**ELECTRICAL BREAKDOWN IN A MARTIAN GAS MIXTURE.** C.R. Buhler^1^, C.I.
Calle^2^, and E. Nelson^3^. ^1^Swales Aerospace, YA-C2-T, Kennedy Space
Center, FL 32899; ^2^Electrostatics and Materials Physics Laboratory,
NASA Kennedy Space Center, YA-C2-T, Kennedy Space Center, FL 32899;
^3^University of Pennsylvania, 3451 Walnut Street, Philadelphia, PA
19104.

**Introduction:** The high probability for dust interactions during
Martian dust storms and dust devils combined with the cold, dry climate
of Mars most likely result in airborne dust that is highly charged. On
Earth, potential gradients up to 5 kV/m have been recorded and in some
cases resulted in lightning \[1\]. Although the Martian atmosphere is
not conducive to lightning generation, it is widely believed that
electrical discharge in the form of a corona occurs.

**Background:** In order to understand the breakdown of gases, Paschen
measurements are taken which relate the minimum potential required to
spark across a gap between two electrodes. The minimum potential is
plotted versus the pressure-distance value for electrodes of a given
geometry. For most gases, the potential decreases as the pressure
decreases. For CO~2~, the minimum in the curve happens to be at Mars
atmospheric pressures (5-7 mm Hg) for many distances and geometries.
However, a very small amount (\<0.1%) of mixing gases radically changes
the curve, as noted by Leach \[2\]. Here, we present the first
experimental results of a Paschen curve for a Mars gas mixture compared
with 100% pure CO~2~.

**Procedure:** The experiments were performed at the Electrostatics and
Material Physics Laboratory at the NASA Kennedy Space Center using a
high vacuum system. Voltages of either positive or negative polarity
were applied at one custom-made brass electrode while the other was
grounded (see Figure 1.). The cylindrical electrodes have a diameter of
5.059 cm (2 in.) with curved edges to eliminate edge effects that create
strong electric fields. Thus the electrodes fixed at 0.1 cm apart
provided the parallel-plate geometry.

##### 

Figure 1. Schematic showing the parallel-plate geometry of the brass
electrodes. High voltage was applied using both polarities.

The chamber was pumped down to 10^-4^ mm Hg and backfilled to \~ 50 mm
Hg with the atmospheric gas. It was pumped down again to remove residual
gases and contaminants. The gases that were used are 100% CO~2~ and a
Mars gas mixture provided by Praxair Inc. that consisted of 95.5% carbon
dioxide, 2.7% nitrogen, 1.6% argon, 0.13% oxygen and 0.07% carbon
monoxide.

**Results:** Measurements were taken by simply increasing the voltage
until the power supply shuts off as a result of the current divergence
during breakdown. Figure 2 below shows the minimum voltage required for
electrical breakdown of the two gases for both positive and negative
polarities.

![](media/image1.wmf){width="3.0805555555555557in"
height="2.9506944444444443in"}

**Figure 2.** Minimum voltage required for breakdown for both gases
using positive and negative polarities as a function of the pressure
times distance relationship. (The error bars are smaller than the
points.)

Clearly, as the pressure is lowered less voltage is required for
breakdown to occur for both gases. At the higher pressures the spark
potential does not seem to depend on the makeup of the gases but depends
strongly on the polarity of the voltage applied. This is mainly due to
the fact that CO~2~ ionizes negatively. A negative potential electrode
releases electrons from the metal that attach to the CO~2~ molecule
easily. These ionized molecules increase in number very rapidly and
eventually form a current path to ground. However, a positive electrode
does not release electrons and breakdown must occur using the more
massive and slower positive ions in the gas. An excellent review of
Paschen breakdown is given by von Engel \[3\] and Cobine \[4\].

However, at lower pressures, a clear difference in breakdown potentials
can be seen. Figure 3 shows how the small concentrations of nitrogen,
oxygen, argon and carbon monoxide greatly alter the Paschen curve for
pressures ranging between 1 and 10 mm Hg. We can see that the minimum in
the CO~2~ curve at around 4-5 mm Hg becomes shifted to 6-7 mm Hg once
the other gases are added. This suggests that even though breakdown
occurs easily at Mars pressures, it may not occur at the minimum in the
Paschen curve for CO~2~. Instead the results seam to indicate that there
may actually be a local maximum near the minimum in the CO~2~ curve.

Once the pressure is lowered below 1 mm Hg (not shown), both the CO~2~
and Mars gas mixture rise again since there are no molecules to ionize
in a hard vacuum.

![](media/image2.wmf){width="3.0805555555555557in"
height="2.941666666666667in"}

Figure 3. Minimum spark potential in the range 1-10 mm Hg for pure CO~2~
and the Mars gas mixture using negative polarity brass electrodes at 0.1
cm apart.

It is interesting to note that at around Mars pressures, the blue spark
seen at higher pressures turns into a purple-blue corona. At pressures
less than 2 mm Hg, there is no longer a well-defined spark between the
cylinders, instead there is only the corona breakdown to the gas around
the charged electrode.

It should be noted that Paschen curves are experimental and depend on
several factors such as gas content, pressure, distance, geometry, wind
velocity, electrode composition, etc. Thus true Paschen measurements
should only be performed on Mars. However, we have shown that
differences can be seen experimentally on Earth and that using a Mars
mixture gas should be more accurate than pure CO~2~.

**Discussion:** It is expected from such low values given by the Paschen
curve that discharge should occur very frequently on Mars. Even though
discharge in any form has not been detected on Mars, Eden and Vonnegut
\[5\] showed that sand in a low pressure CO~2~ environment can
triboelectrically charge above the Paschen limit and result in glow
discharges and sparks. In their experiment a small amount of sand (\~ 50
g) was placed in an evacuated flask and backfilled up to 10 mm Hg with
CO~2~. It was then shaken by hand in the dark to observe the low
intensity discharges. The dominant colors observed were blue and red in
their experiment.

We recently redid this experiment but replaced CO~2~ with the Mars gas
mixture and replaced the sand with JSC Mars-1 Martian simulant \[6\].
Dry simulant (baked at 180^o^C for several days) was placed inside the
evacuated flask under vacuum. After backfilling to 10 mm Hg, a stopper
was used to enclose it before being removed from the chamber. After a
few minutes in the dark room, the flask was shaken vigorously and
several discharges were observed. Approximately 90% of the sparks
observed were green and the rest were blue/purple. This was in stark
contrast to Eden and Vonnegut's result in which the colors observed from
red to blue.

**Acknowledgments:** The authors would like to thank Nancy Zeitlin of
NASA Kennedy Space Center for project management support.

**References:** \[1\]. Crozier, W. D., "The Electric Field of a New
Mexico Dust Devil", *JGR*, **69**, 5427 (1964). \[2\] Leach, R. N., Sand
and Dust on Mars, *NASA Conference Publication 10074*, 36 (1991). \[3\]
von Engel, A., *Ionized Gases*, American Institute of Physics, New York,
1994. \[4\] Cobine, J. D., *Gaseous Conductors: Theory and Engineering
Applications*, Dover, New York, 1958. \[5\] Eden, H. F. and B. Vonnegut,
"Electrical Breakdown Caused by Dust Motion in Low-Pressure Atmospheres:
Considerations for Mars", *Science*, **180**, 962-963 (1973). \[6\]
Allen, C., Jager, K., Morris, R., Lindstrom, D., Lindstrom, M., and
Lockwood, J., "JSC Mars-1: A Martian Soil Simulant" *Proceedings of the
Conference American Society of Civil Engineers*, Albuquerque, NM,
469-476 (1998).