Electrically charged lunar dust near shadowed craters can
get lofted above the surface and jump over the shadowed region, bouncing
back and forth between sunlit areas on opposite sides, according to new
calculations by NASA scientists.
The research is being led by Michael Collier at NASA's Goddard Space
Flight Center, Greenbelt, Md., as part of the Dynamic Response of the
Environment At the Moon (DREAM) team in partnership with the NASA Lunar
Science Institute (NLSI), managed at NASA's Ames Research Center,
Moffett Field, Calif.
"The motion of an individual dust particle is like a pendulum or a
swing," says Collier. "We predict dust can swarm like bees around a hive
over partially shaded regions on the moon and other airless objects in
the solar system, such as asteroids. We found that this is a new class
of dust motion. It does not escape to space or bounce long distances as
predicted by others, but instead stays locally trapped, executing
oscillations over a shaded region of 1 to 10 meters (yards) in size.
These other trajectories are possible, but we now show a third new
motion that is possible." Collier is lead author of a paper on this
research published October 2012 in
Advances in Space Research.
This effect should be especially prominent during dusk and dawn,
according to the team, as regions become partially illuminated while
features like mountains and crater rims cast long shadows.
"The dust is an indicator of unusual surface electric fields," says
William Farrell of NASA Goddard, a co-author on the paper and lead of
the NLSI DREAM team. "In these shaded regions, the surface is negatively
charged compared to the sunlit regions. This creates a locally complex,
larger electric field with separate positively and negatively charged
regions, called a dipole field, over the shaded region. The dust
performed its swinging motion under the influence of this dipole. Such a
surface process occurring on the moon at the line where night
transitions to day, called the terminator, might also occur at small
bodies like asteroids. It might be a fundamental process occurring at
airless rocky bodies."
There is evidence that dust actually moves this way over the lunar
surface. "There are hints for this type of dust swarm in Surveyor
images. A twilight was observed over the landed platforms during dusk
and dawn. This was surprising at first because the moon does not have a
dense enough atmosphere to scatter light when the sun is below the
horizon. It was long considered to be light scattered from lifted dust.
This model suggests the dust is really leaping or swarming overtop a
large number of shaded regions that would exist along the lunar
dusk/dawn line, called the lunar terminator. It's a natural fit. Charged
lunar dust transport is also believed responsible for the Apollo 17
Lunar Ejecta and Meteorites (LEAM) experiment's observation of highly
charged dust near the terminator," adds Collier.
To our eyes, the moon has no apparent activity and seems dead.
However, because it has almost no atmosphere, the moon is exposed to the
solar wind, a thin stream of electrically conducting gas called plasma
blown off the surface of the sun at around a million miles per hour. The
effects of sunlight and the solar wind generate a bustle of unseen
commotion at the moon. On the day-lit side, sunlight knocks negatively
charged electrons off the surface, giving it a positive charge. On the
night side or in shadow, electrons from the solar wind rush in, giving
the surface a negative charge.
The exact mechanism for launching lunar dust is not uniquely known.
Micro-meteoroid impacts can transfer energy to the surface to launch
particulates. Also, a rough surface has small, localized concentrations
of electric fields that could lift dust electrostatically from the
surface. The pendulum motion then happens because sunlit areas on the
moon tend to get positively charged, while shaded areas become
negatively charged. Since like charges repel each other, a positively
charged dust grain in a sunlit area gets pushed away from the positively
charged surface. If there were no negatively charged area nearby, the
dust grain would rise straight up. However, since opposite charges
attract, the positively charged dust gets pulled toward the negatively
charged crater floor, bending its path over the crater. Dust launched
from the sunlit area with just the right speed will pass over the shaded
floor of the crater to the sunlit area on the other side, where the
positively charged surface there will reflect it back over the crater
again. When many particles do this, the model predicts there should be a
swarm or canopy of dust over the crater.
If there were no complications, the particle could continue to bounce
between sunlit areas on opposite sides of the crater indefinitely.
However, in reality, things like differences in crater rim height,
roughness on the crater floor, and interference from the solar wind that
weakens the electric field produced by the surface charges can alter
the particle's path. These perturbations cause the dust to eventually
either fall into the crater or be launched away. "This model provides a
natural explanation for the observation of dust ponds inside craters on
the asteroid Eros," says Collier.
"Calculating how these complications will affect the path of a dust
particle on the moon and around asteroids are good areas for future
research," says Collier. "Additionally, we're not sure how many
particles get charged and move like this -- is it something like one in a
thousand, one in a million, or one in a billion? We'd like to do more
studies to see how likely it is that a particle will behave this way.
Since most of the lunar surface is covered in dust, even one in a
billion would still be significant." The team is also planning on
examining Apollo-era images to evaluate possible evidence for dust
canopies over shadowed craters.
The team includes Collier, Farrell, and Timothy Stubbs, also at NASA Goddard. The research was funded by the NLSI.
For more information about the DREAM team visit:
http://ssed.gsfc.nasa.gov/dream/
NLSI is a virtual organization funded by NASA's Science Mission
Directorate and the Human Exploration Office in Washington, which
enables collaborative, interdisciplinary research in support of NASA
lunar science programs. The institute uses technology to bring
scientists together from around the world and is composed of
competitively selected U.S. teams and several international partners.
