A revised calibration function and results for the Phoenix mission TECP relative humidity sensor

Abstract

A new calibration function for the humidity sensor in the Thermal and Electrical Conductivity Probe (TECP) on the Phoenix (PHX) Mars mission has been developed. Two changes are incorporated: (1) it is now cast in terms of frost point (T f ) rather than relative humidity (RH), and (2) flight data, taken when the atmosphere is independently known to be saturated, are included in the calibration data set. Daytime (6:00 h-19:00 h) frost points ranged from 194 K to 209 K; the nighttime frost point ranged from 179 K to 206 K. The response of the sensor was smooth and continuous throughout. Daytime humidity exhibited large, high-frequency variance driven by turbulence, whereas nighttime humidity varied smoothly with the temperature of the atmosphere. Nighttime saturation of the atmosphere begins at L s 101°, (Martian solar day (sol) 55), which is earlier than reported by either Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) or solid-state imager (SSI). Early mornings are the most humid part of the sol after L s 113°(sol 80), due to sublimation of surface ice that precipitates overnight. H 2 O is removed from the atmosphere into the regolith, mostly during the late afternoon, although this continues into the evening. The ground ice exposed by Phoenix operations masks the naturally occurring process in the early evening and may cause the atmosphere immediately around the lander to saturate somewhat earlier in the evening than it otherwise would have. The average H 2 O vapor density is close to the summertime value expected for equilibrium with ground ice. A discrepancy between the H 2 O column calculated from TECP data and the column measured by CRISM and SSI is likely due to comparable timescales between turbulent mixing through the planetary boundary layer and adsorptive drawdown of H 2 O. We find that RH is mostly < 5% (daytime) or > 95% (nighttime), and the transition between the two extremes is extremely rapid. TECP Humidity MeasurementsThe TECP H 2 O measurement was based on a General Electric Panametrics MiniCap 2 polymer capacitive humidity sensor. The flight sensor was one of four, each mounted in one of the four flight model TECPs that were built and calibrated. The flight instrument was selected from among them based on performance and stability over all six measurement types during calibration. The humidity sensor was mounted internally, on ZENT ET AL.PHOENIX HUMIDITY RESULTS 626 PUBLICATIONS

The Modern Near-Surface Martian Climate: A Review of In-situ Meteorological Data from Viking to Curiosity

“…Values of temperature and RH covered in the pre-flight calibration (Zent et al 2009) overlap only partially with the environmental conditions found at the Phoenix landing site ). The calibration function was revised in 2016 to correct for inaccuracies at the lowest temperatures observed at the Phoenix landing site (Zent et al 2016), but the new processed RH values are not available in the PDS yet.…”

The Effect of Mars‐Relevant Soil Analogs on the Water Uptake of Magnesium Perchlorate and Implications for the Near‐Surface of Mars

“…From these two quantities, the water vapor pressure can be derived. The TECP onboard PHX measured RH at various heights, but not consistently at the surface, making it difficult to integrate into the model (Zent et al, , ). For PHX, the simulation includes an ice table at a depth of 0.1 m (Smith et al, ) that exchanges water vapor with the atmosphere.…”

“…In the case of these subsurface modeled conditions, the solar longitude ( L s ) = 90°, which is shown here because the simulated diurnal cycle permits for the deliquescence of Mg (ClO 4 ) 2 . Figure b shows all in‐air data from the PHX TECP instrument averaged over 1‐hr intervals (Zent et al, ). The measurement height varied, as the robotic arm of PHX was moved up and down over the course of the measurements.…”

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

“…The bimodal behavior of the water vapor pressure shown in Figure 6 is not unique to sol 55 (see Figure 4) and may be explained by the north-facing lander workspace and shadowing from the lander and/or the TECP itself, resulting in a temporarily lowered ground temperature (Zent et al, 2010) and less sublimation of exposed water ice in the workspace, lowering the water vapor pressure measured by the TECP 3 cm above the ground. Figure 7 compares the water vapor pressure values resulting from our TECP recalibration in yellow, the original preflight calibration in blue (Zent et al, 2010), and a postflight calibration in orange (Zent et al, 2016;current PDS values). While during nighttime, our calibration shows values that are in excellent agreement with those of the revised 2016 calibration, during daytime, our values are closer to those of the original calibration.…”