Scientists have found clues dust devils on Mars might
have high-voltage electric fields, based on observations of
their terrestrial counterpart. This research supports the
vision for space exploration by helping to understand
challenges the martian environment presents to explorers,
both robotic and, eventually, human.
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. Images
and information about the research are available at:
