Near-Surface Humidity at the Phoenix Landing Site as Measured by the Thermal and Elec- trical Conductivity Probe (TECP)

Abstract

Background: The Phoenix Mars Lander is the first spacecraft to explore a sub-polar latitude on Mars where subsurface ice is present [1]. It is also the first with the ability to do direct measurement of the atmospheric humidity in the boundary layer within 2 meters of the surface [2]. A relative humidity device (GE Panametrics MiniCap 2 polymer relative humidity sensor) in the body of the TECP (Thermal and Electrical Conductivity Probe) permitted frequent monitoring of atmospheric humidity at the end of the Phoenix robotic arm. Relative humidity (RH) and temperature data at the sensor are used to compute vapor pressures. Atmospheric temperature from the meterology (MET) mast can then be used to compute true atmospheric RH. When the TECP is inserted into the soil for thermal properties analysis, the humidity sensor is approximately 5 cm from the regolith surface. Data & Analysis: Polymer humidity sensors are non-linear in their response to relative humidity close to 0 and 100% [3]. TECP sensor data with a relative humidity (with respect to water) less than 1.6% are not well fit by the calibration function and are rejected. To reduce instrumental noise, individual data points at 1 s intervals are averaged into 10 s bins, comparable to or shorter than the response time of the sensor. These methods result in 1,465 data points taken with the TECP in the soil, and 5,972 points with the TECP in air. Figure 1 shows all data converted into vapor pressure plotted against local mean solar time. The daytime averaged H2O pressure is steady throughout the mission (LS 76 to 147) at ~1.8 Pa. Daytime RH was initially <5%, increasing to ~10% late in the mission. Subsequent to about Sol 70, nighttime condensation was observed by lidar [4] and SSI surface images, consistent with high relative humidities. As the season progressed, temperatures dropped and saturation occurred earlier in the evening.