Aqueous Perchlorate Liquid Solutions at the Phoenix Landing

Introduction: The possibility of liquid water or brines on the surface of Mars is one of the most important topics in Mars research [1]. Recent results from NASA’s polar lander, Phoenix, have indicated the presence of perchlorates (ClO4 ) in the soil surrounding the landing site [2]. It is likely that these ions associate with either sodium or magnesium [3], which would make them ideal candidates for liquid brines on the surface [4, 5]. Very little is known regarding the behavior of these salts and their solutions in water, especially at low temperatures. Through evaporation experiments and geochemical modeling, we have studied the stability of perchlorates in a simulated Martian environment, and applied these results to Phoenix observations. Experimental: Experiments were performed in a Martian simulation chamber using previously described methods [1]. Samples of NaClO4 or Mg(ClO4)2 solutions at various concentrations (~20 to 50 wt%) are placed into a chilled CO2 atmosphere (~263 K), which is then pumped down to Martian pressure (7 mbar). Using a precision balance, the mass loss rate was then measured and converted into evaporation rate. Results: At temperatures between 256 and 269 K, evaporation rates of sodium perchlorate (Fig. 1A) range from 0.07 mm h -1 (55 wt% concentration at 258 K) to 0.49 mm h -1 (20 wt% concentration at 266 K). As seen in previous studies, evaporation rate is dependent on both temperature and concentration of the solution [1]. For instance, the evaporation rate of a 55 wt% solution ranges from 0.07 to 0.19 mm h -1 over a temperature range of 11 K. The evaporation rate is also dependent on perchlorate concentration: at 266 K, the 20 wt% sodium perchlorate evaporates at 0.49 mm h -1 , while the 55 wt% evaporates at 0.14 mm h -1 . Magnesium perchlorate behaves in a similar manner (Fig. 1B). Evaporation rates range from 0.29 mm h -1 (20 wt% at 267 K) to 0.06 mm h -1 (49 wt% at 260 K). Again, evaporation rates are dependent on both temperature and concentration. For instance, at 264 K, the 20 wt% magnesium perchlorate evaporates at 0.29 mm h -1 , while a 49 wt% solution evaporates at 0.07 mm h -1 . In addition, temperature plays a suppressive role as seen in the 49 wt%: evaporation rates range from 0.17 mm h -1 (272 K) to 0.06 mm h -1