Results of the Phoenix Relative Humidity Sensor Recalibration

We show results of the recalibration of the Thermal and Electrical Conductivity Probe (TECP) relative humidity (RH) sensor of the Phoenix Mars lander [Zent et al., 2009]. Due to uncertainties in its pre-flight calibration, which partially overlapped the environmental conditions found at the Phoenix landing site [Tamppari et al., 2010], only the raw, unprocessed output of the TECP RH sensor is available in NASA's Planetary Data System (PDS). The sensor's calibration was revised in 2016 to correct for inaccuracies at the lowest temperatures [Zent et al., 2016], but the new processed RH values were not posted in the PDS. We have been using a spare engineering unit of the TECP to recalibrate the sensor in the full range of Phoenix landing site conditions in the Michigan Mars Environmental Chamber (MMEC) [Fischer et al., 2016]. We compare raw output data of the engineering unit in the MMEC with that of the flight unit from the preflight calibration. We observed that the engineering unit's RH sensor output was shifted to higher values compared to the flight unit's output at the same conditions of temperature and humidity. Based on this shift, we use a translation function that fits the in-situ measurements of the flight unit into the engineering unit output space. To improve the accuracy of this function, we use additional observations corresponding to saturated conditions when near-surface fog was observed [Whiteway et al., 2009], as well as observations around noon when the RH is expected to be below 5%. The entire range of conditions observed on the Martian surface is covered in our recalibration. The raw output of the sensor is used to obtain a new calibration function. This allows us to obtain high-level RH data at Martian polar conditions. The recalibrated data will be posted in the PDS. REFERENCES: Fischer, E., et al. (2016), Astrobiology, 16, 12, doi: 10.1089/ast.2016.1525. Tamppari, L. K., et al. (2010), J. Geophys. Res., 115, E00E17, doi:10.1029/2009JE003415. Whiteway, J. A., et al. (2009), Science, 325, 68, doi: 10.1126/science.1172344. Zent, A. P., et al. (2009), J. Geophys. Res., 114, E00A27, doi:10.1029/2007JE003052. Zent, A. P., et al. (2016), J. Geophys. Res. Planets, 121, 626-651, doi:10.1002/2015JE004933.