Analyzing Hydrated Salt Solutions at the Phoenix Landing Site

Introduction: In 2008, NASA’s Phoenix lander arrived in the northern arctic plain of Mars. Its objective was to perform analyses on the subsurface water ice, ultimately characterizing the geology, climate, and habitability of the Martian polar regions. Two of the instruments on board were TECP (Thermal Electric Conductivity Probe), which recorded temperature and relative humidity measurements, and WCL (Wet Chemisty Lab), which deposited soil into a leaching solution and recorded the resulting ion concentrations. The most interesting of these ions were perchlorate salts (ClO4 ) [1]. The presence of salt significantly lowers the eutectic temperature of water, or the lowest possible temperature to have any remaining liquid. While perchlorates have some of the lowest documented eutectic temperatures, some are dramatically lower than others [2]. NaClO4 for example has a eutectic temperature of 236 K (Fig. 1A), while Mg(ClO4)2 can be liquid down to 206 K (Fig. 1B). Relative humidity (RH) also affects the stability of liquid water. When the water vapor pressure of the salt is less than that of the air, deliquescence occurs. Deliquescence is when the salt reacts with adsorbed water from the air to create a solution. The temperature and RH at which this happens are important properties of