The case of the electric martian dust devils
NASA NEWS RELEASE
Posted: April 20, 2004
NASA and university researchers discovered dust devils on Earth have unexpectedly large electric fields, in excess of 4,000 volts per meter, and can generate magnetic fields as well. Like detectives chasing down a suspect, the scientists attached instruments to a truck and raced across deserts in Nevada (2000) and Arizona (2001). They drove through dust devils to get measurements as part of the Martian Atmosphere and Dust in the Optical and Radio (MATADOR) activity. The Arizona observations included a fixed base camp with a full suite of meteorological instruments.
Dust devils are like miniature tornadoes. They are about 10- to-100 meters wide with 20-to- 60 mph (32-to-96 kph) winds swirling around a hot column of rising air. "Dust devils are common on Mars, and NASA is interested in them as well as other phenomena as a possible nuisance or hazard to future human explorers," said Dr. William Farrell of NASA's Goddard Space Flight Center (GSFC) in Greenbelt, Md.
"If martian dust devils are highly electrified, as our research suggests, they might give rise to increased discharging or arcing in the low-pressure martian atmosphere, increased dust adhesion to space suits and equipment, and interference with radio communications," Farrell said. He is the lead author of the paper about this research published today in the Journal of Geophysical Research. "Complex tracks, generated by the large martian dust devils, are commonly found in many regions of Mars, and several dust devils have been photographed in the act of scouring the surface," said MATADOR Principal Investigator Dr. Peter Smith of the University of Arizona, Tucson, Ariz.
"These martian dust devils dwarf the five-to-10 meter terrestrial ones, can be greater than 500 meters in diameter and several thousand meters high. The track patterns are known to change from season to season, so these huge dust pipes must be a large factor in transporting dust and could be responsible for eroding landforms," Smith said. "Two ingredients, present on both Earth and Mars, are necessary for a dust devil to form: rising air and a source of rotation," said Dr. Nilton Renno of the University of Michigan, Ann Arbor, Mich., a member of the research team and expert in the fluid dynamics of dust devils. "Wind shear, such as a change in wind direction and speed with altitude, is the source for rotation. Stronger updrafts have the potential to produce stronger dust devils, and larger wind shear produces larger dust devils," Renno said.
Dust particles become electrified in dust devils, when they rub against each other as they are carried by the winds, transferring positive and negative electric charge the same way you build up static electricity if you shuffle across a carpet. Scientists thought there would not be a high-voltage, large-scale electric field in dust devils, because negatively charged particles would be evenly mixed with positively charged particles, so the overall electric charge in the dust devil would be in balance.
However, the team's observations indicate smaller particles become negatively charged, while larger particles become positively charged. Dust devil winds carry the small, negatively charged particles high into the air, while the heavier, positively charged particles remain near the base of the dust devil. This separation of charges produces the large-scale electric field, like the positive and negative terminals on a battery. Since the electrified particles are in motion, and a magnetic field is just the result of moving electric charges, the dust devil also generates a magnetic field.
If martian dust grains have a variety of sizes and compositions, dust devils on Mars should become electrified the same way as their particles rub against each other, according to the team. Martian dust storms, which can cover the entire planet, are also expected to be strong generators of electric fields. The team hopes to measure a large dust storm on Earth and have instruments to detect atmospheric electric and magnetic fields on future Mars landers.
The team includes researchers from NASA's GSFC, Glenn Research Center, Cleveland and Jet Propulsion Laboratory, Pasadena, Calif.; University of Arizona, Tucson; University of California, Berkeley; SETI Institute, Mountain View, Calif.; University of Washington, Seattle; University of Michigan, Ann Arbor; and Duke University, Durham, N.C.