Identity of the Perchlorate Parent Salt(s) at the Phoenix Mars Landing Site Based on Reanalysis of the Calcium Sensor Response

In 2008 the first comprehensive wet chemical analyses of the martian soil were performed by the Wet Chemistry Laboratory (WCL) on board the Phoenix Mars lander [1-4]. In addition to being dominated by Ca²⁺ and Mg²⁺ carbonates, SO₄⁼, and other minor soluble species, the soil contained about 0.6 %wt perchlorate (ClO₄^-). Its discovery on Mars has broad implications for that planet's water cycle, astrobiology, human habitability, and possible formation of liquid brines. One remaining important question raised by the WCL discovery of ClO₄^- on Mars is the identity of the parent perchlorate salt. New refined analysis of the Phoenix WCL sensor data and laboratory analyses and equilibrium modeling, indicate that the identity and ratios of the ClO₄^- parent salts are different than initially proposed. The analyses performed by the Phoenix WCL, along with previous mission results [5], indicated that K⁺, Na⁺, Mg²⁺, Ca²⁺, or Fe^2/3+, either singly or as a mixture, were possible candidate counter cations that could be associated with the amount of ClO₄^- present in solution. If we assumed 100% association of the ClO₄^- in the soil, then Ca²⁺ or Mg²⁺ could account for 100% in each case, since there is in effect a more than sufficient supply of these two counter cations. For other cations, measured concentrations limit the ClO₄^- salt potentially present, with Na⁺ accounting for < 30%, K⁺ < 5%, and Fe^2/3+ < 3%. Clearly, Mg(ClO₄)₂ or Ca(ClO₄)₂ could constitute as much as 100% of the perchlorate salt present in the soil, either alone or in combination. To definitively identify the parent salt of the ClO₄^- detected by WCL, we have; (1) reexamined the original Phoenix WCL data in terms of the perchlorate and calcium sensor response; (2) performed a set of laboratory analyses using a WCL testbed to better understand the behavior of the sensor responses on Mars; and (3) have modeled possible chemical equilibrium and the effect of the perchlorate parent salt on other minerals present. The new analyses further constrain and provide a clear indication that the dominate parent salt in the soil at the Phoenix landing site is Ca(ClO₄)₂, with little or no contribution by Mg or Na-perchlorate salts. [1] Kounaves et al. (2010) J. Geophys. Res., 114, E00A19. [2] Hecht et al. (2009) Science, 325, 64-67. [3] Kounaves et al. (2010) Geophys. Res. Let., 37, L09201 [4] Quinn et al. (2011) Geophys. Res. Lett., 38, L14202. [5] Gellert et al. (2004) Science, 305, 829-32.