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Dust Gets A Charge In A Vacuum

Boulder - July 14, 2000 -
A small layer of dust suspended several feet above the moon's surface that was first photographed by the Lunar Surveyor spacecraft in the 1960s and later observed by Apollo astronauts has been a puzzle to some planetary scientists.

But a University of Colorado at Boulder research team is on the verge of explaining the odd dust phenomenon, which also may occur on asteroids, the rings of planets and even around spacecraft.

The new results can help explain dust transport on the surface of the moon, said CU-Boulder doctoral student and experiment team member Amanda Sickafoose. The research also may help scientists mitigate problems related to the contamination of spacecraft, scientific instruments and space suits by dust as NASA contemplates future missions to the moon and Mars.

Researchers have assumed the lunar dust levitation is caused by ultraviolet photons from the sun ejecting electrons from isolated grains of dust, giving each a positive charge. The same radiation also is thought to be knocking electrons off the moon's surface rocks, causing the electrons to bounce upward and negatively charge dust grains near the surface, said CU-Boulder physics Professor Scott Robertson.

The end result is thought to be that negatively charged dust particles fall back to the surface while a small belt of positively charged particles float several feet above the surface. The positively charged particles are thought to be held in place in a layer less than a foot across by gravitational forces pulling the particles toward the moon's surface and the repelling force of the positively charged moon.

The team from CU-Boulder's Laboratory for Atmospheric and Space Physics designed an experiment to confirm the first step of the theory that included dropping individual, pepper grain-sized particles of zinc, copper and graphite through an evacuated chamber illuminated by UV light from an arc lamp. The grains fell about a foot into a device known as a Faraday cup, which directly measured the electrical charge of each grain.

Each dust particle received roughly the amount of positive charge the research team expected, said Robertson. In a follow-up experiment, a zirconium plate was placed near the falling grains to simulate the effect of moon rocks emitting photoelectrons. That experiment produced negatively charged dust particles, as expected, said Sickafoose.

A paper on the subject was published in the June 26 issue of Physical Review Letters. The paper was authored by Sickafoose and LASP Research Associates Joshua Colwell, Mihaly Horanyi and Robertson.

"The original theory seemed to satisfy most planetary scientists when it was proposed some time ago," said Robertson, also a research associate at CU-Boulder's Laboratory for Atmospheric and Space Physics. "But no one seems to have bothered to do a rigorous experimental check."

The LASP team's final goal is to simulate the effect of floating dust by levitating particles over a substrate simulating the moon's surface in the laboratory, said Sickafoose, who is pursuing a doctorate in the astrophysical and planetary sciences department.

"By understanding how and why these dust particles charge, scientists can find ways to better protect telescope lenses, spacecraft instruments and astronauts from the negative effects of charged space dust," she said.

The experiments were devised after CU researchers modified an apparatus originally assembled at CU-Boulder by University of Northern Colorado faculty member Bob Walch several years ago. "CU-Boulder is one of the few places in the country that has the equipment necessary to carry out these type of experiments," said Sickafoose.

  • University of Colorado-Boulder

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