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The Phoenix Mars Lander Wet Chemistry Laboratory (Wcl): Understanding the Aqueous Geochemistry of the Martian Soil S

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The Phoenix Mars Lander Wet Chemistry Laboratory (Wcl): Understanding the Aqueous Geochemistry of the Martian Soil S International Workshop on Instrumentation for Planetary Missions (2012) 1005.pdf THE PHOENIX MARS LANDER WET CHEMISTRY LABORATORY (WCL): UNDERSTANDING THE AQUEOUS GEOCHEMISTRY OF THE MARTIAN SOIL S. P. Kounaves, In-Situ Planetary Chemical Analy- sis Laboratory, Department of Chemistry, Tufts University, Medford, MA 02155 ([email protected]). Introduction: In May of 2008, the Phoenix Mars Lander descended on the northern plains of Mars. The goals of the Phoenix Lander were, to acquire and ana- lyze samples of soil and ice, to investigate the presence of water/ice, to determine the chemistry and mineralo- gy of the soil, to identify potential chemical energy sources available to support life, to analyze for organ- ics, and to identify the potential of the geochemical environment to preserve paleontological evidence [1]. Figure 1. The Wet Chemistry Laboratory (WCL) as part of On the Phoenix deck were several instruments, in- MECA (left) and a single WCL unit (right. cluding four Wet Chemistry Laboratory (WCL) units - that were to perform the first wet chemical analysis of was the discovery of 0.7% perchlorate (ClO4 ), most soil on Mars [2]. Each of the four WCL units, shown likely present as either the Mg/Ca-(ClO4)2 salt [3,4]. in Figure 1, consisted of a lower "beaker" containing The presence of such levels of perchlorate, and most an array of sensors to analyze the chemical properties likely on a global scale, has led to the realization that of the soil/water mixture, and an upper "actuator" that the mass spectrometer on Phoenix and on both 1974 added the soil, water, and reagents, and provided for Viking 1/2 landers would not have been able to detect stirring of the mixture. The array of sensors in the any organics. This discovery has rekindled the possi- beaker consisted of solid state and PVC-membrane bility for the presence of both organics and life on based ion selective electrodes (ISE) that analyzed for Mars. Reviewed here are the major findings of the wet 2+ 2+ + + + - - - - chemistry analyses performed by WCL and their impli- Ca , Mg , K , Na , NH4 , Cl , Br , I , NO3 , pH, and = cations for martian geochemistry and habitability. SO4 , and special electrodes for conductivity and oxi- The ionic species directly measured by the sensors dation-reduction potential (Eh). Also included were anodic stripping voltammetry (ASV) for determination and originally reported [3,4], were the soluble compo- of heavy metals (e.g., Cu2+, Cd2+, Pb2+), chronopotenti- nents and represented a portion of the total amount of any specific chemical elements present in the sample ometry (CP) for independent determination of chloride, = - and did not reflect other species such as CO3 , HCO3 , bromide and iodide, and cyclic voltammetry (CV) for = identifying and analyzing possible reversible and irre- and SO4 , that may have been present and were slowly versible redox couples. The actuator consisted of a or totally insoluble, or not measurable by any sensors. 25mL titanium tank that contained deionized water To better understand the identity of the added soil and plus ionic species at ~ 10-5 M concentrations for the its soluble mineral composition, equilibrium modeling initial sensor calibrations, a drawer that would accept ~ of the soil/solution mixture with Geochemist’s Work- 1 cm3 of soil through a screened funnel from the robot- bench (GWB) was used to provide guidance and for ic arm, and a stirrer motor. The actuator also included a additional laboratory experiments with synthetic Mars reagent dispenser that held five “crucibles”, a second samples. calibration reagent, an acid, and three packed with The models and laboratory analyses have revealed = a very complex system with the final species distribu- BaCl2 for the titrimetric determination of SO4 . During surface operations, three ~ 1 cm3 soil sam- tion dependent on several variables, including ionic ples, one from the surface (Rosy Red) and two from the strength, pH, precipitation, ion adsorption, the partial top of the ice table approximately 5cm in depth (Sor- pressure of the CO2 in the WCL headspace, and the ceress 1 and Sorceress 2), were successfully added and rate of equilibration. To differentiate between possi- analyzed in three of the four WCLs. In general, the ble, probable, and confirmed chemistry, has require soluble equilibrium concentrations were found to be extensive formulation and testing with both simple dominated by Mg2+, and Na+ at the mM levels, with aqueous leachate simulants and eventually more realis- sub-mM concentrations of Ca2+, Cl-, and K+. The pres- tic Mars simulants. Recent additional data analyses ence of these salts in addition to the moderate pH (7.7) have confirmed the presence of sulfate in the soil, the redox potential of the soil/water mixture, and the prob- and Eh (253mV), has broad implications for Mars' geo- chemistry and habitability. However, most surprising able identity of the parent perchlorate salts. International Workshop on Instrumentation for Planetary Missions (2012) 1005.pdf 1.2 The comparison of the Mars soil with a simulant, 1 formulated according to the results of the WCL and a - .8 Fe(SO4)2 GWB model, shows that when it is dissolved in water, .6 .4 yields ion concentrations almost identical to those ob- Hematite Eh (volts) Eh FeSO .2 4 served on Mars. These results give confidence that the 0 soluble composition and parent salts of the soil at the Pyrite –.2 Phoenix site are reasonably constrained. –.4 0 2 4 6 8 10 A long standing question about the martian soil has pH been its redox potential (Eh) on contact with water. The Figure 2. The titration for soluble sulfate on Mars by addi- measured Eh of the soils in WCL solution has been tion of barium chloride [5], and the Eh-pH diagram showing recently reported to be 253 ±6mV at a pH of 7.7 ± 0.1. the phase stability fields for a soil-solution mixture based on −3 −9 A difference between E and pH changes suggested a Table 1 data and 10 > [Fe2+] > 10 [6] (left) h contribution from species not measured with the WCL, = The total soluble sulfate (SO4 )T present in the soil and experimental simulations confirmed that ppm soil 2+ was determined by addition of Ba (as BaCl2) to two levels of metal peroxides or similar oxidants could samples, Rosy Red on sols 30/34 and Sorceress-2 on explain this difference. Although these soils contain - sols 107/116, and from a blank on sol 96. As shown in high levels of ClO4 (a strong oxidant at temperatures > Figure 2, the concentration of the added Ba2+ in Sor- 200°C), and possibly low levels of more chemically ceress-2 sample remained relatively constant until the reactive oxidants, the Eh of the soil is moderate and end of sol 116 when it rapidly increased, indicating that within the range of that expected for habitable soils [6]. = it was no longer being precipitated by any SO4 present The next generation Mars chemical analysis lab 2− in solution. At that point the (SO4 )T was equal to half will build on the heritage and demonstrated success of − = of the ΔCl . The SO4 in solution is thus 5.9 (±1.5) the Phoenix WCL, and take advantage of recent im- = mM, and equivalent to 1.4 (±0.5) wt % SO4 in the soil. provements in both sensor and lab-on-a-chip technolo- There are several sulfate mineral phases that are gy. As part of an MER-class rover it will provide the plausible candidates. These include K-, Na-, Fe-, Mg-, ability to perform wet chemical analyses while ranging and Ca-sulfate. Soluble Fe-sulfate candidates can be over a wide variety of geological surfaces, materials, eliminated since any Fe2/3+ >10-4M would have been soil chemistries, and over the lifetime of a long term detected due to “poisoning” effects on the ISEs. Both mission. The sensor array and chemical analyses could K and Na were present at too low of concentration and be tailored to include parameters and substances of would have only accounted for a minor fraction of their high priority to scientific analyses, sample return, and respective mineral phases. The most likely sulfate the health of future human explorers. mineral phases in the soil are thus MgSO4 or CaSO4 (or Table 1. Most Likely Species in Solution & Soil [5]. a mixture of the two). Equilibrium calculations with Equilibrium Conc. in GWB show that addition of BaCl2, coupled with disso- Species Conc. in Martian Soil = Solution (mM) (wt %) lution of SO4 , would have only resulted in an increase 2+ 2+ of Mg and a decrease of Ca if a MgSO4 phase were CaCO3 (calcite) Saturated 3-5 (TEGA) added, as was observed during the sol 107 analysis. MgCO3 (magnesite) Saturated ≥1.8 (GWB) MgSO4 (epsomite) Dissociated 3.3 (GWB) The addition of soluble CaSO4 would have caused an − 2+ 2+ ClO4 2.5 0.6 increase in Ca and no change in Mg , which was not + Na 1.4 0.08 observed. This suggested that the major fraction of Cl− 0.40 0.04 SO 2- was added as a MgSO phase. The titration re- K+ 0.40 0.04 4 4 2+ Mg 6.4 – sults and equilibrium modeling also allowed a more 2− SO4 3.9 – − accurate assessment of other species and their concen- HCO3 5.4 – trations. Table 1 shows the most likely composition of MgSO4(aq) 1.2 – 2+ the martian soil at the Phoenix landing site. The level Ca 0.75 – 0.17 – of dominant salts also has a direct bearing on the ques- CaSO4(aq) tion of whether, under appropriate conditions, water References: [1] Smith P.
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