Microscopy, Electrochemistry, and Conductivity Analyzer
 

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Renamed the Microscopy, Electrochemistry, and Conductivity Analyzer (MECA) for Phoenix, the Mars Environmental Compatibility Assessment instrument was delivered to the MSP'01 project and then kept in bonded storage at JPL. The MECA package is comprised three instruments: the Wet Chemistry Laboratory(WCL), the Optical (OM) and Atomic Force (AFM) Microscopes, and a Thermal and Electrical Conductivity Probe (TECP). The MECA box contains the four WCL cells and the two microscopes. The TECP is mounted on the robotic arm. The chute (front left) provides soil to the microscope station; the right four openings provide soil to the WCL. A block diagram of the MECA package is shown below.

 

MECA block diagram


 

Wet Chemistry Laboratory (WCL)

 



The WCL has four single-use modules 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. Each beaker assembly (Thermo Electron) is instrumented with an array of ion selective electrodes (ISE) and electrochemically-based sensors. This array of sensors allows for the determination of a wide variety of inorganic anions, cations, selected heavy metals (via ASV), and electrochemical parameters, including:

 

  • Conductivity
  • pH (3 sensors)
  • Cl- (2 sensors)
  • Br- (2 sensors)
  • I- (2 sensors)
  • NO3-
  • SO4= (using Ba ISE)
  • K+
  • Ca2+
  • Mg2+
  • NH4+
  • Na+
  • Pb/Cu/Cd/Zn/Fe (ASV)
  • Cyclic Voltammetry
  • ORP (redox potential)
  • Temperature
  • Li Reference (3 sensors)

Each WCL actuator assembly (Starsys Research) includes a tank containing 30 ml of a calibration/leaching solution, and a sample loading drawer which will hold approx. 1 cm3 of soil. As the door closes, a brush removes excess soil, and a coarse screen rejects particles larger than 3 mm. Completing the assembly is a stirrer and a dispenser that releases a reagent pellet for calibrating the solution. In addition to being flight qualified for the MSP'01 mission, the WCL has been rigorously and successfully tested after being frozen for over 18 months, thawed and refrozen dozens of times, and after sitting on the shelf for over a year at room temperature.[1] The figures below show the upper dispenser/stirrer unit (left) and the lower beaker assembly containing the sensor array (right).

To operate the WCL, the water reservoir is heated to melting before puncturing a metal seal. Head pressure forces the water from the tank to the beaker. After sensor calibration at 0C, the RA places soil in the sliding drawer collector, the drawer is closed, and the chamber and cell are resealed. The temperature in-side the WCL is then raised to 250.5C. The sensors are read immediately and then, as the stirrer agitates the solution, serially at intervals.

The WCL was developed at NASA's Jet Propulsion Laboratory (Pasedena CA) in collaboration with Thermo Electron (Beverly, MA), Starsys Research (Boulder, CO), and Tufts University, (Medford MA).

____________________

 

[1] "Mars Surveyor Program '01 Mars Environmental Compatibility Assessment wet chemistry lab: A sensor array for chemical analysis of the Martian soil", S. P. Kounaves, S. R. Lukow, B. P. Comeau, M. H. Hecht, S. M. Grannan-Feldman, K. Manatt, S. J. West, X. Wen, M. Frant, and T. Gillette, J. Geophys. Res., 2003, 108(E7), 5077-89 [Abstract] [Full Text PDF]

 

 


 

Optical And Atomic Force Microscopes

 

Illuminated by red, green, blue, and ultraviolet LEDs, the fixed-focus 6x optical microscope (4 m/pixel) represents an optimal trade between resolution and depth of field. Its 256512 CCD array acquires a 12-mm image, sharing the RAC electronics.

 

The AFM is contributed by a Swiss consortium led by the University of Neuchatel. Run by a dedicated microcontroller, the AFM, uses a micromachined tip array and a low-voltage electromagnetic scanner that can view a 40-m40-m area.

 



The fixed-focus, 6X optical microscope and AFM assembly.

Optical-microscope image. ................... AFM image.

To operate the microscope, samples are deposited from the RA to a segment of the wheel protruding from a horizontal slot in the MECA enclosure. A scraping blade removes excess material as the wheel is withdrawn into the enclosure. A sample stage rotates substrates from their horizontal load positions into their vertical imaging positions. The wheel on the sample stage is also used for focusing and AFM approach. The wheel holds sixty-nine 3-mm-diameter substrates (ten sets of six materials, and one utility set of nine tools and calibration standards). The substrates are designed to distinguish different adhesion mechanisms. They include magnets, sticky polymers, and "buckets" for bulk sampling. Several tools on the sample wheel service the AFM. A silicone pad is used to clean tips. Another tool cleaves irrecoverable tips to expose a fresh cantilever.


 

Thermal and Electrical Conductivity Probe (TECP)

TECP, adapted from the commercial KD-2 (Decagon Devices), replaces the original MECA electrometer. A four-pin probe determines electrical conductivity by a two-pin LC-approach and a redundant four-pin van der Pauw technique. Thermal conductivity is measured by a pulse-decay method using a heater and a thermocouple pair. The patch plate passively exposes 92 samples to the environment, to determine how dust adhesion depends on conductivity, electric and magnetic fields, texture, and other material properties. To operate the patch plate, the RA initially deploys a latch to expose the patches. After a period of exposure, the RA deposits soil and subsequently "jostles" the spring-loaded plate. RAC returns images after initial deployment; before, during, and after soil delivery; and at the conclusion of the mission.