The Phoenix Wet Chemistry Laboratory (WCL)

During the summer of 2008, the Phoenix Mars Lander acquired and analyzed samples of soil and ice, to investigate the presence of water in all its phases and the historical record preserved in the chemistry and mineralogy of the soil. It also addressed biohabitability by, identifying potential chemical energy sources available to support life, analyzing for organics, and identifying the potential of the geochemical environment to preserve paleontological evidence.

To analyze the chemistry of the soil, Phoenix carried with it a Wet Chemistry Laboratory (WCL) consisting of four identical cells (Figure 1), each comprised of a lower "beaker" containing a set of chemical sensors designed to analyze the chemical properties of the soil, and an upper "actuator" for adding soil, water, reagents, and stirring. The beaker contained an array of sensors consisting of solid state and PVC-membrane based ion selective electrodes (ISE) that analyzed for inorganic anions and cations, including Ca2+, Mg2+, K+, Na+, NH4+, Cl-, Br-, I-, NO3-, ClO4-, and SO4=. The array also included electrodes for pH, conductivity, oxidation-reduction potential (Eh), anodic stripping voltammetry (ASV) for heavy metals (such as Cu2+, Cd2+, Pb2+, Hg2+), chronopotentiometry (CP) for independent determination of chloride, bromide and iodide, and cyclic voltammetry (CV) for identifying and analyzing possible reversible and irreversible redox couples.

The upper assembly consisted of a sealed, Teflon-coated, titanium leaching solution reservoir (water plus ionic species for initial sensor calibration), a sample drawer designed to receive the soil through a screened funnel from the robotic arm, remove excess soil, and deposit 1 cm3 of soil into the beaker containing 25 mL of water; a stirrer motor with impeller; and a reagent dispenser that held five crucibles consisting of a second calibration reagent, an acid, and three packed with barium chloride for determination of sulfate. A complete description of the WCL has been published, both as part of Phoenix mission [1] and the previous cancelled 2001 MSP mission [2].

i Figure 1. . . . . . . . . . . . .

In June of 2008, sol 30 on Mars, the Wet Chemistry Laboratory (WCL) performed the first wet chemical analysis of a soil on another planet to determine its soluble components (Figure 2). The WCL's first analysis of a 1 cc soil sample on sol-30, produced major new scientific findings that have changed the way we view the aqueous geochemistry of Mars.  The data from this array of ISE sensors provided new scientific insights into the history of the planet, its potential for supporting microbial life, and its atmospheric chemistry. The analyses on three soil samples, two from the surface and one from 5cm depth, revealed a slightly alkaline soil with a pH of ~7.7 (±0.3), an average conductivity of ~1.4 (±0.5) mS/cm for the 1:25 soil/solution mixture, and the presence in solution of Ca2+, Mg2+, K+, Na+, Cl-, SO4= , and most unexpectedly, ClO4- [3,4].

More recent analyses have also shown that the soil contains at least 1.3 (±0.5) wt% SO4= [5], that the redox potential (Eh) of the soil in the WCL test solution was 253 (±0.5) mV [6], and that the parent salts of the perchlorate are 60% Ca(ClO4)2 and 40% Mg(ClO4)2 [7].

The best estimate we have for the concentrations of the species present in the solution and in the martian soil are given in
Table 1 [5] .

Figure 2

[1] S. P. Kounaves, et al., "The MECA Wet Chemistry Laboratory on the 2007 Phoenix Mars Scout Lander",
J. Geophys. Res., 114, 2009,
E00A19, doi:10.1029/2008JE003084. Abstract - Full Text PDF

[2] S. P. Kounaves, et al., "Mars Surveyor Program '01 Mars Environmental Compatibility Assessment Wet Chemistry Lab: A Sensor Array for Chemical Analysis of the Martian Soil", J. Geophys. Res., 2003, 108(E7), 5077-89. Abstract - Full Text PDF

[3] S. P. Kounaves, et al., "Wet Chemistry Experiments on the 2007 Phoenix Mars Scout Lander: Data Analysis and Results" J. Geophys. Res., 2010, 115, E00E10, doi:10.1029/2008JE003084 Abstract.- Full Text PDF

[4] M. H. Hecht, S. P. Kounaves, et al., "Detection of Perchlorate and the Soluble Chemistry of Martian Soil at the Phoenix Lander Site", Science, 2009, 325, 64-67. Abstract - Full Text PDF

[5] S. P. Kounaves, et al., "Soluble Sulfate in the Martian Soil at the Phoenix Landing Site",
Geophys. Res. Lett., 2010, 37, L09201, doi:10.1029/2010GL042613. Abstract - Full Text PDF

[6] R. C. Quinn, J. D. Chittenden, S. P. Kounaves, and M. H. Hecht, "The Oxidation‐Reduction Potential of Aqueous Soil Solutions at the Mars Phoenix Landing Site" Geophys. Res. Lett., 2011, 38, L14202, doi:10.1029/2011GL047671. Abstract - Full Text PDF

[7] S. P. Kounaves, et al., "Identification of the Perchlorate Parent Salts at the Phoenix Mars Landing Site and Possible Implications", Icarus, 2014, 232, 226-231, doi:10.1016/j.icarus.2014.01.016 Full Text

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