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

Abstract

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.

Keywords: Mars; Phoenix; TECP; relative humidity; water cycle; water vapor.

Figure 1

Sketch of the Michigan Mars Environmental Chamber (MMEC). It can simulate the entire range of atmospheric pressure, temperature, and relative humidity encountered at the Phoenix landing site. The MMEC has been used to augment the calibration of the relative humidity sensors onboard the Mars Science Laboratory, Mars 2020, and ExoMars 2020 missions (Hieta et al., 2019).

Figure 2

The thermal and electrical conductivity probe (TECP) preflight calibration data (red) only partially overlaps the recorded relative humidity (RH) measurements at the Phoenix landing site (light gray). We use the output of a TECP engineering unit (blue) that matches the environmental conditions of the preflight calibration (red) in terms of T _b and T _f and additional known landing site conditions (green) to transform the in situ measurements range (light gray) into the range of the engineering unit (dark gray). We then use the entire output of the engineering unit (symbolized by the arrows) to cover the entire range of T and RH conditions (dark gray) to calibrate the engineering unit and find a recalibration for the flight unit.

Figure 3

Temporal coverage of the thermal and electrical conductivity probe relative humidity (RH) sensor as a function of local true solar time and sol number, with solar longitude color‐coded. In‐soil measurements were taken on sols 46–47, 54–55, 69–71, 86, 98, 103–104, 111, 119, 122–124, and 149–150. On the remaining sols, atmospheric RH measurements were conducted at heights ranging from 0 to ~2.2 m.

Figure 4

The recalibrated thermal and electrical conductivity probe relative humidity sensor measurements at the Phoenix landing site color‐coded by L _s as water vapor pressure (top) and frost point temperature (bottom) over local time.

Figure 5

The recalibrated thermal and electrical conductivity probe relative humidity based on frost‐point and board temperature measurements at the Phoenix landing site color‐coded by L _s as local relative humidity at the sensor location.

Figure 6

Recalibrated Phoenix thermal and electrical conductivity probe measurements on sols 55 and 56 with error bars based on instrument errors and errors due to the upper‐bound assumption for water vapor pressure at the highest temperatures.

Figure 7

Comparison of our calibration of the thermal and electrical conductivity probe relative humidity sensor with previous calibrations.

Figure 8

Comparison of the maximum diurnal water vapor pressure values throughout the Phoenix (PHX) mission obtained using the results of our recalibration (dark green), the previous postflight calibration (orange; Zent et al., 2016), and from precipitable water vapor column abundance retrievals at the PHX landing site by Compact Reconnaissance Imaging Spectrometer for Mars (CRISM; cyan) and PHX Surface Stereo Imager (SSI, blue; Tamppari et al., 2010). Also shown are water vapor pressure values derived around noon by the Mars Science Laboratory/Chemcam instrument (red; McConnochie et al., 2018). For the sake of clarity, PHX/thermal and electrical conductivity probe values (dark green and orange) shown in this figure correspond to averages over ΔL _s = 5° bins, and therefore absolute maximum values shown here are slightly lower than in Figure 4. CRISM and SSI measurements were taken at ~14:00 Local Mean Solar Time (LMST) and between 13:00 and 17:00 LMST, respectively.

Figure 9

Day/night ratio comparison of water vapor pressure between the Phoenix (PHX, red) and the Mars Science Laboratory (MSL, blue) mission. At both landing sites and for every L _s, the ratio is always >1, indicating higher daytime than nighttime values. At the PHX landing site, the ratios are one order of magnitude larger than at the MSL site, indicating larger atmosphere‐regolith H₂O exchange.

Figure 10

Stability diagram of NaClO₄, Mg (ClO₄)₂, and Ca (ClO₄)₂ with superimposed values of Phoenix (PHX) relative humidity (RH) and temperature values at the thermal and electrical conductivity probe (TECP) location (blue) and at 2 m height (orange) and Mars Science Laboratory (MSL)/ Rover Environmental Monitoring Station values at the ground (yellow) and at 1.6 m height (purple). RH values shown here are converted to be with respect to liquid water for comparison with the brine stability lines, not with respect to water ice as measured by the instruments. For each salt, the colored thick‐dashed line represents the deliquescence relative humidity at which the various salts form aqueous solutions. Results from previous lab experiments of deliquescence of Ca, Mg, and Na perchlorates are shown in colored empty circles (Gough et al., 2011; Nuding et al., 2014). For reference, the eutectic temperature isotherm of Ca (ClO₄)₂ (solid black at ~199 K) and two isobars (dashed black) showing water vapor pressure values of 0.005 (minimum measured by the TECP) and 1.4 Pa (maximum measured by the TECP) are shown.