ALBERT

Description: A new calibration function for the humidity sensor in the Thermal and Electrical Conductivity Probe (TECP) on the Phoenix 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 ( R H ) and 2) flight data, taken when the atmosphere is independently known to be saturated, is included in the calibration data set. Daytime (6:00h – 19:00h) frost points ranged from 194K – 209K; the nighttime frost point ranged from 179K - 206K. 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°, (sol 55), which is earlier than reported by either CRISM or 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 PBL and adsorptive drawdown of H 2 O. We find R H is mostly 〈 5% (daytime) or 〉 95% (nighttime), and the transition between the two extremes is extremely rapid.