Relative humidity laboratory measurements in Martian atmospheric conditions

1 Introduction

The Finnish Meteorological Institute (FMI) has developed relative humidity instruments for several Mars lander missions. Calibration of these instruments can be challenging due to the required temperature, pressure and humidity conditions that can be reliably simulated by only a few laboratories. Humidity measurements in Martian conditions have been previously performed for these instruments in the FMI’s laboratory and the Michigan Mars Environmental Chamber (MMEC) at the University of Michigan. A new measurement campaign was performed at the Planetary Analog Simulation Laboratory (PASLAB) of the German Aerospace Center (DLR) during autumn 2020 and spring 2021. The campaign included three ground reference models (REF) of FMI’s relative humidity instruments: REMS-H of the MSL Curiosity rover [1], MEDA HS of the Mars 2020 Perseverance rover [2] and METEO-H developed for the ExoMars 2022 surface platform [3]. All three instruments are based on HUMICAP® capacitive thin-film polymer sensors by Vaisala as well as Vaisala’s ASIC technology. REMS-H has three HUMICAP sensor heads, while MEDA HS and METEO-H both contain two newer types of HUMICAP sensors which provide a larger dynamic capacitance range and include integrated resistive PT1000 sensors for measuring the temperature of the sensor head. The REF models of each instrument are identical to the flight models and have gone through testing and calibration campaigns at FMI together with the flight models.

Figure 1: REMS-H (left) of the MSL Curiosity rover and MEDA HS (right) of the M2020 Perseverance rover. METEO-H for ExoMars 2022 is identical to MEDA HS. The electronics and sensor heads are located on a PCB inside a cylindrical Faraday shield. The Faraday shield is covered with PTFE membrane filter to protect the sensor from dust.

The main purpose of the measurement campaign at the DLR was to obtain calibration data in multiple humidity points in CO₂ within the instruments’ operational temperature and pressure ranges. The instruments have been calibrated at the FMI using two humidity points in CO₂, resulting in satisfactory quality but a notable calibration uncertainty. The new multi-point data are used for complementing the calibration of MEDA HS and METEO-H flight models, as well as to check REMS-H calibration.

2 Test setup

The DLR PASLAB facility, designed to simulate Martian near-surface atmospheric conditions, is used for habitability-related investigations and sensor studies [4][5]. In this measurement campaign the environmental conditions were controlled in terms of gas type, pressure, temperature and relative humidity. The setup is described in Figure 2. The instruments were enclosed in the same measurement chamber as has been used in the FMI’s laboratory (see Figure 3), and the chamber was connected to the environmental control system. As part of the DLR’s own experiment, another chamber with humidity sensors was connected to the system for simultaneous measurements, with one sensor also placed inside the FMI’s chamber for comparison.

The measurements were performed at stable temperature points between -70 °C and -30 °C and pressure points ranging from 5.7 hPa to 9.8 hPa in CO₂ gas. At each temperature and pressure combination, a sequence of stable humidity points was run by an automatic program, increasing the humidity in steps from the driest point to the highest humidity and back. Some continuous measurements were also performed in changing humidity. At the end of the campaign, a different gas mixture imitating the true Martian atmospheric composition was tested to investigate possible effects on the sensor behavior.

Figure 2: Setup of the environmental control and measurement system at the DLR PASLAB. Credit: DLR.

Figure 3: Cutaway illustration of the measurement chamber with the three FMI's ground reference models inside.

3 Campaign outcome

The measurement campaign was completed in May 2021 with successful outcomes. All planned temperature and pressure combinations were performed once or twice with a sufficient range of humidity points. However, at -30 °C temperature only points up to about 30 %rh could be achieved due to system limitations. We obtained a comprehensive set of stable humidity point data for each of the humidity sensors, as well as some data in changing humidity. Figure 4 shows the stable humidity points measured by one HUMICAP sensor from each instrument. The results are as expected: the temperature dependence is observed as a clear spread of capacitance readings and the pressure effect as a slight dispersion. Using the Martian-like gas composition did not seem to affect the sensor behavior compared to pure CO₂.

Figure 4: Selected measurement points in different temperature and pressure combinations shown for one HUMICAP sensor of each REF model: MEDA HS (upper left), METEO-H (upper right) and REMS-H (lower center). The y-axis represents the reference relative humidity in the measurement chamber and the x-axis the sensor output capacitance. Measurements in the Martian atmospheric gas composition are marked with green stars.

4 Conclusions

The humidity measurement campaign at the DLR PASLAB was performed between September 2020 and May 2021. As an outcome we obtained valuable datasets for MEDA HS, METEO-H and REMS-H ground reference models consisting of multiple relative humidity points over the sensors’ range from 0 %rh to 100 %rh in low pressure CO₂. The results were in line with previous laboratory measurements of these instruments.

References

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[2] Rodriguez-Manfredi, J.A. et al.: The Mars Environmental Dynamics Analyzer, MEDA. A suite of environmental sensors for the Mars 2020 mission. Space Sci Rev 217:3, 48 (2021).

[3] Vago, J. et al.: ESA ExoMars program: The next step in exploring Mars. Sol Syst Res 49, 518-528 (2015).

[4] Lorek A. and Koncz A.: Simulation and measurement of extraterrestrial conditions for experiments on habitability with respect to Mars. In Habitability of Other Planets and Satellites, vol. 28, 145–162 (2013).

[5] Lorek A. and Majewski J.: Humidity measurement in carbon dioxide with capacitive humidity sensors at low temperature and pressure. Sensors 18. (2018).