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 0°C, 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 25±0.5°C.
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 µm/pixel) represents an optimal
trade between resolution and depth of field. Its 256×512
CCD array acquires a 1×2-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-µm×40-µm 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.
