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

摘要

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_2O 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_2O vapor density is close to the summertime value expected for equilibrium with ground ice. A discrepancy between the H_2O 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_2O. We find that RH is mostly 95% (nighttime), and the transition between the two extremes is extremely rapid.