MECA Wet Chemistry Laboratory (WCL)
The MECA instrument suite is a component of the Mars '07 Phoenix investigation, which will also return data from a Thermal and Evolved Gas Analyzer (TEGA), three cameras, and a meteorology suite (MET). Phoenix is motivated by the goals of (1) studying the history of water in all its phases, and (2) searching for habitable zones. Samples of surface and subsurface soil and ice will be delivered to MECA and TEGA from a trench excavated by a Robot Arm (RA), while MECA's Thermal and Electrical Conductivity Probe (TECP) will be deployed in soil and air by the Robot Arm. The Robot Arm Camera (RAC) will document the morphology of the trench walls, while the Surface Stereo Imager (SSI) and the Mars Descent Imager (MARDI) establish a geological context. Throughout the mission, MET will monitor polar weather and local water transport.
The MECA instrument suite is composed of an Atomic Force Microscope (AFM), a Thermal Electrical Conductivity Probe (TECP) and a Wet Chemistry Laboratory (WCL). This data set description catalog file will describe the WCL. A complete description of the WCL can be found in [KOUNAVESETAL2008].
As part of the overall Phoenix science goals, the specific objective of the WCL is to characterize the aqueous chemical properties of soil samples as delivered by the Lander's robotic arm from the surface and at least two depths. To fulfill this goal, each WCL cell will determine:
hydrogen ion activity (pH),
redox potential (Eh/ORP),
solution electrical conductivity (EC),
concentration of selected soluble inorganic ionic species, and
possible redox couples and/or electrochemically mediated reactions.
Calibration of the WCL instrument is discussed in the WCL Calibration Report, which can be found in the Calibration folder of the MECA Non-imaging data archive.
Most WCL sensors will have three separate calibration steps prior to their use on Mars. Each individual electrochemical sensor was first calibrated prior to integration into the beaker by laboratory measurement in several standard solutions using commercial electronics. The second calibration was performed with the same solutions after beaker integration, using flight-like analog electronics and a laboratory digital controller. The exceptions were the bromide and iodide sensors, which could not be tested after integration without contaminating the chloride sensor. The final two-point calibration will occur on Mars. In general, the ion selective electrodes exhibit classic logarithmic Nernstian behavior over the specified measurement range. Error analysis indicated that the best results for calculating unknown sample concentrations are obtained by using the average slope obtained from all the preflight calibrations anchored at the TS21 point.
Operational Considerations and Operational Modes
MECA's wet chemistry laboratory (WCL) comprises four single-use modules, each consisting of a beaker assembly and an actuator assembly. The modules mix soil samples with a leaching solution in a pressure vessel for electrochemical analysis. The scientific objective of the WCL is to determine the total pH, redox properties, and concentration of the principal aqueously solvated components of the acquired soil samples.
Chemical data is returned by 26 distinct sensors, some redundant, lining the walls of each beaker. These measure: Temperature; pH (3); conductivity; oxidation-reduction potential; the anions chloride (2), bromide, and iodide; cations sodium, potassium, calcium, magnesium; and barium, used in a sulfate titration. Also included are electrodes for cyclic voltammetry, anodic stripping voltammetry, and chronopotentiometry (3). Lithium electrodes (2) are used as a reference relative to the known concentration of lithium salts in the solution. Sensors for nitrate, ammonium, dissolved oxygen and carbon dioxide, which for various reasons do not provide a quantitative measure of soil composition, are used only for context. A heater is imbedded in the base of the beaker to maintain water temperature during operation.
Each WCL actuator assembly (AA) includes a tank containing 26 ml of a calibration and leaching solution, a sample loading drawer with a capacity of ~1.0 cm3, temperature and pressure sensors, heaters, a stirring mechanism, and a device to dispense up to five small crucibles into the solution. The AA is responsible for soil, water, and solid reagent addition as well as stirring and two-zone internal heating (tank and drawer). Telemetry returned by the AA includes internal cell pressure, water storage tank and sampling drawer temperatures, and certain limit switch positions.
Pre-amplifier circuitry for the electrochemical sensors is embedded in the beaker walls. Analog to digital conversion (12-bit) is performed on a heavily-multiplexed Analog Board which interfaces to an FPGA on the primary MECA control and measurement electronics (CME) board. The FPGA also generates waveforms for the voltammetric and potentiometric sensors, performs temperature control, and operates actuators. Also returned in telemetry is a reading from an external temperature sensor located on the base of the microscopy sample stage.
Each WCL experiment lasts two days (Sol A and Sol B), not necessarily sequential. After an initial post-landing checkout, preparation for a chemical experiment starts with melting the frozen leaching/calibration solution in the storage tank and delivering it to the beaker by actuating a puncture mechanism. Sensors are calibrated in that solution, and then calibrated again after addition of a crucible containing small quantities of specific salts. The combined ion concentration from the initial solution and from the crucible, less than 10-4 molar for most ions, establishes the detection floor. The final step is to open the sampling drawer and receive a sample from the robotic arm. The total sample volume is estimated with an accuracy of 0.25 cc (maximum 1 cc) from images acquired by the robot arm camera. For the remainder of the day, the concentration of major anions and cations are monitored as well as key indicators such as pH, redox potential, conductivity, and cyclic voltammetry, stirring when appropriate and maintaining a constant temperature of 5C. At the end of the day the solution is allowed to freeze in the beaker.
A second day (Sol B) of measurement begins with thawing of the solution in the beaker, determining the sensor baseline, and adding an acid-containing crucible to lower the pH. Monitoring continues as on the first day. The final activity is the sequential addition of three crucibles of barium chloride. A sulfate titration is performed by monitoring the barium and chloride levels as the crucible contents slowly dissolve.