Experiments in a chamber under Martian-like conditions in a University of
Oregon lab suggest that small sparks may be triggered by friction under
normal atmospheric conditions.
Friction caused by dry Martian dust particles making contact with each other
may produce electrical discharge at the surface and in the planet’s
atmosphere, according University of Oregon researchers.
However, such sparks are likely to be small and pose little danger to future
robotic or human missions to the red planet, they report in a paper
published in the journal Icarus.
Viking landers in the 1970s and orbiters since then detected silts, clays,
wind-blown bedforms and dust devils on Mars, raising questions about
potential electrical activity.
Scientists have sought to determine experimentally if large electrical
storms and lightning were possible and whether static electricity generated
by particles of the planet’s mostly basaltic rock striking vehicles or,
eventually, visiting humans in protective gear would pose hazards.
Using volcanic ash as a stand-in for Martian dust, researchers in the lab of
UO volcanologist Josef Dufek found that electrical discharges in Martian
dust devils and storms are indeed possible. However, the discharges would
likely be small given weak electrical fields, close to 20 thousand volts per
meter, supported by the Martian atmosphere.
Earth’s atmosphere, by comparison, can withstand electrical fields reaching
3 megavolts per meter, producing spectacular thunderous lightning storms
common and sometimes deadly in the southeast United States, said Joshua
Méndez Harper, a research engineer in the Oregon Center for Volcanology in
the Department of Earth Sciences.
“Our experiments, and those of others before us, suggest that on Mars it is
easy to get sparks when you agitate sand or dust,” Méndez Harper said.
“However, it may be difficult, even in large dust storms or within dust
devils, to get very large discharges or conventional lightning because the
Martian atmosphere is bad at storing charge.”
Such anticipated triboelectric or frictional processes are experienced often
on Earth by way of socks sliding across carpeting and then touching a
doorknob or sticking a balloon on a window after rubbing it on human hair.
Martian dust devils, he said, may appear to sparkle, crackle, or faintly
glow as they roll across Mars’ desiccated landscape but with discharges
probably so small that they may not be visible except through detection of
their radio waves.
Previous experiments to determine if spark discharges could occur were
inconclusive because particles were swirled in a way that put them in
contact with the walls of the testing enclosures. Some experiments used
particles of materials not found on Mars. These contacts may have led to
charging not characteristic of a Martian dust storm.
“We set out to determine whether the sparks observed in previous works were
representative of Mars or merely experimental artifacts,” Méndez Harper
said.
At the UO, Méndez Harper, Dufek and George McDonald, a postdoctoral
researcher at Rutgers University, got around the wall-exposure limitation
using a vertical glass tube comparable in size to a water bottle measuring
some 4 inches in diameter and 8 inches in length.
They created triboelectric charging by colliding particles of basaltic ash
from Mexico’s Xitle volcanic eruption about 2,000 years ago.
Collisions in the sealed tubes occurred at frictional velocities expected to
occur during a light Martian breeze, without the particles touching the
outer walls and in a pressurized, atmospheric pressure of 8 millibars of
carbon dioxide, similar to that found on the Martian surface.
The Mexican basalt used in the project is similar to Martian basalt, as
detected by rovers in the Pathfinder and Mars Exploration Rover missions and
the dust analogs developed by NASA’s Jet Propulsion Laboratory.
As a comparison, the research team conducted experiments in which the
particles were allowed to make contact with surfaces foreign to anticipated
conditions on Mars. Sparks occurred in both sets of experiments, but the
addition of an artificial wall changed the polarity of the discharges.
“We were interested in pursuing this work because of the number of new
missions to Mars and the potential of constraining observations,” said
Dufek, a professor in the Department of Earth Sciences and director of the
Oregon Center for Volcanology. “Quantifying charging and discharging
behavior has a bearing on the transport of dust in the atmosphere and has
long been studied in relation to modulating chemical reactions, including
synthesizing organic compounds.”
NASA’s Mars mission that landed February 18 includes the Perseverance rover
and Ingenuity robotic helicopter.
The low energy of discharge on Mars as indicated by the new experiments
means these effects are unlikely to impact mechanical operations, Dufek
said.
Nevertheless, Jezero crater, the landing site for Perseverance, seems to
regularly experience dust storms in the autumn and winter. That, McDonald
said, may provide opportunities for rudimentary observations of
electrostatic phenomena.
One of the objectives of the Perseverance mission is to assess past
environmental conditions. Evidence for a more substantial atmosphere in the
past would have a bearing on the planet’s electrical environment and how it
has changed over time.
“The big takeaway from this study is that Mars may be an electrically active
place, although in ways quite different than the Earth,” Dufek said. “The
fact that analog Mars dust readily charges up to the point of discharge even
when grains did not rub against other surfaces suggests that future
colonists may find a world modified by static electricity in subtle ways.”
Reference:
Detection of spark discharges in an agitated Mars dust simulant isolated
from foreign surfaces by Joshua Méndez Harper, Josef Dufek and George D.
McDonald, 11 December 2020, ICARUS.
DOI: 10.1016/j.icarus.2020.114268
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