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. Each 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, 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  and the previous cancelled 2001 MSP mission .
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 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 5 cm 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, perchlorate (ClO4-) [3,4]. Analyses of the WCL data have also shown that the soil contained at least 1.3 (±0.5) wt% of soluble SO4= , that the redox potential (Eh) of the soil/water mixture in the WCL was 253 (±0.5) mV , and that the parent salts of the ClO4- are most likely a mixture of 60% Ca(ClO4)2 and 40% Mg(ClO4)2 . The best estimate we have for the concentrations of the species present in the solution, and in the martian soil at the Phoenix site, are given in Table 1 of reference .
The presence of perchlorate on Mars has also been confirmed by its detection in the Mars meteorites EETA79001  and Tissint , and by the Sample Analysis at Mars (SAM) instrument on the Curiosity rover . It has been shown that the perchlorate on Mars is likely produced by the action of UV irradiation on chloride-bearing mineral surfaces . During this process several ClOx intermediates such as hypochlorite (ClO-), chlorite (ClO2-), chlorate (ClO3-), and chlorine dioxide (ClO2) gas, as well as radicals such as ClO-, O2-, ‧OCl, ‧Cl, and ‧OH are also likely generated [12,13]. These intermediate products can alter or destroy organic compounds, with only highly refractory and/or those well protected from UV surviving.
Figure 2. First of three successful deliveries to the first WCL unit on sol-30
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Last Updated 10/19/2023