Paschen Breakdown in the Martian Atmosphere
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.
Although the Martian atmosphere is not conducive to lightning generation,
it is widely believed that electrical discharge in the form of a corona occurs.
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 CO2, 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. Here,
we present the first experimental results of a Paschen curve for a Mars gas
mixture compared with 100% pure CO2.
Voltages of either positive or negative polarity were applied at one custom made brass electrode while the other was grounded . 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 parallelplate geometry.
The chamber was pumped down to 104 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% CO2 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. Measurements were taken by simply increasing the voltage until the power supply shuts off as a result of the current divergence during breakdown. The figure below shows the minimum voltage required for electrical breakdown of the two gases for both positive and negative polarities.
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 CO2 ionizes
negatively. A negative potential electrode releases electrons from the metal
that attach to the CO2 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 in books by von Engel and Cobine.
However, at lower pressures, a clear difference in breakdown potentials can be seen. The figure below 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 CO2 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 CO2. Instead the results seam to indicate that there may actually be a local maximum near the minimum in the CO2 curve. Once the pressure is lowered below 1 mm Hg (not shown), both the CO2 and Mars gas mixture rise again since there are no molecules to ionize in a hard vacuum.
It
is interesting to note that around Mars pressures, the blue spark seen at
higher pressures turns into a purpleblue corona. At pressures less than 2
mm Hg, there is no longer a welldefined 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 CO2.