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Workshop on Mars 2001 2526.pdf

THE MARS ENVIRONMENTAL COMPATIBILITY ASSESSMENT (MECA) WET EXPERIMENT ON THE MARS ’01 LANDER S. M. Grannan1, M. Frant2, M. H. Hecht1, S. P. Kounaves3, K. Manatt1, T. P. Meloy4, W. T. Pike1, W. Schubert1, S. West2, X. Wen2 1NASA Jet Propulsion , 4800 Oak Grove Drive, Pasadena, CA, 91109 2Orion Research, Inc., 500 Cummings Center, Beverly, MA, 01915 3Department of Chemistry, TuftsUniversity, Medford, MA, 02155 4Mineral Processing Department, West Virginia University, Morgantown, WV, 26507

Introduction. The Mars Environmental chlorine-containing salts, presumed to be Compatibility Assessment (MECA) is an and ). Salts are formed by the instrument suite that will fly on the Mars following processes: Surveyor 2001 Lander Spacecraft. MECA is · the -based weathering of rocks; sponsored by the Human Exploration and · the action of volcanic gases; and Development of Space (HEDS) program and · biological activity. will evaluate potential hazards that the dust and soil of Mars might present to astronauts and Salts will therefore accumulate wherever the their equipment on a future human mission to drainage water evaporates, wherever volcanic Mars. Four elements constitute the integrated gases act upon the soil, and potentially in areas MECA payload: a microscopy station, patch where microbial activity is found. Characteristic plates, an electrometer, and the wet chemistry salts will be formed by each of the above laboratory (WCL). processes. An analysis of the salts present at a given location can potentially provide The WCL consists of four identical cells, each information on the geochemical history of Mars, of which will evaluate a sample of Martian soil and in particular the history of liquid water on in water to determine conductivity, pH, redox the planetary surface. potential, dissolved CO2 and O2 levels, and concentrations of many soluble including A single wet chemistry cell has a total mass of sodium, potassium, magnesium, , and ~600 g including an upper Actuator Assembly the halides. In addition, cyclic voltammetry will built by Starsys Research and a lower Soil be used to evaluate reversible and irreversible Analysis Beaker built by Orion Research, and oxidants present in the water/soil solution. will consume approximately 2-5 W of power Anodic stripping voltammetry will be used to during operation with a peak power requirement measure concentrations of trace metals including of 15 W. Each cell will evaluate a 1 cc soil lead, , and cadmium at ppb levels. sample combined with 30 cc of water using 25 Voltammetry is a general electrochemical sensors arrayed around the perimeter of the cell. technique that involves controlling the potential The primary analytical tool of the wet chemistry of an electrode while simultaneously measuring laboratory is the -selective electrode (ISE). the current flowing at that electrode. In addition, microelectrodes will be used to perform voltammetric analyses of the solution The WCL experiments will provide composition. All of the sensors are compact and information on the corrosivity and reactivity of rugged and are not subject to radiation damage. the Martian soil, as well as on soluble components of the soil which might be toxic to Actuator Assembly. The actuator assembly human explorers. They will also guide HEDS shown in Figure 1 is used to deliver (1) soil, (2) scientists in the development of high fidelity solution, and (3) a calibration pellet to the soil Martian soil simulants. In the process of analysis beaker as well as to (4) mix the acquiring information relevant to HEDS, the soil/water solution. WCL will assess the chemical composition and (1) A paraffin actuator with a 25% volumetric properties of the salts present in the Martian soil. expansion at T ~ 72 C is used to extend the sample drawer over a distance of 0.75” so Salts in the Martian soil. Based on results from the two Viking landers and Mars that the robot arm can deposit a 1 cc soil Pathfinder, the Mars surface soil appears to sample into the drawer. This actuator consist of ~10% salts (dominated by sulfur- and requires 13 -15 W for approximately 3 Workshop on Mars 2001 2526.pdf

MECA: S. M. Grannan et al.

minutes. When the power to the actuator is turned off, a spring is used to retract the Sensor (Qty) Type Measurement drawer and create a vacuum seal between Technique the inside of the unit and the Martian Conductivity 4-electrode, Conductimetric ambient. A brush is used to remove soil Cell planar chip for measuring particles from the edges of the drawer. total ionic (2) A second paraffin actuator is used to drive a content puncture needle through a burst disk at the pH (2) Polymer Ion Selective base of the water tank. The over-pressure in membrane Electrode (ISE) the tank relative to the pressure in the base (Potentiometric) of the unit drives 30 cc of water into the pH Iridium ISE analysis chamber. This actuator requires dioxide (Potentiometric) 13-15 W for ~ 2 minutes. Membrane- 3-Electrode, Cyclic (3) A third paraffin actuator is used to puncture Covered CV 0.25-mm Voltammetry a kapton disk and drop a calibrant pellet Electrode gold cathode (CV) for measuring O with a mass of 0.2 g into the soil/water 2 and other volatile solution. oxidants (4) A MicroMo 8 mm motor is used to drive a Platinum 1.0-mm ISE mixing paddle at 2 Hz in order to stir the Macro- disc (Potentiometric) soil/water solution. electrode for measuring redox potential Gold Macro- 0.25-mm Cyclic electrode disc Voltammetry (CV) for evaluating oxidants and reductants Gold Micro- Planar chip, Anodic Stripping electrode 512 10-mm Voltammetry Array (MEA) elements (ASV) for trace metal detection / Solid-state ISE pellet (Potentiometric) Cadmium “ ISE (2) “ ISE Bromide “ ISE Iodide “ ISE Lithium (3) Polymer ISE, used as membrane reference Figure 1. Actuator Assembly built by Starsys Sodium “ ISE Research for the Wet Chemistry Experiment. Potassium “ ISE Magnesium “ ISE Soil Analysis Beakers. Complementary to Calcium “ ISE the Viking experiments, the WCL analysis Ammonium “ ISE beakers will characterize the properties of the / “ ISE water/soil solution rather than evolved gases, perchlorate using a suite of potentiometric, voltammetric, Perchlorate or “ ISE and conductimetric sensors. These sensors will bicarbonate provide information on the toxicity and Carbon Membrane- ISE reactivity of soluble components of the Martian Dioxide covered gas soil in preparation for an eventual manned sensor mission to Mars. Table 1 summarizes the Table 1. Wet Chemistry Experiment sensors. sensors contained in each MECA Wet Chemistry Experiment analysis beaker.