Improving relative humidity measurements on Mars: New laboratory calibration measurements

Abstract. In this paper we present new calibration measurements that have been performed with the ground reference models of the relative humidity instruments of the MSL, Mars 2020 and ExoMars missions. All instruments are based on capacitive sensor head technology, and they are developed, manufactured and tested by The Finnish Meteorological Institute (FMI). Calibration of capacitive humidity sensors for the Martian environment has been a challenging task and special facilities are needed in order to create Martian conditions including all relevant environmental parameters: low pressure, low temperature, carbon dioxide environment, and especially humidity that can be accurately controlled and measured. A measurement campaign was performed at DLR PASLAB (Planetary Analog Simulation Laboratory) determining relative humidity calibration data sets for REMS-H, MEDA HS and METEO-H instruments in temperatures from -30 °C down to -70 °C in low pressure CO₂. In addition to the stable point humidity calibration measurements in CO₂, the instrument performance was tested with the actual Martian atmosphere composition, and during long continuous measurements. The new calibration data set has already been used in the flight calibration of the MEDA HS instrument resulting in successful calibration and excellent accuracy. The results from this campaign will further improve relative humidity measurements on Mars by providing the means to reanalyse the current calibration of the REMS-H flight model, and by allowing more accurate comparison between the two instruments currently on the Martian surface.