The robotic landing missions to Mars require a better understanding of the electrostatic response of the materials used in landing crafts and equipment when exposed to wind-blown dust or to surface dust and sand particles. MECA (Mars Environmental Compatibility Assessment) is the name of the science package that was slated to fly on the 2001 Marie Curie Mars Lander to study the properties of Mars for future human exploration. JPL in collaboration with the Electrostatics and Surface Physics Laboratory, developed an instrument for this package called the MECA Electrometer that was designed to characterize the electrostatic and triboelectric properties of the Martian soil. This instrument consists of five electrometer sensors covered with different insulators designed to make contact with the Martian soil. Due to the misshaps of the Mars Polar Lander and Mars Polar Orbiter, the landing part of this mission was cancelled and the MECA Electrometer never flew. However, a more advanced version of these sensors, embedded in a rover wheel, is currently being developed in our laboratory.
The MECA Electrometer (above) was designed primarily to investigate the electrostatic interaction between the surfaces of insulating materials and the soil on the surface of Mars in preparation for human exploration. The two openings shown above the five insulators in the electrometer photo are the local electric field sensor (ELF) on the left, and the ion gauge (IG) on the right. The five small circular patches shown are the five types of insulators. The insulating materials were selected for their use on previous space missions. The five insulators selected for the MECA Electrometer were: Fiberglass/Epoxy, Polycarbonate (known as LexanTM), Polytetraflouroethylene (TeflonTM), Rulon JTM, and Polymethylmethacrylate (LuciteTM or PMMA).
The five insulating materials of the tribroelectric sensor array are placed above metal electrodes that are connected to five independent electrometer circuits, one circuit for each type of insulator. The tribo sensors are housed inside a case made of titanium with a volume of ~50 cm3, a total mass of ~50 g, and power consumption of <250 mW. The electrometer, which will be attached to the robot arm, as shown below, is used to measure the triboelectrically-induced charge after the Triboelectric Sensor Array is rubbed through the Martian soil and pulled away from the surface. The electrometer will also measure the electric field strength above the Martian soil using the Electric-Field Sensor as well as atmospheric ion currents using the Ion Gauge.
The tribo sensor circuit has an output voltage, Vout, that is proportional to the electric charge that develops on the surface of the insulator. The circuit is shown schematically in the figure below. The overall gain of the tribo circuit (not shown) is 4. Thus, Vout = 4Q/C, where the fixed capacitor C = 1 nF. So, the charge Q on the surface of the insulator must be given by Q = Vout / (4x10e9 ). Hence, the tribo circuit's gain is 0.25 nC/V as measured at the output. If the A/D converter has a 2 mV per bit resolution, then the device can detect an amount of electric charge as small as 0.5 pC, which is numerically equivalent to 3.1x10e6 electrons or protons.
Testing the triboelectric sensor array involved measuring the degree to which the insulator surfaces became electrically charged when rubbed over Martian soil simulant (JSC Mars-1) at CO2 pressures and temperatures similar to those near the surface of Mars. The electrometer, which was kept electrically insulated, swings freely from a metal plate. This plate is connected to a pneumatic piston system that lifts and drops the assembly, as shown below. After 5 seconds of contact with the soil, the electrometer is slid along the surface under its own weight for 4-5 seconds. This is done by attaching the assembly to a motorized pulley system. Once the electrometer reaches the end of its motion, it is lifted off the surface and returns to the starting position . All of these movements are controlled by a LabView program which stores the electrometer position along with the data. Data was taken using the Parallax Basic Stamp IITM controlled by a PC laptop running LabViewTM.
Experiments were performed in a Martian pressure of 5-7 torr CO2 environment in a LabView controlled vacuum chamber. The JSC Mars-1 Martian regolith simulant was baked out for several days beforehand to remove as much moisture as possible. After the soil was placed inside the vacuum chamber, the system was evacuated below 1 torr and then backfilled with the atmospheric gas. The soil was placed in a grounded metal container that contained liquid N2 lines to cool the simulant. Thermocouples were then placed in contact with the soil to monitor its temperature.
The figure below shows a typical experimental data run in
which the soil was kept at room temperature but under Martian atmospheric
conditions. There is a background signal of ~30 mV before rubbing takes place.
While rubbing the polymers onto the simulant, a certain amount of charge is
deposited onto the sensors depending on the polymer material. In response
to being rubbed over the Martian simulant, Fiberglass and Lucite tend to charge
positively, Teflon and Rulon J tend to charge negatively, while Lexan does
not tend to charge very much. The peaks and dips in the figure correspond
to repeated movements of the electrometer.
At 18 seconds the electrometer is placed onto the soil simulant but no charge is detected due to simple contact. At 21 seconds the electrometer starts to be dragged over the simulant under its own weight for about 4 seconds. During this stage, charge, created by friction is exchanged between the polymer material and the soil. In many cases the particles adhere to the surface and the resulting charge layer is measured. Charge leakage into the bulk of the insulators may also be measured. However, the maximum amount of charge actually exchanged is not detected due to the charge double layer that is formed during contact.
Once the electrometer is lifted from the soil, charge separation of the double layer takes place. Now the total amount of charge on the surface of the insulator is no longer masked by nearly equal amounts of charge on the surface. Again, once the electrometer is placed in contact with the soil at 31 seconds, the double layer is again formed and the combined electric field from the two layers cancels. The remaining charge detected has either made its way into the bulk, or is simply still attached to the particles adhering to the polymer's surface. The difference in voltage between contact and non-contact is referred to as the step voltage.This process was repeated until the charge on the insulators remained unchanged. Note that only two contacts were needed to produce a significant change in the amount of charge on the surface.
Generally speaking, a triboelectric series could be formed and the placement of the regolith simulant could be added. From positive to negative, the following arrangement is conceived: Lucite, Fiberglass, Lexan, Mars-1 Regolith Simulant, Teflon, and Rulon J. A more detailed triboelectric series that also includes the simulant is shown below.