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Microscopy, Electrochemistry, and Conductivity Analyzer (MECA)

built by the Jet Propulsion Laboratory

MECA characterizes the soil of Mars much like a gardener would test the soil in his or her yard. By dissolving small amounts of soil in water, the wet chemistry lab (WCL) determines the pH, the abundance of minerals such as magnesium and sodium cations or chloride, bromide and sulfate anions, as well as the conductivity and redox potential. Looking through a microscope, MECA examines the soil grains to help determine their origin and mineralogy. Needles stuck into the soil determine the water and ice content, and the ability of both heat and water vapor to penetrate the soil.

Two views of one of the four wet chemistry laboratories. Soil is delivered through the funnel in the foreground, and added to water which is stored in the shiny metal tank. The bottom section contains electrochemical seensors. Image credit: MECA Team/JPL.

MECA contains four single wet chemistry labs contains four single-use beakers, each of which can accept one sample of martian soil. Phoenix's RA will initiate each experiment by delivering a small soil sample to one beaker, which is ready and waiting with a pre-warmed and calibrated soaking solution. Alternating soaking, stirring, and measuring, the experiment continues until the end of the day. After freezing overnight and thawing the next morning, the experiment continues with the addition of four crucibles containing solid reagents. The first contains an acid to tease out carbonates and other constituents that are better dissolved in an acidic solution. The other three crucibles contain a reagent to test for sulfate.

The cartoon above shows an atomic force microscope chip (indicated as AFM-Chip) with 8 sensors and cantilevers. Only one at a time is used for imaging and if the tip becomes worn out or dirty, the cantilever can be broken off by a command from Earth.
Animated Version
Image Credit: University of Basel Institute of Physics.

The optical and atomic-force microscopes complement MECA's wet chemisty experiments. With images from these microscopes, scientists will examine the fine detail structure of soil and water ice samples. Detection of hydrous and clay minerals by these microscopes may indicate past liquid water in the martian arctic. The optical microscope will have a resolution of 4 microns per pixel, allowing detection of particles ranging from about 10 micrometers up to the size of the field of view (about 1 millimeter by 2 millimeters). Red, green, blue, and ultraviolet LEDs will illuminate samples in differing color combinations to enhance the soil and water-ice structure and texture at these scales. The atomic force microscope will provide sample images down to 10 nanometers - the smallest scale ever examined on Mars. Using its sensors, the AFM creates a very small-scale "topographic" map showing the detailed structure of soil and ice grains.

The thermal and electrical conductivity probe (TCP) with four metal sensing needles mounted in a plastic head. Image credit: MECA Team/JPL.

Prior to observation by each of the microscopes, samples are delivered by the RA to a wheel containing sixty-nine different substrates. The substrates are designed to distinguish between different adhesion mechanisms and include magnets, sticky polymers, and "buckets" for bulk sampling. The wheel is rotated allowing different substrate-sample interactions to be examined by the microscopes.

The thermal and electrical conductivity probe (TCP) as mounted on the robotic arm.
Image credit: MECA Team/JPL

MECA's final instrument, the thermal and electrical conductivity probe, will be attached at the "knuckle" of the RA. The probe will probably consist of three small spikes that will be inserted into the ends of an excavated trench. In addition to measuring temperature, the probe will measure thermal properties of the soil that affect how heat is transferred, providing scientists with better understanding of surface and atmospheric interactions. Using the same spikes, the electrical conductivity will be measured to indicate any transient wetness that might result from the excavation. Most likely, the thermal measurement will reflect ice content and the electrical, unfrozen water content.

The MECA team in the Alps near Neuchatel, Switzerland before a microscopy workshop.(Image Credit: Michael Hecht)

MECA is being built by a team at the Jet Propulsion Laboratory, led by Dr. Michael Hecht. The atomic force microscope is contributed by a Swiss consortium led by the University of Neuchatel, the Orion chemical beakers are being developed by Thermo Electron Corporation, the chemistry actuator assemblies are being built by Starsys Research Corporation, the thermal and electrical conductivity probes are being built by Decagon Devices, Inc., the microscope sample wheel is being designed by Transfer Engineering and Manufacturing, Inc., and the optical microscope is being designed by the University of Arizona. MECA was originally developed for the cancelled Mars Surveryor 2001 Lander mission.