**A WHEEL ELECTROMETER TO MEASURE ELECTROSTATIC FIELDS ON THE MARTIAN
SURFACE.** C.I. Calle^1^, C.R. Buhler^2^, J.G. Mantovani^3^, E.E.
Groop^1^, M.G. Buehler^4^, and A.W. Nowicki^5^ ^1^Electrostatics and
Materials Physics Laboratory, NASA Kennedy Space Center, YA-C2-T,
Kennedy Space Center, FL 32899; ^2^Swales Aerospace, YA-C2-T, Kennedy
Space Center, FL 32899; ^3^Florida Institute of Technology, 150 West
Boulevard, Melbourne, FL 32901; ^4^Jet Propulsion Laboratory, California
Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109;
^5^Dynacs Inc., Kennedy Space Center, FL 32899.

**Introduction:** Surface sand and dust electrification is a highly
probable phenomenon in the low pressure, low humidity environment of
Mars. Surface soil and dust particles on Mars may become
electrostatically charged due to incident UV radiation reaching the
surface. Recent experiments have shown \[1\] that this process can
electrify the soil and provide a photoelectron sheath that levitates
charged particles about one meter above the surface, a phenomenon which
was possibly seen on the Moon and that may also exist on Mars \[2\].
Contact charging may also occur due to collisions between wind-blown
dust particles and stationary surface particulate matter. It has been
suggested that the formation of soil agglomerates and sand dune
formation may be attributed to the electrostatic properties of the
Martian soil \[3\]. To date, there has been no experiment on the surface
of Mars to directly measure the amount of charge contained on the
surface dust and soil particles \[4\].

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. This technology could perhaps be applied to different types of
sensors that require the mobility provided by a rover's wheel.

**Instrument Design:** The Wheel Electrometer Sensor (WES) will consist
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 (Figure 1(a)). FIDO is an advanced vehicle that
is used in technology definition and field tests for future NASA Mars
Program.

There will be two basic types of sensors. The first type of sensor is
the Electric Field sensor (ELF) that will measure the regolith surface
charge density as the rover wheel rolls over the Martian surface. The
second type of sensor is the Triboelectric sensor (TRIBO) which will
measure the amount of electrical charge that develops on a wheel-mounted
insulator material through frictional contact as the rover wheel rolls
over the Martian regolith.

![](media/image1.wmf)

\(a\) (b)

Figure 1. The Wheel Electrometer System (WES) shown on the FIDO wheel.
(b) Cross-sectional view of the ELF and TRIBO modules. The ELF is
recessed to allow measurement of an undisturbed soil plug that has not
been compressed or rubbed by the wheel rim.

Figure 1 (a) 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 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 (Figure 1(b)).

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. Our studies of the electrostatic properties of Martian regolith
simulant JSC Mars-1 at NASA KSC indicate that the electrometer response
to rubbing an insulator over the simulant is significantly different for
different insulators \[5\]. 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.

**Preliminary Results:** A prototype of the WES with four TRIBO sensors
was built in our laboratory to test the concept in a simulated Martian
environment using JSC Mars-1 simulant soil \[6\] (Figure 2). 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/G-10, and Lexan disks of 2.0 cm in diameter and 0.7 cm thick.

Figure 3 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 Mars-1 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.

**Acknowledgments:** The authors would like to thank Nancy Zeitlin of
NASA Kennedy Space Center for project management support and Peter
Richiuso of NASA KSC for support with design and fabrication.

![](media/image2.png)

Figure 2. Prototype Wheel Electrometer System.

![](media/image3.wmf){width="3.0805555555555557in"
height="2.9604166666666667in"}

**Figure 3.** Charge generated on the four polymers capping the
electrometer sensors on the prototype WES.

**References:** \[1\]. Sickafoose, A.A., J.E. Colwell, M. Horányi, and
S. Robertson, "Dust Levitation in a Plasma Sheath Near a Surface" *LPS
XXXIII,* 1743 (2002). \[2\] Kolecki J. and M. Siebert, \"Overview of
Mars System-Environment Interactions,\" 27th AIAA Plasmadynamics and
Lasers Conference, New Orleans (1996). \[3\] Kolecki, J. and G.B
Hillard, Electrical and Chemical Interactions at Mars Workshop, *NASA
Conf. Publ. 10093*, 9 (1991). \[4\] Farrell, W.M., Kaiser, M.L., Desch,
M.D., Houser, J.G., Cummer, S.A., Wilt, D.M., and Landis, G.A.,
"Detecting Electrical Activity from Martian Dust Storms", *JGR*,
**104**, pp. 3795-3801 (1999). \[5\] Calle, C.I., C.R. Buhler, J.G.
Mantovani, E.E. Groop, M. D. Hogue, and A.W. Nowicki, "Experimental
Results of a Mission-Ready Triboelectric Device for Mars Robotic
Missions," *Proc. Electrostatics Society of America-Electrostatics
Society of Japan*, 106 (2002). \[6\] Allen, C., Jager, K., Morris, R.,
Lindstrom, D., Lindstrom, M., and Lockwood, J., "JSC Mars-1: A Martian
Soil Simulant" *Proc. Conf. American Soc. Civil Engineers*, Albuquerque,
469-476 (1998).