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Ground Calibration Measurements of Humidity Sensors for Mars Missions

Presentation #118.03 in the session Mission-supporting Practices, Modeling, and Data (Poster)

Published onOct 23, 2023
Ground Calibration Measurements of Humidity Sensors for Mars Missions

The Finnish Meteorological Institute (FMI) has provided atmospheric relative humidity (RH) instruments for several Mars missions, the first one being REMS-H [1] onboard Curiosity rover followed by MEDA HS [2] onboard Perseverance. Both instruments are currently active and working nominally, providing daily readings of relative humidity and derived water vapour volume mixing ratio (VMR). A relative humidity instrument was also delivered for METEO on the ExoMars 2020 but the mission was suspended in 2022.

Calibration of capacitive humidity sensors for the Martian environment has proven to be a challenging task. The capacitive sensors, that have been used in all in situ measurements so far, are sensitive not only to relative humidity but also other variables like temperature, low pressure and carbon dioxide (CO2)[3]. Special facilities are needed in order to create Martian conditions including all relevant environmental parameters, and especially to control and measure the humidity inside the measurement chamber with sufficient accuracy. Building upon the experience from previous missions, a test program was planned for the humidity sensor of Mars 2020 using extensive ground reference model measurements to supplement the flight model calibration [2]. An appropriate facility for the supplementary ground reference measurements was identified as the DLR PASLAB (Planetary Analog Simulation Laboratory)[3].

We present the experiment setup, the test program and the results of the campaign at DLR PASLAB, encompassing RH calibration data sets of three relative humidity instruments at temperatures from -30°C to -70°C in Martian pressure CO2 gas. The results have already been used in formulating the flight calibration of MEDA HS [2] and now we provide equivalent results for REMS-H and METEO-H. The results from this campaign will further improve the near-surface Martian relative humidity measurements by providing the means to reanalyze the current calibration of the REMS-H flight model, and by allowing more accurate comparison between the two active instruments on Mars.

References: [1] Harri, A.-M. et al.: Mars Science Laboratory relative humidity observations: Initial results, Journal of Geophysical Research: Planets, 119, 2132–2147, 2014. [2] Hieta, M. et al.: MEDA HS: Relative humidity sensor for the Mars 2020 Perseverance rover, Planetary and Space Science, 223, 105 590, 2022. [3] Lorek, A. and Majewski, J.: Humidity Measurement in Carbon Dioxide with Capacitive Humidity Sensors at Low Temperature and Pressure, Sensors, 18, 2615, 2018.

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