Microscopy, electrochemistry, and conductivity analyzer
The Microscopy, Electrochemistry, and Conductivity Analyzer (MECA) is an instrument package originally designed for the canceled Mars Surveyor 2001 Lander mission. It consists of a wet chemistry lab (WCL), optical and atomic force microscopes, and a thermal and electrical conductivity probe. The Jet Propulsion Laboratory built MECA. A Swiss consortium led by the University of Neuchatel contributed the atomic force microscope.
Using MECA, researchers examined soil particles as small as 16 μm across; additionally, they attempted to determine the chemical composition of water-soluble ions in the soil. They also measured electrical and thermal conductivity of soil particles using a probe on the robotic arm scoop.
This instrument presents 6 of 69 sample holders to an opening in the MECA instrument to which the robotic arm delivers the samples and then brings the samples to the optical microscope and the atomic force microscope. Imperial College London provided the microscope sample substrates.
The optical microscope, designed by the University of Arizona, is capable of making images of the Martian regolith with a resolution of 256 pixels/mm or 16 micrometers/pixel. The field of view of the microscope is a 2 mm × 2 mm (0.079 in × 0.079 in) sample holder to which the robotic arm delivers the sample. The sample is illuminated either by 9 red, green and blue LEDs or by 3 LEDs emitting ultraviolet light. The electronics for the readout of the CCD chip are shared with the robotic arm camera which has an identical CCD chip.
The atomic force microscope has access to a small area of the sample delivered to the optical microscope. The instrument scans over the sample with one of 8 silicon crystal tips and measures the repulsion of the tip from the sample. The maximum resolution is 0.1 micrometres. A Swiss consortium led by the University of Neuchatel contributed the atomic force microscope.
The wet chemistry lab (WCL) sensor assembly and leaching solution were designed and built by Thermo Fisher Scientific. The WCL actuator assembly was designed and built by Starsys Research in Boulder, Colorado. Tufts University developed the reagent pellets, barium ISE, and ASV electrodes, and performed the preflight characterization of the sensor array.
The robotic arm scooped up some soil and put it in one of four wet chemistry lab cells, where water was added, and, while stirring, an array of electrochemical sensors measured a dozen dissolved ions such as sodium, magnesium, calcium, and sulfate that leached out from the soil into the water. This provided information on the biological compatibility of the soil, both for possible indigenous microbes and for possible future Earth visitors.
All of the four wet chemistry labs were identical, each containing 26 chemical sensors and a temperature sensor. The polymer Ion Selective Electrodes (ISE) were able to determine the concentration of ions by measuring the change in electric potential across their ion-selective membranes as a function of concentration. Two gas sensing electrodes for oxygen and carbon dioxide worked on the same principle but with gas-permeable membranes. A gold micro-electrode array was used for the cyclic voltammetry and anodic stripping voltammetry. Cyclic voltammetry is a method to study ions by applying a waveform of varying potential and measuring the current–voltage curve. Anodic stripping voltammetry first deposits the metal ions onto the gold electrode with an applied potential. After the potential is reversed, the current is measured while the metals are stripped off the electrode.
The MECA contains a Thermal and Electrical Conductivity Probe (TECP). The TECP, designed by Decagon Devices, has four probes that made the following measurements: Martian soil temperature, relative humidity, thermal conductivity, electrical conductivity, dielectric permittivity, wind speed, and atmospheric temperature.
Three of the four probes have tiny heating elements and temperature sensors inside them. One probe uses internal heating elements to send out a pulse of heat, recording the time the pulse is sent and monitoring the rate at which the heat is dissipated away from the probe. Adjacent needles sense when the heat pulse arrives. The speed that the heat travels away from the probe as well as the speed that it travels between probes allows scientists to measure thermal conductivity, specific heat (the ability of the regolith to conduct heat relative to its ability to store heat) and thermal diffusivity (the speed at which a thermal disturbance is propagated in the soil).
The probes also measured the dielectric permittivity and electrical conductivity, which can be used to calculate moisture and salinity of the regolith. Needles 1 and 2 work in conjunction to measure salts in the regolith, heat the soil to measure thermal properties (thermal conductivity, specific heat and thermal diffusivity) of the regolith, and measure soil temperature. Needles 3 and 4 measure liquid water in the regolith. Needle 4 is a reference thermometer for needles 1 and 2.
The TECP humidity sensor is a relative humidity sensor, so it must be coupled with a temperature sensor in order to measure absolute humidity. Both the relative humidity sensor and a temperature sensor are attached directly to the circuit board of the TECP and are, therefore, assumed to be at the same temperature.
