A system of embedded sensors that can be incorporated into the wheel of any future mission rover would provide for a simple and fairly unobtrusive way to measure the distribution of electrostatic fields on the Martian surface and to measure variations in soil electrostatic response. The Wheel Electrometer Sensor (WES) consists of two types of sensor modules. These sensors will be attached just beneath the rover wheel in such a way that each sensor will be exposed to the Martian regolith either by line of sight through a small amount of Martian atmosphere or by direct contact with the regolith. For purposes of visualization, we will describe how the WES might be incorporated into one of the wheels of the Field Integrated Design & Operations (FIDO) rover shown below. FIDO is an advanced vehicle that is used in technology definition and field tests for future NASA Mars Program.

The figure on the left shows the FIDO wheel with the both the ELF and TRIBO sensor modules. The five TRIBO sensors utilize a different insulator material and are backed by independent miniature electrometers while the ELF is a bare sensor recessed back from the wheel surface. The ELF and TRIBO sensors are actually based on the same MECA technology. Each type of sensor simply measures the amount of charge that is induced on a metal electrode that has been exposed to some external distribution of electrostatic charge and has sensitivities that are achieved by adjusting circuit component values and the sensor area.

In the case of the ELF sensor, the source of the charge would be any charged soil particles that may be present on the Martian surface at the time the rover wheel rolls over it. The ELF sensor electrode will be recessed several centimeters radially inward from the outer surface of the wheel through a hole in the wheel. This will ensure that the ELF directly measures any naturally occurring charge that may be present on a small patch of undisturbed Martian regolith as the wheel rolls forward. The ELF will provide an output voltage that is directly proportional to the amount of charged regolith that the sensor "sees" through the hole. The regolith's surface charge density will be determined using the charge measurement and the known hole area. As the rover travels across the Martian surface, the local surface charge density will be mapped using the ELF measurements. These data will provide direct measurements of the presence of electrically charged particles on the Martian surface.

The TRIBO sensor module will have five independent sensors. The electronic circuitry for each sensor is identical, but a different insulator material will cover the electrometer sensor electrode of each sensor. As the rover wheel rolls over the Martian regolith, each of the five different insulators will make contact with the surface. The electrostatic response to contact charging of each insulator with the regolith will provide data regarding how the regolith fits into the triboelectric series.

A prototype of the WES with four TRIBO sensors was built in our laboratory (left) to test the concept in a simulated Martian environment using JSC Mars1 simulant soil. The prototype wheel is 12.7 cm in diameter and has a length of 10.3 cm. The four TRIBO sensors have a diameter of 1.84 cm with a concentric guard and a shield. The sensors are capped with Teflon, Lucite, Fiberglass/G10, and Lexan disks of 2.0 cm in diameter and 0.7 cm thick.
The figure below shows preliminary data obtained with the prototype WES in dry air at 9% relative humidity and at atmospheric pressure. The prototype wheel was rolled along a 60 cm tray containing JSC Mars1 Simulant. The four insulators acquire different electrostatic charges when in contact with this simulant. The sharp peaks observed in the graph are due to the initial contact with the soil. Repeated contacts show an increase in the charge exchanged between simulant and insulator. Several runs were taken prior to the one generating the data presented here. The insulators and simulant were exposed to an ionizer to neutralize their surface charges before this run but no cleaning was performed. Thus, this procedure is fairly close to an actual procedure that could be used on a flight instrument. Atmospheric ions would neutralize the insulators during long periods of rover inactivity.