By simple visual observation, it is known that volcanoes exist on Mars. Lightning (electrical discharge) has been observed in the ash plumes of erupting terrestrial volcanoes [e.g., Anderson et al., 1965; Hobbs et al., 1981]. However, direct measurements of the electrical properties of these lightning strokes are extremely difficult and dangerous to make. Additionally, there have been no observations of volcanic activity on Mars in recorded history. While geologic features have shown that eruptions have occurred on Mars in the past, and electrical discharges associated with these eruptions may have contributed to the global electric circuit, electrical discharges associated with volcanic eruptions are not a possible generator driving the Martian global electric circuit at the present time.
Some of the most recent results from the Mars Pathfinder mission may hold some clues pointing to a possible current source. Schofield et al. [1997] reported observations of changes in pressure, temperature, and wind direction which indicate the passage of a convective, small-scale vortex, or "dust devil," over the Mars Pathfinder location. They report a readily decreasing atmospheric pressure in conjunction with rotating wind directions, and an abrupt change in wind direction coincident with a minimum in pressure. This period also coincides with a temporary decrease in the power generation by the solar panels attributed to obscuring dust carried by the dust devil. Unfortunately, the cameras were not operational as the dust devil past the site.
A similar report was made by Ryan and Lucich [1983] using data from Viking Landers 1 and 2. They looked at records covering an extended time interval and found that Martian dust devils occur most commonly during the northern hemisphere spring and summer (both landers were located in the northern hemisphere). They also reported that the core diameters of the dust devils observed ranged from 10 m to nearly 1 km and had wind speeds of a few tens of meters per second. Dust devils were found to occur during the warmest part of the Martian day when the lower most atmosphere is convectively unstable.
Using Viking Orbiter images, Thomas and Gierasch [1985] looked at the optical properties of Martian dust devils. Assuming a particle size of 10 micrometers and particle density on the order of 3 g/cm3, they calculated that a typical Martian dust devil 2 km high with a diameter of 200 m contained approximately 3000 kg of dust, or, equivalently, 2 x 1015 dust particles. Additional surveys of Viking orbiter high resolution images have found about 100 dust devils with funnel diameters up to 1 km and heights up to 6.8 km [Lewis, 1995].
Measurements of terrestrial vortex phenomena may help us understand what these observations may imply about the electrical environment of Mars. Frier [1960] was one of the first to measure the electrical properties of dust devils in the Sahara Desert. Using a field mill, he recorded a decrease in the potential gradient of 540 V/m, i.e., the measured electric field had a value of a few hundred V/m oppositely directed from the fair weather electric field, as the dust devil passed within a few hundred meters of the observation site. He thus interpreted the charge structure of the dust devil to be opposite that of a thunderstorm, namely, a negative charge aloft above a positive charge at lower altitudes. Assuming this dipolar structure, Frier [1960] calculated an electric dipole moment of 5.7 x 10-3 C/m based upon the strength of the electric field, the height of the dust devil, and his distance away from it. However, this calculated dipole moment could not reproduce the time evolution of the changes in the observed electric field unless he adjusted the transverse velocity of the dust devil to be significantly lower that the 4 m/s he estimated by eye. One possibility for this discrepancy that he suggested was that the dipole moment was changing as the dust devil moved.
Later airborne and ground based observations by Bradley and Semonin [1963] and Crozier [1964] disagreed with Frier's [1960] interpretations. During the spring and summer months of 1962, Bradley and Semonin [1963] flew aircraft through several large dust whirls in east central Illinois. All of the days in which they recorded dust devils were dry and dusty. They found negative charge aloft at ~150 m altitude associated with the dust devils, and a change in the vertical electric field by as much as 1 kV/m. Neither the airborne observations of Bradley and Semonin [1963] nor the ground based measurements of Crozier [1964] in the New Mexican desert showed evidence of a dipolar structure, only negative charge aloft. Based on their measurements, Bradley and Semonin [1963] estimated the space charge density to be between a few hundred to >103 electronic charges per cm3. Crozier's [1964] observations suggested a much higher space charge density, on the order of 106 charges per cm3.
Harris [1967] reported observations of the electrical properties of larger scale dust storms during the Harmattan Season in west Africa. During the Harmattan Season, which typically lasts from December to March in west Africa, the local atmosphere is dry and dusty and dust storms occur frequently. Harris [1967] reported a reversal of the fair weather electric field with a magnitude of up to 5 kV/m at midday associated with dust storm activity. The dust particles carried by the wind were basically quartz crystals (silica) with sizes ranging from 0.1 to 1 micrometers.
Kamra [1969, 1977] also observed kV/m reversals of the fair weather electric field associated with dust storms in India. Kamra [1969] also claims to have observed lightning in a dust storm. He also noted that the clay minerals and silica carried by the winds were negatively charged. Though his measurements did not indicate this, he postulated that a positive space charge may have been present at higher altitudes. However, if these dust storms are simply larger versions of Bradley and Semonin's [1964] dust whirls, then their airborne observations make Kamra's [1969] postulation of positive space charge located above the negatively charged dust unlikely.
Now that we have found an atmospheric phenomenon that occurs on Mars which has an analogue in Earth's environment, we need to develop a mechanism for electrifying these dust particles that will fit with the recorded observations. In 1950, Kunkel was investigating the electrification of dust particles upon dispersion into a cloud. He found that homogeneous dust mixtures when rapidly blown out of a small container (similar in mechanism to Millikan's oil drop experiment) had no net charge and that the sign of the charge on individual particles was not dependent upon particle size. However, when he repeated the experiment with a mixture of quartz (an insulator) and platinum (a metal), the quartz particles acquired a net negative charge. Consistently, when an insulator and metal were in contact and then rapidly separated by being blown out of the small vessel, the insulator acquired a net negative charge.
The mechanism by which this occurs is called triboelectrification and is more commonly referred to as "static electricity." Triboelectrification occurs when two dissimilar materials are placed into contact and then separated. Weak chemical bonds form when the surfaces touch. When the surfaces in contact are separated, the bonds rupture, and any asymmetrical bonds will tend to leave imbalanced charges behind [Beaty, 1995]. What determines which material will become positive and which will become negatively charged is their relative position on the triboelectric series. In agreement with Kunkel's [1950] findings, silicon is further toward the negative end of the triboelectric series than metals such as platinum [Beaty, 1995]. Therefore, when quartz (SiO3) is put into contact with platinum, and then the two materials are separated, quartz would be expected to acquire a net negative charge.
The triboelectric charging mechanism agrees with the findings of Harris [1967] and Kamra [1977] who both reported negatively charged silica dust grains in association with dust storms. Winds may blow these dust grains across the heterogeneous surface, allowing the grains to accumulate a net negative charge before convective motion moves them aloft. Similarly, once aloft, silica grains may collide with grains of other composition again acquiring a net negative charge while the other grains acquire a net positive charge. In order for a dust storm to be net negatively charged, quartz grains must out number grains of other composition, possibly due to a systematic variation in the size of dust grains accompanied by gravitational settling.
There is experimental evidence that triboelectric charging of dust can occur on Mars. Kolecki and Landis [1996] describe the charge state of a moving Sojourner Rover wheel in a Martian environmental chamber. Within a few revolutions in the simulated Martian environment, the wheel charged to nearly 350 V positive. Over time, the potential of the wheel relaxed to about 100 V. Kolecki and Landis [1996] also noted that the track behind the moving wheel became negatively charged. These results were explained in terms of triboelectric effects in the dust as it came into contact with and was compacted by the wheel. When contact between the metal wheel and insulating dust was broken, the dust gained a net negative charge. As a result of this work, discharge points were added to the Pathfinder rover antenna base as a precaution against electrostatic charging [Kolecki and Landis, 1996].
However, the global electric circuit of Mars, if such exists, would be largely devoid of Earth-made metal objects. Questions about Mars' global electric circuit must be answered with information about the Martian environment. On Mars, 43% of the surface material is made up of silicates in the form of quartz, SiO3 [Banin et al., 1991]. The second most abundant surface material is Fe2O3 (18%), followed by oxides of sulfur, aluminum, magnesium, and calcium (each less than 10%) [Banin at al., 1991]. Of these, the most abundant, SiO3, also gains the most negative charge when separated from contact with the others.
In order to determine the net charging of the dust, grain size as a function of composition becomes important. If SiO3 is the only surface mineral which forms tiny grains that are easily moved by winds, then Martian dust will charge negatively as it is blown across the surface. Collisions between SiO3 dust and SiO3 surface material should result in a zero net charge transfer [Kunkel, 1950], while SiO3 dust colliding with other surface minerals will transfer negative charge to the dust. Alternately, if SiO3 does not readily form small dust grains while the other surface materials do, the SiO3-depleted dust would charge positively while being blown across SiO3 surface material. If all of the surface minerals formed small dust grains in proportion equal to their surface concentrations, then there would be no net charge transfer to the wind blown dust.
The question cannot be solved without more direct measurements of the composition or electrical properties of Martian dust. In the absence of such data, we will again look to Earth and assume that similar processes occur on Mars. Nearly all of the terrestrial observations of dust devils and dust storms have indicated the presence of a negative charge aloft. Therefore, we will assume that this is the charge distribution found in Martian dust devils. This may not be the case due to varying surface composition, but this question cannot be adequately answered otherwise unless additional in situ measurements are made.
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Variation in Conductivity with Height
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The Martian Global Electric Circuit