The Exo-Mars Experiment MiniHUM

Kurzfassung

The water content of soils significantly influences their chemical, physical and biological properties. In respect of Mars the thin layer of the upper millimetres of the Martian surface are of particular interest since this soil interacts directly with the diurnally varying atmospheric humidity, which can reach saturation during night and early morning. Adsorption/desorption of water in the soil and freezing of it can be a consequence. Therefore, near-surface measurements of the atmospheric content of water vapour will allow to investigate the interaction between the atmosphere and the adsorbed water, which is deposited in the upper soillayer. This should be an important point in respect of exobiology and the exploration of Mars. The Institute of Planetary Research at the German Aerospace Center and the SMB Dr. Wernecke & Partner are jointly developing a humidity sensor system in preparation for the ExoMars mission. Its goal is to obtain first in-situ measurements of diurnal and seasonal variations of the near-surface atmospheric water vapour content. By consequent miniaturization the MiniHUM Team was able to reduce size and weight of the coulometric sensor in such a way that the system, weighting 180g, will be one of the smallest instruments on board of the ExoMars mission. The instrument itself consists of two different units: HUM and ASS. The humidity sensor HUM is a combined unit of the coulometric sensor (QSE) and a capacitive (CPS) humidity sensor with an integrated thermocouple. The unit will be placed inside a small sensor housing (55 x 48 x 9 mm³) on the outer solar panel of the lander structure. For measuring the humidity under Martian conditions the principles of coulometric and capacitive measurement are the most appropriate. The coulometric principle is based on the ability of Diphosphorpentoxid (P2O5) to adsorb environmental water vapour almost completely, generating finally HPO3 in solution. In case of a sufficiently applied DC voltage this leads to a resulting electric current due to charge separation in the solution. The resulting current is directly related to the amount of adsorbed water, as described by Faraday´s law. This offers the possibility to quantitatively measure absolute humidity in the dew-point range between –90° C and -40°C. The capacitive humidity sensor uses the humidity dependence of some polymeric dielectrics to measure the relative humidity content of environmental gas from 5% r.h to 80% r.h. By using the two different principles of measurement it is possible to ensure a wide working range but also redundancy and a cross-reference for each sensor. The Atmosphere Saturation Sensor (ASS) will be mounted externally and consists of a high sensitive resistant thermometer for independent determination of the frost point temperature. The measurement of the frost point temperature will give an additional and independent way to determine absolute humidity, specifically at that point of phase transition (cf. Ryan and Sharman, 1981). This information can also be used for calibration purposes of the humidity sensors.