource utilization on the Moon has gained significant interest for future lunar missions. Of particular interest is the presence of water, 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: In-situ water ice detection and quantification can be performed by employing permittivity sensors [4]. They measure a material’s complex electric permittivity, which is related to the bulk capacitance of the material mixture. The lunar surface material is a combination of regolith, vacuum, and – if present – water ice. Due to the significant difference between the (static) relative permittivities of water/ice (typically ε_r ~ 80 [5]), lunar regolith (typically ε_r ~ 5 [6]), and vacuum (ε_r ~ 1), a measurement of this property can be used to constrain a sample’s composition and in particular to derive its water ice content. In addition to the permittivity measurement, knowledge of the temperature at the measurement location is essential to accurately evaluate and interpret the sensor data, as the permittivity of the lunar regolith is temperature-dependent, especially in the presence of a water ice fraction [7]. The capabilities of permittivity sensors for space science have already been demonstrated successfully on missions such as Cassini-Huygens [8] and Rosetta [9]. Due to the extremely low conductivity of lunar surface materials, simplified instrument concepts can be employed on the Moon.

The Rover Permittivity Sensor (RPS): RPS is developed as a contribution of ESA to an upcoming UAE rover mission to the polar region of the Moon [10]. It is designed for in-situ investigation of the regolith in the shallow subsurface and is an evolution of the permittivity sensor used in the PROSPECT instrument package [11]. Mounted on a rover wheel, it allows the determination of the regolith’s density, porosity, and water ice fraction at various locations along the rover track. The sensor comprises an electronics unit inside the rover body, which is connected via a slip ring to the electrodes on the rover wheel. The temperature measurement required to evaluate the data of the permittivity sensor is performed by two distinct sensors. A resistive temperature detector (RTD) for direct temperature measurement is integrated into one electrode and comes into contact with the regolith at the electrode-soil interface to provide a direct measurement. An additional temperature measurement is performed using an infrared sensor mounted at the front of the rover.

Figure 1: Prototype of a measurement electrode with an integrated resistive temperature detector attached to a rover wheel.

The RPS Infrared Temperature Measurement Unit (TU): The infrared temperature measurement unit determines the temperature of the undisturbed regolith surface in front of the rover. It therefore provides important contextual data for the permittivity measurement in combination with the RTD integrated in the electrode. To reduce the effects of IR radiation from the rover itself and avoid the exposure of the sensor element to direct sunlight, the IR temperature measurement unit is mounted to the rover body at an angle that minimizes the effect of these error sources.

The main element of the IR temperature measurement unit are thermopile sensors, which consist of multiple thermocouples connected in series. The voltage at the sensor output depends on the temperature difference between the area exposed to the incoming IR radiation and the substrate of the chip. A precise knowledge of the substrate temperature is essential to determine the absolute temperature of the IR radiation. Commercial thermopiles typically have an integrated negative temperature coefficient (NTC) thermistor to determine the substrate temperature. The infrared temperature measurement unit uses an external RTD instead of an NTC thermistor, which is more suitable for measurements at low temperatures, which are expected during operation on the Moon.

A performed feasibility study demonstrated the sensors’ compatibility with cryogenic temperatures and the linearity and sensitivity of the signal. The tests were performed at temperatures between - 130°C and - 40°C, representative of the surface temperature range expected during operation on the Moon. Based on its outcome, we selected the most promising sensor for a detailed study, including the investigation of its signal-to-noise level at different temperatures, the repeatability of the measurement, the variation between different sensors of the same series, and different designs of the TU. We will also investigate the sensitivity of the TU regarding regolith properties determining its emissivity, such as the material composition, grain size distribution, and temperature.

Figure 2: Prototype of the IR temperature measurement unit.

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] Gscheidle, C. et al. (2024) Frontiers in Space Technologies 4. [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] Nurge, Mark A. (2012) Planetary and Space Science 65.1 76-82. [8] Fulchignoni, M. et al. (2002) Space Sci. Rev. 104, 395–431. [9] Seidensticker, K. J. et al. (2007) Space Sci. Rev. 128, 301–337. [10] Trautner, R. et al. (2024) European Lunar Symposium. [11] Trautner, R. et al. (2021) Measurement Science and Technology 32.12 125117.