The MECA wet chemistry laboratory on the 2007 Phoenix Mars Scout Lander

To analyze and interpret the chemical record, the 2007 Phoenix Mars Lander includes four wet chemistry cells. These Wet Chemistry Laboratories (WCLs), part of the Microscopy, Electrochemistry, and Conductivity Analyzer (MECA) package, each consist of a lower "beaker" containing sensors designed to analyze the chemical properties of the regolith and an upper "actuator assembly" for adding soil, water, reagents, and stirring. The beaker contains an array of sensors and electrodes that include six membranebased ion selective electrodes (ISE) to measure Ca²⁺, Mg²⁺, K ⁺, Na⁺, NO₃^-/ClO₄ ^-, and NH₄⁺; two ISEs for H⁺ (pH); a Ba²⁺ ISE for titrimetric determination of SO₄ ^2-; two Li+ ISEs as reference electrodes; three solid crystal pellet ISEs for Cl^-, Br^-, and I^-; an iridium oxide electrode for pH; a carbon ring electrode for conductivity; a Pt electrode for oxidation reduction potential (Eh); a Pt and two Ag electrodes for determination of Cl^-, Br^-, and I^- using chronopotentiometry (CP); a Au electrode for identifying redox couples using cyclic voltammetry (CV); and a Au microelectrode array that could be used for either CV or to indicate the presence of several heavy metals, including Cu²⁺, Cd²⁺, Pb²⁺, Fe^2/3+, and Hg²⁺ using anodic stripping voltammetry (ASV). The WCL sensors and analytical procedures have been calibrated and characterized using standard solutions, geological Earth samples, Mars simulants, and cuttings from a Martian meteorite. Sensor characteristics such as limits of detection, interferences, and implications of the Martian environment are also being studied. A sensor response library is being developed to aid in the interpretation of the data.