The Phoenix Wet Chemistry Laboratory

The 2007 Phoenix Mars Scout lander will acquire and analyze 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 regolith. It will also address 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 and interpret the chemical record, Phoenix carries with it four single-use independent wet chemistry cells. The Wet Chemistry Labs (WCL), part of the Microscopy, Electrochemistry, and Conductivity Analyzer (MECA) package, each consists of a lower "beaker" containing a set of chemical sensors designed to analyze the chemical properties of the regolith and an upper "actuator", for adding soil, water, reagents, and stirring. The beaker contains an array of sensors consisting of solid state and PVC-membrane based ion selective electrodes (ISE) that will analyze for inorganic anions and cations such as Ca²⁺, Mg²⁺, K⁺, Na⁺, NH₄⁺, Cl^-, Br^-, I^-, NO₃^-, ClO₄^-, and SO₄⁼. The array also has special electrodes for pH, conductivity, and oxidation-reduction potential (Eh). Also included is 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 consists of a sealed, Teflon-coated, titanium leaching solution reservoir (water plus ionic species for initial sensor calibration), a 1 cc soil sample drawer designed to receive the soil through a screened funnel from the robotic arm, then remove excess soil, and deposit it into the beaker; a stirrer motor with impeller; and a reagent dispenser that holds five crucibles consisting of a second calibration reagent, an acid, and three packed with barium chloride for determination of sulfate. The WCL sensors and analytical procedures have been calibrated and are being characterized using a variety of standard solutions, geological Earth samples, Mars simulants, and cuttings from a Martian meteorite. Sensor response characteristics such as limits of detection, interferences, and other constraints imposed by the Martian environment are also being studied. A sensor response library is being developed to aid in the interpretation of the data. The data from the WCL analyses will provide a variety of chemical and physical parameters to help us better understand the geochemistry of Mars, its biopotential, and its history.