MECA Thermal and Electrical Conductivity Probe (TECP)
The MECA instrument suite is a component of the Mars '07 Phoenix investigation, which will also return data from a Thermal and Evolved Gas Analyzer (TEGA), three cameras, and a meteorology suite (MET). Phoenix is motivated by the goals of (1) studying the history of water in all its phases, and (2) searching for habitable zones. Samples of surface and subsurface soil and ice will be delivered to MECA and TEGA from a trench excavated by a Robot Arm (RA), while MECA's Thermal and Electrical Conductivity Probe (TECP) will be deployed in soil and air by the Robot Arm. The Robot Arm Camera (RAC) will document the morphology of the trench walls, while the Surface Stereo Imager (SSI) and the Mars Descent Imager (MARDI) establish a geological context. Throughout the mission, MET will monitor polar weather and local water transport.
The MECA instrument suite is composed of an Atomic Force Microscope (AFM), a Thermal Electrical Conductivity Probe (TECP) and a Wet Chemistry Laboratory (WCL). This data set description catalog file will describe the TECP. A complete description of the TECP can be found in [ZENTETAL2008].
The scientific objectives of the TECP are:
To provide ground-truth for orbital surface thermal measurements and input parameters for thermal models by directly measuring the thermal properties of Martian regolith.
To measure the concentration and nature of water in martian regolith in solid, 'non-frozen,' liquid, and vapor states.
To determine changes in the reservoirs of water when soil is freshly exposed.
To characterize the movement of water in and out of the soil by measuring atmospheric humidity, temperature, and wind speed above the surface.
Calibration of the TECP instrument is discussed in the TECP Calibration Report, which can be found in the Calibration folder of the MECA Non-imaging data archive.
Operational Considerations and Operational Modes
An end-effector on the Phoenix Robotic Arm, the TECP is a probe of soil physical properties including temperature, thermal conductivity and diffusivity, electrical conductivity and permittivity, as well as atmospheric temperature, humidity, and wind speed. These measurements are made with four conical needles, three of which contain electrical heaters and thermometers, and a hygrometer sensor mounted separately in the body of the TECP.
Three of the four parallel needles contain a thermocouple and a heater. The two needle pairs are used as electrodes for regolith electrical properties measurements. The same needles also serve as heating elements and thermometers for regolith thermal properties and wind speed measurements. The needles can be inserted into the soil for thermal and electrical measurements or positioned above the surface for atmospheric temperature, and wind speed measurements. Regolith thermal properties (including temperature, thermal conductivity, thermal diffusivity, volumetric heat capacity, and thermal inertia) as well as wind speed are derived from the heating and cooling behavior of the needles before and after a known amount of heat is added. Regolith electrical properties, including electrical conductivity and dielectric permittivity, are measured with capacitance and resistance sensors coupled to the regolith through the sensing needles. Atmospheric water vapor concentration is measured with a calibrated capacitance hygrometer mounted near a temperature sensor on the TECP printed circuit board, but exposed to the atmosphere through a particulate filter.
The humidity sensor determines the capacitance of the thin film hygrometer, which is a calibrated function of the relative humidity at the film surface. By measuring the film temperature with the adjacent temperature sensor, the result can be converted to absolute humidity. Under the assumption that gradients in vapor pressure are small, external relative humidity can be determined by comparison of the TECP result with the MET temperature sensors.
Measurement electronics are contained in the body of the TECP, and include a 12 bit A/D converter, two phase detectors, three resistance bridges, and a digital shift register. The A/D converter and shift register communicate through a serial interface to an FPGA on the primary MECA control and measurement electronics (CME) board.
TECP will measure the temperature, thermal conductivity and volumetric heat capacity of the regolith. It will also detect and quantify the population of mobile H2O molecules in the regolith throughout the polar summer, by measuring the electrical conductivity of the regolith, as well as the dielectric permittivity. In the vapor phase, TECP is capable of measuring the atmospheric H2O vapor abundance, as well as augment the wind velocity measurements from the meteorology instrumentation. TECP is mounted near the end of the 2.3 m Robotic Arm, and can be placed either in the regolith material or held aloft in the atmosphere.
TECP thermal and electrical properties measurement quality depends on proper needle placement by the RA. Non-linear insertion, partial insertion, and lateral movement all affect data quality negatively. Thermal properties measurements can also be negatively impacted by non steady state thermal conditions, and the TECP should therefore be allowed to equilibrate to its thermal environment before making thermal properties measurements.
There are no requirements for TECP to measure wind velocity. However, application of a protocol similar to the thermal properties experiments has shown some promise in characterizing wind speeds, particularly in high-wind regimes where the wind-sock associated with the Meteorology package is not capable of measuring wind as well. The procedure involves heating needle one for a period up to 70 seconds, and then following the cooling curve for another 90 seconds. Preliminary analysis of the heating and cooling data acquired at Mars-like pressures in a wind tunnel at the University of Michigan suggest that both peak temperatures and the cooling time constant are sensitive to wind velocity. Characterization of the TECP response to wind velocity is ongoing, and the determination of whether or not to try wind velocity measurements on the Martian surface will be made once those analyses are complete.