Geoscience Reference
In-Depth Information
5. Electric Discharges
Lightning have been detected on every planet with an atmosphere, except
Mars. On Saturn, Uranus, and Neptune, atmospheric electric activity has
been recorded in the radio VLF band.
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Due to the high opacity of the Venus,
Saturn, Uranus, and Neptune atmospheres, which hamper the observation
of lightning, candidate planets to image atmospheric electrical discharges
are Mars and Jupiter.
5.1.
Observation of Mars lightning
Due to the prevalence of Martian dust devils and dust storms, an under-
standing of the underlying physics of electrical discharges in Martian
dust clouds is critical for future Mars exploratory missions. Mars low-
atmospheric pressure and arid, windy environment suggest that the dust
near the surface of Mars is even more susceptible to triboelectric charg-
ing than terrestrial dust. Electrical discharges on Mars should occur more
frequently but at lower intensities than those seen on Earth.
On the Earth, lightning discharges occur in association with desert sand-
storms and volcanic ash plumes. In these events, electric charge separation
begins when dust particles collide or brush past each other in the turbulent
air. In a Martian dust cloud the electric potential should remain near zero.
Variable winds and/or gravitational settling, however, may separate parti-
cles by size, building substantial electric fields within the cloud. When these
fields reach a critical value, a lightning discharge occurs. Dust storms have
been observed on Mars to develop and spread over the entire planet and may
last for months. Smaller storms are also known to occur and should provide
an ideal environment to search for Martian lightning. On Mars lightning
may be very different from terrestrial lightning due to the low-atmospheric
pressure. Some authors have suggested diffuse glows or flashes, filamentous
discharges, or small arcs.
The most likely candidate for the creation of electrostatic charges and
fields is triboelectric charging of dust, i.e., the friction between blown
dust particles and the ground (or between) dust particles with each
other. Terrestrial experience demonstrates that electric fields of 5-15 kV/m
are not uncommon during dust storms and dust devils.
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Olhoeft
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sug-
gests that Martian lightning will be a diffuse flash (similar to sum-
mer heat lightning). Because of Mars low-atmospheric density, electrical
discharges occur at lower electric potential than on Earth, and there-
fore, should be more frequent. The breakdown electric field on Mars is
