EUGENE, Ore. -- Feb. 19, 2021 -- 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 online and scheduled to appear in the March 15 print issue of 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 Martin 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 Feb. 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."
The National Science Foundation funded the research through a grant to Dufek. Méndez Harper was supported by a Blue Waters Graduate Fellowship.
About Josef Dufek: https://pages.uoregon.edu/jdufek/
Department of Earth Sciences: https://earthsciences.uoregon.edu/
Oregon Center for Volcanology: https://pages.uoregon.edu/volcanologycenter/
About Blue Waters: https://bluewaters.ncsa.illinois.edu/blue-waters-overview