Relative Humidity on Mars: New Results From the Phoenix TECP Sensor

In situ measurements of relative humidity (RH) on Mars have only been performed by the Phoenix (PHX) and Mars Science Laboratory (MSL) missions. Here we present results of our recalibration of the PHX thermal and electrical conductivity probe (TECP) RH sensor. This recalibration was conducted using a TECP engineering model subjected to the full range of environmental conditions at the PHX landing site in the Michigan Mars Environmental Chamber. The experiments focused on the warmest and driest conditions (daytime) because they were not covered in the original calibration (Zent et al., 2010, https://doi.org/10.1029/2009JE003420) and previous recalibration (Zent et al., 2016, https://doi.org/10.1002/2015JE004933). In nighttime conditions, our results are in excellent agreement with the previous 2016 recalibration, while in daytime conditions, our results show larger water vapor pressure values. We obtain vapor pressure values in the range ~0.005–1.4 Pa, while Zent et al. (2016, https://doi.org/10.1002/2015JE004933) obtain values in the range ~0.004–0.4 Pa. Our higher daytime values are in better agreement with independent estimates from the ground by the PHX Surface Stereo Imager instrument and from orbit by Compact Reconnaissance Imaging Spectrometer for Mars. Our results imply larger day‐to‐night ratios of water vapor pressure at PHX compared to MSL, suggesting a stronger atmosphere‐regolith interchange in the Martian arctic than at lower latitudes. Further, they indicate that brine formation at the PHX landing site via deliquescence can be achieved only temporarily between midnight and 6 a.m. on a few sols. The results from our recalibration are important because they shed light on the near‐surface humidity environment on Mars. Key Points We have recalibrated the relative humidity sensor of the Mars Phoenix lander We obtain water vapor pressure values in the range ~0.005–1.4 Pa, while in previous recalibrations, values in the range ~0.004–0.4 Pa Our results show a two‐order‐of‐magnitude diurnal variation of water vapor pressure, suggesting a strong atmosphere‐regolith interchange Plain Language Summary We present our recalibration of Phoenix's humidity sensor. This recalibration was conducted with a copy of the sensor subjected to the environmental conditions at the Phoenix landing site. Our experiments focus on the warmest and driest conditions because they were not covered in previous calibrations. Our recalibration shows daytime water content values on